Physics and Radiobiology of Nuclear Medicine

PET Scanners 187

Detector 1 Detector 2

A Pulse 1 Pulse 2

Pulse 1

1 2

Coincidence

Detector 2 Detector 1

B Pulse 2

12

Coincidence

Detector 1 Detector 2

C Pulse 1 Pulse 2

21

Coincidence

Fig. 13.3. Two time signals with time width tdefine the coincidence timing window
of 2t.(A)Pulse 2 (dashed line) of time with tarrives at the opposite detector just
after the arrival of Pulse 1 (solid line) of time width tat one of the detector pair,
overlapping only at the edge and two events are counted in coincidence.(B)Pulse
2 of time width tarrives at one of the detector pair exactly prior to the arrival of
Pulse 1 of time width tat the other detector, overlapping only at the edge and two
events are counted in coincidence.(C)Pulse 2 overlaps Pulse 1 partially or com-
pletely depending on the time of arrival of the pulses at the detector pair, and two
events are counted in coincidence. Both A and B indicate that the timing window
of the coincidence circuit must be at least 2tto detect all events in coincidence.