Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

septa that separate the individual xenon detectors can also be made quite thin, andthis improves the geometric efficiency by reducing dead space between detectors.The geometric efficiency is the fraction of primary x-rays exiting the patient thatstrike active detector elements.The long, thin ionization plates of a xenon detector are highly directional. Forthis reason, xenon detectors must be positioned in a fixed orientation with respectto the x-ray source. Therefore, xenon detectors cannot be used for fourth-generationscanners, because those detectors have to record x-rays as the source moves overa very wide angle. Xenon detectors can be used only for third-generation systems.Xenon detectors for CT are ionization detectors-a gaseous volume is surroundedby two metal electrodes, with a voltage applied across the two electrodes(see Chapter 20). As x-rays interact with the xenon atoms and cause ionization (positiveatoms and negative electrons), the electric field (volts per centimeter) betweenthe plates causes the ions to move to the electrodes, where the electronic charge iscollected. The electronic signal is amplified and then digitized, and its numericalvalue is directly proportional to the x-ray intensity striking the detector. Xenondetector technology has been surpassed by solid-state detectors, and its use is nowrelegated to inexpensive CT scanners.A solid-state CT detector is composed of a scintillator coupled tightly to a photodetector.The scintillator emits visible light when it is struck by x-rays, just as inan x-ray intensifying screen. The light emitted by the scintillator reaches the photodetector,typically a photodiode, which is an electronic device that converts lightintensity into an electrical signal proportional to the light intensity (Fig. 13-15).This scintillator-photodiode design of solid-state CT detectors is very similar inconcept to many digital radiographic x-ray detector systems; however, the performancerequirements of CT are slightly different. The detector size in CT is measuredin millimeters (typically 1.0 X 15 mm or 1.0 X 1.5 mm for multiple detectorarray scanners), whereas detector elements in digital radiography systems are typi-Clarge acceptanceanglevisible lightscintillatorFIGURE 13-15. Solid-state detectors comprise a scintillator coupled to a photodiode.X-rays interact in the scintillator, releasing visible light, which strikesthe photodiode and produces an electric signal proportional to the x-ray fluence.For a single (T linear detector array, the detectors are slightly widerthan the widest collimated x-ray beam thickness, approximately 12 mm.