Monte Carlo simulation of the scatter contribution in a 3D whole-body PET

Scatter contamination is one of the main reasons for image degradation in 3D Positron Emission Tomography (PET). The knowledge about the amount of scatter and its distribution is a prerequisite for performing an accurate scatter correction. One concern is the scatter contribution from activity outside the field-of-view (FOV) and multiple scatter. We examined the scatter distribution for various phantoms using Monte Carlo simulations. The simulations were performed for a whole-body PET system (ECAT EXACT HR⁺, Siemens/CTI). The scanner has an axial FOV of 15.5 cm and a ring diameter of 82.7 cm. With (without) interplane septa the scatter contribution is up to 40(65)% (for a line source in a 40 cm cylinder) of the total counts. The scatter fraction varies significantly with the axial position. The results show also that for an accurate scatter correction, both activity and scatter media outside the FOV have to be taken into account. Furthermore it could be shown that there is a considerable amount of multiple scatter which has a different spatial distribution from single scatter. Therefore multiple scatter cannot be corrected by simply rescaling the single scatter component