Increasing the axial extent in volume imaging PET scanners: Is there a limit? a Monte Carlo study

A Monte Carlo simulation program for the PENN-PET (University of Pennsylvania positron emission tomography) scanner design is presented. The program is flexible enough to allow one to vary the geometry and study the effects of increasing the axial extent of the scanner. The authors present a series of Monte Carlo studies comparing two volume imaging scanners currently being used which differ only in their axial extent (9.0 cm and 13.5 cm). They also present predictions of the performance of a similar body scanner with a 25.6 cm axial extent and a head-only scanner, also having a 25.6 cm axial extent but with a reduced radius which greatly increases the solid angle of the detectors. The studies show the effects of increasing the axial extent on the trues, scatters, randoms, and overall system sensitivity. It is shown that the scatter fraction does not change significantly as the axial extent of the scanner is increased, especially when an appropriate definition for scatter is taken into account. The ratio of randoms/trues decreases with increasing axial extent as a function of the activity