Early pile-up rejection methods were based on measuring the length of a pulse. If the pulse did not return close to baseline level within the time expected given the decay time of NaI(Tl), it was assumed that pile-up of two pulses had occurred and the event was rejected, resulting in the loss of both γ rays. This improved image quality but resulted in an effective increase in system dead time, because many events were rejected at high counting rates.
Many gamma cameras now incorporate circuits that continuously monitor the decay of a pulse and use a method based on pulse-tail extrapolation for pile-up correction. Consider two γ-ray interactions that occur close together in time and create overlapping pulses. When the second γ ray arrives, the decay of the pulse created by the first γ ray immediately deviates from the expected exponential decay and the gamma camera signal is switched to a second amplifier circuit. Estimator circuitry in the first amplifier circuit completes the signal from the first γ ray by extrapolating the remainder of the tail of the pulse with an exponential function based on the decay time