Virtual PMTs: Improving centroid positioning performance near the edges of a gamma camera detector

The radiation detector used in typical gamma cameras and NaI-based positron emission tomography (PET) scanners effectively positions events over most of its crystal area, but the spatial resolution begins to suffer at the edges. One reason is that the measured light distribution is truncated or otherwise asymmetric, and the centroid positioning algorithm no longer optimally weights the photomultiplier tube (PMT) signals in this case. As a result, events less than a certain distance from the edge must be rejected, reducing geometric efficiency and complicating image reconstruction for PET systems with multiple fixed detectors. Short of discarding the centroid algorithm altogether, a computationally simple way to improve the positioning is to define a strip of physically nonexistent, or virtual, PMTs around the actual PMTs and to assign realistic signal values to them event by event based on the signals of nearby real PMTs. If the centroid algorithm now also includes the virtual PMTs, improved spatial resolution is obtained due to better weighting of the PMT signals. With minimal modification to the detector, the virtual PMT method was implemented and tested on an ADAC UGM C-PET detector and found to significantly improve spatial resolution near the detector edges.