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

The low coincidence fraction (ratio of trues to singles detected) and a limited count rate capability reduce performance of hybrid PET systems as compared with dedicated PET. Axial collimation (i.e., 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 2D 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. The GEANT Monte-Carlo simulation code was modified to model a Marconi IRIX triple-head hybrid PET system in the 0°-90°-270° geometry and used to investigate varying axial collimator designs. Basic PET system parameters such as scatter fraction, global trues to singles (T/S) ratio, axial profiles of the T/S ratio, and energy spectra were examined. Comparisons to an experimentally acquired line source phantom showed that the GEANT Monte-Carlo accurately modeled the Marconi IRIX camera. The T/S ratio was observed to increase with increasing septal spacing from 0.77 cm 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 non-uniform with a strong peak towards the axial center of the field-of-view. Using the hybrid parallel-fan beam axial collimator provided a more desirable uniform axial T/S profile. In addition, for a given singles rate, a larger proportion of singles was measured within the photopeak energy window as septal spacing decreased. This observation can have implications on the recorded randoms rate for some PET systems.