Medical Physics

Residents Training Program

Department of Medical Imaging

King Abdulaziz Medical City- Riyadh

X-RAY GENERATION

X-Ray Production

electron

Bremsstrahlung radiation

x-ray

Bremsstrahlung Process

A large potential difference is applied between two electrodes with opposite charges.

Electrons released by the cathode travel towards to anode and gain kinetic energy before they impact with the target.

The vast majority of energy is converted to heat in the target material.

Approx. 1% of this energy is used to produce x-rays.

The loss of kinetic energy to form x-rays is called the Bremsstrahlung process.

Higher energy x-rays are produced by electrons that move close to the nucleus of the atom.

The maximum x-ray energy will be produced when a direct impact occurs between the electron and the nucleus of the atom.

The bremsstrahlung spectrum shows the range of energies that x-rays produced by the Bremsstrahlung process can have.

An unfiltered spectrum will be a wedge shape relationship between the number of photons and the x-ray energy.

The highest x-ray energy will be determined by the kVp.

The x-ray tube will act as a filter and preferentially absorb the low energy x-rays.

The most probable x-ray energies in a filtered Bremsstralung spectrum are about 1/2 to 1/3 of the maximum energy

kVp

X-ray Energy

Nu

mb

er

of

X-r

ays

Pro

du

ce

d

X-Ray Production Characteristic radiation

1. Electron knocks out

Inner Shell electron,

leaving an Inner Shell “hole”

2. Outer Shell electron

moves in to fill the

Inner Shell “hole”

3. X-ray is emitted with

energy equal to the

difference in energy

of the Inner and Outer

shells.

Characteristic X-ray Production:

When the energy of the incident electrons exceeds the binding energy of the inner shell electrons of the target material.

The following electron cascade will produce characteristic x-rays.

A variety of x-rays can be produced depending on the shell in which the electron was removed and the shell from which the hole was filled.

The transitions are denoted by the shell in which the hole was created and the transition of the electron to fill the hole.

(e.g. Ka = L K , Kb = M K)

Characteristic x-rays other than the K-shell x-rays are almost always completely absorbed the the x-ray tube of most diagnostic x-ray systems and therefore can be ignored.

X-ray Energies

Measured in units of electron volts (eV)

Diagnostic x-ray energies in the thousands of electron volts (keV)

Maximum x-ray energy produced by Bremsstrahlung processequals the maximum energy of the incident electron.

Maximum electron energy equals the peak potential difference between anode and cathode

(Peak Potential = kVp).

Characteristic Radiation Bremsstrahlung Radiation

X-ray energy X-ray energy

X-ray energy

Bremsstrahlung Radiation +

Characteristic Radiation

Bremsstrahlung Radiation +

Characteristic Radiation + Filter

(Tungsten

k-shell 58-67keV)

X-ray Spectrum

kVp mA

mA

kVp

kVp

X-ray Tube and Tube Housing

Oil

HT Cable

Radiolucent

Window

Oil Expansion

Diaphragm

Thermal

Switch Stator Glass Envelope

Oil

X-ray Tube

Cathode

Block

Focusing

Cup Filament Target

Rotor

Rotor/Anode

Support

Stator or

Induction Motor

Bearings

Anode

Stem Anode

Filament:

• Thermionic emission of electrons at >2200 C

• Usually made from Tungsten (W)

• High melting point (3380C)

• Coil approx. 1-2cm long and 2-5mm diameter

• Only 1% of energy goes to produce electrons

Anode:

• Target usually made from Tungsten (W)

• (Mammography uses Molybdenum (Mo))

• High melting point (3380C), High Z (74)

• Heat eliminated by rotating anode & copper anode block

Focal Track

(about 6mm wide)

Anode Disk

Rotating at

> 3000 rpm

Anode Angle

Typical 12 to 15°

Heel Effect

This refers to the reduction in x-ray beam intensity on the anode side of the beam.

This is due to the fact that x-rays must pass through a greater thickness of the anode to pass out of the x-ray tube.

Electron

Beam

100%

75%

Heel Effect

Heel Effect 10-20

electron

Intensity of x-rays in the

x-ray beam is not the same.

Intensity will decrease

as we move closer to the

anode.

Inverse Square Law

The decrease in intensity is proportional to the square of the distance from the x-ray source.

I1 X2

X1

I2

e.g. if,

X1 = 1m and X2= 2m

Then,

X1

X2

2

I2 =

= 1

2

2

= 1

4

I1

I1

I1

X-ray Generators

Alternating Current (AC) power source

Transformers used to increase or decrease voltage

Direct Current (DC) power requirements

Filament (electron production)

Low Voltage (10 V)

High Current (4 A)

Tube Potential (electron acceleration)

High Voltage (50,000-120,000 Volts)

Low Current (100-1000 mA)

Power converted from AC to DC

High Frequency Generators &

Waveform Rectification and Smoothing

Transformers

Transformers are electrical devices that increase, decrease or isolate electrical current and voltage using electromagnetic induction.

The transformer has an iron core and is wound with 2 coils of insulated wire.

An electrical current flowing in the primary coil will induce a current in the secondary coil.

The voltage in either coil will be determined by the following Law of Transformers: V p = N p V s N s

A transformer that results in an increase in the voltage of the secondary coil is called a step-up transformer.

A transformer that results in an decrease in the voltage of the secondary coil is called a step-down transformer.

e.g. (anode cathode circuit)

Voltage of Primary = 50 Volts

Ratio of turns in Primary:Secondary = 1:1000

50 = 1 hence Vs = 50 x 1000 = 50,000 V Vs 1000

e.g. (filament circuit)

Voltage of Primary = 50 Volts

Ratio of turns in Primary:Secondary = 5:1

50 = 5 hence V s = 50 = 10 V V s 1 5

Transformers

In order to calculate the current flowing through each

circuit we must use the following law:

Vp Ip = Vs Is

Where

Vp = Voltage in primary circuit

Vs = Voltage in secondary circuit

Ip = Current in primary circuit

Is = Current in secondary circuit

e.g. (anode cathode circuit)

Voltage of Primary = 50 Volts

Voltage of Secondary = 50,000 Volts

Current of Primary = 10A

50 x 10 = 50,000 x Is

Is = 500 = 0.01A = 10mA 50,000

e.g. (filament circuit)

Voltage of Primary = 50 Volts

Voltage of Secondary = 10 Volts

Current of Primary = 10A

50 x 10 = 10 x Is

Is = 500 = 50A 10

Autotransformers

Autotransformers are constructed so that both the number of turns in the coils of both the primary and secondary coils are selectable.

This allows for a range of voltages to be selected Autotransformers are commonly used in most modern x-ray systems.

Rectification

Smoothing Result

Diodes and Rectifiers

A diode is a simple electrical device that allows current to flow in only 1 direction.

Most modern x-ray systems use solid state diodes.

Diodes are used in a rectifier circuit to allow the flow of electrons in only 1 direction.

Rectification is done on the secondary electrical circuit.

Half-wave rectifiers allow current to flow in one direction but only half of the time.

Full-wave rectifiers allow current to flow in one direction for the the entire AC cycle.

Half-wave

Rectification

Full-wave

Rectification

Multi-Phase Generators

The output of a single phase generator will have a waveform with peaks and troughs even after smoothing.

Multi-phase generators are used to smooth out the resultant waveform.

3-phase power supplies are often used with 3 single phase power sources with each 120 ahead of the next.

If the 3-phase input power is rectified we have a 6-pulse power output.

It is also possible to create a 12 pulse signal with a different combination of rectifiers.

0 Phase 120 Phase 240 Phase

Sum of 3 Single Phases Rectified 3-Phase

Output

High Frequency Generators

An Inverter Generator can be used to create an even smoother output power supply than the 3-phase generator.

Simple rectification and smoothing of a single or 3-phase input is first performed to produce an almost DC waveform.

An Inverter is used to convert this waveform into a high frequency AC waveform.

The high frequency waveform is then rectified and smoothed to produce the final output.

The final output is almost a continuous voltage output.

Voltage Ripple

Ripple is the amount of variation in the voltage supply to the x-ray tube.

Voltage Ripple (%) = V max – V min x 100 ------------------------ Vmax

The lower the voltage ripple the more the waveform is like a Constant Potential, which is the ideal power supply to the x-ray tube.

[ ]

Voltage Ripple

Generator Type Typical Waveform kV Ripple

Single Phase

1-pulse

100%

Single Phase

2-pulse

100%

3-Phase

6-pulse

13-25%

3-Phase

12-pulse

3-10%

Medium/High

Frequency

4-15%

Constant Potential <2%

Switches and Timers

Most modern x-ray machines use electrical switches to switch the x-ray tube on and off.

Electrical timers can switch a tube on and off with an accuracy of 1msec.

An x-ray tube can also be switched on and off auto matically using a phototimer (also called automatic exposure control).

A phototimer uses an ionization chamber to detect x-rays passing through the patient before they enter the detector.

The tube is switched off once a certain x-ray intensity is reached.

Factors Affecting X-ray Emission

There are 3 terms that are used to describe the output of the x-ray tube: Quality, Quantity and Intensity.

There are 6 major factors that influence the output of an x-ray tube: target material, applied voltage potential, tube current, applied generator waveform, exposure time and beam filtration.

1. Target Material:

The target material affects the quantity of bremsstrahlung

x-rays produced and the quality of the characteristic

x-rays produced.

2. Applied Voltage Potential:

kVp will determine the maximum energy of the

bremsstrahlung x-rays produced and therefore affects

beam quality. Increasing kVp will also greatly increase

the quantity of x-rays produced. Intensity is proportional

to the square of kVp.

I kVp2

3. Tube Current:

The tube current controls the number of electrons

accelerating towards the anode and hence will also

affect the number of x-rays produced. The quantity of

x-rays produced is proportional to the tube current.

4. Generator Waveform

The waveform will determine the average applied potential

to the anode/cathode. Hence this will also affect the

quality of the x-rays produced.

100% Ripple

5% Ripple

5. Exposure Time:

Exposure time will determine the total time for x-ray

production. Hence this will be directly related to the

quantity of x-rays produced. Quantity is proportional to

the product of tube current and exposure time (mAs)

6. Beam Filtration:

Filtration modifies the quantity and quality of the x-ray

output by selectively removing the low energy x-rays

from the spectrum.

Summary

Quantity is proportional to target material, kVp, mA

and exposure time.

Z target x kVp2 x mAs

Quality is determined by kVp, generator waveform and tube filtration.

Factors Affecting X-ray Emission

Heat Loading

Simple unit of Heat Loading is the Heat Unit (HU)

HU = kVp x mA x Exposure Time

Must multiply HU by 1.35 for 3-Phase and High Frequency

Generators and 1.4 for Constant Potential Generators

An Anode Cooling Chart is usually supplied with any x-ray tube, and this chart can be used to predict the heat dissipation of the tube.

We can convert HU to Energy in Joules simply,

1 HU = 1.4 Joule.

Power Ratings

The power rating refers to the maximum energy able to be delivered by the generator.

Loads are usually calculated for a 100 kVp and 0.1 second exposure.

Power (kW) = 100kV x Amax (for 0.1 sec exposure)

Typically

2-Pulse Single Phase < 50kW

6-pulse 3 phase <100kW

12-pulse 3 phase <150kW

High Freq 50-100kW

Single Exposure Rating Chart

This chart shows the allowed combinations of kV, mA

and exposure time for a single exposure for a specific

x-ray tube and generator at a particular room temp.

When reading a chart;

Find the intersection of the kV and exposure time

Determine the corresponding mA

Compare the maximal mA allowed to the mA you need.