Physics and Radiobiology of Nuclear Medicine

PET Scanners 185

the opposite detector. This uncertainty in detection time is called the timing
resolution or coincidence timing window. The timing resolution results from
the difference in pulse formation in the detector primarily due to statisti-
cal variations in gain and scintillation decay time of the detector. Further-
more, there is a time delay in the arrival of one photon relative to the other,
because of the difference in distances traveled by the two photons, partic-

A

B

Y

X

C

D

PM tubes

8 × 8 grooves

cut into a block of LSO or

BGO crystal

Fig. 13.1. A schematic block detector is segmented into 8 ×8 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 g-ray interaction occurs, and the sum of the four pulses determines if it falls
within the energy window of 511 keV.