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We describe a PET detector module that provides three levels of depth-of-interaction (DOI) information. The detector is a 9 x 9 array of 2 mm x 2 mm x 12 mm deep phoswich crystal elements, each consisting of 4 mm long LSO (entrance layer), GSO (middle layer) and BGO (exit layer) crystals joined optically together end-to-end. The BGO exit layer is directly coupled to a miniature position-sensitive photomultiplier tube (PSPMT). Delayed charge integration, a method that exploits differences in the light decay times of these scintillators, is used to determine the layer-of-interaction. DO1 accuracy, measured by scanning a slit source of 5 1 1 keV radiation along the length of the module was 86% for the LSO layer, 80% for the GSO layer and 84% for the BGO layer. Energy resolution at 511 keV was 19% for LSO, 21% for GSO and 40% for BGO. Apparent gain differed between layers in the ratios 2.7: 1.9: 1 .O (LS0:GSO:BGO). Crystal separation was good between crystals in the LSO layer, acceptable between crystals in the GSO layer and poor between crystals in the BGO layer due, primarily, to the pronounced spatial non-linearity of the PSPMT. The delayed charge integration method, however, does appear suitable for obtaining multi-level depth information when DO1 effects are particularly significant, e.g. in very small ring diameter PET scanners for small animal imaging. ; Was supported, in part, by a grant from CICYT (Spanish Government). S. S. was supported by a grant from the National Research Council. ; Publicado

We assembled a compact detector module comprised of an array of small, individual crystals of lutetium oxyorthosilicate: Ce (LSO) coupled directly to a miniature, metal-can, position-sensitive photomultiplier tube (PSPMT).We exposed this module to sources of 511-keV annihilation radiation and beams of 30- and 140-keV photons and measured spatial linearity; spatial variations in module gain, energy resolution, and event positioning; coincidence timing; the accuracy and sensitivity of identifying the crystal-of-first-interaction at 511 keV; and the effects of intercrystal scatter and LSO background radioactivity. The results suggest that this scintillator/phototube combination should be highly effective in the coincidence mode and can be used, with some limitations, to image relatively low-energy single photon emitters. Photons that are completely absorbed on their first interaction at 511 keV are positioned by the module at the center of a crystal. Intercrystal scatter events, even those that lead to total absorption of the incident photon, are placed by the module in a regular "connect-the-dot" pattern that joins crystal centers. As a result, the accuracy of event positioning can be made to exceed 90%, though at significantly reduced sensitivity, by retaining only events that occur within small regions-of-interest around each crystal center and rejecting events that occur outside these regions in the connect-the-dot pattern. ; The work was supported in part by a grant from CICYT (Spanish Government). The work of S. Siegel was supported by a grant from the National Research Council. ; Publicado