GATE simulations of human and small animal PET for determination of scatter fraction as a function of object size

In 2D-mode positron emission tomography (PET), scatter is either compensated by approximate methods based on the existing emission data or ignored altogether as the magnitude of the scatter fraction (SF) is within an acceptable range (10-20%). In clinical 3D PET studies, however, attenuation and sophisticated scatter correction methods are required along with CT or radionuclide transmission scans. With the growing interest in small animal imaging, these correction methods are being translated to small animal scanners, but there is little scientific information about the requirements associated with smaller size objects and scatter geometries. In this study, we focused on the magnitude of the scatter through a series of scatter fraction measurements. To determine the scatter as a function of object size, Monte Carlo simulations were performed using GATE (Geant4 application for emission tomography). Models of the ECAT HR+ PET scanner (included in the GATE package) and the Siemens Inveon small animal scanner (generated by the first author) were used. Simulations were performed for several digital phantoms including the NEMA, XCAT and MOBY phantoms over a wide range of sizes. The simulations on the Inveon scanner of the NEMA-like phantoms indicated that for cylindrical objects less than 4 cm diameters (encompassing small rats and all mice), the scatter fraction was lower than 15%. In addition, the scatter fraction values for MOBY phantoms up to 5 cm in diameter and 7.7 cm in length were also less than 15%. On the other hand, the scatter fraction was more than 25% for even the smallest size human NEMA-like and XCAT phantoms. These results suggest that sophisticated scatter correction methods may not be required for the indicated sizes of mice and rats.