Light sharing in multi-flat-panel-PMT PEM detectors.

Large are a detectors, such as those used in positron emission mammography (PEM) and scintimammography, utilize arrays of discrete semtillator elements mounted on arrays of position sensitive photomultiplier tubes (PSPMT). Scintillator elements can be packed very densely (minimizing area between elements), allowing good detection sensitivity and spatial resolution. And, while new flat panel PSPMTS have minimal inactive edges, when they are placed in arrays significant dead spaces where scintillation light is undetectable are created. To address this problem, a light guide is often placed between the detector and PSPMT array to spread scintillation light so that these gaps can be bridged. In this investigation we studied the effect of light guides of various thickness on system performance. A 10x10 element array of LYSO detector elements was coupled to the center of a 2x2 array of PSPMTs through varying thicknesses (1 to 4 mm) of UV glass. The spot size of the imaged elements and distortions in the regular square pattern of the imaged scintillator arrays were evaluated. Energy resolution was measured by placing single elements of LYSO at several locations of the PSPMT array. Spatial distortions in the images of the array were reduced by using thicker light guides (3-4 mm). Use of thicker light guides, however, resulted in reduced pixel resolution and slight degradation of energy resolution. Therefore, some loss of pixel and energy resolution will accompany the use of thick light guides (minimum of 3 mm) required for optimum identification of detector elements.