Depth of interaction in scintillation crystal by light collimation for the design of "universal nuclear medicine imager"

Abstract

The detector head for a dual headed Universal Nuclear Medicine Imager (UNMI) is designed. It is based on position sensitive (PS) area detectors fitted with a thick (0.75) Nal(Tl) crystal. In order to make it universal, a means for determining the depth of interaction (DOI) in the crystal is devised. Crystal thickness is divided into two layers, each 3/8 thick, by insertion of a thin (0.03) quartz (glass) layer; these layers added to the external lower and upper surfaces of crystal, too, form two internal collimator (LC) channels by their reflectively (specular) polished couplings. These DOI channels, according to the light refraction/reflection law direct the light reflected from quartz layers (as exceeding a critical angle) through NaI(Tl) layers to the external parts of LC system; these are connected via quartz optical cables to the DOI determining PM tubes. DOI electronics including a coincidence circuit, connecting signals from conventional PS and DOI PM tubes arrays, identifies a DOI layer and shifts an, x, y, address to the particular DOI memory stack. The UNMI design is likely to retain spatial resolution of PS detectors-gamma cameras for low energy single photon emitters and to improve spatial resolution in PET studies done by PS detectors fitted with thick NaI crystals, enabling a proper correction for parallax error.