Block detector

Figure 2.2. A schematic block detector is segmented into 8x8 elements, and four PM tubes are coupled to the block for pulse formation (Reprinted with the permission of The Cleveland Clinic Center for Medical Art Photography 2009. All Rights Reserved).

A modification of the basic block detector has been made such that each PM tube straddles over four quadrants of four different blocks (Fig. 2.4). The technique of quadrant sharing permits the use of larger PM tubes and reduces the total number of PM tubes used in the PET system. This design improves the spatial resolution relative to the basic design, but has the disadvantage of increasing the dead time. 

In PET scanners with block detectors, each detector pair is not connected by coincidence circuitry for practical difficulty, so the detectors are grouped together into banks or sets, which are then connected in coincidence opposite to each other. Essentially each detector in a set is connected by a coincidence circuit with a time window to a set of opposite detectors (both in plane and... 

Figure 2.3. A typical commercial block detector (8 x 8) attached to four square PM tubes (bottom) and a packaged module top), developed and manufactured by Siemens Medical Solutions USA. (Courtesy of CPS Innovations, Knoxville, TN, USA. currently Siemens Medical Solutions USA, Inc.).

What are block detectors and why are they preferred to single detectors in a PET scanner ... 

The common number of PM tubes per block detector in a PET scanner is... 

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 3.2. A schematic block detector is segmented into 8x8 elements, and four PM tubes are coupled to the block for pulse formation. The pulses from the four PM tubes (A, B, C, and D signals) determine the location of the element in which 511-keV 7-ray interaction occurs, and the sum of the four pulses determines if it falls within the energy window of 511 keV. (Reprinted with the permission of The Cleveland Chnic Center for Medical Art Photography 2009. All Rights Reserved).

Localization of detector The use of block detectors instead of single detectors causes an error (Re) in the localization of the detector by X, Y analysis and it may amount to 2.2mm for BGO detectors (Moses and Derenzo, 1993). However, it can be considerably minimized by using better light output scintillators, such as LSO. 

Moses WW, Derenzo SE (1993). Empirical observation of performance degradation in positron emission tomographs utilizing block detectors. J Nucl Med 34 101P... 

Cellular profiles for NOvA block detector. [Data from Richard Talaga,... 

S. Lashkari, et al., "The Influence of Crystal Material on Interonystal Scattering and the Parallax Effect in PET Block Detectors A Monte Carlo Study," 2008, pp. 633-636. 

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