Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

FIGURE 13-13. Multiple detector array computed tomographic (CT)scanners use several, closely spaced, complete detector arrays. With notable translation (nonhelical acquisition), each detector array acquires aseparate axial CT image. With helical acquisition on a multiple detectorarray system, table speed and detector pitch can be increased, increasingthe coverage for a given period of time.sition protocol is increased as well. Also with multiple detector arrays, the notion ofhelical pitch needs to be redefined. This topic is discussed later in the chapter.Xenon detectors use high-pressure (about 25 atm) nonradioactive xenon gas, inlong thin cells between two metal plates (Fig. 13-14). Although a gaseous detectordoes not have the same detection efficiency as a solid one, the detector can be madevery thick (e.g., 6 cm) to compensate in part for its relatively low density. The metalsmall acceptanceangleFIGURE 13-14. Xenon detectorarrays are a series of highlydirectional xenon-filled ionizationchambers. As x-raysionize xenon atoms, thecharged ions are collected aselectric current at the electrodes.The current is proportionalto the x-ray fluence.