A discrete scintillation camera module using silicon photodiode readout of CsI(Tl) crystals for breast cancer imaging

We characterize a 3x4 element imaging array consisting of 3x3x5 mm(3) CsI(TI) scintillator crystals individually read out by 3x3 mm(2) PIN silicon photodiodes. The array is a prototype for larger modules (16x16 element) for use in single photon breast cancer imaging. The photodiode output signals are amplified with a 16 channel custom IC (<3 mm on a side), after which a "Winner Take All" (WTA) custom IC (<3 mm on a side) identifies the crystal of interaction based on relative signal amplitudes. The compact nature of these readout electronics will simplify the construction of larger imaging arrays. The photodiodes were developed for low leakage current (similar to 50 pA) and yield a total electronic noise of 390 e-full width at half maximum (fwhm) at a shaping time of 8 mu s, with signal levels of 6600 e-for the 140 keV emissions of Tc-99m. Array pixels demonstrate an average room temperature energy resolution of 10.7+/-0.6% fwhm for these 140 keV gamma rays. We observe an intrinsic spatial resolution of 3.3 mm fwhm for a 2.5 mm diameter Co-57 beam on the face of the crystal array, and a system resolution of 5.9 mm fwhm for a 2 mm diameter uncollimated Tc-99m source viewed through a high resolution hexagonal hole collimator (1.5 mm hole diameter, 32 mm length, 4300 events/mCi/sec) at an imaging distance of 5 cm.