Characterization of imaging gamma detectors for use in small animal SPECT

In this paper we investigate the basic properties of an imaging gamma ray detector built using a square 2×2 array of Hamamatsu H8500 position sensitive photomultiplier tubes (PSPMTs) coupled to a pixelated NaI(Tl) crystal array. The measured imaging properties are compared with those of a detector containing an identical crystal array, but based on a single Hamamatsu R3292 PSPMT resulting in a round field of view. For both detectors, the crystal array contains 1.4 mm × 1.4 mm × 6 mm crystals with 1.6 mm center-to-center spacing. Specifically, we present the results of investigations of the basic properties, including intrinsic spatial resolution, detection efficiency, energy resolution, and sensitivity uniformity. Intrinsic spatial resolution was measured at three different locations on the detector surface giving an average of 1.8 mm FWHM for both detectors. Energy resolution was the same for both detectors and was 15% FWHM at 140 keV and 31% at 31 keV. The measured detection efficiency at 140 keV was ∼35% and ∼33% for the square and round detectors respectively. The detector response non-uniformity, defined as the ratio of the standard deviation of pixel values to the average pixel value in the central 75% of the field of view, was 8% for the square detector and 23% for the round one. These measurements indicate that the performance of the two detectors is similar, with the exception of sensitivity uniformity, in which the square detector performance is superior. The results of initial testing of a detector with a 3×4 H8500 PSPMT array coupled to 1.2 mm × 1.2 mm × 6 mm NaI crystal array are also presented. The intrinsic spatial resolution of the 3×4 detector was measured to be about 2.1 mm FWHM. The energy resolution was 22.7% at 140 keV. The measured detection efficiency at 140 keV was ∼27% The detector response nonuniformity was measured to be about 40%. Although the sensitivity non-uniformity of the 3×4 detector was not as good as that of the other two detectors, the ability to more readily scale the field of view of the square PSPMT-based detector, makes this design a promising one for use in small animal SPECT.