2 NEW STRATEGIES TO INCREASE THE SIGNAL-TO-NOISE RATIO IN POSITRON VOLUME IMAGING

Positron volume imaging (PVI) systems detect many more scattered events than do multi-slice positron emission tomographs (MS-PET). This higher scatter fraction is offset by significantly improved sensitivity to true events. We present two new hardware techniques which improve the scatter fraction of PVI systems. 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. By using a low-energy threshold which is proportional to the line of response's (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 back-scattered, 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, we propose using a notched energy acceptance window, which includes the photo-peak and the back-scatter peak, but discards the scattered events in between.