Effect of the rotational orientation of circular photomultipliers in a PET camera block detector design

This is a study of the effects of geometric asymmetries in circular photomultipliers (PMT) on the design of PET position-sensitive block detectors. The dynodes of linear-focus circular PMTs are asymmetric relative to the axis of the photocathode, despite the rotational symmetry of the photocathode. Hence, regional photocathode differences that appear in the anode signal, affect the decoding characteristics of position-sensitive block detectors. This orientation effect, as well as the effect of introducing light diffusers, were studied in a EGO-block detector design using the PMT-quadrant-sharing configuration. The PMT studied is the Philips XP-1911 (19 mm diameter). Seven symmetrical and representative orientations of the four decoding PMTs were investigated, as well as one asymmetric orientation. Measurements included block-composite pulse-height spectra and crystal decoding maps. Two orientation effects were observed: (a) distortion variation in decoding maps, and (b) variation in decoding resolution. The introduction of circular plastic pieces, used as light diffusers, proved to be useful because they improved the decoding of crystals on the periphery of the detector block and minimized distortion in the decoding map. Optimal PMT orientation for the PMT-quadrant-sharing design, was obtained from these measurements, as well as for conventional block designs.