Digital
radiography is a
form of radiography that uses x-ray–sensitive plates to directly
capture data during the patient examination, immediately transferring it to a
computer system without the use of an intermediate cassette. Advantages
include time efficiency through bypassing chemical processing and the ability
to digitally transfer and enhance images. Also, less radiation can be
used to produce an image of similar contrast to conventional
radiography.
Instead of X-ray
film, digital radiography uses a digital image capture device. This gives
advantages of immediate image preview and availability; elimination of costly
film processing steps; a wider dynamic range, which makes it more forgiving for
over- and under-exposure; as well as the ability to apply special image
processing techniques that enhance overall display quality of the image.
Contents
- 1Detectors
- 1.1Flat panel detectors
- 1.2Other direct digital detectors
- 1.3Phosphor plate radiography
- 2Industrial usage
- 2.1Security
- 2.2Materials
Flat panel
detector used in digital radiography
Flat panel
detectors (FPDs) are the most common kind of direct digital detectors.They are
classified in two main categories:
1. Indirect
FPDs Amorphous silicon (a-Si) is the most common material of
commercial FPDs. Combining a-Si detectors with a scintillator in the
detector’s outer layer, which is made from caesium iodide (CsI)
or gadolinium oxysulfide (Gd2O2S), converts
X-rays to light. Because of this conversion the a-Si detector is considered an
indirect imaging device. The light is channeled through the a-Si photodiode
layer where it is converted to a digital output signal. The digital signal is
then read out by thin film transistors (TFTs) or fiber-coupled CCDs.
2. Direct
FPDs. Amorphous selenium (a-Se) FPDs are known as “direct”
detectors because X-ray photons are converted directly into charge.
The outer layer of the flat panel in this design is typically a
high-voltage bias electrode. X-ray photons create electron-hole pairs in
a-Se, and the transit of these electrons and holes depends on the potential of
the bias voltage charge. As the holes are replaced with electrons, the
resultant charge pattern in the selenium layer is read out by a TFT array,
active matrix array, electrometer probes or microplasma line addressing.
Other direct
digital detectors
Detectors based
on CMOS and charge coupled device (CCD) have also been
developed, but despite lower costs compared to FPDs of some systems, bulky
designs and worse image quality have precluded widespread adoption.
A high-density
line-scan solid state detector is composed of a photostimulable barium
fluorobromide doped with europium (BaFBr:Eu) or caesium bromide (CsBr)
phosphor. The phosphor detector records the X-ray energy during exposure and is
scanned by a laser diode to excite the stored energy which is released and read
out by a digital image capture array of a CCD.
Phosphor plate
radiography
Phosphor plate
radiography resembles the old analogue system of a light sensitive
film sandwiched between two x-ray sensitive screens, the difference being the
analogue film has been replaced by an imaging plate with photostimulable
phosphor (PSP), which records the image to be read by an image reading device,
which transfers the image usually to a Picture archiving and communication
system (PACS). It is also called photostimulable phosphor (PSP)
plate-based radiography or computed radiography (not to be
confused with computed tomography which uses computer processing to
convert multiple projectional radiographies to a 3D image).
After X-ray
exposure the plate (sheet) is placed in a special scanner where the latent
image is retrieved point by point and digitized, using laser light
scanning. The digitized images are stored and displayed on the computer screen. Phosphor
plate radiography has been described as having an advantage of fitting within
any pre-existing equipment without modification because it replaces the
existing film; however, it includes extra costs for the scanner and replacement
of scratched plates.
Initially
phosphor plate radiography was the system of choice; early DR systems were
prohibitively expensive (each cassette costs £40-£50K), and as the 'technology
was being taken to the patient', prone to damage. Since there is no
physical printout, and after the readout process a digital image is obtained,
CR has been known as an indirect digital technology, bridging the gap between
x-ray film and fully digital detectors.
Industrial usage
Security
EOD (Explosive
Ordnance Disposal) training and material testing. A 105 mm shell is
radiographied with battery powered portable X-ray generator and flat panel
detector.
Digital
radiography (DR) has existed in various forms (for example, CCD and amorphous
Silicon imagers) in the security X-ray inspection field for over 20 years and
has largely replaced the use of film for inspection X-rays in the Security
and nondestructive testing (NDT) fields. DR has opened a window
of opportunity for the security NDT industry due to several key advantages
including excellent image quality, high POD (probability of detection),
portability, environmental friendliness and immediate imaging.
Materials
Nondestructive
testing of materials is vital in fields such as aerospace and electronics where
integrity of materials is vital for safety and cost reasons. Advantages of
digital technologies include the ability to provide results in real time.
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