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 ph otomultipliers (PMT) on the design of PET position-sensitive block det ectors. The dynodes of linear-focus circular PMTs are asymmetric relat ive to the axis of the photocathode, despite the rotational symmetry o f the photocathode. Hence, regional photocathode differences that appe ar in the anode signal, affect the decoding characteristics of positio n-sensitive block detectors. This orientation effect, as well as the e ffect of introducing light diffusers, were studied in a BGO-block dete ctor design using the PMT-quadrant-sharing configuration. The PMT stud ied is the Philips XP-1911 (19 mm diameter). Seven symmetrical and rep resentative orientations of the four decoding PMTs were investigated, as well as one asymmetric orientation. Measurements included block-com posite 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 pl astic pieces, used as light diffusers, proved to be useful because the y improved the decoding of crystals on the periphery of the detector b lock and minimized distortion in the decoding map. Optimal PMT orienta tion for the PMT-quadrant-sharing design was obtained measurements, as well as for conventional block designs.