The 2 signals from the photomultiplier tubes are fed into circuitry that defines the time difference

1
Pet scanners consist of rings of detectors,
composed of photomultiplier tubes connected to
scintillating crystals, that circle the patient.
Here we see 2 such opposing detectors in one ring.
The 2 signals from the photomultiplier tubes are
fed into circuitry that defines the time
difference between signals