Investigation of scattered radiation in 3D whole-body positron emission tomography using Monte Carlo simulations

The correction of scattered radiation is one of the most challenging tasks in 3D positron emission tomography (PET) and knowledge about the amount of scatter and its distribution is a prerequisite for performing an accurate c orrection. One concern in 3D PET in contrast to 2D PET is the scatter contr ibution from activity outside the field-of-view (FOV) and multiple scatter. Using Monte Carlo simulations, we examined the scatter distribution for va rious phantoms. The simulations were performed for a whole-body PET system (ECAT EXACT HR+, Siemens/CTI) with an axial FOV of 15.5 cm and a ring diame ter of 82.7 cm. With (without) interplane septa, up to one (two) out of thr ee detected events are scattered (for a centred point source in a water-fin ed cylinder that nearly fills out the patient port), whereby the relative s catter fraction varies significantly with the axial position. Our results s how that for an accurate scatter correction, activity as well as scattering 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. This means that multi ple scatter cannot be corrected by simply rescaling the single scatter comp onent.