Discrete scintillator coupled mercuric iodide photodetector arrays for breast imaging

Multi-element (4×4) imaging arrays with high resolution collimators, size matched to discrete CsI(Tl) scintillator arrays and mercuric iodide photodetector arrays (HgI₂ PDA) were developed as prototypes for larger 16×16 element arrays for breast imaging. The compact nature of the arrays allows detector positioning in close proximity to the breast to eliminate activity not in the line-of-sight of the collimator, thus reducing image background. Short collimators, size matched to 11.5×1.5 mm² scintillators show a factor of 2 and 3.4 improvement in spatial resolution and efficiency, respectively, compared to high resolution collimated gamma cameras for the anticipated compressed breast geometries. Monte Carlo simulations, confirmed by measurements, 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 demonstrated that an increase in the ratio of scintillator area to length corresponds to an improvement in light collection. Electronic noise was 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 1×1×2.5 mm³ and 2×2×4 mm³ CsI(Tl) scintillators demonstrated energy resolutions of 9.4% and 8.8% FWHM at 140 keV, respectively, without the spectral tailing observed in standard high-Z, compound semiconductor detectors. Line spread function measurements matched the scintillator size and pitch, and small, complex phantoms were easily imaged