Two new strategies to reduce the scatter fraction in positron volume imaging

Two hardware techniques which improve the scatter fraction of positron volume imaging (PVI) systems are presented. The first uses the fact that Compton scattered rays scattered through a large angle have a lower energy. The range of angles through which rays can be scattered and still be in the scanner´s field of view (FOV) depends on the radius of the line of response (LOR). By using a low-energy threshold which is proportional to the LOR radius, the scatter fraction at the edges of the image is substantially reduced. The second technique is based on the observation that in NaI crystals many of the 511-keV rays are backscattered, and the scattered photon then escapes undetected. Systems using NaI crystals typically use a relatively high energy threshold to take advantage of their good energy resolution. Instead, the authors propose using a notched energy acceptance window, which includes the photopeak and the backscatter peak, but discards the scattered events in between