Digital Radio Graphs

8/6/2019 Digital Radio Graphs

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DIGITAL

RADIOGRAPHY

Moderator:Mr. Ram Singh (Lecturer)

Deptt. Of Radio-Diagnosis & Imaging

P.G.I.M.E.R

Presented By:Krishna Sharma

B. Sc. Med. Tech. (X-Ray) 3rd Year Student

Deptt. Of Radio-Diagnosis & Imaging

P.G.I.M.E.R 1


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DEFINITION

:

It is defined as the image acquired from thenumeric that is discrete binary digits by

use of computers .Whereas in conventional radiography

information is represented in the analog orcontinuous form rather than a discretefashion.

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ANALOG IMAGE DIGITAL IMAGE

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INTRODUCTION:

In 1982, the first computed Radiography System was

developed by the Fugi Film Corporation. This used

Photo-stimulable Phosphors as a image receptor.

In 1990, Direct Capture Radiography (DR) or FlatPanel System started , which used Amorphous Silicon

or Selinium as detectors.

As the newer technology are being introduced, there

has been progressive & evolutionary growth in the

medical field. The pathological conditions can be

diagnosed more clearly, acurately & in comparatively

less time, resulting in increased patient cure rate.5


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Cont

Digital technology has increased the image processing

speed & decreased cost to the patient where totally

electronic radiographic image detection, storage &

display are beginning to replace film in a no. of

procedure.

More importantly radiographic image stored in a

digital memory can be manipulated in many ways that

have been impossible with film. Such an imagemanipulation enables the radiologist to isolate the

image information i.e. which can not be recognized on

a conventional radiograph.

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CONVENTIONAL

METHOD:Since the clinical use of x-rays in 1895,majority of

radiographic examinations have been carried out by

the conventional method.The beam is projected through the patient and the

transmitted beam, which has information about the

body structures, is made to strike the cassette

containing the film and the intensifying screens. This

way the latent image is produced.

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The latent image can be made visible and permanent by processing it with suitablechemicals.

This conventional method of obtainingradiographs has dominated the field of

radiography for many years. But it has beenrealized that the film-screen system has its ownlimitations.

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LIMITATIONS OF CONVENTIONAL

RADIOGRAPHY:

1. After the film has been exposed,the information

contents cannot be enhanced.

2. If the radiograph is too dark or too light,it has to berepeated. This results in extra exposure to the patient.

3. The completion of the examination is delayed as the

film has to be processed to convert the latent image

into a permanent one.

4. A magnifying glass may be required to see very

small structures in detail.

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5. Copied radiographs have an inferior quality as

compared to original ones.

6. The film is a physical object and so it requires

considerable space for storage.

7. Films can only be in one place at a time and they

also get deteriorated with passage of time.

8. Film can not be stored for longer time when

required.

9. Faults in the film can lead to repeat theexamination.

10. Dynamic range of the x-ray film is limited.

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COMPUTED RADIOGRAPHY:

PRINCIPLE:

-In the C.R. system we use an imaging platemade of a photostimulable phosphor.

-The cassette is exposed to x-rays in a similarfashion as the conventional cassette.

-The latent image is produced in the phosphorlayer of the imaging plate.

-Then the cassette is transferred to the readersystem where the imaging plate is scanned witha red helium-neon (633nm) beam.

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- This stimulates luminescence proportional to the x-

ray energy absorbed. These light signals are

converted into electrical signals by using

photomultiplier tubes. - These electrical signals are converted into digital

information by an ANALOG TO DIGITAL

CONVERTER (ADC).

-The digitized data is transferred to the digital image processor in the computer, from where it can be

processed and viewed on the monitor.

Cont

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VARIOUS DIGITAL TECHNIQUE:

1) Slit scan system.

2) Scan projection system.

3) Image intensifier system.

4) Imaging plate system.5) Flat panel detector system.

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1. SLIT SCAN SYSTEM:

-This system uses single line pencil beam about 5

mm & linear detector system.

- It has mechanically linked x-ray tube, collimator,

opposite the detector array. There are different

possible movement of mechanical linked system &

patient couch movement.

- This system provides the ultimate in scatterrejection & detector efficiency .

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ADVANTAGES:

Scatter radiation almost totally absent.

Detector efficiency is more.

DISADVANTAGES:

Time factor, more exposure time required.

Tube loading increased. Poor resolution.

Bedside radiography is not possible.

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2. SCAN PROJECTION SYSTEM:

Based on CTTechnology, another projection

radiography technique was developed using a

narrow fan beam of X-Rays intercepted bylinear array of detectors called Scan

Projetion Radiography. Basically, SPR

involves the use of the existing CT-Gantry &Computer to generate an image that looks

surprisingly like Conventional Radiography.

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Cont

The X-ray beam is shaped into a fan by collimators that

confine the beam to a 2-10 mm thickness through on

arch of 30

0

4

5

0

. There are two collimators. The pre-patient collimators shape the beam, reduced scatter

radiation & control patient dose. The post-patient

collimators further reduce the scatter radiation.

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PRINCIPLE OF S.P.R.:

X-Ray beam after passing through patient is detected

by detector array. The signals having image

information of the body parts are transferred to the

computer where they get digitized & processed toreconstruct the image.

To obtain a complete image of the

body part, X-Ray tube & detector assembly remains

stationary & the patient is translated through the X-Ray beam or alternatively patient remains stationary,

while the X-Ray tube-detector assembly translates .

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COMPONENTS OF S.P.R. SYSTEM:

1. X-Ray Tube.

2. Pre-Patient Collimator.

3. Post-Patient Collimator.

4. DetectorArray.

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X-RAY TUBE DETECTOR

ASSEMBLY: -X-Ray tube of high heat loading capacity usually

above 1 MHu is required because of long imaging

time & detector efficiency. Usually 20 50 cmbody part of patient is imaged at a translation speed

of 1 2 cm/second

Two types of detector can be used: -

1. A Gas Filled.

2. Scintillation detectors couple to solid state

photodiode.

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A GAS FILLED DETECTOR A gas filled detector consists of a volume of gas

between two electrodes, with a n electricalpotential difference(voltage) applied between the

electrodes.

Ionizing radn prduces ion pairs in the gas. Positive ion attracted to negative

electrode(cathode), elctrons attracted to positive

electrode(anode). In most detectors, cathode is the wall of the

container that holds the gas and anode is a wire

inside the containers.21


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There are 3 types of gas filled detectors in

common use:

Ionization chambers

Proportional counters

Geiger Mueller counter

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Scintillation detectors

Scintillators are used in conventional film

screen radiography, many digital

radiographic receptors, fluoroscopy,

scintillation cameras, most CT scanners andPET scanners.

Scintillation detectors consist of a

scintillator and a device, such as a PMT thatconverts the light into electrical signals.

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ADVANTAGE:

1. High radiographic contrast.

2. Low contrast detectibility.

3. Image manipulation.

DISADVANTAGE:

1. Poor spatial resolution.

2. Scanning time is more.

3. More radiation dose to the patient.

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IITV-SYSTEM:

-It uses large field & high quality image

Intensifier Television System. The scanning

lines should be at least 1024.

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ADVANTAGES:

1. Radiation dose in 25% less than the conventionalRadiography.

DISADVANTAGES:

1. A spatial resolution is poor.

2. Different units required for different types ofwork.

3. Bedside Radiography is not possible.

4. Dedicated unit should have at least 1024 scanningline

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COMPUTED RADIOGRAPHY (CR)

Digital way of doing general radiography

with Conventional X-ray machines. It is a process of capturing radiographic

data from a conventional X-ray machine and

processing the data digitally to produce crispand high quality radiographic images.

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COMPONENTS OF THE C.R.

SYSTEM:

Image Reader

Image Processor.Cassette With Imaging Plate

Image Recorder.

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THE IMAGING PLATE:

-It consists of a polyester base over which a layerof photostimulable phosphor (europium doped barium fluoro bromide crystals- BaFBr:Eu 2) is

coated.-A protective layer composed of fluorinated

polymer material is applied over it. A supportinglayer which prevents the reflection of light is also

applied.-Next is the backing layer. This prevents the

scratching on the imaging plates during storage andtransfer. Therefore it has a protective action.

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-The next is the bar-code label which contains

the number assigned to the imaging plate.

-This bar-code provides a mechanism forassociating each imaging plate with patient

identification, related examination and

positioning information.-The imaging plate is flexible and less than

1mm thick.

Cont

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OTHER CHARACTERISTICS

OF IMAGING PLATE:

-It retains the image for 24 hours, but some

degradation may occur with passage of time.

-Imaging plate shows a linear response to the

intensity of x-ray exposure over a broad range.

-It shows superior performance capability i.e. it

provides more information.

-It is available in the same sizes as conventional

cassettes.

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-High resolution imaging plates are also

available which help in reducing the radiationdose to the patient considerably.

-Imaging plates are reusable and thousands of

exposures can be made on it.

Cont.

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THE IMAGE READER:

-The image reader converts the continuousanalog information (latent image) into a digitalformat.

-In the reader the imaging plate is scannedsequentially by a red helium-neon [633nm] laserbeam.

-The laser beam induces photostimulable

luminescence from the phosphor. The intensityof the emitted luminescence is proportional to theamount of x-ray energy absorbed in the crystallayer.

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-This emitted light is directed by highly efficientlight guides to the photomultiplier tubes, where itis converted into electrical signals.

-The electrical signals are sampled and digitizedby an A.D.C.

-The digital data is stored on the hard disk of a

work station from where it can be processed,viewed, printed or distributed via a network toperipheral stations.

Cont..

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-The image reader has a capacity to read 110

plates per hour.-Therefore one reader can serve several

radiographic rooms and the data input isstored on an easy image workstation.

Cont

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THE IMAGE

RECORDER:

The work station provides a DICOMcompliant output which maybe directedto a laser printer for hard copies, ornetworked to other viewing stations.

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-The image displayed on the monitor can be

manipulated in a variety of ways: contrast

enhancement, edge enhancement, black/white

reversal etc.

-The process of filing the images does not require

separate rooms and is relatively easier.

-The acquired image can be transferred to manymonitors for viewing in separate places.

Cont.

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LIMITATIONS OF THE C.R.

SYSTEM:-Lesser spatial resolution as compared toconventional radiography.

-C.R. systems are not inherently low dose

systems as compared to the conventional rareearth screen-film systems.

-Radiological technologists receive no direct

feedback on the accuracy of their selection ofexposure factors as the resultant images are ofconsistent quality regardless of the exposure.This may lead to undesirable and undetected

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DIRECT RADIOGRAPHY:

FLAT PANEL DETECTOR SYSTEMS:

-This system uses x-ray detectors of

photoconductive materials such as amorphous

Se or Si for direct acquisition of projection

radiographs.

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METHODS:

Essentially, two methods have been

developed for direct captureradiographs: -

1. Indirect Method.

2. Direct Method.

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1. INDIRECT METHOD:

-Here we use CsI scintillation phosphors coated over

an active matrix array of amorphous silicon

photodiodes.

-The x-ray beam emerging from the patient interacts

with the cesium iodide producing light.

-This light interacts with the amorphous silicon

producing electrical charge.-Thin film transistors store the signal until

read out, one pixel at a time.

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INDIRECT DETECTORS:

Uses two step process :Uses two step process :

X rays

Visible light

Electronic signals

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DIRECT METHOD:

Convert XConvert X--rays into electronicrays into electronic

signals.signals.

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2. DIRECT METHOD:

-In this case we do not use the phosphor coating,thus eliminating the intermediate light producingstep.

-Hence amorphous selenium directly acts as thex-ray detector.

-The x-ray beam directly interacts with a thinlayer of amorphous selenium creating electron-

hole pairs, which being charged, travel directly tothe electrodes.

-From here, the charge pattern is read out to formthe image.

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-The advantage of the amorphous seleniumapproach is that there is no light spreading in

the phosphor and so there is improved spatialresolution.

-On the other hand, the cesium iodide phosphor has a high detective quantum

efficiency and so it results in lower radiationdose.

Cont

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CONSTRUCTION & WORKING

OF FLAT PANEL DETECTOR

SYSTEM:

-The physical dimensions of the detector array

are 40 x 50 x 4 cms with 2560 x 3072 pixelmatrix.

-The array consists of a glass substrate ontowhich a layer of amorphous silicon is

evaporated.

-The matrix is covered with a cesium iodidescintillator layer.

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-The amorphous silicon is structured in amatrix of individual photo sensors andswitching elements, either a thin film

transistor or a diode which allows theconnections of the sensor with the read outline in column direction.

-Thin film transistors or switching diodes are

controlled via address lines in the horizontaldirection, in order to read out the single chargevalues of photodiodes.

Cont..

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-These signals are multiplexed and convertedinto digital signals by an A.D.C. inside thedetector housing.

-The 2-D image data is directly transferred tothe image processing computer via an opticfiber link.

-So the image is available in digital formshortly after the exposure has been made.

Cont..

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CHARACTERISTICS OF

AMORPHOUS SILICON:

-It is a good photo detector in thin film form.

-Its easy to deposit on large glass substrates.

-They are very sensitive to light with an

efficiency close to 100%

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ADVANTAGES OF FLAT

PANEL DETECTOR SYSTEM

-Less radiation dose to the patient.

-The examination becomes quick as nocassettes have to be fetched from the storagearea, taken to the examination site, or to theprocessing unit after exposure.

-Radiography as well as fluoroscopy can beperformed.

-Post processing can be done.

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DISADVANTAGES OF F.P.D.

SYSTEMS:

-Different equipment is required for

different kinds of work.

-They are quite costly.

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DIGITAL FLUOROSCOPY:

-It provides real time viewing of anatomic

structures. As maximum image detail is

required, so image brightness must be high.-Image intensifier was developed to replace

the conventional fluoroscopic screen.

-With the introduction of computertechnology into fluoroscopy,digital images

with better detail can be obtained.

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EQUIPMENT:

-D.F. requires the same fluoroscopy

equipment in addition to a computer, 2 video

monitors, and a more complex operatingconsole.

-A high voltage generator.

-A video system.

-A charge couple device.

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ADVANTAGES:

D.F permits high speed digital image

acquisition,processing & display

-Better image quality.

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DEVELOPMENTS IN D.F.

SYSTEM:

-Flat panel detector system has replaced the

I.I.T.V. SYSTEM.

-X-rays passing through the patient are

converted into electrical signals by the

F.P.D.s. These are then passed through the

amplifier and ADC where they are converted

into digital signals.

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-The digital image data is directly transferred

to an image storage PC via an optic fiber link

at the rate of 30 frame/s

-This system permits high speed digital

image acquisition, processing and display.

Images are of excellent resolution.

Cont..

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SIMILARITIES B/W DIGITAL &

CONVENTIONAL RADIOGRAPHY

Same X-Ray tube & generator system isrequired in both, for exposing the patient.Selection of similar required exposure factorsma, Kv etc.

Both required, accurate positioning of patientfor deferent projections.

Latent image is produced in both & then laterprocessed.

Use of Collimators, Cones, Grids required inboth. Imaging Plates like film cassette can be

transported easily to distant areas like OT,Patient Bedside.

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DIFFERENCE B/W DIGITAL &

CONVENTIONAL RADIOGRAPHY:

1. No use of radiographic film.

2. The latent image on IP is scanned by laserbeam, than digitized & sent to computerfor processing.

3. IP is capable of much wider exposure

latitude then conventional.4. IP can be reused after erasing unlikely

conventional.

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Cont.

1. Digital image is displayed as a combination ofrows & column called matrix, where as inconventional, it is made up of minute strands ofblack metallic silver.,

2. Size of matrix i.e. no. of pixel affect imagequality. In conventional, it is grain size ofemulsion in film or speed of screens.

3. Once image is converted to digital data,computer can performed post processing imageenhancement like subtraction contrast, edge-contrast.

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IMAGE QUALITY IN C.R.:

1. RESOLUTION:

- Digital Radiography: - 2.5 lines pair/mm.

- Conventional Radiography: 6-10 line

pair/mm.2. DENSITY:

- Digital Radiography: -

a) Provide wider exposure latitude.b) Improve visualization of anatomical

structure.

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3. CONTRAST:

Primary dependent on Kv & can beincreased by use of Collimators, Cone, Diaphragm

& Grid.

4. NOISE:

- Signal that contributes no useful diagnostic

information.

- Degrades the quality.

- In digital radiography, it is dependent.

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QUALITY

CONTROL:Three levels of system performance for quality

control and system maintenance: -

1. Routine: Technologist level- no radiation measurements.

2. Full inspection: Physicist level

- radiation measurements; non-invasive

adjustments.3. System adjustment: Vendor service level

- hardware and software maintenance.

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PERIODIC QUALITY CONTROL:

-Daily (technologist): -

General inspection.

Film processor / Laser printer.

Erase imaging plates.

Verify digital interfaces and network

transmission.

-Weekly (technologist): - Test phantom images.

System cleanliness.

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Cont

Monthly (Technologist): -

Film processor maintenance (if any).

Inspect and clean image receptors.

Review film retake rate.

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Cont

.Semi-Annually / Annually (Physicist): -

Evaluate image quality.

Acceptance tests to re-establish baseline

values.

Review.

QC records.

Service history.

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CONCLUSION:

With the advent of computed radiography

diagnostic radiology is advancing towards a film

less system

The replacement of film by detectors and storagedevices eliminated several inherent drawbacks

of conventional radiography and decreases the

radiation exposure to the patient and radiographer

Very soon digital imaging will become more

common and affordable for all aspect of

radiography.

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