Investigation of count rate and deadtime characteristics of a high resolution PET system.

Count rate and deadtime characteristics were investigated for a whole body positron emission tomography system by measuring prompt, delayed, and multiple (three or more detectors) coincidence rates and single detector rates as a function of input count rate by imaging a variety of short-lived positron emitting sources as a function of time. Data were collected with cylinder, ring, and point sources for a range of energy thresholds and fields of view. The largest source of deadtime loss involved processes that led to multiple coincidences, which are primarily true or accidental events in coincidence with an unrelated event. In measuring the count rate as a function of time for each type of event, components, with decay constants of 1, 2, or 3 times that of the isotope being measured, could be resolved corresponding to 1 (true events), 2 (accidental and some multiple events), or 3 (multiple events) independent nuclear disintegrations, respectively. Analysis of true, accidental, and multiple coincidence and singles count rate data allowed identification and evaluation of the magnitude of the sources of deadtime losses and provided a basis for a deadtime correction from data available from the PET system.