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16.5 Genuine Digital Radiography (DR)
One-step digital radiography
Digital radiography, DR for short, is also known as “direct” radiography to indicate the dif-
ference with CR, which is a two-step, and thus slower process. With DR technology, there is
an immediate conversion of radiation intensity into digital image information.
Similar to common digital photo cameras, the radiographic image is almost immediately
available. Exposure and image formation happen simultaneously, allowing near real-time
image capture, with the radiographic image available for review only seconds after
the exposure. Some systems even provide a true real-time (radioscopic) mode with display
rates up to 30 images per second. This almost instant image formation is the reason that
some consider DR the only “genuine” (true) method of digital radiography. This instant
availability of results offers immediate feedback to the manufacturing process to quickly
correct production errors.
16.5.1 Detector types
Many materials or combinations thereof are sensitive to the impact of ionising radiation.
Over the years a considerable number of them proved to be efficient and commercially viable
to create radiation detectors for NDT applications. As a result a wide variety of detector
types are in use for formation of DR images. Often the application dictates the selection of
a particular detection/sensor system dependent on pros and cons of such a system. The
detectors can be characterised by detection method (direct versus indirect) and by geometry
(linear versus two-dimensional: 2D). All the different detector types that are useful in industri-
al inspection applications have a wide dynamic range similar to CR plates, see figure 8-16.
Direct versus indirect detection
All X-ray detection methods rely on the ioni-
sing properties of X-ray photons when they
interact with matter. In direct detection
(one-step) devices the amount of electric
charge created by the incident X-rays is
directly detected in semiconductor materials.
In indirect (two-step) devices, the X-ray
energy is absorbed by phosphorescent
materials (known as “scintillators”) which
emit visible light photons, and these pho-
tons are then detected by a photo detector
being the second layer, thus being an indirect process.
The different active layers are illustrated in figure 11-16. Because many thousands of ioni-
sed charges can be created by a single X-ray, direct photo detectors must be both very sensi-
tive and able to measure large amounts of charge to produce good image quality. The tech-
nologies for CMOS, scintillator materials and amorphous photo-detectors are relatively
mature and used in many commercially available DR detector products.
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Exposure time and noise
In addition to the wide dynamic range the dose sensitivity (speed) of CR plates is five to
ten times higher, compare point A and B in figure 8-16 at a density of 2 (see also figure
27-16). This allows for shorter exposure times or weaker sources, reducing the unsafe
radiation area. Unfortunately, if a source with lower energy is chosen this will result in
reduction of the image quality. Iridium192, with a lower energy than Cobalt60, requires
a longer exposure time and this in turn reduces image quality due to the larger quantity
of scattered radiation. For profile radiography applications (sometimes also called on-
stream radiography) Iridium can replace Cobalt for pipes with a diameter up to 6”
(150 mm), with still an acceptable image quality, or even 8” (200 mm) in case of thin wall
pipe. The general rule is: the shorter the exposure time the less the scatter thus the
better the image quality.
Note: CR plates are more sensitive to low energy scatter (more noise) than conventional film.
Careful filtering and collimation of the radiation and control of backscatter are vital to good CR.
Fading
After exposure the intensity of the stored information (cassette closed) naturally decays
over time, resulting in some signal loss. Scanning within 1 hour of exposure provides the
best results; typically 50% of the information is lost after 24 hours, dependent on the manu-
facturer of the plate. Fading is dependent on ambient temperature. To avoid image fading,
scanning of the CR plate should not be delayed longer than necessary. In critical applica-
tions, where signal loss is expected due to delayed scanning, the plates can be exposed with
a higher radiation dose to compensate for this information decay.
Optimisation
To optimise the use of CR imaging plates in
practice, a small handheld terminal as
shown in figure 10-16 has been developed to
superimpose specific project- and exposure
information to the images. To this end the
cassette contains a microchip which can
receive (wireless) information from the ter-
minal. On site and prior to the exposure the
relevant information is sent from this terminal
to the microchip on the cassette. The specific
data is ultimately added to the image in the
CR scanner. Once the data from the microchip
has been erased the cassette is ready for re-use.
Improvements
Due to ongoing efforts for improvement the image quality of the phosphor plate one has
already achieved a level equal to the quality obtainable with a medium-grain conventio-
nal X-ray film, see figure 27-16. In fine-grain films, graininess is only a few microns, while
in current (2008) phosphor plates this is considerably more (approximately 10 microns).
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Fig. 10-16. Terminal for CR imaging plates
Radiation
(photons)
Amorphous silicon array (detector)
light
CsI Scintillator - TFT photodiode array
electrons
Read-out electronics + digitisation
Digital data to workstation
Fig. 11 -16. Schematic of an indirect (two-step)
flat panel detector