DISCRETE SCINTILLATOR COUPLED MERCURIC IODIDE PHOTODETECTOR ARRAYS FOR BREAST IMAGING

Multi-element (4x4) imaging arrays with high resolution collimators, s ize matched to discrete CsI(Tl) scintillator arrays and mercuric iodid e photodetector arrays (HgI2 PDA) were developed as prototypes for lar ger 16x16 element arrays for breast imaging. The compact nature of the arrays allows detector positioning in close proximity to the breast t o eliminate activity not in the line-of-sight of the collimator, thus reducing image background. Short collimators, size matched to less tha n or equal to 1.5x1.5 mm(2) scintillators show a factor of 2 and 3.4 i mprovement in spatial resolution and efficiency, respectively, compare d to high resolution collimated gamma cameras for the anticipated comp ressed breast geometries. Monte Carlo simulations, confirmed by measur ements, demonstrated that scintillator length played a greater role in efficiency and photofraction for 140 keV gammas than cross sectional area, which affects intrinsic spatial resolution. Simulations also dem onstrated that an increase in the ratio of scintillator area to length corresponds to an improvement in light collection. Electronic noise w as below 40 e(-) RMS indicating that detector resolution was not noise limited. The high quantum efficiency and spectral match of prototype unity gain HgI2 PDAs coupled to 1x1x2.5 mm(3) and 2x2x4 mm(3) CsI(Tl) scintillators demonstrated energy resolutions of 9.4% and 8.8% FWHM at 140 keV, respectively, without the spectral tailing observed in stand ard high-Z, compound semiconductor detectors. Line spread function mea surements matched the scintillator size and pitch, and small, complex phantoms were easily imaged.