Simulations of the Effect of Collimation on Count Rates of an LSO PET System

We present a simulation study of the effect of different degrees of collimation on countrate performance of a hypothetical PET scanner with LSO crystals. The simulated scanner is loosely based on the geometry of the Siemens Biograph Hi-Rez scanner. System behavior is studied with a photon tracking simulation package (SimSET). We investigate the NEMA NU2-2001 count rate and scatter fraction behavior for systems with different amounts of collimation, which is acheived by adding septa to the fully-3d system as in clinical use. We study systems with 2,5,11, and 40 septa. The effect of collimation is studied for three patient thicknesses. The resulting count rate curves for true, scattered, and random coincidences as well as noise equivalent count rates are compared for the different collimation cases. Improved countrate performance with partial collimation is seen. However, except for the largest diameter phantom, the NEC rate increase is seen at higher activities than those used clinically. The NEC countrate versus activity curves for the LSO systems are also compared to those from a BGO system where partial collimation increases NEC countrate over a clinically relevant activity range.