2

Table of Contents

What is Power Quality? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

The Healthcare Imaging System Environment:The Importance of Quality Diagnostic Imaging . . . . . . .4

Types of Imaging Systems.............................................................................................................................4X-Ray Imaging Systems..............................................................................................................................4Ultrasound Imaging Systems .....................................................................................................................8Computed Tomography (CT) Imaging Systems ........................................................................................10Magnetic Resonance Imaging (MRI) Systems .........................................................................................12Nuclear Imaging Systems.........................................................................................................................14

Why Do Imaging Systems Require Quality Power?.....................................................................................15Image Artifacts Caused by Poor Power Quality........................................................................................18

Imaging Systems: The Backbone for Diagnostic Services for Healthcare Facilities.....................................19

Power Quality for Support Equipment in Imaging System Suites: An Expanding Concern .......................20

Power Quality for Imaging Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21

Specifying Power Quality Performance for New Imaging System Installations ..........................................22Addressing Power Quality at the Installation Planning Stage ..................................................................23

Improving Power Quality for Existing Imaging Systems..............................................................................26What Are Imaging System Manufacturers Doing about Power Quality? .................................................26

Establishing a Partnership with Imaging System Manufacturers ................................................................31

System Compatibility Testing of Imaging Systems ......................................................................................32

The PQ Checklist: Planning, Purchasing, Installing, and Maintaining Imaging System Equipment.............33Planning for Additional Equipment ...........................................................................................................33Purchasing Additional Equipment .............................................................................................................34Installing Additional Equipment ................................................................................................................35Maintaining Equipment ............................................................................................................................35

Identifying Power Quality Problems in Imaging System Suites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35

Types of Audits ............................................................................................................................................36

Performing an Audit in Your Own Suite........................................................................................................36Assembling an Audit Plan.........................................................................................................................36Preparing for the Audit .............................................................................................................................37Conducting the Audit................................................................................................................................38Interpreting the Audit Results ..................................................................................................................40

Is My Imaging System Vulnerable to PQ Problems? Questions for the Facility Engineer, Imaging Department Director, and Imaging System Operators ..................................................................40

PQ Monitoring for Imaging Suites: Answering the Six Basic Questions.....................................................41

Common PQ Problems in Imaging Suites ...................................................................................................42Grounding.................................................................................................................................................43Transformers.............................................................................................................................................44Electrical Connections ..............................................................................................................................47

Meeting the Power Quality Challenges of Imaging System Suites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .47

Power Quality and the National Electrical Code...........................................................................................48

Impacts of Testing Emergency Power Systems on Imaging Systems.........................................................48

Power Conditioning for Imaging Systems....................................................................................................49

Financial Impacts of PQ Problems on Imaging Systems .............................................................................51

What is Power Quality?For utilities, customers, and their loads alike, power quality is the concept of powering and

grounding electronic equipment in a manner that is suitable to the operation of that equip-

ment. A brief reduction in voltage, as in the case of a voltage sag at the service entrance of a

hospital that reaches a branch circuit powering a diagnostic imaging system (DIS), does not

constitute a power quality problem for a healthcare provider. An imaging system may or may

not react to a sag. If the performance of the system is affected by the sag in any way to interrupt

the imaging process, or introduce artifacts into the images, then a power quality problem

occurred. The impact of a sag upon an imaging process may not be obvious at first, even if the

imaging process continued during the sag. Electrical disturbances such as sags and surges can

affect the bottom-line performance of either the imaging system manufacturer, the healthcare

provider, or the underwriter.

Power quality problems are caused by

the effects of electrical disturbances

combined with unfavorable equip-

ment immunity to disturbances.

Disturbances that alter imaging sys-

tem performance can originate inside

or outside a healthcare facility, or

inside or outside an imaging suite or

department. Facility power and

ground circuits that have no errors

and meet state and local codes can

provide very suitable paths for distur-

bances to reach imaging systems.

Wiring and grounding errors do not

have to be present for imaging system

power quality problems to occur.

Common wiring and grounding errors

only intensify electrical disturbances

making them more damaging to imag-

ing system components.

In power quality, the voltage supplied

to medical equipment includes three

properties: amplitude, frequency, and

waveform. The electrical disturbances

occur whenever any or all of these

properties vary from normal. Imaging

systems may be susceptible to

changes in amplitude, frequency, and

waveform.

Amplitude. Although the amplitude of

the voltage at the point of use (i.e.,

the facility electrical system-to-imag-

ing system interface) depends upon

3

the requirements of the equipment, voltage

should remain constant. To operate reliably,

some imaging systems require less than a

five-percent variation in voltage amplitude. A

minor decrease in voltage usually does not

damage imaging equipment but may cause

equipment to lock up and data to be scram-

bled or lost. An increase in voltage, especially

if combined with transients, may destroy

electronic components (e.g., power supplies

and amplifiers) and cause equipment to mal-

function.

Frequency. Although utility-supplied elec-

tricity rarely deviates from 60 hertz, transfer-

ring from utility power to emergency-genera-

tor power may cause a brief shift in frequen-

cy or slow drift as the generator operates.

However, frequency variations are more like-

ly to disturb imaging equipment than low-

powered medical equipment.

Waveform. The waveform of equipment volt-

age and current should be as sinusoidal as

possible. For example, non-sinusoidal volt-

ages can cause data to be lost in medical

imaging systems and other medical equip-

ment that depend upon data processing and

storage. Non-sinusoidal voltages can also

cause biomedical equipment such as ventila-

tors to malfunction, imaging systems to mal-

function, and lights in operating rooms to

flicker. Additionally, current drawn by typical

healthcare equipment may be distorted, pro-

ducing harmonic currents, which entails

other power quality problems such as the

overheating of neutral conductors and trans-

formers, possibly leading to widespread

equipment failures and even a risk of fire.

The Healthcare ImagingSystem Environment:The Importance ofQuality DiagnosticImagingImaging systems have grown to be very criti-

cal to patient care as they provide investiga-

tive and diagnostic devices for healthcare

professionals. Almost all healthcare facilities

have an imaging system in most available

modalities. The complex nature of imaging

systems requires that quality power be pro-

vided to their power distribution units (A

PDU in an imaging system interfaces the sys-

tem to facility power and distributes power

to other subsystems in the imaging system).

Imaging systems are becoming more complex

and powerful in the amount of detailed data

they gather from the patient. These increas-

ing complexities warrant the need for contin-

ued compatibility engineering studies of

imaging systems and improvements in power

quality to imaging suites.

Types of Imaging Systems

The various types of imaging systems used in

the healthcare industry are referred to as

modalities. Among the diagnostic imaging

systems in use today, X-ray and ultrasound

were among the first modalities used in

healthcare facilities. Computed tomography

(CT), magnetic resonance imaging (MRI),

nuclear imaging, and positron emission

tomography (PET), are four other common

modalities of imaging technologies in use

today.

X-Ray Imaging Systems

X-ray imaging systems produce X-ray images

of an object. In the healthcare industry, the

object is usually the part of the body under

investigation. Traditionally, X-ray images are

stored on a special film for viewing later by

medical staff. Many of today’s newer sophis-

ticated X-ray machines can place the images

4

Imaging

systems

have grown

to be very

critical to

patient care

in digital memory for viewing on video moni-

tors or sending patient data to other depart-

ments in a hospital or to another healthcare

facility.

X-ray equipment is a type of medical equip-

ment that requires a significant amount of

power, takes up a significant amount of

space, and includes many subsystems to

operate. An X-ray machine may be portable

or a permanent installation. X-rays that are

used in medical radiography are generated

and produced by electronic equipment that

requires many different internally generated

levels of AC and DC voltages. The simplified

block diagram shown here depicts the major

components of an X-ray system.

An X-ray machine uses an X-ray tube that

requires a controllable high DC voltage.

Other components of the X-ray machine may

include a patient-positioning system, a sys-

tem to position the X-ray source, and a film-

handling system. Portable X-ray machines

include a set of batteries with an internal

battery charger to allow the machine to be

operated in remote locations.

The purpose of an X-ray machine is to con-

vert electrical energy into X-ray (X-radiation)

energy as specified by the X-ray technician,

and to direct the X-rays toward the area of

interest within the patient. The power supply

converts the electrical energy provided by

the healthcare facility electrical system into

high-voltage power, which the X-ray genera-

tor requires to operate the X-ray tube. The

power supply converts AC electrical power

provided at 120, 208, 220, or up to 277 volts

into a waveform more suitable for the opera-

tion of the high-voltage X-ray generator. It

also provides other forms of power for the X-

ray tube and other components of the X-ray

system.

The high-voltage generator converts electri-

cal energy that the X-ray tube requires to

produce the X-ray beam. This is accom-

plished by imposing high voltage between

the negative cathode and the positive anode

of the tube. Whenever electrons are products

of the negatively charged cathode and accel-

erated toward the anode, they strike the posi-

tively charged target area of the anode, and

produce X-rays. Most of the energy used in

the production of X-rays is converted into

heat in the target. A small fraction is typically

converted into X-radiation.

The heart of any X-ray is the high-voltage

generator and the X-ray tube. The X-ray

machine requires many power electronic

components and systems. Before some exam-

ples of X-ray machines are discussed, some

additional information about generators and

tubes is necessary. The purpose of the X-ray

generator is to provide a controlled amount

of power to the X-ray tube that is required to

produce X-rays, and to supply power to aux-

iliary components of the X-ray system.

Without controlled power, X-radiation can-

not be controlled. Although a standard AC

receptacle may provide sufficient power for

some X-ray applications, such as mobile

radiography, dental and podiatry radiogra-

phy, and some mammography, this power

must be reshaped and precisely controlled

for use in X-ray machines.

With the exception of portable generators,

most X-ray generators are hard-wired directly

into the healthcare facility electrical system,

using 208 volts or higher, and most require a

three-phase voltage supply. This voltage is

used to produce the thousands of volts that

must be applied to operate the X-ray tube.

The ideal type of voltage to be applied to the

5

Typical Block Diagram of an X-Ray Machine

The heart of

any X-ray

is the

high-voltage

generator

and the

X-ray tube.

X-ray tube is a constant (zero ripple), or DC

voltage. The high-voltage generator is used to

generate this voltage.

The two principal components in a basic

generator are the transformer and the rectifi-

er. An auto-transformer is used in the first

stage to change the voltage that is applied to

a step-up transformer in the second stage by

a factor that is variable. The second principal

component of an X-ray generator is the

power rectifier that is used to rectify the

stepped up high voltage.

The X-ray tube acts as a rectifier, because it

allows current to flow only when the cathode

is negative and the anode is positive. In some

X-ray machines, during the first half-cycle of

the stepped up high voltage, the cathode is

negative with respect to the anode; this

allows current to flow and X-rays to be pro-

duced. During the second half cycle, the

cathode is positive and the anode is negative;

therefore, no current flows through the tube

and no X-rays are produced. This type of rec-

tification circuit is called a self-rectified X-

ray circuit. The illustration below shows this

circuit.

While this scenario demonstrates a simple

(and inexpensive) method of producing X-

rays, it is clearly inefficient because half of

the electrical power is not used. If the voltage

applied to the X-ray tube were a constant

voltage, with the cathode always negative

and the anode always positive, the efficiency

of X-ray production would be greatly

improved. Specialized power rectifier circuits

made possible with advancements in power

electronics are employed in modern X-ray

machines to bring about this improvement. A

voltage sag or momentary interruption in the

AC input to the X-ray machine may impact

the operation of the high-voltage generator.

If the disturbance occurred in the positive

half of the applied high-voltage, then the

production of the X-rays would be more

severely impacted.

In the next level of sophistication in X-ray

generator circuits, a high-voltage full-wave

rectifier is inserted between the secondary of

the high-voltage step-up transformer and the

X-ray tube. The illustration above shows the

full-wave rectifier X-ray circuit. In this circuit

diagram, electrons can only flow in one

direction through the rectifier as in a con-

ventional full-wave rectifier.

The use of this rectifier circuit allows X-rays

to be produced during both half-cycles of the

sinusoidal voltage that is input to the trans-

former. Because current flows and X-rays are

produced during both half-cycles, the effi-

ciency of the system is substantially

6

Self-Rectified X-Ray Circuit

Full-Wave Rectifier X-Ray Circuit

improved. While the full-wave circuit pro-

vides a substantial improvement in efficiency

by using both halves of the AC power cycle,

the resulting waveform is not at all close to

the constant voltage that is optimal for the

operation of the X-ray tube. The full-wave

rectifier circuit has 100 percent ripple. A dis-

turbance that reduces the input voltage here

would have a greater impact on the circuit

than on the self-rectified circuit.

The next improvement in X-ray generator

waveforms uses three-phase AC power. In

principle, three full-wave rectified circuits

are connected to three-phase AC power, with

the output of all circuits going to the X-ray

tube. The result is a waveform with six-pulses

per cycle (two for each half of the three phas-

es) and about 13 percent ripple.

A further enhancement of this design uses

two sets of three-phase circuits, slightly out

of phase with each other. The resulting three-

phase, twelve-pulse waveform has only about

3 percent ripple, and is extremely close to the

desired constant voltage. The illustration

(bottom left) shows the types of waveforms,

their respective ripple factors, and the kilo-

volt peak (kVp) waveform applied to the X-

ray tube.

Nearly all diagnostic X-ray generators manu-

factured prior to 1980 used the 60 Hz fre-

quency from the healthcare facility electrical

system to produce the kilovolt peak (kVp)

waveform applied to the X-ray tube. For sin-

gle-phase X-ray equipment, the kVp wave-

form varies as the 60 Hz frequency. For three-

phase X-ray equipment, the kVp waveform

varies at a frequency that is a combination of

three or more 60 Hz lines, separated from

each other by slight delays. The result is a

smoother kVp waveform.

The use of even higher frequencies to gener-

ate a very smooth kVp waveform would allow

the X-ray tube to operate at near optimal

performance. Advancements in power elec-

tronics have enabled X-ray equipment manu-

facturers to use higher frequencies to gener-

ate these waveforms. High frequencies are

generated by an electronic oscillator, which

are then applied to a power converter. The

use of advanced power electronics systems in

X-ray systems can increase their susceptibili-

ty to electrical disturbances; thus the need

for compatibility testing to determine where

immunity improvements are necessary.

With this type of X-ray rectification circuit,

manufacturers are able to build high-fre-

quency generators that are much smaller and

more cost-effective than single-phase or

three-phase generators and achieve tighter

regulation of the kVp waveform and the mil-

liamp tube current by the addition of closed-

loop, electronic feedback circuits. Stability of

closed-loop feedback circuits are mandatory

during an electrical disturbance upon the

input of an X-ray machine.

7

Types of Waveforms and Ripple Factors for X-Ray

Systems

The use of

advanced

power

electronics

systems in

X-ray systems

can increase

their

susceptibility

to electrical

disturbances

The frequencies used in high-frequency X-

ray generators are in the range of 3 kHz to as

high as 100 kHz. The efficiency of transform-

ers at these frequencies is much greater than

at 60 Hz. An important result is that the

high-tension transformer need not be as

large as a conventional single- or three-phase

transformer. Regulation of the kVp waveform

and tube current in high-frequency genera-

tors is accomplished by the use of a closed-

loop feedback circuit. The kVp feedback cir-

cuit senses the difference between the tube

kVp and the kVp set by the X-ray machine

operator. The result is a voltage that is pro-

portional to the difference. This voltage is

input to a voltage-controlled oscillator

(VCO). The VCO provides a correction to the

original frequency used to generate the kVp

and therefore can provide close regulation of

the selected kVp. The illustration below

shows the simplified schematic of a high-fre-

quency X-ray generator.

While there are some limitations to this

design, high-frequency generators are gener-

ally smaller, less expensive, and better regu-

lated than conventional designs. The

advancements made in generator design are

the result of advancements made in power

electronics. As a result, they are becoming

increasingly popular in many diagnostic

imaging applications. In order to reduce

operating costs associated with X-ray sys-

tems that use high-frequency generators,

compatibility engineering studies should be

carried out to determine their immunity to

common electrical disturbances.

Ultrasound Imaging Systems

When comparing different types of medical

equipment, especially those with similar

functions, it is important to describe some

basics of their operation.

Ultrasound equipment is often thought of as

imaging equipment. Although this is true, it

is classified as diagnostic equipment.

Sound is mechanical energy, and thus

requires a medium such as a gas, a liquid, or

a solid for propagation. In contrast, electro-

magnetic energy such as that used in mag-

netic resonance imaging (MRI) systems does

not require a medium in which to travel.

Another difference is that while the velocity

of an electromagnetic wave remains constant

when traveling through different media, the

velocity of a sound wave varies. Also, sound

cannot be transmitted through a vacuum as

can electromagnetic energy. Sound becomes

more directional in its travel, and the beam

narrower, as the frequency increases. The

beam may be reflected, refracted or transmit-

ted when crossing the boundary between two

media of different densities, and may be

attenuated or absorbed within the media.

The penetration of sound waves into a medi-

um becomes less as the frequency increases,

which is the opposite behavior of X-rays.

Ultrasound techniques work on the principal

of sound or pressure waves traveling through

a medium, causing the molecules of the

medium to vibrate. The molecules are alter-

nately compressed and expanded; thus the

motion is transmitted from molecule to mol-

ecule across the medium. The ultrasound

spectrum begins at 20,000 Hz, where the

audio spectrum ends. Diagnostic ultrasound,

however, is confined to the range from 50,000

8

High-Frequency X-Ray Generator

Hz to 25 MHz. The velocity at which the wave

travels through any medium is dependent

upon the compressibility of the medium.

Wave velocity and compressibility are

inversely proportional by the inverse of the

density of the medium. The opposite of com-

pressibility is hardness. Gases have the high-

est compressibility; liquids are next, followed

by solids. While sound waves of many differ-

ent types may be generated in solids, only

longitudinal waves are transmitted through

liquids. At any given frequency, the velocity

of a sound wave will increase as the wave

passes from a gas into a liquid into a solid.

The velocity of sound in body tissue is very

close to that for water, about 1,540 meters

per second. This is not surprising, because

the structure of body tissue is primarily liq-

uid.

Ultrasound scanners are types of electronic

medical equipment that require several sepa-

rately packaged electric systems to operate.

These systems include one or more transduc-

ers, a computer mainframe, the cathode ray

tube (CRT) display device (which may be

integral or separate from the mainframe), a

keyboard, a video recording device, an addi-

tional display device, and a printer. The

transducers are external devices that are

used to transmit and receive the ultrasonic

waves. Ultrasound scanners may also include

circuitry to allow monitoring of electrocar-

diograph (ECG) signals.

Most ultrasound scanners will contain one or

more combination power supplies because

DC source voltages with low-noise (derived

from linear power supplies) are required to

power sensitive analog circuits and because

DC source voltages with some switching

noise (derived from SMPSs) can be used to

power digital circuits. A combination power

supply consists of a switch-mode power sup-

ply and one or more linear power supplies

with their AC voltages derived from the same

power transformer. To power the mainframe,

an original equipment manufacturer (OEM)

combination power supply will more than

likely be used. To power each transducer, a

combination power supply manufactured by

the ultrasound scanner manufacturer will

more than likely be used. The circuitry in

both the mainframe and the transducers usu-

ally require several dual DC source voltages

and thus each power supply will contain sev-

eral outputs. If the CRT is internal to the

mainframe, an OEM DC-to-DC converter will

normally be used to convert a low DC voltage

from the mainframe SMPS to a high DC volt-

9

Surgical team discussing images captured using an ultrasound imaging system.

age that can be used to drive the CRT. The

block diagram above illustrates an example

of an ultrasound system.

In this example, the power management sys-

tem consists of the DC-to-DC buck convert-

ers, one boost converter, one low-voltage

drop-out (LDO) supply, a DC-to-DC con-

troller, a supply voltage supervisor circuit,

and a voltage reference. An electrical distur-

bance incident upon an ultrasound system

will be felt at the AC inputs of each of these

power supply systems. The response to a dis-

tinct sag, for example, will be different for

each supply. Integrating embedded solutions

into each supply will help assure that each

subsystem in the ultrasound machine

remains operative during the disturbance as

opposed to installing a whole-system UPS on

the AC input.

Computed Tomography (CT) Imaging

Systems

Computed tomography (CT) is a type of X-

ray radiographic imaging system that pro-

vides digitally-enhanced sectional views of

the patient. Tomography is a radiographic

technique that selects a level in the body and

blurs out structures above and below that

plane, leaving a clear image of the selected

anatomy. In a CT system, the motion of an X-

ray tube must be precisely controlled.

Electromechanical devices such as servos,

positioners, etc., are used to rotate and posi-

tion the complex X-ray tube in a CT scanner.

This is accomplished by moving the x-ray

tube in the opposite direction from the imag-

ing device around a stationary fulcrum defin-

ing the plane of interest. X-ray tube move-

ments can be linear, curvilinear, circular,

elliptical, figure eight, hypocycloidal, or

trispiral. CT is a type of tomography in which

transverse planes of tissue are swept by a

pinpoint radiographic beam and a computer-

ized analysis of the variance in absorption

produces a precise reconstructed image of

that area. This technique has a greater sensi-

tivity in showing the relationship of struc-

10

Block Diagram of an Ultrasound System

Embedded

solutions

will help

assure

that the

ultrasound

machine

remains

operative

during the

disturbance

tures than conventional radiography (X-ray

machines).

The reconstruction of the internal structure

of an object from its projections is neither

new nor novel. Thousands of solid-state X-

ray detectors measure the X-ray beams from

the patient. The data from the detectors is

digitized, processed, and analyzed by a large

computer system. There were many investi-

gators in the area of image reconstruction of

X-ray projections. However, it was not until

1970 that Godfrey Hounsfield, a British engi-

neer working for EMI Limited in England,

produced the first X-ray scanning machine,

the EMI brain scanner, which basic power

electronic components made possible. For

his work in the development of the CT scan-

ner, Hounsfield was awarded the Nobel Prize

for Medicine in 1979. It has been said that

there has been no comparable discovery of

this magnitude in radiology since Roentgen

discovered X-rays in 1895.

CT Scanning Techniques:

The Reconstruction Process

An ordinary X-ray system takes pictures by

passing X-rays through the body and detect-

ing them with photographic film. The differ-

ent tissue and body structure of the body

attenuate the X-ray beam differently and

thus vary the intensity of the X-radiation as a

function of location. These changes in inten-

sity are what the photographic film responds

to. The CT scanner, on the other hand, con-

sists of an X-ray tube, X-ray detectors with

photomultiplier tubes, A/D converters, com-

puter systems, and a video display as shown

in the figure below.

Power electronic components and systems

are used throughout the CT scanner. High-

range to very high-range AC-to-DC SMPSs

are embedded throughout the scanner to

provide power to the computer processor,

the data acquisition system (DAS), memory

storage devices and management systems,

and control systems. A high-voltage DC

power supply similar to the ones used in

modern X-ray systems are used to provide a

high voltage to the X-ray tube. A separate

high-voltage power supply is used to provide

power to operate the CRTs used in the CT

scanner. If a separate viewing console is

required, an additional SMPS is also used to

11

Simplified CT Scanner Block Diagram

power this device. Through the use of power

electronics and electromechanical devices,

the X-ray tube rotates around the patient to

transmit a series of X-ray beams through the

patient that are detected by thousands of

solid-state X-ray detectors. The detectors

measure the attenuation of the fan-shaped X-

ray beam by structures within the patient.

These detectors collect the information from

each projection. The information is then dig-

itized and analyzed by a computer to recon-

struct cross-sectional CT images.

Each power supply used in a CT scanner to

power one or more subsystems is exposed to

electrical disturbances from the healthcare

facility electrical system. Because each of

these power supplies are different designs

and have different loadings, they will

respond differently to common disturbances

such as voltage sags and momentary inter-

ruptions. If their compatibility with the pub-

lic power system and the healthcare facility’s

electrical environment is acceptable, then

each supply should continue operating to

some specified ride-through level. The ability

of the high-voltage generator to survive such

disturbances is critical to the life of the scan-

ner. Compatibility engineering studies can be

used to determine the response of each sup-

ply and its associated system to common dis-

turbances.

Magnetic Resonance Imaging (MRI)

Systems

Magnetic resonance imaging (MRI) is an

imaging technique used primarily in the

healthcare industry to produce high quality

images of the inside of the human body.

MRI is based on the principles of nuclear

magnetic resonance (NMR), a spectroscopic

technique used by scientists to obtain micro-

scopic chemical and physical information

about molecules. The technique was called

magnetic resonance imaging rather than

nuclear magnetic resonance imaging (NMRI)

because of the negative connotations associ-

ated with the word nuclear in the late 1970’s.

MRI started out as a tomographic imaging

technique; that is, it produced a thin-slice

image of the human body with NMR signals.

MRI has advanced beyond a tomographic

imaging technique to a volume imaging tech-

nique. This section presents an overview of

some of the electrical systems used in a typi-

cal MRI system.

Imaging is the production of a picture,

image, or shadow that represents the object

being investigated. MRI is a type of radiogra-

phy using electromagnetic energy. Certain

atomic nuclei with an odd number of neu-

trons, protons, or both are subjected to a

radio frequency pulse, causing them to

absorb and release energy. The resulting cur-

rent detected by a set of radio frequency coils

passes through a radio frequency receiver

and is then transformed electronically into

an image. This technique is valuable in pro-

viding soft-tissue images of the central nerv-

ous and musculoskeletal systems. Other MRI

techniques allow visualization of the vascular

system without the use of contrast agents.

However, agents are available for contrast

enhancement.

Hardware Overview

The following illustration depicts a schematic

representation of the major systems on a

magnetic resonance imager and a few of the

major interconnections. This overview briefly

states the function of each component.

The components of the imager are located in

a shielded room that protects the signal gen-

eration and detection circuits of the RF coil

system. Without a shielded room, electro-

magnetic noise from inside and outside the

hospital environment would corrupt the

images and introduce unwanted imaging

artifacts. Improper installation of metallic

subsystems that come in contact with the

shielded room and RF leaks in the shielded

room can also produce artifacts.

12

Each power

supply used in

a CT scanner

is exposed to

electrical

disturbances

from the

healthcare

facility

The magnet above and below the total mag-

net system produces the baseline magnetic

field (Bo) for the imaging procedure. Within

the magnet system are the gradient coils for

producing a gradient in this magnetic field

(Bo) in the x, y, and z directions. The power

for gradient coils is generated by the gradient

power supply and amplifier, which contains a

high-range DC-to-DC SMPS that powers elec-

tronic circuitry used to generate the current

for the gradient coils.

The bore of the MRI system, surrounded by

the magnet of the MRI system, where the

patient rests on a movable table, contains the

gradient coils and RF coils. Within the gradi-

ent coils is the radio-frequency (RF) coil. The

RF coil produces another magnetic field (B1)

necessary to rotate the magnetic spins by 90°

or 180°. The RF coil also detects the signal

from the spins that originate within the body.

The RF power for the RF coils is generated by

the RF power supply, which contains a very

high-range DC-to-DC SMPS that powers the

RF power transistors used to switch the RF

power to the RF coils. The main power supply

for the MRI system is provided via a power

distribution unit (PDU), the heart of the MRI

system with respect to power management.

The PDU provides power to the computer,

the magnet power supply, the gradient power

supply, the RF power supply, the digitizer, the

patient table, and the RF detectors. The PDU

is the first to see a disturbance and can allow

that disturbance to travel to other power

supplies within the MRI system.

The patient is positioned within the magnet

by a computer controlled patient table that

contains a DC stepper drive. The table has a

positioning accuracy of 1 mm, for example.

The scan room is surrounded by an electro-

magnetically shielded room, which also con-

fines the RF energy. The shield prevents the

high power RF pulses from radiating out

through the healthcare facility. It also pre-

vents external radiated emissions, such as

various RF signals generated by television

and radio station transmitters and radio

communications equipment from within the

facility, from being detected by the imager.

Some scan rooms are also surrounded by a

magnetic shield, which prevents the magnet-

ic field from extending too far into the

healthcare facility. In newer magnet systems,

the magnet shield is an integral part of the

magnet system.

On the data setup and processing side of the

MRI is the heart of the imager, a very large

13

Major Electrical System Components of a Typical MRI System

The PDU is

the first to

see a

disturbance

and can

allow that

disturbance to

travel to other

power

supplies

within the

MRI system.

computer system. It controls all of the com-

ponents on the imager. The RF components

under control of the computer are the RF sig-

nal generator, which generates the RF signals

and the RF pulse programmer. The generator

produces a sine wave of the desired RF fre-

quency. The pulse programmer reshapes the

RF pulses. The RF amplifier increases the

power of the pulses from milliwatts to kilo-

watts. The computer also controls the gradi-

ent pulse programmer, which sets the shape

and amplitude of each of the three gradient

magnetic fields. The gradient amplifier

increases the power of the gradient pulses to

a level sufficient to drive the gradient coils.

The array processor, located on some

imagers, is capable of performing a two-

dimensional Fourier transform in fractions of

a second. The computer off-loads the Fourier

transform to this faster device.

The operator of the imager gives input to the

computer through a control console. An

imaging sequence is selected and customized

from the console. The operator can see the

images on a video display located on the

console, can manage the memory storage of

the images, or can make hard copies of the

images on a film printer.

Nuclear Imaging Systems

Nuclear medicine can be defined as the prac-

tice of making patients radioactive for diag-

nostic and therapeutic purposes. The

radioactivity is injected intravenously,

rebreathed, or ingested. It is the

internal circulation of radioactive

material that distinguishes nuclear

medicine from diagnostic radiology

and radiation oncology in most of

its forms. The radioactivity is

detected from outside the body

without trauma to the patient.

Diagnostic nuclear medicine is suc-

cessful for two main reasons: (1) It

can rely on the use of very small

amounts of materials thus usually

not having any effect on the processes being

studied, and (2) The radionuclides being

used can penetrate tissue and be detected

outside the patient. Thus, the materials can

trace processes or “opacify” organs without

affecting their function.

Radionuclides that exit the patient must be

detected to be of use to the medical staff in

diagnosing a patient’s condition. Detection is

accomplished through the use of a variety of

radiation detectors. Imaging systems that

employ detectors are called sophisticated

imaging devices. Extensive research and

development has gone into the development

of radiation detectors with the goal of

improving the image.

Radiation detection is accomplished by

employing a transducer and associated elec-

tronics as shown in the figure below.

Radiation interacts in the transducer,

depositing energy by exciting or ionizing

transducer atoms. The transducer is physi-

cally coupled to an electronics stage, where

the subtle effect within the transducer is

converted into a measurable electronic sig-

nal. This signal is processed, analyzed, and

counted by the digital computer system

which is the main part of the nuclear imag-

ing system. The output from the electronics

stage is fed into a display or storage device

for interpretation.

The types of detectors encountered in

nuclear medicine are gas-filled detectors,

scintillation detectors, and semiconductor

14

Stages of Radiation Detection

detectors. Each one of these detectors

requires the use of a power supply system to

bias the detector electronics such that the

radiation may be detected. The power supply

also biases the detection stages such that the

electronic signal may be produced.

As an illustration of a detector used in scin-

tillation, consider the figure above. This dia-

gram shows a schematic of a photomultiplier

tube. Light photons entering the glass

entrance window impinge on the photocath-

ode. The result is emission of one electron

for approximately every five light photons.

The electron produced is accelerated toward

a dynode chain. The accelerated electron has

sufficient kinetic energy to liberate approxi-

mately five additional electrons when it

strikes each dynode. The effective electron

gain at the collecting anode is 106 to 108.

The output of the photomultiplier is a signal

with a specific characteristic shape whose

amplitude is proportional to the number of

photons entering the photomultiplier or the

energy deposited in the crystal.

In the figure above, the high-voltage power

supply biases the tube using a series of resis-

tors. The output at the anode is capacitively

coupled to a pre-amplifier where the signal is

prepared for processing and analysis. Similar

to the other imaging systems, each power

supply system in a nuclear imaging system is

exposed to the quality of power present with-

in the healthcare facility. Abrupt changes in

the input voltage may shorten the life of NMI

detectors and the electronic systems used to

bias and control them. Electrical distur-

bances incident upon a system may also alter

calibration settings. Compatibility engineer-

ing studies can be used to determine the

response of each supply and its associated

system to common disturbances.

Why Do Imaging Systems RequireQuality Power?

Diagnostic imaging systems (DISs) are a

compilation of many integrated subsystems

each of which is given life through the quali-

ty of power from the healthcare facility elec-

trical system. Each subsystem is designed for

a specific modality and integrated into over-

all imaging system designs. Integrating sub-

systems is necessary in imaging system

design because DISs are complex and image

processing intensive, and DIS manufacturers

can save development, design, and imple-

mentation costs by not re-inventing subsys-

tems that already exist. Their complexity and

data-intensiveness requires the use of vast

amounts of data processing power using

many central processing units (CPUs) and

memory storage devices (MSDs) among other

subsystems. Examples of other subsystems

that are critical to DISs include power sup-

plies, amplifiers, and electromechanical

devices.

Billions of data bits are collected and

processed each second to reconstruct high-

resolution images representative of organs,

tissues, cellular structures and other physio-

logical and anatomical parts of the human

15

Basic Diagram of a Photomultiplier Tube Used in Nuclear Medicine

Diagnostic

imaging

systems (DISs)

are a compi-

lation of

many

integrated

subsystems

body. DISs are used several times per hour in

a healthcare facility not only probe the body

slice by slice to display reconstructed images

for physicians, but also to store images in

MSDs that are a part of the overall imaging

system. Stored images are later used by the

physician who ordered the test and other

specialists to diagnose a patient’s condition.

More commonly now, images are transmitted

via patient data networks within a healthcare

complex and via the Internet to other geo-

graphical areas when they must be read by

other physicians.

Semiconductor devices play many very

important roles in all DISs. Power electronic,

small signal, and integrated circuit semicon-

ductor devices are used throughout all DIS

subsystems. Early X-ray machines and com-

puted axial tomography (CAT) are among the

few electrical/electronic systems that

required the use of high power vacuum X-ray

tubes to generate X-ray images. All early DISs

used more individual subsystems and

devices requiring lots of interconnecting

cables, equipment cabinets and floor space

in healthcare facilities. Although some basic

protection devices such as surge protectors

were starting to become available for end-

use equipment when early DISs were

designed, they were used on a limited basis

to protect DISs from electrical disturbances.

During the design of early DISs, active power

factor correction (PFC) techniques did not

yet exist, and designers did not use passive

techniques to address the highly non-linear

line input currents that DISs drew from the

electrical systems of healthcare facilities.

Similarly, designers used basic energy storage

components to achieve minimal filter

requirements in basic power supplies. The

lack of protection devices, current correction

techniques, and optimal energy storage in

DISs placed them at high exposure to shut-

downs and damage from non-destructive and

destructive electrical disturbances. Today,

DIS manufacturers recognize that many field

failures of early DISs probably occurred

because their lack of awareness of power

quality, system compatibility engineering,

and the types of problems that disturbances

could cause to DISs.

Advancements in the design, packaging, and

application of the devices in each semicon-

ductor category provided designers with

semiconductors of higher power, increased

switching speeds, and enhanced data storage

densities in smaller device package sizes,

allowing DISs the ability to drive more power

into capturing better images and processing

more data in a shorter time frame. Moreover,

hybrid semiconductor technologies— tech-

nologies that integrated advanced control

techniques with low-loss fast power switch-

ing devices—allowed equipment designers to

generate, control and manage larger amounts

of power at higher frequencies. These

advances allowed DIS designers to achieve

DIS designs with higher imaging and storage

performance. This performance is required

to meet physicians’ demands for DISs that

could reveal more details about the human

body with faster image processing times. But,

as devices continued to advance, they also

continued to generate more heat and require

more electrical power to operate as imaging

and data processing power increase.

16

Although the sensitivity of early DISs to elec-

trical disturbances was unknown to design-

ers, installers, end-users, and maintainers,

high performance DISs, which utilized the

most sophisticated semiconductor technolo-

gies available, end users suspected that DISs

were becoming more sensitive to electrical

disturbances as their designs advanced. This

increasing sensitivity resulted from the

increased use of semiconductor devices com-

bined with complex electronic control sub-

systems that required higher input power

and that were less forgiving of electrical dis-

turbances.

This trend occurred until manufacturers of

subsystems—namely, power supplies and

subsystems that included power supplies on

their front end—began to integrate some

minimal level of immunity to common elec-

trical disturbances in their designs. However,

because physicians have been focused on

obtaining higher resolution images in shorter

time frames to decrease diagnostic time, this

trend of improving immunity did not

progress at the same rate as DIS power com-

plexity, power requirements, and imaging

performance.

Similarly, healthcare facility engineers have

been focused on trying to provide the utili-

ties (electricity, electromagnetically quiet

environments, water, air conditioning, heat-

ing, and medical gases) to operate DISs and

efficient utilization of space in imaging

suites. And, imaging department directors

have been focused on managing imaging

departments—staffing each DIS with trained

personnel, maintaining patient flow, and

archiving and providing access to patient

imaging data. For these reasons, limited

emphasis has been placed on characterizing

the immunity of DISs to electrical distur-

bances and providing quality power to imag-

ing systems.

The immunity levels that exist in today’s DIS

subsystems exist because of immunity

requirements integrated into some PDUs and

OEM power supplies as required by interna-

tional standards. Immunity improvements in

some PDUs and power supplies resulted from

an increased level of awareness of power

quality and system compatibility engineering

applied in other industries that required

compatible systems such as manufacturing

and personal computing. Although some

improved levels of immunity to disturbances

are included in some power supplies, ride-

through performance (i.e., voltage tolerance

envelopes) is not known for DISs as a whole.

In other words, DIS manufacturers are not

aware of the weakest links in the ability of

their systems to ride through common elec-

trical disturbances such as voltage sags and

voltage swells.

DISs manufacturers are continuously intro-

ducing more advanced features and

improved imaging performance to continue

meeting the needs of physicians. With

increases in semiconductor density, thermal

management, system flexibility, and high

imaging performance, today’s programmable

logic devices provide the system on a chip

(SOC) capabilities to drive next generation

imaging systems. Although increased integra-

tion within DISs equates to higher imaging

performance in smaller-sized systems, the

immunity of DISs to electrical disturbances

is still a concern.

The illustration below is an example of a data

system used in a modern DIS. Each of these

17

Example of Complex Data Filtering, Alignment, Buffering, and Imaging

Processing in DISs

Awareness

of weak links

in imaging

systems will

improve their

ride-through

performance

cards requires a DC power supply capable of

providing steady-state DC voltage and pulsed

DC current. One supply may be used to

power all cards in a system or individual sup-

plies may be used. DC current requirements

vary among imaging system modes and

among modalities. As processing speeds con-

tinue to increase, the power requirements of

DC power supplies also increase. Immunity

issues will still remain a concern even if

power supplies are designed to provide high

frequency output currents embedded on

individual data cards.

The data acquisition card, which filters

incoming data, is the most cost-sensitive sys-

tem card. Usually a diagnostic imaging sys-

tem will consist of multiple data acquisition

cards (in some cases up to 20 cards per sys-

tem). Once the data is filtered, it is sent to

the data consolidation card for buffering and

data alignment. Once the data has been

aligned, it is sent to the image/data process-

ing card, which is the most algorithm-inten-

sive card in the system because it performs

all the heavy-duty filtering and image recon-

struction. Any damage to these cards caused

by voltage transients or electromagnetic

fields that migrate through the system may

cause card damage, image artifacts, lost

imaging data, or malfunction of

the imaging system.

Image Artifacts Caused by

Poor Power Quality

Signal intensity artifacts are

often encountered during mag-

netic resonance (MR) imaging

and during the use of other

modalities for image construc-

tion. Too frequently, these arti-

facts are severe enough to

degrade image quality.

Occasionally, they interfere with

the interpretation of the

patient’s condition by radiolo-

gists and other physicians. Prior

to carrying out power quality

investigations and the consideration of com-

prehensive compatibility testing of imaging

systems, imaging system designers, end

users, and service technicians believed that

most artifacts were caused by non-power-

related phenomena. Injecting disturbances

into the AC power inputs of imaging systems

of various modalities may very well reveal

that the propensity for disturbances to cause

artifacts may be higher than originally

expected.

Before examples of power quality-related

artifacts with an MR system are shown, some

discussion on imaging artifacts included

here. Signal intensity artifacts inherent in

local coil imaging include intensity gradient

and local intensity shift artifacts. The latter

can be minimized but not eliminated with

optimal coil design and tuning. Improper

coil or patient positioning can produce sub-

tle or, in some cases, severe signal intensity

artifacts, and each is easily corrected. Signal

intensity artifacts and image degradation can

also occur in a perfectly functioning coil if

protocols are not optimized. Failure of

decoupling mechanisms can produce signal

intensity artifacts that will not respond to

protocol optimization and will worsen with

gradient imaging. Improper coil tuning man-

18

Example of Signal-Intensity Artifact Caused by an Electrical Disturbance

Incident upon an MR System during a Thunderstorm

Signal

intensity

artifacts

interfere

with the

interpretation

of the patient’s

condition by

radiologists

and other

physicians

ifests as a shading artifact that can mimic

other findings. Signal degrading artifacts may

be caused by a ferromagnetic foreign body in

the imager. Signal intensity artifacts can also

result from performing ultrafast imaging with

coils that were not designed for this type of

imaging or from MR imaging system mal-

function. Familiarity with the various causes

of signal intensity artifacts is necessary to

maintain optimal image quality and are typi-

cally required as part of any MR imaging

quality assurance program. Optical image

quality also involves knowledge of how elec-

trical disturbances create artifacts and how

to avoid them.

The black and white image (previous page)

illustrates a signal intensity artifact caused

by system malfunction. This is a sagittal T1-

weighted brain MR image obtained in a pedi-

atric patient that demonstrates a signal

intensity artifact caused by shim coil mal-

function that resulted from an electrical dis-

turbance incident upon the AC power input

to the MR system during an electrical storm.

The color images (above) illustrate an image

artifact (left) caused by electrical noise pres-

ent on the input line of a power distribution

unit that delivers power to various parts of

an imaging system. When the noise was

removed, the artifact was not present in the

next series of images.

Imaging Systems: The Backbone forDiagnostic Services for HealthcareFacilities

Practicing medicine in today’s society would

not be possible without the use of imaging

studies. As one might expect, imaging studies

are carried out through the use of diagnostic

imaging systems (DISs). Without DISs, physi-

cians could not be alerted of a patient’s

underlying serious illness. Clinicians would

not be able to make an accurate diagnosis in

a timely manner critical to a patient’s condi-

tion. Physicians would have no way of gener-

ating follow-up images to determine if a

patient’s treatment is working.

Common DISs such as CT and MRI have

become indispensable technologies and are

used to generate critical patient data at sur-

prisingly increasing speeds. As medical

researchers probe further into the body and

as DIS manufacturers develop more powerful

systems, continual improvements are being

made to imaging techniques. Many more

imaging system applications in clinical and

research settings are also being developed.

In the past ten years, DISs have become an

integral part of every healthcare facility. In

major healthcare research institutions, it is

not uncommon for hundreds of physicians,

researchers and support staff to provide mis-

sion-critical care, education, research, and

prevention in radiology, nuclear medicine,

19

Example of Image with Artifact (Left) from Poor Power Quality and without Artifact (Right)

Common

DISs such as

CT and MRI

have become

indispensable

technologies

and various types of diagnostic imaging. As

many as 500 CT scans and 100 MRI scans

may be performed daily in a single health-

care research institution.

Any medical imaging director or technician

who has ever been responsible for providing

imaging services to a hospital or operating

an imaging system has experienced the con-

sequences of imaging system downtime and

the effects of this downtime on the hospital’s

ability to serve the community. Most health-

care facilities that provide imaging services

only have one system for each modality. And,

as the demand for imaging services increases

and as new applications for imaging are

developed, hospitals and imaging centers are

having to install additional systems in the

widely used modalities. An interruption to

the imaging services at a facility will impact

the patient flow process whether the inter-

ruption is brought about by having to reboot

a system just once a day or having a system

go down for a few hours or days.

If a system cannot remain operational or if a

system cannot be restarted, then the systems’

field service engineer from the imaging man-

ufacturer must be called in to resolve the

problem. In most instances, a problem that

requires a service engineer can be resolved

within a 24-hour period. However, depending

upon the severity of the problem, there are

several instances when a system problem

cannot be resolved for several days, and

these cases are usually related to power qual-

ity.

If the service engineer addressing a problem

is trained to conduct a power quality audit,

then he or she may be able to identify a

wiring or grounding problem that may be

contributing to the system shutdown. Service

engineers are being limited as to how deep

they can probe into a facility electrical sys-

tem. There are risks associated with arc flash

hazards when opening up an electrical panel

and probing around in the panel. These risks

now limit them only to investigating the

main panel that provides power to the imag-

ing system.

A wiring or grounding problem may be easily

correctable by the facility electrician without

reaping havoc on the quality or reliability of

power to an imaging system. On the other

hand, a wiring or grounding problem may

require significant modifications to the facili-

ty electrical system which cannot be made in

the course of a day or two. Such modifica-

tions must be carefully scheduled to prevent

interrupting the operation of other critical

equipment in the healthcare facility.

Temporary branch circuits may even need to

be installed to eliminate a power quality

problem for an imaging system and provide

quality power to the system until a perma-

nent solution can be identified and imple-

mented in a facility while following applica-

ble electrical codes and requirements.

Power Quality for SupportEquipment in Imaging SystemSuites: An Expanding Concern

The need for imaging systems, especially

MRI and CT systems, has grown dramatically

in healthcare facilities. Although advances in

MRI and CT system technologies have

occurred in the last several years, the

increasing number of applications for MRI

and CT studies have spawned a number of

related problems. Imaging studies conducted

with MRI systems can be lengthy, requiring

healthcare professionals to sedate some

patients including children and those who

cannot tolerate the procedure. Patient move-

ment during a study can result in undesir-

able image quality. Moreover, patients who

are critically ill, immobile or under distress

may not be able to undergo a study or may

cause image quality to be compromised.

Poor image quality and the cancellation of

studies reduce the efficiency of the health-

care facility and the imaging department.

A patient who undergoes an MRI procedure

is under little risk. However, the need to

20

Poor image

quality

and the

cancellation

of studies

reduce the

efficiency

of the

healthcare

facility

and the

imaging

department

sedate patients for an imaging study may

increase the risk of life-threatening adverse

events resulting from sedation or the use of

anesthesia. If a patient develops a problem

or if a condition worsens, the patient may

not be able to respond or alert an imaging

system operator about the problem, a cardiac

distress, or other changes in the patient’s

physiological status. A patient requiring an

imaging study may also be dependent upon a

ventilator, which also requires the use of

patient monitoring equipment, thus the need

for support equipment in the imaging suite

and quality power to operate the equipment.

When patients are sedated or when anesthe-

sia is used, cardio-respiratory parameters

must be monitored during the imaging study

to provide a standard of care equivalent to

that provided in the operating room.

Organizations specializing in the safety of

imaging have developed guidelines requiring

healthcare professionals to monitor critically

ill patients and patients who receive sedation

or anesthesia during an MRI procedure.

Most equipment used to monitor a patient’s

status or to provide respiratory assistance

cannot be used in an MRI environment

because of the characteristics of strong mag-

netic fields. Items, including electronic med-

ical equipment, containing ferrous-based

materials can become dangerous projectiles

possibly injuring patients and healthcare

professionals and causing damage to the

imaging system. To overcome this magnetic

field compatibility problem, manufacturers

of patient monitors and ventilators have

designed equipment that can be used in MRI

suites.

Traditionally, MRI-compatible patient moni-

toring and other equipment used in an imag-

ing suite is not an integral part of the imag-

ing system, and is powered via a branch cir-

cuit separate from that powering the imaging

system. Ventilators are designed to be mobile

stand-alone equipment. When a patient

monitor or ventilator is needed, equipment

compatible with the MRI system is brought

into the suite and placed near the patient.

Monitors that are not an integral part of the

imaging system and ventilators must be pow-

ered from a 120-volt branch circuit internal

to the imaging suite. However, healthcare

facility designers and operating engineers,

imaging department managers, and system

operators may not realize that monitors and

ventilators used in imaging suites should

also be powered from sources of quality

power. Malfunction of support equipment

caused by poor power quality in an imaging

suite may also pose interruptions to the

imaging department.

Regardless of whether the circuit is fed from

the emergency power source, monitors and

ventilators must function when needed and

require quality power to meet the needs of

the healthcare professionals. Quality power

may be provided by an individual UPS rated

to handle a typical load consisting of a moni-

tor and ventilator. Or, a larger dedicated UPS

may be used to provide quality power to

every 120-volt receptacle in the imaging

suite.

On the other hand, some imaging systems

may contain patient monitoring equipment

integral to the system. Such monitors will be

fed from the circuit(s) powering the imaging

equipment inside the suite. If this circuit is

derived from a quality power source, then the

monitoring equipment will also be fed from

quality power. Imaging department managers

should consult with the healthcare facility

designers and engineers to ensure that quali-

ty power is provided to all critical areas of an

imaging suite.

Power Quality forImaging SystemsAchieving power quality for imaging systems

cannot be written into a given system speci-

fication or imaging suite design that stands

alone. Power quality for imaging systems

21

Power

quality

for imaging

systems

must be

achieved

through

well-defined

specifications

must be achieved through well-defined spec-

ifications that are based on the characteris-

tics of the real healthcare electrical environ-

ment where the system will be installed, the

expectations of the imaging department and

the imaging system manufacturer, and the

immunity (or susceptibility) of the imaging

system to common electrical disturbances

such as sags and surges. Presently, imaging

system manufacturers know little about how

their systems will react to electrical distur-

bances other than the impact of such distur-

bances on their systems costs millions of dol-

lars in the aggregate cost of spare parts,

repair labor, and downtime. Manufacturers

are eager to learn more about how their

equipment responds to disturbances and

how to integrate embedded solutions into

the power distribution, power supply, and

critical systems to improve overall perform-

ance and reliability. Once equipment per-

formance is known and matched closely to

the electrical environment of the healthcare

facility, most of the obstacles in purchasing,

planning, installing, operating, and main-

taining imaging systems in real electrical

environments will be removed.

Specifying Power QualityPerformance for New ImagingSystem Installations

Record numbers of new imaging systems are

delivered to healthcare facilities each year.

Larger permanent systems such as MRI and

CT require well-engineered, careful, and

step-wise installations. An installation

error—on the facility electrical system side or

on the point-of-use side of the main discon-

nect switch at the imaging system—will fos-

ter power quality problems. While mobile

systems, also available in MRI and CT

(portable systems are also typically used for

ultrasound and X-ray) do not require as com-

plex installation procedures, the quality of

power on circuits used to power these sys-

tems in still vitally important. Some of the

new systems are installed in place of older

ones at existing imaging suite sites, some are

new site installations, and some are used in

geographic areas where natural disasters

have occurred to provide imaging services to

communities where healthcare facilities are

inoperable. Remanufactured systems in all

modalities are also used and installed in

healthcare facilities. Regardless of the situa-

tion, specifying the level of power quality is

crucial to the successful startup of a new sys-

tem and its continued reliable operation dur-

ing the course of its life.

Because the successful startup and operation

of imaging systems is highly dependent upon

the quality of power delivered to the system,

the installation procedure, from beginning to

end, must be carried out with power quality

in mind. Knowledge of installation guidelines

that encompass all facets of power quality

and following those guidelines will help

designers, manufacturers, installers, service

people, and end users reduce installation

time and significantly reduce downtime

caused by power quality problems.

Unfortunately, comprehensive guidelines

that address all of the causes and effects of

power quality problems and recommenda-

tions to avoid imaging system problems do

not yet exist, with any imaging system manu-

facturer. Fairly speaking, while manufactur-

ers have learned a lot about the do’s and do

not’s about basic power quality, installation

planning guidelines and system specifica-

tions have not reached maturity. The system

22

Installation of an Imaging System

Comprehensive

guidelines

that address

all the causes

and effects of

power quality

problems and

recommenda-

tions to avoid

imaging system

problems do

not yet exist

compatibility concept, developed by EPRI

Solutions, when applied to imaging system

design will be critical in helping to develop

these guidelines. As the players—end-use

customers, manufacturers, power quality

researchers, and utilities—in the industry

learn more about power quality, the number

of power quality-related shutdowns in imag-

ing suites will decrease.

Addressing Power Quality at the

Installation Planning Stage

Addressing each facet of power quality at the

installation planning stage will minimize the

number of power quality-related shutdowns

experienced by imaging systems. If one were

to obtain a copy of the electrical system

specifications and installation requirements

for the facility electrical system for any imag-

ing system modality, one would find some

similarities and also some distinct differ-

ences that impact the power quality related

operation of imaging systems.

Key Areas of Power Quality.

There are five key areas of power quality that

should be addressed at the beginning of the

site and installation planning stage. Table 1

below list these five areas, the activity that

should be carried out beneficial to the start-

up and continued operation of the imaging

system, and the questions that should be

answered to ensure that each area is properly

addressed. Addressing these areas will help

manufacturers, installers, facility engineers,

and end users avoid many power quality

problems that end up causing malfunctions

and damage to imaging systems.

23

Power QualityArea to Examine

Activity Beneficial to Startup and ContinuedOperation of Imaging System

Question to Answer

1. Voltage Quality Over a typical year’s period, obtain knowledge of

what voltage disturbances to expect on a circuit pow-

ering an imaging system and on circuits power imag-

ing system accessories.

Will the amplitude, frequency, waveshape, and rate of

occurrence of the disturbances that reach the imaging

system affect the startup and operation of the system?

2. Wiring Over the entire length of the power and neutral cir-

cuits delivering power to the imaging system, avoid

all wiring errors, including those associated with dis-

connects, panels, circuit protection devices, trans-

formers, and splices.

Has the entire length of each segment of the entire

power and neutral circuit been physically inspected for

errors? Have wiring practices in question been identi-

fied as errors? Have all wiring errors been resolved?

3. Grounding Over the entire length of the grounding system that

provides a safety and equipment ground for the imag-

ing system, avoid all grounding errors, including

those associated with the facility, panels, subpanels,

disconnects, and transformers.

Has the entire length of each segment of the grounding

system been physically inspected for errors? Have

grounding practices in question been identified as

errors? Have all grounding errors been resolved?

4. Energy Storage Determine if there is a need for providing a minimum

level of energy storage to a new imaging system, and

if previous or other systems in the facility utilize

energy storage.

Has energy storage been considered? If so, what are

the driving factors? Has an energy storage system been

included in the site plan? If it has not been considered,

should it be?

5. Immunity Determine if the manufacturer specifies the immuni-

ty of the imaging system to disturbances such as

short duration variations and transients.

If immunity to disturbances is known, to what distur-

bance and to what levels is the system immune? Are

the immunity criteria available for review? If no immuni-

ty criterion has been established, then ask the manu-

facturer about immunity testing.

Table 1. The Five Key Areas of Power Quality and Their Benefit to Diagnostic Imaging Systems

Including Power Quality Criteria in

Imaging System Specifications.

The level of detail with respect to power

quality criteria that is included within the

electrical specifications of imaging systems

varies with manufacturer. Some manufactur-

ers take the more proactive position of

including more criteria than others. Others

may include only the basic electrical specifi-

cations for use during the site planning

stage. Research conducted by EPRI and oth-

ers are helping manufacturers to improve

how to include power quality criteria in their

specifications.

Minimum Requirements.

Regardless of the approach, there are five

minimum criteria that should be included in

any imaging system specification. Table 2

outlines these minimum criteria and the

basic description of each requirement.

With respect to line voltage, most imaging

systems will require a three-phase voltage

source, require a four- or five-wire system,

will operate at 50 or 60 hertz line frequency,

and will operate within somewhat of a broad

range of steady-state AC line voltages. It is

standard practice for imaging system manu-

facturers to require a three-phase line volt-

age to be delivered to the system’s main

power distribution unit (PDU). However, not

all subsystems that are part of an overall

imaging system require three-phase power.

Within a PDU, three-phase and single-phase

power is divided up according to the individ-

ual voltage and power requirements for each

subsystem as needed. Unlike some end-use

equipment, line frequency operation at 50 or

60 hertz does not imply that the system will

operate at any frequency between these val-

ues. For example, the minimum require-

ments may specify a ± 1 hertz or a ± 3 hertz

range where the system can be operated

above or below the nominal line frequency

without malfunction. Many imaging systems

are designed to operate within a nominal

voltage range characteristic of healthcare

facility electrical systems found in the United

States and in other countries. Some manu-

facturers will impose a limit on the lower

steady-state operating voltage range, for

example 200 to208 volts, which may not be

an acceptable operating voltage for some

systems operated in the United States. At any

of the acceptable higher nominal voltage

24

Minimum PowerQuality SpecificationRequirement

Description of Requirement

Line Voltage Number of phases

Line frequency

Nominal operating voltage range

Line Current Demand Current requirement at minimum acceptable line voltage and at maximum

acceptable line voltage

Power Source

Configuration

Type of transformer connection: wye-connected or delta-connected

Power Demand Maximum power demand and continue power demand

Power Grounding Appropriately sized dedicated grounding conductor originating from the facili-

ty’s primary power source (main switchgear), following through each panel,

bonded to the surface of each panel with one non-insulated grounding block,

through the main disconnect panel, and terminating at the power distribution

unit of the imaging system.

Table 2. Minimum Power Quality Specification Requirements for an Imaging System

operating points—380, 400, 415, 480 volts

AC—most systems can be operated at 10%

above or below these values.

With respect to line current demand, systems

operated at lower line voltages will require

higher current demands, of course, resulting

in the requirement for larger sizes for power

and neutral conductors. (The use of larger

power conductors may also require the use of

larger ground conductors.) If current demand

values for maximum momentary operation

are provided, these values will be larger than

those during continuous operation. Larger

currents are usually required at system start-

up, during image capture, and/or during a

change from one system operating mode to

another.

With respect to power source configuration

and power demand, the type of transformer

used to power the imaging system and its

size is critical to powering the system cor-

rectly. In healthcare facilities, wye-connected

or delta-connected transformers may already

be in place for providing power to other

equipment or an existing imaging system.

Ideally, when a lower line voltage must be

stepped up to provide a higher line voltage to

an imaging system, a delta-wye transformer

configuration should be used. In this appli-

cation, the delta side should be connected to

the low input voltage and the wye side

should be used to power the imaging system.

As a minimum, the maximum power demand

data from the system’s specifications should

be used to size the transformer instead of the

continuous demand data. If the continuous

demand data is used, the transformer will be

undersized, and the power demanded by the

imaging system will collapse (distort) part of

the line voltage during image capture.

Several important design considerations

must be addressed to prevent transformers

that are in the power conductor path from

contributing to a power quality problem with

an imaging system. These considerations

include transformer sizing, transformer ori-

entation (using the transformer as a step-up

or step-down device), connection and use of

neutral and ground conductors, and ground-

ing. Further guidance on these issues is dis-

cussed in some of the following sections.

Providing proper power and signal grounding

within an imaging system (design and instal-

lation) is one of the most critical system

requirements. In the steady-state (i.e., when

the system is powered up and ready for use),

inadequate grounding will reduce the quality

of the images captured during a scan and will

place the system at a higher exposure level

for damage resulting from electrical distur-

bances. In the dynamic-state (i.e., when the

system is scanning) when a disturbance is

occurring, inadequate grounding will result

in higher levels of voltage transients whether

the transient was a true voltage surge (origi-

nating from outside the imaging suite), a

capacitor switching transient, or an embed-

ded transient on the leading or lagging tail of

a voltage sag or momentary interruption.

With respect to power grounding, providing

the lowest possible ground impedance from

the point where the facility’s power ground is

established to the point where the PDU is

grounded will result in the best power quality

performance. The total resistance of a

grounding system used in an imaging system

consists of the sum of the resistances of each

part of the grounding system including the

resistance of each series grounding conduc-

tor, grounding block, grounding lug, and the

grounding electrode at the main service

entrance.

But practically, the grounding impedance

requirement for the facility ground to be a

few ohms or less as required by some imag-

ing system manufacturers can be difficult to

achieve. As a part of the power quality audit

(See Identifying Power Quality Problems in

Imaging System Suites), the total resistance

and ground current of the grounding system

may be measured, identifying issues related

to ground conductor sizing, ground conduc-

tor terminations, and facility grounding sys-

25

Providing

proper power

and signal

grounding

within an

imaging

system is

one of the

most critical

system

requirements

tems that may need improvement to lower

ground currents and lower the total ground

resistance for the imaging system.

As a standard guideline for facility grounds,

the NEC in Article 250 requires that a single

electrode grounding system having a resist-

ance of more than 25 ohms be augmented by

one additional NEC-approved electrode.

Grounding to the building steel (beam) at the

main disconnect panel to the imaging system

as a supplemental ground is acceptable to

the NEC, and local codes should be checked.

However, the use of a ground rod at the main

disconnect panel to the imaging system is

not recommended by imaging system manu-

facturers, and will create power quality prob-

lems for imaging systems.

Optimal Requirements.

There are a number of additional require-

ments that may be included in the electrical

specifications and site and installation plan-

ning stages that will have a definite positive

impact on the power quality for imaging sys-

tems. As imaging system manufacturers con-

tinue to learn more about how their systems

respond to electrical phenomena, improve-

ments to electrical specifications and site

and installation planning guides are also

being made. Continued in depth power qual-

ity research and testing of how these systems

respond to electrical disturbances will foster

this process, resulting in systems that are far

more robust than ever thought. In addition

to the minimum requirements outlined in

Table 2, the optimal requirements are out-

lined in Table 3 on the following page. These

requirements are only listed here as a guide-

line for helping end-use customers and man-

ufacturers, and for utilities to help their cus-

tomers avoid power quality problems with

imaging systems.

Table 4, on page 28, outlines the categories

and typical characteristics for power system

electromagnetic phenomena (i.e., electrical

disturbances in power quality) as defined by

the IEEE Standard 1159-1995 (R2001). The

categories shaded in orange represent the

typical characteristics recognized by imaging

system manufacturers who require more

than the minimum, but not the optimum

power quality requirements for their systems.

Improving Power Quality for ExistingImaging Systems

When owners and operators of multi-million

dollar imaging systems complain about sys-

tem malfunctions and shutdowns, it is in the

best interest of imaging system manufactur-

ers to listen. In response to the growing num-

ber of power quality problems with imaging

systems, manufacturers have learned that

electrical disturbances do affect the opera-

tion of their systems and are listening to cus-

tomer concerns about system shutdowns.

Listening to customers complain about the

shutdown of existing systems and identifying

improvements that will reduce the number

and frequency of shutdowns is vital to the

imaging system manufacturer–healthcare

organization relationship. Some healthcare

organizations strive to keep existing systems

online for as long as they can as a backup

system or if they are unable to purchase a

new system when new systems become avail-

able.

What Are Imaging System Manufacturers

Doing about Power Quality?

With the continuing demand for imaging sys-

tems that capture a larger number of more

detailed images in a shorter imaging time,

manufacturers are focused primarily on the

performance of subsystems (software and

hardware) that allow them to produce imag-

ing systems with higher imaging perform-

ance. Physicians and researchers are interest-

ed in systems that can reveal more detailed

information about cellular structures, com-

plete scans in a shorter time, and allow them

to probe further into the human body (and

other forms of life) before a decision is made

to perform an invasive or other procedure.

26

Manufacturers

have learned

electrical

disturbances

do affect the

operation

of their

systems

27

Optimal PowerQuality SpecificationRequirement

Description of Requirement

Voltage Transients System will continue to function when transients incident upon main power conductors:

impulsive1 (electrical fast transients) – oscillatory (ring wave, combination wave, capacitor

switching) – level of withstand voltage should be 6 kV for ring and combination wave tran-

sients and 2.0 p.u. for capacitor switching transients from 400 Hz to 5 kHz.

Voltage Variations: Short

Duration - Interruptions

System will continue to function when short duration voltage variations incident upon

main power conductors: momentary interruptions of 0.5 cycles < d < 3 seconds and mag-

nitude < 0.1 p.u.; temporary interruptions of 3 seconds < d < 1 minute and magnitude <

0.1 p.u.

Voltage Variations: Short

Duration - Sags

System will continue to function when short duration voltage variations incident upon

main power conductors:

1) SEMI F47 Requirement: instantaneous sags of 0.05 < d < 0.2 seconds and magnitude

> 50%; 0.02 < d < 0.5 seconds and magnitude > 70%; momentary sags of 0.5 < d < 1

second and magnitude > 80%.

2) SEMI F47 Recommendation: instantaneous sags of 0 < d < 0.02 seconds and magni-

tude > 0%, 0.02 < d < 0.2 seconds and magnitude > 50%, 0.02 < d < 0.5 seconds and

magnitude > 70%; momentary sags of 0.5 < d < 3 seconds and magnitude > 80%; tempo-

rary sags of 3 < d < 10 seconds and magnitude > 80%, 10 < d < 100 seconds and magni-

tude > 90%

Voltage Variations: Short

Duration - Swells

System will continue to function when long duration voltage variations incident upon main

power conductors: instantaneous swells of 0.5 < d < 30 cycles up to 1.8 p.u.; momentary

swells of 30 cycles < d < 3 seconds up to 1.4 p.u.; temporary swells of 3 seconds < d < 1

minute up to 1.2 p.u.

Voltage Variations: Long

Duration Variations

System will continue to function when long duration voltage variations incident upon main

power conductors: sustained interruptions of d > 1 minute and magnitude of 0.0 p.u.;

undervoltages of d > 1 minute and magnitude of 0.8 to 0.9 p.u.; overvoltages of d > 1

minute and magnitude of 1.1 to 1.2 p.u. of d > 4 hours and magnitude up to 1.5 p.u.

Voltage Distortion System will continue to function with a distorted voltage signal composed of primarily 3rd,

5th, and 7th harmonics at 8%, 4%, and 1%, respectively; so that the overall Vthd is

approximately 10% applied to a line voltage at 110%, 100%, and 90% of Vnominal with

harmonic content.

Voltage Notching System will continue to function with a voltage notch centered at 90° on the line-voltage

wave form with the following notch parameters: Width: 5°, Depth: 20% Vnominal.

Voltage Fluctuations System will continue to function with a 60 Hz sine wave fluctuating supply voltage modu-

lated with a square wave with peak-to-peak magnitude of 1%, 3%, and 5% Vrms of rated

supply voltage at a frequency of 5, 10, and 15 Hz, synchronized with the zero crossings of

the supply voltage.

Frequency Variations System will continue to function with a 60 Hz sine wave of varying frequency from 56 Hz

to 64 Hz.

Table 3. Optimal Power Quality Specification Requirements for an Imaging System

1Impulsive transients incident upon power conductors for subsystems and control conductors within system are

not considered here.

28

CategoriesTypical Spectral

ContentTypical Duration

Typical VoltageMagnitude

1.0 Transients

1.1 Impulsive

1.1.1 Nanosecond 5 nsec rise < 50 nsec

1.1.2 Microsecond 1 µsec rise 50 nsec – 1 msec

1.1.3 Millisecond 0.1 msec rise > 1 msec

1.2 Oscillatory

1.2.1 Low Frequency < 5 kHz 0.3-50 msec 0-4 p.u.

1.2.2 Medium Frequency 5-500 kHz 20 µsec 0-8 p.u.

1.2.3 High Frequency 0.5-5 MHz 5 µsec 0-4 p.u.

2.0 Short Duration Variations

2.1 Instantaneous

2.1.1 Sag 0.5-30 cycles 0.1-0.9 p.u.

2.1.2 Swell 0.5-30 cycles 1.1-1.8 p.u.

2.2 Momentary

2.2.1 Interruption 0.5 cycles-3 sec < 0.1 p.u.

2.2.2 Sag 30 cycles-3 sec 0.1-0.9 p.u.

2.2.3 Swell 30 cycles-3 sec 1.1-1.4 p.u.

2.3 Temporary

2.3.1 Interruption 3 sec-1 minute < 0.1 p.u.

2.3.2 Sag 3 sec-1 minute 0.1-0.9 p.u.

2.3.3 Swell 3 sec-1 minute 1.1-1.2 p.u.

3.0 Long Duration Variations

3.1 Interruption, Sustained > 1 minute 0.0 p.u.

3.2 Undervoltages > 1 minute 0.8-0.9 p.u.

3.3 Overvoltages > 1 minute 1.1-1.2 p.u.

4.0 Voltage Imbalance Steady State 0.5-2%

5.0 Waveform Distortion

5.1 DC Offset Steady State 0-0.1%

5.2 Harmonics 0-100th Steady State 0-20%

5.3 Interharmonics 0-6 kHz Steady State 0-2%

5.4 Notching Steady State

5.5 Noise Broadband Steady State 0-1%

6.0 Voltage Fluctuations < 25 Hz Intermittent 0.1-7%

7.0 Power Frequency Variations < 10 sec

Table 4. Categories and Typical Characteristics of Power System Electromagnetic Phenomena from

IEEE 1159-1995 (R2001)

Source: IEEE Standard 1159-1995 (R2001)

Imaging system manufacturers can combat

power quality problems at two basic levels—

the design level and the installation level.

With much experience in imaging system

malfunctions and damage to equipment

caused by power quality problems, combat-

ing these problems at the installation level

for existing systems in the field has almost

become a standard operating procedure for

some manufacturers. Field engineers that

routinely service and repair imaging systems

have witnessed all types of subsystem fail-

ures caused by power quality problems. A

thorough knowledge of what happens to an

imaging system as a result of an electrical

disturbance is key to a later understanding of

how and why these systems respond to simi-

lar disturbances in the power quality labora-

tory.

Although imaging performance is a primary

function of how well and how fast internal

features of the body are reconstructed, the

imaging process requires compatibility

between continuous acceptable quality

power and the imaging system that does not

alter or interrupt the process. Continuous

quality power does not imply that the power

is free from electrical disturbances.

Compatibility implies predictable perform-

ance of the imaging system in real electrical

environments with real power in the health-

care facility. Sine waves that contain voltage

variations such as a 5-cycle sag to 80 % volt-

age, for example, whether caused by electri-

cal operations internal or external to a

healthcare facility, should have no impact

upon the imaging capture or reconstruction

process. Waveforms that contain a 1-kHz

capacitor switching transient at 1.8 per unit

(a voltage that is 1.8 times its nominal value)

characteristic of operating utility line capaci-

tors that provide voltage stability for the dis-

tribution system and correct the power factor

also should not impact the imaging process.

Compatibility of imaging systems with the

public power system is important at both the

design and installation levels to provide rea-

sonable protection against all types of elec-

trical disturbances, especially sags, swells,

and transients. A sound installation of a sys-

tem that has a low immunity to sags and

surges is no better than a poorly installed

hardened system that has high immunity to

these disturbances.

Like many other manufacturers of electronic

systems, imaging system manufacturers in

the last several years have become more

knowledgeable about protecting their sys-

tems from voltage surges and transients at

both the design and installation levels. This

does not come by surprise—most equipment

shutdowns and interruptions were thought to

29

Region of Instantaneous Sags, Momentary Sags, and Temporary Sags for Voltage Sag Immunity

Testing of Imaging Systems

Successful

imaging

requires

compatibility

between

continuous

quality power

and the

imaging

system

be caused solely by surges, before the days of

understanding more about how voltage sags

and momentary interruptions (i.e., distribu-

tion power quality) really impact imaging

system operation. Designers of PDUs and

subsystem power supplies for use in imaging

systems and other medical devices have inte-

grated some level of surge protection at vari-

ous circuit levels. In some cases, that level of

protection may be not enough while in oth-

ers it may be too much for a specific circuit

level. Although some level of surge protec-

tion is included in most systems, an engi-

neering analysis to determine the appropri-

ate coordination between overcurrent pro-

tection devices, SPDs, and SPD size and loca-

tion for each type of system requiring protec-

tion will result in improved imaging system

performance and more reliable protection

against surges and transients. Combining

this concept with the expected environmen-

tal exposure to surges in a healthcare facility

will reveal whether the SPDs can provide

enough protection throughout the life of the

imaging system. In all, an imaging system

failure should not result from the failure of a

overvoltage protection device such as an SPD

failure, nor should a functional system be

placed at high exposure to failure from

surges and transients because of an SPD fail-

ure.

Providing immunity protection against volt-

age sags and momentary interruptions is

more challenging whether at the design or

installation level. Imaging system operators

and department directors, along with health-

care facility engineers, and even imaging sys-

tem manufacturers will admit that nothing

has been done on the design level to provide

protection against voltage sags and momen-

tary interruptions—now the most common

type of electrical disturbances traveling into

healthcare facilities across the main discon-

nect of an imaging system.

Although the manufacturers of some power

supplies used in imaging systems may have

as much as a SEMI F47 compliance against

voltage sags, knowing the immunity perform-

ance of individual power supplies is not

enough to define total system performance

and reduce the cost of system downtime and

repairs. Imaging system manufacturers, end

users in the healthcare industry, and utilities

need to know the “whole system” response to

sags and interruptions. The complex design

and dynamic operating modes of imaging

systems present various load profiles to

internal power supplies and varying load

profiles to the PDU. These load profiles will

result in a varying immunity to sags and

interruptions. Knowing the sag and interrup-

tion immunity at the minimum and maxi-

mum “whole system” load will be important

depending upon the criticality of the system

and its intended use. Including a SEMI F47-

compliant power supply in an imaging sys-

tem design that has unknown immunity to

voltage sags and momentary interruptions

will reap little, if any, benefit in preventing a

system from shutting down or initiating a

system reboot. Each power supply in the

power chain, including the PDU, plays a role

in “whole system” immunity and must con-

tinue to function during a common sag or

interruption.

At the installation level, healthcare facility

engineers and field service engineers from

the imaging system manufacturer can initiate

and carry out proactive and reactive meas-

ures to reduce system shutdowns and dam-

age to equipment from electrical distur-

bances. These measures are discussed in the

following two sections.

Proactive Approach

As some medical imaging system manufac-

turers learn more about power quality in

healthcare facilities and why electrical dis-

turbances affect the reliable operation of

their equipment, more proactive measures

for preventing power quality problems with

imaging systems will be implemented at the

design level. One might ask, “What is driving

their interest in taking a proactive

30

Proactive

measures for

preventing

power quality

problems

with imaging

systems

should be

implemented

at the

design level

approach?” Well, some imaging system man-

ufacturers have learned that minor wiring

and grounding errors such as improperly

mounted ground bus bars in a panel,

reversed neutral and ground conductors, and

redundant grounds to a system can cost

them hundreds of thousands of dollars in

service labor hours and installed replace-

ment parts. Moreover, they have learned that

most of these problems could have been

identified and corrected in existing electrical

systems before a new system was installed if

field engineers knew what to look for in an

electrical system of a healthcare facility.

As imaging system manufacturers operate

pre-installation programs targeted towards

identifying and correcting wiring and

grounding errors that can contribute to or

cause power quality problems, they are

requiring that all new installation sites have

a proactive power quality audit. All errors or

potential problems that may interfere with

the installation process or impact the relia-

bility of an imaging system are formally doc-

umented and brought to the attention of the

electrical engineer or electrician at the

healthcare facility. Once the electrical engi-

neer or electrician reports that these prob-

lems have been resolved, then a follow-up

audit is conducted at the site to verify that

the errors and potential problems have been

corrected before the installation is allowed to

proceed. Some manufacturers have even

instituted a program and reserve the right as

the servicing organization to cancel a service

contract and shutdown an imaging system to

prevent its operation until significant errors

and potential problems can be resolved.

Reactive Approach

The complexity of imaging systems warrants

a network connection from each system back

to a manufacturer’s online customer service

center. At the service center, online experts

for each modality are on standby to help

their customers resolve system problems.

Network connections are used to help opera-

tors with system operating problems, reset

systems, and upload computer code among

other services when needed. If an imaging

system problem can be resolved via an activi-

ty carried out through a network connection,

then a field service engineer may be able to

avoid an unnecessary trip to a healthcare

facility.

Following a reactive approach, field service

engineers from imaging system manufactur-

ers are routinely called into imaging depart-

ments to analyze and resolve imaging system

problems that are cannot be resoled remotely

(via a telephone or network connection).

While a system problem requiring a field

engineer may not require that the perform-

ance of the AC power system be evaluated,

inspecting the condition of each component

of the AC power system may be included in

the site visit and the system’s overall evalua-

tion. A “red flag” will be raised by the field

engineer if a frequent system malfunction or

failure can be attributed to the malfunction

or failure of a power-related subsystem such

as an internal amplifier or power supply.

Similarly, if a specific part is frequently

removed and replaced, even if it is not direct-

ly related to any part of the system’s power

supply, then the system’s evaluation will

include inspection of the healthcare facility’s

AC power system from the system’s main dis-

connect panel back to the facility’s service

entrance.

Establishing a Partnership withImaging System Manufacturers

The cooperative effort between healthcare

facilities, equipment vendors, equipment

manufacturers, and electric utilities to pre-

vent and resolve power quality problems in

healthcare facilities has continued to grow.

Healthcare facilities are looking more and

more to EPRI and their utility to help them

resolve power quality problems in various

parts of their facilities. This is especially true

in imaging suites where power quality prob-

lems have the potential to significantly

31

Healthcare

facilities are

looking more

and more to

EPRI and their

utility to help

them resolve

power quality

problems in

various parts

of their

facilities

impact imaging suite operations.

Utilities have always offered assistance to

customers in emergencies, as well as promot-

ed new energy-efficient technologies to

improve productivity and reliability. As prob-

lems associated with new technologies were

revealed, many utilities established power

quality programs that invested in power

quality research to assist utility customers

with resolving equipment-compatibility

problems. In the area of diagnostic imaging,

new higher-performance technologies in all

modalities are frequently being introduced

into healthcare settings. The introduction of

these new technologies continues to foster

the need for power quality engineering serv-

ices for the healthcare industry.

Electric utilities especially recognize the

necessity of providing power quality services

to their healthcare customers. These services

enable healthcare staff to select the proper

power-conditioning equipment, develop

specifications for purchasing medical equip-

ment, establish correct installation guide-

lines, and plan facility renovations or the

construction of new facilities.

Each of these efforts is extremely important

to the successful installation and operation

of a diagnostic imaging system. Not all

installations will require a power quality mit-

igation device, but those that have this

requirement will need the assistance of

power quality professionals to help make

that selection process a successful one. In

other cases, procurement specialists in a

healthcare facility will not know how to

interpret or what to look for in an electrical

specification for an imaging system. What

should be included? What is important? What

parameters are not good enough? What level

of compatibility between the imaging system

and the power system can be expected? Is the

electrical environment where the system is to

be installed matched to these specifications?

Regarding the level of detail in installation

guidelines, some differences are present

among the various imaging system manufac-

turers. More room for errors exist when a

facility is renovating an imaging suite or

modifying an area for a new suite where a

suite previously did not exist. Power quality

professionals can assist the healthcare facili-

ty in making sure that these requirements are

met.

As power quality professionals learn more

about the electrical environment of a health-

care facility and as imaging system manufac-

turers strive to learn more about the per-

formance of their systems when subjected to

electrical disturbances, stronger relation-

ships between power quality researchers and

medical imaging system manufacturers are

formed. These relationships boil up the

interest level of the manufacturers in efforts

to reduce the impact that disturbances have

upon their equipment.

Building strong relationships between

healthcare facilities, equipment vendors,

equipment manufacturers, and electric utili-

ties offers many benefits. These benefits

include reducing or eliminating controllable

electrical disturbances, managing common

uncontrollable electrical disturbances,

encouraging equipment manufacturers to

design and build robust equipment immune

to most common electrical disturbances, and

avoiding citations and penalties from regula-

tory agencies.

System Compatibility Testing ofImaging Systems

System compatibility testing of imaging sys-

tems is necessary to determine the response

of imaging systems to common electrical dis-

turbances that originate inside and outside a

healthcare facility. The System Compatibility

Research Project, created by EPRI Solutions –

PEAC Laboratory, has been used to charac-

terize the performance of hundreds of end-

use devices including semiconductor fabrica-

tion tools, adjustable speed drives, electronic

ballasts, programmable logic controllers,

32

System

compatibility

testing of

imaging

systems is

necessary to

determine

the response

of imaging

systems to com-

mon

electrical

disturbances

consumer electronics systems, power sup-

plies (including those for medical devices),

and many others. Each end-use device is

characterized using a pre-engineered Test

Protocol for System Compatibility. For med-

ical imaging systems, EPRI Solutions and

imaging system manufacturers are develop-

ing a series of Test Protocols for System

Compatibility – Diagnostic Medical Imaging

Systems. Application of these protocols will

describe compatibility tests and identify the

response of a medical imaging system to var-

ious types of electrical disturbances includ-

ing voltage sags, voltage transients, voltage

distortion, voltage fluctuations, and other

disturbances. EPRI Solutions is presently

working with imaging system manufacturers

to determine the immunity of various modal-

ities to common electrical disturbances.

The PQ Checklist:Planning, Purchasing, Installing, andMaintaining Imaging SystemEquipment

Pre-installation checklists for imaging sys-

tems are vital to the successful operation of

all systems. Complex modalities such as MRI,

CT, vascular, and other require lengthy

installation checklists. For systems such as

MRI where a heavy magnet system is also

required, specific checklist items for the

magnet system must be carried out. As an

example, listed are the items that may be

found on a manufacturer’s pre-installation

checklist for an MRI system:

n Vibration study

n Magnetic field study

n Structural steel requirements

n Acoustic levels

n Magnet room floor loading

n Environmental controls

n Air conditioning

n Power quality

n Water cooling

n Magnet room anchors

n Multiple MR system site requirements

n Main distribution power

n Room ventilation and exhaust fan

n Telephone or broadband connections

n Cryogenic ventilation

n Radio-frequency shielding

n Lighting

n Dust-free environment

From this list, one can see that there are

many requirements including power quality

for a specific type of MRI system. Although

the list of requirements will vary with imag-

ing system modality, there are several

requirements such as power quality that are

now common to all checklists regardless of

modality. One should not confuse this check-

list with a checklist intended to guide an

auditor through the power quality audit

process when auditing an existing imaging

system installation or auditing a site for a

new installation.

Healthcare facilities routinely procure and

install equipment. Imaging systems are

updated on an as needed basis with an effort

to utilize the most advanced imaging equip-

ment possible, matched to the needs of the

community and medical staff. To reduce

power quality problems between imaging

equipment and the intended electrical envi-

ronment, equipment-procurement proce-

dures should include the following steps:

Planning for Additional Equipment

n The healthcare facility may want to

begin a sound in-house power quality

program with the purchase of a

power-line monitor. A monitor can be

used to conduct an on-site power

quality survey to identify potential

power quality problems with sensitive

medical and imaging system equip-

ment. Consider tapping the expertise

33

Power

quality is

common to

all checklists,

regardless of

modality

of your local electric utility and other

power quality professionals to devel-

op this program. Determine the char-

acteristics of your facility’s electrical

system: Can it tightly regulate equip-

ment voltage? Is voltage to equipment

continuous? Does high-wattage imag-

ing equipment create electrical dis-

turbances in the facility wiring? Your

local utility can also provide site-spe-

cific characteristics such as expected

voltage regulation and electrical dis-

turbances for the area where the facil-

ity is located.

n Request the performance require-

ments of existing imaging equipment

from the imaging system manufactur-

er. For new equipment, request that

performance requirements be identi-

fied through power quality/system

compatibility testing. How suscepti-

ble is each type of imaging equipment

to common electrical disturbances

such as voltage sags and transient

overvoltages?

n Set your expectations for the perform-

ance of new equipment. New, more

advanced imaging systems should

perform better than the older sys-

tems, but some type of power quality

mitigation system and/or wiring and

grounding correction may be

required. And, then ask your utility

for help in specifying facility design

features that enhance compatibility

between the imaging systems and

their intended electrical environ-

ment.

n Repair all wiring and grounding prob-

lems identified in a power quality

audit or other electrical work within

the imaging suite and the facility.

n Identify all areas where portable and

mobile (in a semi-trailer) imaging

systems may be used and the special

power requirements of such equip-

ment.

n With assistance from your local utility,

identify appropriate power quality

mitigation devices for portable and

mobile imaging systems.

Purchasing Additional Equipment

n Disclose to equipment manufacturers

the power quality characteristics of

the electricity and wiring where the

new imaging equipment will be

installed. A power quality audit and a

power-line monitoring activity may

be required to fully characterize the

electrical environment in the imaging

suite.

n For all new imaging equipment, speci-

fy the voltage range (required voltage

regulation), frequency, and ride-

through performance (if known)

expected by the healthcare facility.

n Purchase equipment with an input

voltage rating matched to the voltage

at the installation site. Purchase high-

quality power transformers with new

equipment when the voltage ratings

of the equipment do not match the

available voltage at the installation

site.

n Make sure that power quality mitiga-

tion devices are designed for compat-

ibility with imaging equipment. This

process will involve compatibility

testing between the mitigation device

and the imaging system.

n Make sure that all imaging and power-

conditioning equipment and installa-

tions comply with all applicable

codes, standards, and consider meet-

ing recommended practices.

n To reduce susceptibility to common

electrical disturbances, select the

highest input voltage rating for equip-

ment known to be sensitive to com-

mon electrical disturbances. Operate

imaging systems at their highest rated

input voltage.

34

Repair

all wiring

and grounding

problems

identified

in a power

quality audit

Installing Additional Equipment

n Use high-performance wiring and

proper grounding techniques speci-

fied in the IEEE Standard 602-1996,

White Book (Recommended Practice

for Electric Systems in Healthcare

Facilities) and the IEEE Standard

1100-2006, Emerald Book (Powering

and Grounding Electronic

Equipment).

n For circuits connected to sensitive

electronic equipment, use single-

point grounding, locate equipment as

electrically close to the source as pos-

sible, and make sure that neutral con-

ductors (if needed for the specific

wiring configuration) are at least the

same size as phase conductors.

n When adding grounding conductors to

an existing facility, run the grounding

conductors parallel to the existing

power and neutral conductors to

reduce stray electromagnetic fields

and avoid other related power quality

problems.

n When installing high-wattage imaging

equipment in an existing facility,

monitor the input voltage at the pro-

posed installation site for electrical

disturbances before initiating the

installation, if possible. The monitor-

ing period should be a minimum of

30 days, but monitoring across each

season will reveal more information.

Maintaining Equipment

n Implement a “clean and tighten” pro-

gram for the healthcare facility and

make sure that all associated electri-

cal distribution panels and equip-

ment (including power conditioning

and mitigation equipment) associated

with powering the imaging system is

included in the program.

n If battery-based UPSs are used for

imaging systems, make sure that a

battery maintenance program is

included in the imaging suite. Failed

batteries cannot provide ride-through

power in the event of a voltage sag or

momentary interruption.

n Regularly review equipment perform-

ance and continue the relationship

between healthcare facility staff, utili-

ty representatives, equipment ven-

dors, equipment manufacturers, and

equipment service companies.

n Document all imaging system prob-

lems. Include patient schedules, the

location of equipment, the symptoms,

suspected causes, time and date of

occurrence, and any other related

events.

n Checking disturbance logs against

utility records and facility activities

which can help in revealing the

source of electrical disturbances.

These logs can also be used to specify

future equipment purchases and

determine correct installation meth-

ods.

Identifying PowerQuality Problems inImaging System SuitesImaging systems require quality power to

function as expected by the customer. Some

of the small imaging systems such as ultra-

sound machines require only one dedicated

120-volt branch circuit while others such as

the larger MRI systems require three-phase

power at 480 volts at tens of kVAs. Lower

power systems such as the ultrasound

machines obviously do not require complex

facility electrical systems to deliver power to

the machine. However, providing 480 volts at

100 kVA requires switchgear, properly rated

and installed transformer, emergency discon-

nect systems, and several branch circuits. In

addition, complex imaging systems require

as many as 12 additional branch circuits to

35

Imaging

systems require

quality power

to function

as expected

by the

customer

support the auxiliary systems necessary for

imaging system operation.

Regardless of the simplicity or complexity of

the facility electrical system required to sup-

port an imaging system, some type of power

quality problem will arise at some point

within the life of the system. The occurrence

of a power quality problem suggests that an

audit is needed. Conducting a power quality

audit of an imaging system is an activity that

must be carefully planned and carried out

with safety in mind and to avoid damage and

interruption to the imaging system or sur-

rounding critical loads in a healthcare facili-

ty. The next few sections will familiarize the

reader with some of the basics of a power

quality audit focused on an imaging system.

Types of Audits

A power quality audit of an imaging system

may be conducted for various reasons. The

customer or manufacturer may require that

an audit be conducted prior to the installa-

tion of a new system where no system was

previously installed. Or, an older system may

be slated for updating with a new system.

These types of audits are referred to as pre-

installation audits.

In some cases, it is not possible to conduct

an audit on all of the electrical equipment

upstream of the main disconnect at the

imaging system before a new system is

installed. Some manufacturers and cus-

tomers require that an audit be conducted

after a system is installed, especially if one

cannot be conducted prior to installation for

one reason or another. In this case, a post-

installation audit is conducted after the sys-

tem has been completely installed and

turned over to the customer.

In cases with installed imaging systems, a

reactive audit may be conducted for a num-

ber of reasons. The manufacturer may

request that an audit be conducted. A cus-

tomer service manager or a field service

engineer may initiate the request. The direc-

tor of an imaging suite or the facility engi-

neer at the healthcare facility may also

request an audit be conducted. Other rea-

sons for conducting an audit may surface

including frequent system malfunctions and

excessive usage of spare parts on an imaging

system where the system and site may be

escalated to a critical level where an audit is

then mandatory. In any event, regardless of

who conducts a power quality audit or the

reason for conducting it, the results usually

reveal staggering problems that need to be

corrected to improve system reliability.

Performing an Audit in Your OwnSuite

Although some extensive training is required

to effectively conduct a power quality audit

on an imaging system, imaging system man-

ufacturers, and facility engineers in health-

care facilities may desire to conduct their

own audit outside the audits that may be

conducted by trained personnel from power

quality professionals in the utility industry

and power quality researchers. Those that

have extensive experience in conducting

audits will likely be more effective in identi-

fying problems.

Assembling an Audit Plan

Before an audit can be conducted, an audit

plan must be assembled. This plan is usually

assembled by a power quality professional

with imaging systems experience or an audi-

tor with experience in the imaging systems

industry. Several important components of

the audit plan must be identified before the

audit can take place. These components

include:

n Identification of imaging system

under investigation

• Manufacturer

• System identification information

• Modality (CT, MRI, X-ray, PET,

36

Those that

have extensive

experience in

conducting

audits will

likely be

more effective

in identifying

problems

Nuclear, other)

• Type of system: permanent, mobile,

or portable

• Problem and malfunction details

(from imaging system operator)

n Location of problem system

• System owner (name of healthcare

facility)

• System contact person (familiar

with imaging system operation)

• Facility engineer at healthcare

facility

n Reason for audit

• Pre-installation

• Post-installation

• Reactive

n Identification of audit team members

• Power quality engineer and/or

auditor

• Technical representative from imag-

ing system manufacturer

• Facility engineer

• Facility electrician or maintenance

personnel

• Imaging suite director

• Imaging system operator

Preparing for the Audit

Before a power quality audit can be conduct-

ed on an imaging system, the imaging suite

director and facility engineer in the health-

care facility must clearly understand the

audit process. They must know: (1) when the

audit should be started, (2) how long it will

take, and (3) how it might impact imaging or

other healthcare services. Because the audi-

tor must carry out work within the imaging

suite (i.e., near the console, inside the com-

puter room, and/or magnet room), entry into

these area is required and should be sched-

uled around patient schedules if the audit

must be conducted during business hours.

Auditors will find that imaging suite directors

and facility engineers in the healthcare facili-

ty will be more than willing to assist if they

realize that the audit will help them improve

the reliability of their imaging systems. Their

goal will be to assist the auditors while not

interrupting any healthcare services.

Because each audit is somewhat tailored to

the electrical design and specific imaging

system in the healthcare facility, not all

audits are conducted exactly the same way.

Everyone knows that no two hospitals or

healthcare facilities and imaging suites are

designed and built the same way. Moreover,

most of their modifications are not well doc-

umented on the original drawings, especially

the electrical one-line diagrams. Many of the

imaging suites have different layouts includ-

ing various equipment placements. Some

audits involve inspecting more electrical dis-

tribution panels, more branch circuits and

conductors, more pull boxes, and more

wiring and grounding connections than oth-

ers.

After the lead auditor or other representative

has visually inspected the site, he or she will

be able to provide a summary of what areas

of the facility and what electrical equipment

should be included in the audit. This infor-

mation will be helpful to the facility engineer

as electricians must be scheduled to open

electrical closets, electrical panels, and pro-

vide access to areas where panels, conduits,

conductors, and grounds are located.

Not all of the facility electrical system will

need to be audited. However, because of the

importance of the wiring and grounding

methods used in ensuring quality power, the

branch and feeder circuitry along with the

grounding system for the imaging system

under investigation will need to be traced

from the main disconnect panel of the sys-

tem all the way back to the service entrance

where the main source is derived and where

the facility ground is established.

37

Each audit is

tailored to the

electrical

design and

specific

imaging

system in the

healthcare

facility

Conducting the Audit

Once the audit plan has been assembled and

preparations have been made to carry it out,

the audit can be conducted. The auditor will

begin the process inside the computer equip-

ment room of the imaging suite where the

system main disconnect panel (SMDP) and

associated PDU, power supply, computer,

and auxiliary equipment are located.

The SMDP must be thoroughly inspected. As

a part of the imaging system, manufacturers

provide an SMDP as an option.

Manufacturers prefer that their SMDPs are

used with their systems. Thus, the auditor

will be looking to see if the imaging system

manufacturer’s SMDP is used at the site. If

so, the layout of equipment inside the panel,

its electrical connections and safety systems

will be designed for the specific modality

system and more familiar to a manufacturer’s

auditor than an SMDP provided by another

manufacturer. The auditor will be inspecting

the breakers, fuses, and disconnect switches

to determine if they are rated correctly for

the size of the system they power. The audi-

tor will document whether the SMDP has a

shunt trip or undervoltage detection system.

All phase voltages and currents at the SMDP

will be measured and documented. The audi-

tor will carefully inspect and measure the

ground currents with an emphasis on facili-

ty-side ground current. Load-side ground

current will be measured with the power to

the imaging system in the ‘on’ and then in

the ‘off ’ position and within the system in

the scan mode. Care should be taken in

measuring these currents to ensure that the

appropriate ampere scale is used so that a

meaningful reading is obtained.

The sizes of the phase conductors, neutral

conductor (if present), and ground conduc-

tors are extremely important in providing

quality power to imaging systems. The

American Wire Gauge (AWG) of each phase,

neutral, facility-side ground, and load-side

ground will be documented.

Even if the sizes of the conductors are correct

does not mean that a wiring or grounding

problem is not present. The method of bond-

ing the facility-side and load-side grounds to

each other is also extremely important to the

reliability and power quality of imaging sys-

tems. This method will be documented care-

fully. The auditor will also determine is the

neutral conductor is isolated from the

ground conductor(s) in the SMDP.

Each and every electrical connection—con-

ductors to breakers, conductors to fuses,

conductors to disconnect switches, conduc-

tors to panel lugs, etc.—will be thoroughly

inspected for proper torque, discoloration,

and improper conductor attachment. Some

conductors are tightened down on top of

their insulation. In some cases, not all of the

conductor strands are inserted into the lugs

before tightening, some are cut off—this is

called a conductor with a “haircut.”

Auditors will also inspect cable ducts to

determine if signal cables are physically sep-

arated from the power cables by the appro-

priate distance. In some installations, a

power quality mitigation device will be

installed before or after the SMDP. This

device could be a power-line filter, voltage

regulator, active power-line conditioner

(APLC), uninterruptible power supply (UPS),

surge protective device (SPD) or other

device. In cases where a mitigation device is

installed, its specification, installation, and

operation must also be carefully reviewed to

determine if it is improving or degrading the

quality of power to the imaging system. In

consulting with the imaging system manu-

facturer, one may find that the mitigation

device (especially in the case of UPSs) is not

compatible with the imaging system. Power

quality monitoring may be useful in deter-

mining this.

With respect to the ground conductors, the

auditor will determine the route that these

conductors take back to the facility electrical

system. The type of conduit supporting the

38

The method of

bonding the

facility-side

and load-side

grounds to each

other is also

extremely

important to

the reliability

and power

quality of

imaging

systems

phase, neutral, and ground conductors will

also be documented along with the current

flow through and impedance of the ground

conductor for the main facility back at the

service entrance and/or transformer loca-

tion. Digital photographs of various facets of

the audit will compliment the auditing

process and help the auditor to discuss the

results with other power quality professionals.

Once the audit process is completed at the

SMDP inside the computer equipment room

for the imaging system, the auditor and team

will progress to trace the feeder and branch

circuits (phases, neutral, and ground) back

through the facility to the next upstream

piece of electrical equipment. That equip-

ment may be a electrical distribution sub-

panel, step-up transformer, step-down trans-

former, isolation transformer, utility trans-

former, UPS, emergency generator, automat-

ic transfer switch (ATS), or main switchgear

(see the illustration above).

Regardless of the type of equipment, the

auditor should make it a practice to allow the

facility engineer or electrician to remove any

covers or doors from the equipment. This

practice is necessary to prevent the auditor

from accidentally tripping a circuit breaker

or causing a fault on the system that will

affect the imaging system and other critical

equipment in the healthcare facility. (Before

any equipment covers are removed, the audi-

tor, facility engineer, and other’s near the

equipment should wear personal protective

equipment (PPE) to protect them from arc

flash hazards.)

At each upstream electrical device, the audi-

tor will record the following information:

n Identification of electrical equipment

type

n Location of electrical equipment in

healthcare facility and in the facility

electrical system

n Manufacturer of electrical equipment

n Healthcare facility identification code

n Current rating

n List of breakers with rating greater

than 50 amps and load names listed

on the panel

39

Block Diagram and Example of Levels of Electrical Distribution System Powering an Imaging System

in a Healthcare Facility

Personal

protective

equipment

(PPE) should be

worn to

protect

auditors from

arc flash

hazards

n Physical distance from SMDP

n Transformer configuration (if applica-

ble)

n kVA rating

n Percent regulation

n kW rating

n Type of source feeding panel or other

equipment

All voltages and currents for all phase, neu-

tral, and ground conductors should also be

recorded for each upstream electrical equip-

ment. In addition, the sizes of the conductors

and the integrity of their connections will

also be documented as in the inspection of

the SMDP.

The auditor will continue this process until

the service entrance of the facility is reached

at the secondary of the utility transformer.

This process will entail tracing and photo-

graphing the conduits (containing the phase,

neutral, and ground conductors, if applica-

ble) through facility interstitials, basements,

suspended ceilings, electrical closets, and

electrical equipment rooms. The process will

not be simple as conduits can take routes

through concrete floors, walls, and ceilings.

The auditor should be able to place his or her

eyes on each inch of the conductors outside

the conduits to ensure their continuity

throughout the circuit. If pull boxes, junction

boxes, wiring or cable trays are encountered,

then their covers should be removed to

inspect the integrity of the connections and

conductors. Care should be taken not to

leave electrical panels with covers removed

unattended for safety reasons.

Once the audit is complete, all electrical

panel covers and doors should be secured in

their original position such that no safety

regulations or requirements are violated.

Interpreting the Audit Results

Once the data from the audit is gathered and

the visual inspections and electrical steady-

state measurements are complete, the audit

results may be complied into a form that can

be reviewed by a professional power quality

engineer. The audit results should identify

any potential installation problems with any

of the facility electrical equipment and the

imaging system as well as any unusual volt-

age, current, or impedance measurements

during imaging system operating conditions.

The reviewer will need to be able to quickly

identify the problems such that corrective

measures can be listed, and those that are

likely to correct or improve the problems can

be investigated and later implemented.

Is My Imaging System Vulnerable toPQ Problems? Questions for theFacility Engineer, ImagingDepartment Director, and ImagingSystem Operators

In every case where an imaging system is

found to be suffering from power quality

problems, healthcare facility CEOs, facility

engineers, imaging department directors,

and imaging system operators ask these

questions:

1. Is the healthcare facility vulnerable to

power quality problems?

2. What causes the facility to be vulnera-

ble to power quality problems?

3. After several years of operation, why

are we just now learning that the

facility has this vulnerability?

4. Why does a million dollar imaging

system have to be shutdown for a day,

or longer, for us to know that our

imaging systems are vulnerable to

power quality problems?

5. What is the cause of these power

quality problems to our imaging sys-

tems?

6. How are the problems going to be

solved?

7. Whose responsibility is it to solve the

40

Care should

be taken

not to leave

electrical

panels with

covers

removed and

unattended for

safety reasons

power quality problems with our

imaging systems?

8. How will we know that our imaging

systems will not again suffer from

power quality problems?

9. And, why does the $40,000 UPS that

we purchased with the imaging sys-

tem not protect our imaging from

power quality problems?

These are only the basic, but typical direct

questions, which healthcare facility person-

nel ask after they have experienced only one

shutdown of an imaging system. Often, these

questions are aimed at all parties that were

involved with the imaging system from the

time it was first discussed during the plan-

ning stage to the time when it was commis-

sioned after its installation. These parties

include:

n Imaging suite director

n Facility engineer

n Facility electrician

n Imaging system operator

n Field service engineer (imaging sys-

tem manufacturer)

n Field installation team (imaging sys-

tem manufacturer)

n Electrical contractor

n Installation specialist (consultant)

Identifying answers to these questions may

end up involving each party.

PQ Monitoring for Imaging Suites:Answering the Six Basic Questions

In determining the need for power quality

monitoring at an imaging suite, one might

ask “What are the six basic questions?” that

need to be addressed before utilizing power

quality monitoring. These questions are:

1. How would one know that monitoring

is needed?

2. If monitoring is needed, then how

many monitors should be used?

3. Would they need to be located tem-

porarily or permanently?

4. Where should a monitor be located in

an imaging suite?

5. How can one ensure access to the

monitor for setup and data down-

loading?

6. What is the future for monitoring

imaging systems and suites?

The complex nature, procurement costs,

operational costs, and cost of downtime

associated with diagnostic medical imaging

systems may very well warrant the need for

power quality monitoring. Monitoring may

be put in place as a proactive or reactive

power quality tool. Traditionally, imaging

system manufacturers and power quality

professionals have used monitoring as a

reactive tool to help identify the cause of sys-

tem damage and shutdowns. Monitor usage

has been limited in imaging suites due to

monitor cost and availability.

However, with the advancement and

improved cost effectiveness of monitors, they

are increasingly being used as a proactive

tool. Some manufacturers will place one or

two monitors at a single healthcare facility

with a focus on the imaging suite to capture

power quality data during normal imaging

system operation. System damage and shut-

downs may be more closely correlated to

electrical disturbances. This type of informa-

tion is useful to manufacturers in identifying

the cause of a power quality problem and a

resolution to the problem. If two monitors

are used, one might be located near the serv-

ice entrance or further upstream close to the

point where the 480-volt bus for the imaging

suite is derived on the primary side of a

transformer, for example. The other monitor

might be located at the main disconnect for

the imaging system. In cases where only one

41

In determining

the need for

power quality

monitoring at

an imaging

suite, one might

ask “What are

the six basic

questions?”

monitor is used, it might be located at the

480-volt derivation point or at the imaging

system disconnect.

Several inherent problems exist in locating

monitors in healthcare facility for imaging

system investigations. For monitors that are

contained in larger enclosures, auditors may

have a difficult time finding a place for their

temporary location out of the way of imaging

system operators and field service engineers.

Space in equipment rooms for imaging sys-

tems is usually very limited. The power and

data connection cables for a monitor should

be located such that they do not interfere

with the operation or safety of the main dis-

connect switch.

The monitor setup and data retrieval

processes also present challenges for audi-

tors. If the monitor cannot be setup for

remote operation, then the auditor will need

to revisit the site to make necessary setup

adjustments and download data. Remote

monitoring is becoming more of a standard-

ized feature incorporating user friendly con-

trols. However, most healthcare facilities will

not allow penetration into their firewall for

remote monitor access and control. Most

monitor users are still relying on an analog

telephone connection for remote operation.

In the future, imaging system manufacturers

may incorporate low-cost monitor system

into the front ends of their systems or utilize

digital signal procerssing (DSP) hardware to

implement embedded monitoring at the PDU

level. Some of these systems may only record

voltage to control monitor cost and size. This

would prevent the use of bulky monitors out-

side the system boundaries. Setup and data

management would also be streamlined and

included in the systems operating platform.

Even though an imaging system may contain

a built-in monitor, it may not be activated on

a reactive basis until a system problem

occurs.

Remote monitoring may also find a wide-

spread usage in imaging suites to monitor

several modalities in conjunction with moni-

toring at the main switchgear level. The illus-

tration on page 43 demonstrates a monitor-

ing system designed to capture power quality

data at five imaging system locations plus at

the main switchgear level where the 480-volt

is derived. This type of system would provide

enough data to determine in an electrical

disturbance occurred upstream or down-

stream of the main switchgear or specific

imaging system.

Common PQ Problems in ImagingSuites

Any power quality professional or auditor

with auditing experience knows the basic

types of problems that are encountered in a

power quality investigation. Quality power

containing even minor everyday electrical

disturbances can reap significant problems

on an imaging system if the electrical system

is plagued with wiring and grounding errors.

Wiring and grounding systems with errors

will make any electrical disturbance at the

source worse at the load, thus increasing the

likelihood of more damage to a system.

42

Inside the Main Disconnect Panel for an Imaging System

Imaging system

manufacturers

may incorporate

low-cost monitor

systems into the

front ends of

their systems

Audits on imaging systems and their electri-

cal system infrastructures typically reveal

many of the same types of problems. These

problems are usually associated with the

techniques used to design and install the

wiring and grounding systems that are used

to power and protect an imaging system.

Some examples of typical errors are included

in the sections below.

Grounding

Grounding methods and practices are

extremely important to the reliable operation

and life of an imaging system. Grounds for

imaging systems are required for safety and

help to protect the sensitive electronics

inside the imaging system. Electrical distur-

bances that are created inside or outside the

imaging system require low-impedance

ground paths back to the voltage source to

prevent ground currents from creating high

potentials across electrical devices where

they can cause damage to the system’s elec-

tronics. Power quality auditors on imaging

systems are consistently interested in the

integrity of the grounding systems.

Some imaging system manufacturers are now

requiring pre-installation power quality

audits for all new MRI, CT, cardiac X-ray, and

vascular X-ray imaging systems. With such

programs in place, service record trends

began indicating that many of the damaged

and shutdown systems involved grounding

errors inside the imaging suite associated

with the grounding system all the way back

to the service entrance.

43

Example of Remote Monitoring in an Imaging Suite

Grounding

methods and

practices are

extremely

important to

the reliable

operation

and life of

an imaging

system

Auditors have found that ground conductors

were missing from the branch circuits feed-

ing the imaging systems. In some cases, the

conduit ground (i.e., ground continuity

established by the connection of the conduit

system) was the only ground return path. In

the event of a electrical disturbance such as a

voltage surge or voltage sag, any ground cur-

rent would find it more difficult to return to

the source when it depended upon the con-

duit as a ground conductor. In cases where a

grounding conductor was available, most of

the time it could be traced back to a ground-

ing point in the facility other than the

ground point associated with the phase con-

ductors.

Older healthcare facilities are known for mul-

titudes of renovations, additions, and

changes to the electrical system. These activ-

ities take their toll on the electrical systems.

Some facility engineers in older facilities are

unable to locate their earth-grounding elec-

trode (i.e., ground rod) because it had been

installed for a very long time and no one ever

needed to examine it. When electricians are

unable to identify a reliable ground, they are

inclined to install an additional grounding

electrode for each new piece of imaging

equipment. In cases like this, a facility with

several grounding electrodes in different

locations cannot establish a equipotential

ground reference during an electrical distur-

bance. This often results in failed equipment

caused by potential differences developed

across the various grounding electrodes.

In other cases, auditors found that the

ground for an imaging system had not been

connected to the grounding system of the

facility. Some architects, electrical system

design engineers, and facility engineers even

recommend that isolated grounds be used

when installing new imaging systems. The

use of isolated grounds is not recommended

for these types of installations. Grounding

errors can cause many types of imaging sys-

tem problems including:

n Randomly appearing, intermittent

software errors

n System lock-ups that require imaging

system operators to reset or reboot

the equipment

n Random imaging artifacts that come

and go in intensity and frequency

n Hardware failures

n Interaction with or interference from

unrelated imaging systems in the

facility

The following photograph illustrates an

example of multiple grounding errors found

in a main disconnect panel for a CT imaging

system. Notice from the figure that there are

multiple grounding conductors with some

not connected to any ground bus. In addi-

tion, the facility-side and load-side ground-

ing conductors are connected together

through the use of two lugs. However, these

lugs are mounted on top of a painted surface

inside the disconnect panel. Mounting

grounding bars on top of painted surfaces is

not good practice. This creates a capacitive

effect between the ground bars and the metal

of the panel. Such effects prevent high-fre-

quency voltages and currents in the grounds

from finding a low-impedance path back to

the source. This type of grounding error

44

Example of Grounding Errors Found in a Main

Disconnect Panel of a CT Imaging System

Older

healthcare

facilities are

known for

multitudes of

renovations,

additions, and

changes to the

electrical

system

would make a voltage transient developed at

the source more damaging for the front end

electronics at an imaging system.

Transformers

In providing power to imaging system, trans-

formers are often necessary to match the

source voltage to the input voltage require-

ments for the system. Some of the imaging

systems are designed for multiple input volt-

ages ranging from 200 to 480 volts, 50 and 60

hertz. One-hundred and twenty (120) volt

requirements are also included but are usual-

ly derived from separate sources. Systems

that provide a range of input operating volt-

ages should be powered from the higher volt-

age (i.e., 480 volts). Utilizing the higher volt-

age will result in the use of phase and neutral

(if needed) conductors that have lower cur-

rent ratings. Important to power quality, uti-

lization of the higher voltage will result in

fewer shutdowns caused by voltage sags and

momentary interruptions.

In some imaging suites, the higher voltage of

480 volts is not readily available, meaning

that no 480-volt circuit is within a reasonable

distance of the imaging suite where distribu-

tion subpanels are located. If a new imaging

system is specified for such suites, then a

problem in supplying the correct voltage to

the area is encountered. Most suites have

three-phase 208/120-volt circuits readily

available with the appropriate ampacity rat-

ings.

The options for providing 480-volts to an

imaging suite are: 1) install a new 480-volt

circuit from the main switchgear, possibly

requiring a long run; 2) install a step-up 208-

volt delta to 480/2770-volt wye transformer,

requiring a special order transformer; or 3)

install a step-down 480-volt delta to 208/120-

volt wye transformer with the 208/120-volt

side connected as the primary and the 480-

volt side as the secondary. Options 1) and 2)

are the most costly and present the longest

installation times. Option 3) is the most cost

effective and most widely used, but can pres-

ent significant power quality problems if the

neutral and ground conductors are not con-

figured correctly on the primary and second-

ary sides.

The figure below illustrates an example of

step-down transformer used as a step-up

transformer to provide a 480-volt source to

an imaging system. The commonly provided

208/120-volt secondary voltage throughout

the facility is used as the primary voltage for

this transformer. However, a neutral conduc-

tor is pulled from the secondary of the 480-

volt delta to 208/120-volt wye transformer

and connected to the ‘primary’ of the

208/120-volt wye to 480-volt delta trans-

former. The use of this neutral conductor

(dashed line) should be avoided to prevent

power quality problems with imaging sys-

tems.

The figure on the following page illustrates

another example of step-down transformer

used as a step-up transformer to provide a

480-volt source to an imaging system. The

commonly provided 208/120-volt secondary

voltage throughout the facility is again used

as the primary voltage for this transformer.

However, a conductor is attached from the

neutral of the primary side of this trans-

former to the ground. The use of this neutral-

to-ground conductor (or bond) should be

avoided to prevent power quality problems

with imaging systems.

45

Transformation of Utility Voltage to Secondary Voltage to Imaging System Voltage

with Improperly Connected Neutral Conductor between the Two Transformers

In providing

power to

imaging

systems,

transformers

are often

necessary to

match the

source voltage

to the input

voltage

requirements

for the system

These types of installations can burn up

phase, neutral, and ground conductors dur-

ing a fault condition. How does this happen?

The neutral on the primary side of the trans-

former provides a path for the fault current.

This current in the primary windings of the

transformer sets up a current in the second-

ary windings of the transformer, which in

turn tries to drive the load. Then, these sec-

ondary circulating currents try to “pull up”

the primary phase that is down, and the neu-

tral current rises too high. You can avoid this

problem by eliminating the neutral connec-

tion on the primary winding of the trans-

former.

In other cases an isolation transformer like

that shown in the illustration below, is used

in a facility to isolate the primary from the

secondary. This type of installation seeks to

isolate incoming voltage transients from

reaching the output. In many of these instal-

lations, a neutral conductor is carried from

the primary side of the transformer to the

secondary side. The use of this neutral con-

ductor should also be avoided to prevent

power quality problems with imaging sys-

tems.

Here are a few more precautions to consider

regarding transformers:

n Do not use three transformers to cre-

ate a wye-wye transformer bank.

Because the transformers do not

share the same iron core, the third

harmonic circulating magnetic fields

do not cancel out, creating a pseudo

voltage of more than 300 volts and

damaging any system or piece of

equipment connected to it.

n Do not use an ungrounded delta

transformer to power diagnostic

imaging equipment. This configura-

tion can actually act like a voltage

doubler. In this type of installation,

auditors have documented phase-to-

ground voltage measurements as high

as 960 volts on a 480-volt system. To

46

Transformation of Utility Voltage to Secondary Voltage to Imaging System Voltage with Improperly

Connected Neutral-to-Ground Conductor at ‘Primary’ of the Reversed Step-Down Transformer

Use of an Isolation Transformer in a Imaging System Installation with Improper Use of

a Neutral Conductor from the Primary to the Secondary Side of the Transformer

Incorrect

transformer

installations

can burn up

phase, neutral,

and ground

conductors

during a fault

condition

eliminate this problem, use a wye or

corner-grounded delta transformer to

power diagnostic imaging equipment.

Electrical Connections

Although some power quality professionals

are debating where the line is drawn in the

old rule of thumb that 85 to 90 % of all power

quality problems are caused by wiring and

grounding errors, these errors, when present,

still pose significant problems for the

dynamic loads and sensitive electronics in

diagnostic imaging systems.

Wiring and grounding problems include

improperly grounded electrical equipment

(see Grounding above); undersized phase,

neutral, and ground conductors; improperly

terminated conductors, and loose conductor

connections. Two out of these four problems

will be found in almost every imaging suite

site with most of them containing some loose

connections.

Loose connections present an additional

impedance in the circuit that will impact the

shape of the voltage and current during

steady-state and dynamic conditions when

an electrical disturbance is occurring. A visu-

al inspection of a connection will not be

enough to tell if it is actually loose. Some

conductors may be so loose that when

touched may fall out of their lugs. For this

reason, it is important to check for loose

connections very carefully so as not to trip

equipment off line or run the risk of getting

shocked. Loose connections can be found by

using a 1,000-volt insulated UL- and OSHA-

approved screwdriver (in good condition) to

check the torque of the connections. A facili-

ty electrician or auditor may be able to turn a

lug screw 2 to 6 times on some loose connec-

tions. Tightening all connections, terminals,

and fittings will help to resolve many power

quality problems.

The photograph (right) illustrates a loose

connection that was found during a power

quality audit of an imaging system. This

loose connection was found in the load-side

phase conductors. This site experienced a

number of power quality problems including

failed power supplies and computer boards.

These connections were tightened to the

appropriate torque during the audit.

Meeting the PowerQuality Challenges ofImaging System SuitesThere are a number of challenges that must

be met in order to understand and improve

the power quality of imaging systems. These

challenges are related to the design, installa-

tion, maintenance, and repair of these sys-

tems. Meeting these challenges will allow end

users and manufacturers to reduce the cost

of operating these systems through fewer

service calls, fewer spare parts used, reduced

customer downtime, and improved patient

care services. Because each of these chal-

lenges are related to the technical backbone

of power quality—delivery of electrical power

through a healthcare facility electrical system

on a 24 x 7 x 365 basis, those involved in

addressing these challenges must understand

47

Example of Loose Conductor on Load Side of Fused Disconnect in

Main Disconnect of CT Imaging System

Eighty-five to

ninety percent

of all power

quality

problems

are caused by

wiring and

grounding

errors

the facets of each challenge. These facets

involve application of the National Electrical

Code, methods of providing backup power,

good power quality practices, and the finan-

cial impacts of power quality problems on

imaging systems.

Power Quality and the NationalElectrical Code

The purpose of the National Electrical Code

(NEC) addresses four areas: practical safe-

guarding, adequacy, intention, and relation

to international standards. In practical safe-

guarding, the purpose of the NEC is to pro-

tect “persons and property from hazards aris-

ing from the use of electricity”. In adequacy,

the NEC “contains provisions that are consid-

ered necessary for safety. Compliance there-

with and proper maintenance will result in

an installation that is essentially free from

hazard but not necessarily efficient, conven-

ient, or adequate for good service or future

expansion of electrical use”. In intention, the

NEC “is not intended as a design specifica-

tion or an instruction manual for untrained

persons”. With regard to standards, the NEC

addresses the fundamental principles of pro-

tection for safety as also recognized in safety-

related international standards.

The NEC does recognize the technical areas

of power quality but simply states that poor

power quality resulting from voltage drops

during load startup and distorted output

voltages may affect the operation of a facility.

However, the NEC does not recognize various

types of electrical disturbances nor does it

specify specific wiring and grounding meth-

ods as improving the power quality to a facil-

ity or a load. Although this may be true, the

healthcare facility engineer, imaging system

manufacturer, and the power quality auditor

should strive to ensure that all relevant NEC

requirements and codes, including those that

are recognized by the state and local govern-

ments, are met. In no way should an NEC

violation be instituted to resolve a power

quality problem. In all cases where NEC

requirements and codes have come up

against power quality issues, parties have

always been able to resolve the issues with-

out violating the NEC or standard power

quality practices. Continuing to utilize this

approach will foster improvements in imag-

ing system performance related to power

quality.

Impacts of Testing Emergency PowerSystems on Imaging Systems

Much of the focus on testing emergency

power systems in healthcare facilities has

been on how often the generator should be

tested and whether it should be tested under

actual load or under a real simulated load

(i.e., load bank). Regardless of testing condi-

tions, national standards and state and local

governments require that the generator be

started, up-to-speed, and carrying the

healthcare facility load with 10 seconds.

Imaging systems are used very frequently in

healthcare facilities. The chances of a power

outage occurring during imaging system use

are real. Facility engineers and imaging suite

directors must be assured that their systems

will be functional on emergency power in the

event of an interruption, outage, or natural

disaster.

Other than the issues of generator starting

and fueling, there are three basic concerns

with using emergency generators to power

any sensitive electronic equipment in a

healthcare facility. These are 1) disturbance

generation resulting from crossing over

(transfer) to emergency power from utility

power, 2) voltage distortion when operating

on emergency power, and 3) disturbance

generation resulting from crossing back to

utility power. Imaging systems simply do not

have enough inherent ride-through capabili-

ty to remain operational during the crossover

and during the cross back. The transition

from utility power to emergency power will

create some discontinuities in voltage

including transients and distortions. In some

48

The NEC

does recognize

the technical

areas of

power quality

cases, these discontinuities will damage the

sensitive power supplies on the front ends of

imaging systems. More importantly, imaging

systems will need to be restarted after

crossover or cross back. And, in some cases

by the time a system is restarted and opera-

tional again, the utility power has already

been restored.

During generator operation, voltage distor-

tions and frequency shifts may occur. In

most cases, voltage distortions (as long as

they are not too severe) will have less of an

impact on imaging system operation than

frequency shifts. Generally, imaging systems

can withstand a shift in frequency from ± 1

hertz to ± 3 hertz. Cases have been reported

where imaging system operation from emer-

gency power resulted in damage and shut-

down to imaging systems caused by distur-

bances and frequency shifts.

Power Conditioning for ImagingSystems

Various types of power conditioners may be

used to provide quality power to imaging sys-

tems. Several conditioners installed on the

inputs of imaging systems during laboratory

tests and field installations were unable to

support the dynamic load characteristics of

imaging systems resulting in a collapse of the

voltage and/or shutdown or permanent dam-

age to the conditioner and/or imaging sys-

tem. In some cases, the installation of a

power conditioner actually further degraded

the power quality. For these reasons, it is

imperative for any conditioner to be validat-

ed for proper operation with an imaging sys-

tem before it is specified, procured, and

installed.

Despite the obstacles in validating a uninter-

ruptible power supply (UPS) type conditioner

for use with an imaging system, UPSs have

been the most popular conditioner. Several

issues must be carefully considered during

the selection process. These issues include

n Load sizing

n Dynamic load testing

n Physical sizing

n Installation requirements

n Battery maintenance and reliability

X-ray, MRI, and CT systems present various

problems that must be considered when siz-

ing a UPS for these systems. Each of these

systems has very dynamic load characteris-

tics and voltage regulation requirements that

must be considered. As an example, a typical

CT system will have a continuous power

demand of 20 kVA but a maximum power

demand of 90 kVA from a few milliseconds

up to possibly 10 to 20 seconds. During this

time frame, the input voltage must be within

6% of the nominal line voltage. From this,

one can see that the CT system has a very

dynamic power demand. Interestingly

enough, some imaging systems are even

more dynamic with respect to their power

demands.

As another example, a vascular X-ray genera-

tor capable of delivering 100 kW of energy to

an X-ray tube requires 171 kVA of input ener-

gy. This is also a very dynamic load because

the vascular system will only have a continu-

ous power demand of 5 to 10 kVA. The power

demand for a vascular system could go from

5 kVA to 171 kVA for 10 to 40 milliseconds

and back to 5 kVA. This load cycling may be

repeated up to 12 times per second. These

described loads are very dynamic and in a lot

of cases are not compatible with a number of

UPS system outputs.

To identify the proper UPS, the operating

characteristics and dynamic load behavior of

the imaging system (e.g., CT, MRI, X-ray, etc.)

need to be clearly defined by the manufac-

turer. This information should be provided to

the UPS manufacturer or to the power quali-

ty researchers studying the compatibility in

order to fully understand what type of loads

will be placed on the UPS.

49

Any power

conditioner

should be

validated

for proper

operation with

an imaging

system before

it is specified,

procured, and

installed

Following is a list of the critical operating

characteristics that need to be provided to

the UPS manufacturer for the specific imag-

ing system:

n Continuous power demand

n Instantaneous or maximum power

demand

n The cycle time for these power

demands

• Definition of how long the maxi-

mum load demand will be required

• Definition of how often the maxi-

mum demand will be repeated

n Voltage regulation requirement

n Front-end rectifier design used in the

diagnostic imaging system that will

be requiring power from the UPS, for

example, does the imaging system use

a 6 or 12 pulse design

All diagnostic imaging systems do not have

dynamic power demands. Nuclear medicine,

positron emission tomography (PET), ultra-

sound, and information technology systems

(used to store and manage imaging data)

have a fairly constant power demand. Thus,

as a general rule, it is easier to size a UPS for

these loads based on nameplate power speci-

fication data and an appropriate UPS sizing

philosophy for non-linear loads.

Although this is true, there are a couple of

caveats that need to be considered:

n Ultrasound systems are very sensitive

to radio-frequency (RF) noise. RF

noise, which can compromise the

operation of an ultrasound system,

can be either radiated or conducted.

Radiated noise travels through the air

and conducted noise can travel

through any metallic medium. The

inverters in a UPS can potentially be

a major source of RF noise for the

ultrasound system and cause electro-

magnetic interference problems

resulting in ultrasound imaging prob-

lems.

n Nuclear medicine cameras can have

an imaging head gantry that is driven

by motors that could require large

momentary current demands that

might be larger than the kVA available

from a UPS. Because of this dynamic

load requirement, the UPS will possi-

bly need to be sized significantly larg-

er than the continuous power

demand that the system would

appear to require.

Power conditioners can be validated for use

with imaging systems by conducting special-

ized compatibility testing on the candidate

conditioner electrically paired with the imag-

ing system requiring conditioned power.

Compatibility engineers at EPRI Solutions

have significant experience in this area and

working relationships with most conditioner

manufacturers and some imaging system

manufacturers. Compatibility testing will

identify problem areas specific to the condi-

tioner and the imaging system that may oth-

erwise be resolved with system settings or

design changes.

Some common misconceptions regarding the

use of UPSs with imaging systems included:

n “If the diagnostic imaging system has

a continuous load of 20 kVA, then a 20

kVA UPS should be able to support

the system.” Wrong, the continuous,

average or normal power rating listed

for most diagnostic imaging systems

is the power demand when the sys-

tem is in an idle state. When a patient

imaging procedure is carried out on

the system, the power demand could

significantly increase to many times

larger than the continuous value list-

ed.

n “The load is only ‘momentary’ so the

UPS can be undersized.” Wrong, the

50

Power

conditioners

can be

validated

for use with

imaging systems

by conducting

specialized

compatibility

testing

diagnostic imaging system has volt-

age tolerance variations for which the

system was designed to operate. If a

UPS is undersized, the peak load of

the system will cause the input volt-

age to go out of that tolerance range

and cause image artifacts, system

errors, lockups, and possibly hard-

ware failures.

n “If the diagnostic imaging system

load is 60 kVA, then just doubling the

size of the UPS up to 120 kVA will take

care of the voltage regulation con-

cerns.” Wrong, the ability of the UPS

inverters to handle the dynamic step

load is the deciding factor. Doubling

or tripling the size of the UPS might

not enable the UPS to meet the volt-

age regulation requirements of the

diagnostic imaging system.

Financial Impacts of PQ Problems onImaging Systems

There is no doubt that power quality prob-

lems financially impact the operation of

imaging systems in healthcare facilities.

Financial impacts may be created by several

factors:

n Missed patient schedules – Patients

having to reschedule an imaging test

will have an extended stay in the

healthcare facility.

n Repeated imaging tests – Patients that

must have their imaging tests repeat-

ed due to corrupted imaging files or

artifacts, for example, must have their

tests repeated. This increases the cost

for that patient and reduces the avail-

able time slots for other patients.

n System reboots – Systems that must

be rebooted as a result of shutdown

due to a power quality problem

require the attention of imaging sys-

tem operators and suite directors. The

time required for these reboots is

billed back to the facility and increas-

es overhead costs. Systems that can-

not be rebooted must remain idle

until online or field service engineers

from the imaging system manufactur-

er can resolve the problem.

n Damaged hardware – Systems that

experience damaged hardware as a

result of power quality problems may

be out of service for a few hours up to

a few days or a week. In some cases, it

may be difficult to determine all of

the hardware components that were

damaged by electrical disturbances

until the system can be brought back

up to some operational level.

Additional parts may be required dur-

ing the repair to render the system

fully operational.

In all, it is very difficult to determine the

exact cost associated with imaging system

downtime. Manufacturers typically keep very

good system records which are used to deter-

mine which systems require a significant

number of spare parts and service time. With

imaging tests costing anywhere from a few

thousand dollars to several thousand dollars

and with the rising costs of healthcare, the

cost of system downtime is definitely not

decreasing.

51

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PROPRIETARY NOTICE:THIS DOCUMENT, IN WHOLE OR ANY PART THEREOF, CONTAINS PROPRIETARY INFORMATION, AND ISINTENDED FOR THE SOLE USE, OF SOUTH CAROLINA ELECTRIC AND GAS COMPANY (A SCANA COMPANY).COPYING THIS DOCUMENT, OR ANY PART THEREOF, IS EXPRESSLY FORBIDDEN WITHOUT PRIOR WRITTENAPPROVAL OF SOUTH CAROLINA ELECTRIC AND GAS COMPANY.

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LEGAL NOTICE:THIS REPORT WAS PREPARED BY THE EPRI SOLUTIONS, INCORPORATED (ESI). NEITHER THE EPRI SOLU-TIONS, EPRI, MEMBERS OF EPRI, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM: (A) MAKES ANYWARRANTY, EXPRESS OR IMPLIED, WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS,METHOD, OR PROCESS DISCLOSED IN THIS DOCUMENT; OR (B) ASSUMES ANY LIABILITIES WITH RESPECTTO THE USE OF, OR FOR DAMAGES RESULTING FROM THE USE OF ANY INFORMATION, APPARATUS, METHODOR PROCESS DISCLOSED IN THIS DOCUMENT; OR (C) GRANTS ANY RIGHT, TITLE, LICENSE, OR PERMISSIONTO USE ANY INFORMATION, APPARATUS, METHOD, OR PROCESS DESCRIBED IN THIS DOCUMENT. USE OFINFORMATION, APPARATUS, METHOD, OR PROCESS DISCLOSED IN THIS DOCUMENT MAY INFRINGE PRI-VATELY OWNED RIGHTS.

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52