Dead time correction and counting statistics for positron tomography

Abstract

A correction for loss of events due to dead time in dynamic positron emission tomography (PET) is presented. The model employs a paralysing dead time to describe the behaviour of a tomograph over the range of event rates normally encountered in patient studies (up to 200000 events/s per detector layer). The Donner 280-crystal positron tomograph has a dead time of 1.8 mu s/event for observed count rates less than 200000 events/s. The dead time correction factor is 1.8 at 180000 events/s. The correction is applied to projection data and region of interest analysis of dynamic PET studies, and formulae for the covariances between corrected projection data and between counts in regions of interest in different images from the same dynamic study are established. At 1800000 events/s, the variance of the actual (corrected) number of events in a region containing 3.34*105 actual events is predicted from the model to be 3.86*106 (events)2, more than 10 times the variance that would be expected from a naive assumption of Poisson statistics. These statistical results are verified experimentally. An error of 25% is observed in myocardial flow if dead time compensation is not applied, showing the necessity for this correction.