In a ring tomograph, the accuracy with which the detectors are aligned in the gantry can affect line of response efficiency. Such variations will manifest in different ways depending on the exact design of the tomograph, the detectors and any casing in which the detectors are contained. Frequently, block detectors are mounted in modules or cartridges, each containing
Figure 5.9. Time-window alignment factors for the Siemens/CTI ECAT 951. The factors range in value from 0.872 to 1.120. (From , with permission.)
several units. Misalignments of these modules can have noticeable affects on LOR sensitivity [11, 15]. Some full-ring systems have a "wobble" feature designed to improve spatial sampling - this feature allows the detectors to describe a small orbit about the mean detector position. As a result, it is possible that the transmission sources can rotate about a point which is not actually the centre of the detector ring, and if they are used to perform normalisation measurements, erroneous asymmetries can be introduced into the normalisation coefficients .
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