Coincidence scintillation camera

A disadvantage of the coincidence scintillation cameras is that they have low sensitivity due to low detection efficiency of Nal(Tl) crystal for 511-keV photons, which results in a longer acquisition time. To improve the sensitivity, thicker detectors of sizes 1.6-2.5 cm have been used in some cameras, but even then, coincidence photopeak efficiency is only 3-4%. This increase in crystal thickness, however, compromises the spatial resolution of the system in SPECT mode. Fast electronics and pulse shaping are implemented in modern systems to improve the sensitivity. Also, there is a significant camera dead time and pulse pileups due to relatively increased single count rates in the absence of a collimator in PET mode. Low coincidence count rates due to low... 

Explain how SPECT scintillation cameras can be used in coincidence counting. Discuss the advantages and disadvantages of these cameras in coincidence counting. 

As stated in Chap. 2, in a PET scanner, block detectors are cut into small detectors and coupled with four PM tubes, which are arranged in arrays of rings. Each detector is connected in coincidence to as many as N/2 detectors, where N is the number of small detectors in the ring. So which two detectors detected a coincidence event within the time window must be determined. Pulses produced in PM tubes are used to determine the locations of the two detectors (Fig. 3.2). As in scintillation cameras, the position of each detector is estimated by a weighted centroid algorithm. This algorithm estimates... 

FIGURE 27.16 Two or more opposed scintillation cameras can be used as coincidence detection systems for PET imaging. Valid events are established when the two detectors record events within the 10- to 15-ns timing window. Graded filters and lead septa are placed in ftont of the detectors to limit scattered radiation. (Courtesy of GE Medical Systems.)... 

In a conventional scintillation camera. Anger logic is used to determine the location of an interaction. The tacit assumption in this approach is that only one event is occurring at a time. At the high count rate encountered in coincidence imaging, multiple interactions are likely, and when this occurs, the events are improperly located somewhere between the two true locations. Improved algorithms have been developed that can identify multiple hits and that use a maximum likelihood calculation to correctly determine event locations. 

Patton J. A., Instrumentation for coincidence imaging with multihead scintillation cameras, Semin. Nucl. Med.. October 2000, 30 239-254. 

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