A Monte Carlo simulation study to evaluate septal spacing using triple-head hybrid PET imaging

The low coincidence fraction (ratio of true coincidences to single events detected) and a limited count rate capability reduce performance of hybrid PET systems as compared with dedicated PET. Axial collimation (i.e., using septa oriented perpendicular to the axis of rotation) has been used to reduce acceptance of photons from outside the field of view, reduce scattered events, and allow the use of 2-D image reconstruction algorithms. The major drawback of axial collimation is a reduced coincidence detection efficiency. The goal of this study was to evaluate the effect of axial collimator septal spacing, as well as to evaluate a new hybrid parallel-fan beam axial collimator design which may better shield out of field-ofview (FOV) photons, reduce the number of scattered events, and increase the sensitivity to the central FOV. The GEANT 3.21 Monte Carlo (MC) simulation code was modified to model a Philips IRIX triple-head hybrid PET system in the 0(degree)-90(degree)-270(degree) geometry and used to investigate different axial collimator designs. Both lead and tungsten septa were investigated. Basic PET system parameters such as scatter fraction, global true coincidences to single events (T/S) ratio, axial profiles of the T/S ratio, and energy spectra were examined. Noise equivalent count (NEC) rate curves were plotted. Noise and contrast were evaluated for different collimator designs and reconstruction strategies. Comparisons to an experimentally acquired line source phantom showed that the GEANT 3.21 MC simulation code accurately modeled the Philips IRIX camera. The T/S ratio was observed to increase with increasing septal spacing from 0.77 to 2.27 cm and was approximately constant for larger septal spacing. Widening the septal spacing modified the T/S axial profiles making them more nonuniform with a strong peak toward the axial center of the FOV. Using the hybrid parallel-fan beam axial collimator provided a more uniform axial T/S profile for some septal spacings and improved contrast and decreased the noise in the reconstructed slices. In addition, for a given single events rate, a larger proportion of single events were measured within the photopeak energy window as septal spacing decreased. This observation can have implications on the recorded rand- om rate for some PET systems.