digital radiography-avinesh shrestha

05/03/2023

1

Presented by:Av inesh Sh res tha

B .Sc .M IT (2nd year ) Ro l l No :90

IOM

20 t h Ma rch 2016

INTRODUCTION Historically, digital radiography referred to specialized modalities that produced digital images.(CT, MRI,

Nuclear Medicine, Ultrasound) which began in 1970’s

Since then, the use of digital technology in diagnostic medical imaging is rapidly expanding

Now, Digital radiography (DR) is a term used to describe general radiography when the radiographic images are in digital form and are capable of being displayed on a computer monitor where digital X-ray sensors are used instead of traditional photographic film

Here Radiation detectors produce output as electrical signal which is proportional to the radiation intensity and hence, digital image is acquired.

HISTORY Digital imaging modalities, such as CT, ultrasound, &

nuclear medicine gained widespread acceptance in 1970s.

The first form of digital imaging, Digital Subtraction Angiography, was introduced in 1977 and put to clinical use in 1980.

In CR, storage phosphor image plates were first used to record general radiographs in 1980.

The direct capture of x-rays for digital images was introduced with DDR which used CCD, in 1990.

Introduced in 2001,The flat-panel thin-film transistor detectors could expose & display images in near real time.

ADVANTAGE Principal advantage of DR over film/screen (F/S) radiography is the increased dynamic

range and the feature of Image enhancement/Post manipulation for better interpretation.

Also: Superior gray-scale resolution: uses 256 colors of gray compared to 16-25 shades on film

ADVANTAGE Exposure reduction: 50-80% less radiation with no loss of image quality

Fast: instantaneous viewing and interpretation

Cost effectiveness over time: less film and processing solutions

Environmental: No chemicals to dispose of

Efficiency: More productive and versatility of storing/sending images, Repeat examinations are reduced

Image enhancement/manipulation for better interpretation: Contrast, colorization, magnification, sharpness, and image orientation

DISADVANTAGE

Cost: High set up cost depending on manufacturer and features

Technology changes: System may become obsolete and no longer has support

Artifacts: unique to CR/DR can be introduced in the digital image acquisition and /or retrieval process.

The spatial resolution of DR image recording systems is lower than that of F/S image recording systems however ;The impact of such a lower spatial resolution system on clinical performance is not significant.

Dose-creep: since , exposure latitude is wide , high exposure technique may be used which increases the patient dose which is called dose-creep. This can be reduced by exposure indicator or exposure index which gives the user feedback about the actual dose.

COMPONENTS OF DR

Capture element: in which the X- ray is captured. In CR, it is PSP & in DR it may be CsI., GdSO or a-Se.

Coupling element: which transfers x-ray generated signal to the collection element. It may be a lens or fiber optic assembly, a contact layer or a-Se.

Collection element: may be a photodiode, CCD or TFT. The photodiode & CCD are light-sensitive whereas TFT is charge sensitive.

CHARGED COUPLED DEVICE Originally invented by bell laboratories in 1969

The basic CCD consists of a series of metal oxide semiconductor capacitors with discrete pixel electronics that are fabricated very close together on the semiconductor surface

Is an integrated circuit made of crystalline silicon.

The silicon surface of CCD chip is photosensitive.

CCDs are used as indirect type of digital receptor.

Thermally generated electrons are indistinguishable from photo-generated electrons . They constitute a noise source known as ‘Dark Current’ and it is important that CCDs are kept cold to reduce their number.

CHARGED COUPLED DEVICE

WORKING OF CCD Light – incoming photons falls onto the surface of a pixel.

Result- generates free electrons in the silicon of the CCD due to the photoelectric effect, proportional to the number of photons striking it. These electrons collect in little packets.

Each pixel contains 3 electrodes as shown. The e-s are kept in each pixel because of these electronic barriers (voltage) on each side of the pixel during exposure.

Once the CCD chip has been exposed, the electronic charge that resides in each pixel is read out. Charge is readout in “bucket brigade” fashion ,i.e.; The electronic charge is shifted pixel by pixel by appropriate control of voltage level at the boundaries of each pixel.

CCD Bucket Brigade Analogy

RAIN (PHOTONS)

BUCKETS (PIXELS)

VERTICALCONVEYORBELTS(CCD COLUMNS)

HORIZONTALCONVEYOR BELT(SERIAL REGISTER)

MEASURING CYLINDER(OUTPUT AMPLIFIER)

Exposure finished, buckets(Pixels) now contain samples of rain(Photons) collected as charge.

Conveyor belt starts turning and transfers buckets(Pixels).The Rain(Photons) collected on the vertical conveyer belt as charge is tipped into buckets on the horizontal conveyor.

Vertical conveyor stops. Horizontal conveyor starts up and tips each bucket in turn into the measuring cylinder(output amplifier) .

`

After each bucket has been measured, the measuring cylinder is emptied , ready for the next bucket load.

A new set of empty buckets is set up on the horizontal conveyor and the process is repeated.

Eventually all the buckets have been measured, the CCD has been read out.

ADVANTAGES OF CCD The main advantage of the CCD is the extremely fast discharge time which is

useful in cardiac catheterization where high-speed imaging is critical to visualizing blood flow.

Sensitivity-ability to detect & respond to very low levels of visible light. Hence reduces the patient dose.

Dynamic range- ability to respond wide range of light intensity. Radiation response of CCD is linear.

Size- its size is very small that makes it highly adaptable to DR in its various forms. It measures 1 to 2 cm.

DISADVANTAGE

The principal disadvantage of CCDs is that they are physically small and consequently can image only a small region.

When large FOV is needed to image, it is impossible to image the light into the surface of CCD chip without losing the light photons.

The amount of light lost is proportional to demagnification factor (input area of screen/output area of CCD chip) The assembly of multiple CCD (mosaic CCD) is used to overcome this problem.

PMT Photomultiplier tubes are members of vacuum tubes, are extremely sensitive detectors

of light in the ultraviolet, visible and near infrared ranges of the electromagnetic spectrum.

These detectors multiply the current produced by incidentlight by as much as 100 million times ,in multiple dynodes

stages, enabling individual photons to be detected when the incident flux of light is very low.

Their operation depends on two topics The photoelectric effect, in which incoming photons strike a metal surface, and eject an electron from the surface.

Emission of electrons from a metal following the metal being struck by a previous electron. Providing the incoming electron has sufficient kinetic energy, more than one electron is emitted, the actual number being a strong function of the incident electron's kinetic energy.

Contd…. A photomultiplier consists of five main parts:

a cathode which is coated with a material of low work function An anode to collect electrons A series of dynodes between the cathode and the anode An external power supply which produces an electric field between the cathode and

first dynode, between the dynodes, and between the last dynode and the anode. An external current meter to measure the number of electrons collected at the

anode, and a recorder to collect the information.

PHOTODIODE A photodiode is a semiconductor device capable of photo-detection and

then converting light into either current or voltage, depending upon the mode of operation.

Photodiodes are similar to regular  semiconductor diodes except that they can be exposed (to detect UV or X-rays) to allow light to reach the sensitive part of the device.

There are many types of photodiode (PN photodiode, PIN photodiode, Avalanche photodiode, Schottky photodiode) designed for use but specifically PIN photodiode is used to increase the speed of response.

NEWER COLLECTION ELEMENTS Recent Advancement is focusing on Complimentary-Metal-Oxide

Semiconductors(CMOS) technology and are being used instead of CCD

These are newer design of Transistors.

CMOS is a type of MOSFET(Metal-Oxide-Semiconductor Field- Effect Transistor) which are basic electronic devices used to direct and control logic signals in IC design.

A MOSFET has four terminals: gate, source, drain, and substrate (body), and according to the nature of doping into these terminals MOSFET are classified as n-MOS or p-MOS or C-MOS

Contd…

SUBSTRATE(BODY)

DRAINSOURCE

GATE(Polysilicon

)INSULATOR(SiO)

GDS

BASIC DESIGN OF MOSFET

B

N-MOS

SUBSTRATE(BODY)(p-type)

DRAIN(n-type)

SOURCE(n-type)

GATE(Polysilicon

)INSULATOR(SiO)

GDS

B

SUBSTRATE(BODY)(n-type)

DRAIN(p-type)

SOURCE(p-type)

GATE(Polysilicon

)INSULATOR(SiO)

GDS

P-MOS

B

P-type

N-type

N-type

N-type P-type P-type

GATE(Polysilicon)INSULATOR(SiO)

GATE(Polysilico

n)INSULATOR(Si

O)

GGG

DD

SSDS

N-MOS P-MOS

C-MOS

B B

Coming to CMOS, it is a complementary MOSFET meaning it has both PMOS and NMOS connected somehow to each other. Hence any circuit which uses both PMOS and NMOS is a CMOS circuit.

CMOS VS CCD Modern CMOS sensors use a more specialized technology and advancements have eradicated

the difference in light sensitivity between a CCD and CMOS sensor at a given price point.

The disadvantages of CCD sensors are that they are analog components that require more electronic circuitry outside the sensor, they are more expensive to produce, and can consume up to 100 times more power than CMOS sensors which can lead to heat issues, which not only impacts image quality negatively, but also increases the cost.

Unlike the CCD sensor, the CMOS chip incorporates amplifiers and A/D-converters, which lowers the cost since it contains all the logics needed to produce an image, i.e. ,Every CMOS pixel contains conversion electronics.

Compared to CCD sensors, CMOS sensors have better integration possibilities and more functions. However, this addition of circuitry inside the chip can lead to a risk of more structured noise, such as stripes and other patterns.

Contd… Nowadays flat panel X-ray detector (FPD) components employ an

innovative CMOS sensor design to provide a new level of performance and reliability for X-ray systems.

Its major advantages are high frame rates, low noise, high reliability, no image lag, high spatial resolution, smaller system size. a faster readout, and lower power consumption

Clinical benefits in medical applications include lower radiation dose to the patient without compromising image quality.

CMOS Detectors are more flexible, more stable, more sensitive, and faster than TFT-based flat panel detectors while producing higher resolution images.

FLAT PANEL DETECTORS The flat panel comprises a large number of individual detector

elements, each one capable of storing charge in response to x-ray exposure.

During exposure, charge is built up in each detector element and is held there by the capacitor. After exposure, the charge in each detector element is read out using electronics .

Because each detector element has a transistor(a simple electronic switch that has three electrical connections the gate, the source, and the drain) and the device is manufactured using thin-film deposition technology,so these flat panel systems are also called thin-film transistor (TFT) image receptors.

It may be of Direct or Indirect in nature which categorize the DR into Direct DR and Indirect DR.

FLAT PANEL DETECTORS

Contd…

FLAT PANEL DETECTORS

Each detector element in a flat panel detector has a transistor associated with it; the source is the capacitor that stores the charge accumulated during exposure, the drain is connected to the readout lines(vertical wires) , and the gate is connected to the horizontal wires.

Negative voltage applied to the gate causes the switch to be turned off(no conduction from source to drain), whereas when a positive voltage is applied to the gate the switch is turned on (source is connected to drain).

R1C1(A)

R1C2(B)

R2C2(D)

R2C1(C)

S1 S2

MultiplexerDigitizer

Scan

Con

trol

Charge AmplifierR1

R2

C1 C2

Detector element(charge collector)

R=Gate lines

C=Readout line(Drain)

TFT

SG

D TFT

R1C1(A) R1C2(B)

R2C2(D)R2C1(C)

S1 S2

MultiplexerDigitizer

Scan

Con

trol

Charge Amplifier

R1

R2

C1 C2

illustrates the readout process used for flat panel detector arrays. four detector elements are shown (A through D). Two gate lines (rows R1, R2) and Two readout lines (columns C1,C2) are illustrated.

Detector element(charge collector/source)

R=Gate lines

C=Readout line(Drain)

SG

DTFT

SCANNED PROJECTION RADIOGRAPHY Developed by CT vendors to facilitate patient positioning

after the introduction of 3rd gen CT scanners.

It remains in use with virtually all current multi-slice helical CT imaging systems.

The patient is positioned on the CT couch and then is driven through the gantry while the x-ray tube is energized.

The x-ray tube and the detector array do not rotate but are stationary, and the result is a digital radiograph.

Contd…

In SPR, the x-ray beam is collimated to a fan by pre-patient collimators. Post-patient image-forming x-rays likewise are collimated to a fan that corresponds to the detector array—a scintillation phosphor, usually NaI or CsI—and is coupled to a linear array of CCDs through a fiber-optic light path.

At the present time, SPR is reemerging with some modification as a promising adjunct to digital radiographic tomosynthesis (DRT).

Contd…

A scanned projection radiography of the entire of the body obtained in computed tomography(Courtesy,Colin Bray,Baylor College Of Medicine))

COMPUTED RADIOGRAPHY(CR) Digital radiography that records radiographic images on photostimulable phosphor

plates instead of film/screen image receptors and the image is acquired in Digital Form.

CR cassette with Imaging Plate(IP) Conventional Cassette with Screen-Film

IP

Screen

Film

CR is based on the principle of Photo-stimulable Luminescence.

The commonly used phosphor is barium fluorohalides: BaFBr (85%) and BaFI (15%): Eu (europium). (X-ray + Eu+2 = Eu+3 + e–)

Some light is promptly emitted, but much of the absorbed x-ray energy is trapped in the PSP screen & can be read later. So, also called storage phosphors screen.

Cassette is placed in a reader(Digitizer) to capture and analyze the image data.

COMPUTED RADIOGRAPHY(CR)

The phosphor can be made as flexible screen, which is enclosed in a rugged cassette and is called imaging plate

Imaging plate was first introduced by Fuji, Japan, in 1983 and is similar to that of a screen-film cassette

PSPs are grown as linear filaments in the IP (needle-shaped technology), enhances the absorption of x-rays & limit the spread of stimulated emission.

IMAGING PLATE

PROTECTIVE LAYER: a very thin, tough, clear plastic that protects the phosphor layer from handling trauma.

PHOSPHOR LAYER: the active layer of Ba FX: Eu +2 , traps electrons during exposure

REFLECTIVE LAYER: It sends light in a forward direction when released in the cassette reader.

CONDUCTIVE LAYER: This layer grounds the plate to reduce static electricity problems and to absorb light to increase sharpness.

BASE: is a semi rigid material that provides the imaging sheet with strength and is a base for coating the other layers.

IMAGING PLATE

e

X-ray photon

X-ray interaction with valence electron

Excitation of electron into a metastable statee

He-Ne/solid State Laser

De-Excitation with Photostimulable Luminescence

e

eExcitation Followed by spontaneous De-excitation with Fluorescence

X-ray photon

X-ray interaction with valence electron

Electron Trap(F-centre)(Due to the presence of Eu)

What happens In Imaging Plate??

CR READER(DIGITIZER)

The CR reader represent the mechanical, optical and computer modules.

MECHANICAL FEATURES

When the CR cassette is inserted into the CR reader , the IP is removed & is fitted to a precesion drive mechanism.

The drive mechanism moves the IP slowly along the long axis of the IP, known as slow scan mode.

A deflection device deflects the laser beam back & forth across the IP which is called fast scan mode.

It precisely interrogate each meta-stable e-s of the latent image in the precise fashion.

Its components include the laser, beam-shaping optics, light-collecting optics, optical filters & a photodetector.

The he-ne gas laser is the source of stimulating light which is monochromatic.

The gas laser has been largely replaced by the solid state laser which produces the light of longer wavelength which are less likely to interfere with emitted light.

OPTICAL FEATURES

OPTICAL FEATURES The light beam is focused into the reflector by a lens system that keeps the beam

diameter small (less than100micrometer).

Smaller the laser beam size , higher the spatial resolution of the image.

Emitted light is focused by fiber optic collection assembly & is directed at the photodetector.

Before photo-detection , the light is filtered so that none of the long-wavelength stimulation light reaches the photo-detector. Filtering improves SNR.

COMPUTER CONTROL The output of the photodetector is a time varying analog signal that is

transmitted to a computer system

The time-varying analog signal from the photo detector is processed for amplitude, scale, and compression.

This shapes the signal before the final image is formed.

Then, the analog signal is digitized with the help ADC with attention paid to proper sampling and quantization.

The computer of the CR reader is in control of the slow scan & the fast scan.

IP

Optical filterPMT

He-Ne/Solid state Laser

Light

Guide

Rotating Polygon or Oscillating Mirror

To ADC

Fast sc

an(Laser

Direction

)

Slow scan(IP Direction)

Beam Shaping Optics

INSIDE DIGITIZER

IPBright white fluorescent light

Erasing The IP for reuse

INSIDE DIGITIZER

INSIDE DIGITIZER

DDR

Direct digital radiography, a term used to describe total electronic image capturing.

Eliminates the need for an image plate altogether.

uses a transistor receiver that captures and converts x-ray energy directly into digital signal that is seen immediately on monitor then sent to PACS/ printer/ other workstations for viewing.

DDR

Indirect Conversion

CsI,GdSO scintillatorThin film transistor

CsICharge

coupled device

Direct Conversion

Photoconductora-Se

Direct DR Indirect DR

a-Si photodiode/CCD

(CsI/GdOs)

Basic Difference of Direct DR and Indirect DR

Basic Difference of Direct DR and Indirect DR

INDIRECT DR Indirect Conversion with a CCD.

X-ray energy is converted into light by a scintillator such as Tl doped cesium iodide. The amount of light emitted is then recorded by the CCD, and the light is converted into electrical charges.

It is an array consisting of several CCD chips which forms a detector area similar to that of a flat-panel detector.

CCDs can be used for radiography as part of either a lens-coupled CCD system or a slot-scan CCD system

CCD-based systems were comparable to flat-panel detectors in terms of image quality and allowed slightly superior low-contrast visualization.

Contd… Indirect Conversion with a Flat-Panel Detector Indirect conversion DR systems are “sandwich” constructions

consisting of a scintillator layer, an amorphous silicon photodiode circuitry layer, and a TFT array.

When x-ray photons reach the scintillator, visible light proportional to the incident energy is emitted and then recorded by an array of photodiodes and converted to electrical charges.

These charges are then read out by a TFT array.

The scintillators usually consist of CsI or GdSO .The advantage of CsI-based scintillators is that the crystals can be shaped in thin needles, which can be arranged perpendicular to the surface of the detector.

CsI

Contd… One further advantage of flat-panel detectors is their small size.

Because CsI based flat-panel detectors are highly vulnerable to mechanical load because of their fine structure, these systems cannot be used outside of fixed installations and therefore lack mobility.

Portable flat-panel detector systems make use of GdOS based scintillators, which are as resistant to mechanical stress as are storage phosphors .

Image generation with flat-panel detectors is almost a real-time process, with a time lapse between exposure and image display of less than 10 seconds.

DIRECT DR Direct conversion requires a photoconductor that converts x-

ray photons into electrical charges by setting electrons free .

Typical photoconductor materials include amorphous selenium, lead iodide, lead oxide, thallium bromide, and gadolinium compounds. The most commonly used element is selenium.

Selenium-based direct conversion DR systems are equipped with either a selenium drum or a flat-panel detector.

A-Se is both capture & coupling element.

Selenium is a photoconductor that when exposed to radiation alters its electrical conductivity proportional to the intensity of radiation

Contd…

In the former case, a rotating selenium drum, which has a positive electrical surface charge, is exposed to x-rays. During exposure, a charge pattern proportional to that of the incident x-rays is generated on the drum surface and is recorded during rotation by an ADC. However, because of their mechanical design, selenium drum detectors have no mobility at all.

A newer generation of direct conversion DR systems make use of selenium-based flat-panel detectors.

1. Radiologic Science For Technologists-Stewart Crlyle Bushong

2. Essentials of Radiologic Science-Robert Fosbinder, Denise Orth

3. The Essential Physics for Medical Imaging-Bushberg

4. Digital Radiography Detectors: A Technical Overview

5. Advances in Digital Radiography: Physical Principles and System

Overview1

6. Charged Coupled Device Imaging By Marc Castro

7. CCD and CMOS sensor technology: Technical white paper

REFERENCES