Physical evaluation of 511 keV coincidence imaging with a gamma camera

This work presents an evaluation of a gamma camera operating in coincidence mode. The major limitations of these cameras compared to full-ring PET are the limited count rate and sensitivity. We evaluate the progress in count rate performance which was made by upgrading the system with digital electronics. At the moment most gamma cameras use septa to reduce the number of incident coincidences by limiting the axial acceptance angle. Higher count rates can be achieved by removing the sept, which results in a fully 3D acquisition. The improvement in Noise Equivalent Count rate is measured for two different uniform phantoms. Another limitation is the non-uniform sensitivity due to the varying acceptance angle, which decreases linearly from the center towards the edges in a 180 degrees opposed dual-headed configuration. Sensitivity and uniformity can be improved by going from a dual-headed to a triple-headed configuration. As triple-headed coincidence imaging is not yet available, we calculated the theoretical sensitivity profiles from geometrical considerations.