The document discusses various digital x-ray imaging technologies including computed radiography (CR), charge-coupled device (CCD) detectors, flat panel detectors using thin-film transistor (TFT) arrays, and scintillator-based indirect detection systems. CR uses photostimulable phosphor plates that store x-ray energy, which is later read out using laser stimulation to produce a digital image. Flat panel detectors directly convert x-rays to electrical signals or use indirect conversion with scintillators. Technique factors like voltage, current and exposure time affect image quality and patient dose in digital radiography.

1. JAI HIND
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2. STUDENT:- FACULTY:-
VIRENDRA K.MAURYA Ms.PRIYANKA MAM
M.Sc.(RIT) (INCHARGE & HEAD OF RIT)
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COMPUTED RADIOGRAPHY (CR)

3. COMPUTED RADIOGRAPHY
• Computed radiography (CR) refers to photostimulable
phosphor detector (PSP) systems, which are historically
housed in a cassette similar to a screen-film cassette.
• Traditional scintillators, such as Gd₂O₂ S and cesium
iodide (CsI), emit light promptly (nearly
instantaneously) when irradiated by an x-ray beam.
• When x-rays are absorbed by photostimulable
phosphors, some light is also promptly emitted, but a
fraction of the absorbed x-ray energy is trapped in the
PSP screen and can be read out later using laser light.
•
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4. • For this reason, PSP screens are also called storage
phosphors.
• CR was introduced in the 1970s, saw increasing use in
the late 1980s, and was in wide use at the turn of the
century as many departments installed PACS.
• Most CR imaging plates are composed of a mixture of
BaFBr and other halidebased phosphors, often referred
to as barium fluorohalide.
• A CR plate is a flexible screen that is enclosed in a light-
tight cassette.
• The CR cassette is exposed to x-rays during the
radiographic examination and is subsequently placed in
a CR reader.
• Once placed in the CR reader, the following steps take
place:
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5. • 1. The cassette is moved into the reader unit, and
the imaging plate is mechanically removed from
the cassette.
• 2. The imaging plate is translated vertically in the
(y) direction by rollers across a moving stage and
is scanned horizontally in the (x) direction by a
laser beam of approximately 700 nm wavelength.
• 3. Red laser light stimulates the emission of
trapped energy in a tiny area (x,y location) of the
imaging plate, and blue-green visible light is
emitted from the storage phosphor as energetic
electrons drop down to their ground state.
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6. • 4. The light emitted through photostimulated
luminescence is collected by a fiber optic light
guide and strikes a photomultiplier tube (PMT),
where it produces an electronic signal.
• 5. The electronic signal is digitized and stored as a
pixel value. For every spatial location (x, y) on the
imaging plate, a corresponding gray scale value is
determined that is proportional to the locally
absorbed x-ray energy.
• 6. The plate is exposed to bright white light to
erase any residual trapped energy.
• 7. The imaging plate is returned to the cassette
and is ready for reuse.
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7. • The digital image that is generated by the CR
reader is stored temporarily on a local hard
disk.
• Once acquired, the digital radiographic image
undergoes image processing by the CR reader.
• The image is then typically sent to a PACS for
interpretation by a radiologist and long-term
archiving.
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The readout mechanics of a
CR system are shown. The
imaging plate is translated
through the mechanism by a
series of rollers, and a laser
beam scans horizontally
across the plate.
The rotating multifaceted
mirror causes the laser
beam to scan the imaging
plate in a raster fashion.
The light released by laser
stimulation is collected by
the light guide and produces
a signal in the PMT. The red
light from the laser is
filtered out before reaching
the PMT

9. • the blue light that is emitted from the screen
has shorter wavelength and higher energy per
photon than the stimulating red light.
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10. • Typical imaging plates are composed of about
85% BaFBr and 15% BaFI, activated with a small
quantity of europium.
• The CR screen is amorphous (lacks structure); the
barium fluorohalide crystals are small and are
held together by an inert, optically transparent
binder.
• The nomenclature BaFBr:Eu indicates that the
BaFBr phosphor is activated by europium. This
activation procedure, also called doping, creates
defects in the BaFBr crystals that allow electrons
to be trapped more efficiently.
• The defects in the crystalline lattice caused by the
europium dopant give rise to so-called F-centers
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12. • A. During exposure, x-rays are absorbed in the
storage phosphor and some of the electrons
reach the conduction band, where they may
interact with an F-center, causing the
reduction of europium (Eu) ions.
B. The electrons are trapped in this high-energy,
metastable state, and can remain there for
minutes to months.
C. the F-center releases a trapped electron, the
trivalent europium (Eu⁺³) is converted back to
its divalent state. (Eu⁺²)
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14. Charge-Coupled Device and Complementary
Metal-Oxide Semiconductor detectors
• Charge-coupled device (CCD) detectors form
images from visible light .
• CCD detectors are used in commercial-grade
television cameras and in scientific
applications such as astronomy.
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15. • The CCD chip itself is an integrated circuit made
of crystalline silicon, as is the central processing
unit of a computer.
• A CCD chip has an array of discrete detector
electronics etched into its surface.
• The silicon surface of a CCD chip is
photosensitive—as visible light falls on each
dexel, electrons are liberated and build up in the
dexel.
• More electrons are produced in dexels that
receive more intense light.
• The electrons are confined to each dexel because
there are electronic barriers (voltage) on each
side of the dexel during exposure
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16. . A photograph of a high-resolution
CCD chip is shown
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17. B. The readout procedure in a CCD chip is illustrated.
C. This illustration shows the shift of a given pattern of
exposure down one column in a CCD chip in four (t1–
t4) successive clock cycles
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18. A. A CCD-based DR system is shown. The x-ray is converted to visible light in the
intensifying screen, which propagates through the lighttight enclosure. A small
fraction of the light photons will strike the mirror and be redirected to the lens to
ultimately be detected in the CCD chip.
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19. . B. A CCDbased Time Delay and Integration (TDI) system is shown. As the x-ray tube / slitslot /
detector assembly moves at velocity V (to the left in this figure), the CCD chip is read-out at a
velocity −V (to the right). By synchronizing the velocity of the read out with the scanning motion,
the signal region under an object in the patient travels across the entire field of view of the
detector, and its contrast signal is built up across all of the dexels in the TDI CCD chip
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20. Flat Panel Thin-Film-Transistor Array
Detectors
• Flat panel Thin-Film-Transistor (TFT) array
detectors make use of technology similar to that
used in flat panel displays, and much of this has
to do with the wiring requirements of a huge
number of individual display elements.
• Flat-panel TFT arrays are made of amorphous
silicon, where lithographic etching techniques are
used to deposit electronic components and
connections necessary for x-ray detector
operation.
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21. • The large area TFT array is divided into
individual detector elements (dexels),
arranged in a row and column matrix .
• Electronic components within each dexel
include a TFT, a charge collection electrode,
and a storage capacitor.
• The TFT is an electronic switch that is
comprised of three connections: gate, source,
and drain.
• Gate and drain lines connect the source and
drain of the TFT’s along the row and columns,
respectively.
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22. flat panel detector systems are pixelated descrete detector systems. The
detector array is comprised of a large number of individual detector elements
(dexels). Each dexel has a light sensitive region and a light-insensitive area
where the electronic components are located.
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23. A photomicrograph of an actual TFT system is shown. The electronics
component can be seen in the upper left corner of each dexel (Image
courtesy John Sabol and Bill Hennessy, GE Healthcare
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24. Indirect Detection TFT Arrays
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This diagram shows circuitry for a TFT flat panel
detector system. The readout process is described in the
text

25. Indirect and direct detector TFT–
based x-ray detectors are shown
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26. • Direct Detection TFT Arrays:-
• Direct x-ray conversion TFT arrays use a semiconductor material
that produces electron-hole pairs in proportion to the incident x-ray
intensity.
• Absorbed x-ray energy is directly converted into charge in the
detector
• - there is no intermediate step involving the production of visible
light photons.
• Amorphous selenium (a-Se) is the semiconductor most widely used,
and is layered between two surface-area electrodes connected to
the bias voltage and a dielectric layer.
• The dielectric layer prevents overcharging dexels, which could
damage the TFT array.
• Ion pairs are collected under applied voltage across the solid state
converter (10–50 V/mum of thickness).
• In addition to selenium, other materials such as mercuric iodide
(HgI₂ ), lead iodide (PbI₂ ), and cadmium telluride (CdTe) are being
studied for use in direct detection flat panel systems.
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27. Technique Factors in Radiography
• the principal x-ray technique factors used for
radiography include the tube voltage (the kV), the
tube current (mA), the exposure time, and the x-
ray source-to-image distance, SID. The SID is
standardized to 100 cm typically , and 183 cm for
upright chest radiography.
• In general, lower kV settings will increase the
dose to the patient compared to higher kV
settings for the same imaging procedure and
same body part, but the trade-off is that subject
contrast is reduced with higher kV.
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29. Scintillators and Intensifying Screens
• An indirect x-ray detector system uses a scintillator to
convert the x-ray fluence incident on the detector into
a visible light signal, which is then used to expose the
emulsion in screen-film radiography or a
photoconductor in digital radiographic systems.
• Over the long history of screen-film radiography, a
number of scintillators were developed.
• Most intensifying screens are comprised of fine-grain
crystalline scintillating powders (also called phosphors),
formed into a uniformly thick intensifying screen that is
held together by a binder.
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30. Absorption Efficiency and Conversion
Efficiency
• Because of the dynamics of indirect x-ray
detection described above, there are two
factors that are important in x-ray detectors.
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32. Other Considerations
• CR is often the first digital radiographic
system installed in a hospital, because it can
directly replace screen-film cassettes in
existing radiography units and in portable,
bedside examinations, where the cost of
retakes in both technologist time and money
is high.
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33. Radiographic Detectors, Patient Dose,
and Exposure Index
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34. Dual-Energy Radiography
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35. Scattered Radiation in Projection
Radiographic Imaging
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