Scintillation crystal design features for a miniature gamma ray camera

A miniature scintillation camera with a 5 cm × 5 cm field-of-view (FOV) is being developed to aid in surgical staging of cancer. This paper reports on certain interesting design aspects of the scintillation crystal array. Simulations and measurements of different types of crystal materials revealed that lutetium oxyorthosilicate (LSO), which is ordinarily used in PET, has appealing properties for detecting 140 keV photons. For example, LSO requires only 3 mm of thickness for 95% detection efficiency at 140 keV. This small thickness is appealing for high light collection efficiency, narrow light distribution, and minimizing the LSO volume in order to reduce intrinsic background events produced by ¹⁷⁶Lu (2.6% abundance). Studies of both discrete and sheet crystals revealed that discrete crystal arrays have the advantage of increased spatial linearity and dynamic range compared to crystal sheets. For optimizing sensitivity and spatial resolution, several crystal array conditions were investigated, including ground crystal pixels packed tightly together with internal reflectors and with optical coupling grease. This design focuses the light, but also allows adequate light sharing for positioning with the position sensitive photomultiplier tube (PSPMT) that will be used in the gamma camera.