Digital detectors can
be divided into CR and DR. CR systems use storage-phosphor image plates with a
separate image readout process. DR is a way of converting x-rays into
electrical charges by means of a direct readout process.
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Figure 1: Chart illustrates a digital radiography system. After
image exposure, the imaging data are digitally processed and stored in a digital
archive. A centralized image management system is used for further distribution
of the images to viewing stations, information systems, and electronic patients’
records.
Computed radiography
· The image plates have a detective
layer of photostimulable crystals that contain
different halogenides such
as bromide, chlorine, or iodine (eg, BaFBr:Eu2+)
· Phosphor crystals are cast into plates
of resin material in unstructured scintillators.
· Image plates replace the conventional
films in the cassette.
· Exposure process with storage-phosphor
image plates:
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1. During exposure,
x-ray energy is absorbed and temporarily stored by these crystals by bringing
electrons to higher energy levels.
2. X-ray energy can
be stored for several hours, depending on the specific physical properties of
the phosphor crystals used.
3. The readout
process should start immediately after exposure because the amount of stored energy
decreases over time.
4. The readout
process is a separate step that follows exposure of the image plate. When the
detective layer is scanned pixel by pixel with a high-energy laser beam of a
specific wave length, stored energy is set free as emitted light having a wave
length different from that of the laser beam.
5. This light
collected by photodiodes and converted digitally into an image.
Simplified process :
· The whole readout process for a
14x17-inch image plates takes about 30-40 seconds. Thus, a maximum workload of
90-120 image plates per hour is theoretically possible.
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| Figure 4:
Amorphous selenium-based direct conversion DR systems. (a) Drawing
illustrates a selenium drum-based system. A rotating selenium-dotted
drum with a positive electrical surface charge is exposed to x-rays.
Alteration of the charge pattern of the drum surface is proportional to
the incident x-rays. The charge pattern is then converted into a digital
image by an analog-digital converter(ADC). (b) Drawing illustrates a
selenium-based flat-panel detector systems. Incident x-ray energy is
directly converted into electrical charges within the fixed
photo-conductor layer and read out by a linked TFT array beneath the
defective layer.
Advantage of storage-phosphor systems
· Reduced rates of failed x-ray exposure
· Because CR systems are cassette based,
they can easily be integrated into existing radiographic devices, are highly
mobile, and are easy to use for bedside examinations and immobile patients.
· Flexible in routine clinical use.
· Can easily be replaced by the
radiographer with no need for specialized equipment or service personnel if a
single image plates shows defects.
Disadvantage of storage-phosphor systems
· Spatial resolution is usually lower
than that with conventional screen-film combinations.
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