IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 45, NO. 3, JUNE 1998 487 High Spatial Resolution Gamma Imaging Detector Based on a 5" Diameter R3292 Hamamatsu PSPMT R. Wojcik', S. Majewski', B. Kross', D. Steinbach2, A.GI. Weisenberger ' 'Jefferson Lab., 12000 Jefferson Ave., Newport News, VA 23606 'College of William and Mary Physics Dept., Williamsburg , VA 23 187 Abstract High resolution imaging gamma-ray detectors were developed using Hamamatsu's 5" diameter R3292 position sensitive PMT (PSPMT) [l] and a variety of crystal scintillator arrays. Special readout techniques were used to maximize the active imaging area while reducing the number of readout channels. Spatial resolutions approaching 1 mm were obtained in a broad energy range from 20 to 5 11 keV. Results are also presented of coupling the scintillator arrays to the PMT via imaging light guides consisting of acrylic optical fibers. I. INTRODUCTION The need for high resolution gamma cameras in many fields continues to grow, In nuclear medicine current commercial systems using multiple PMTs and a slab of scintillator can only resolve cancers on the order of a cm whereas, in the new fields of scintimammographyand positron emission mammography, doctors need to see cancers as small as a few mm. In the case of studying gene expression in small animals a resolution of about 1 mm is needed [2]. One commercial solid state imaging system using Cadmium Zinc Telluride [3] is trying to fill the need for higher resolutions, however, the stopping power is limited to energies less than 200 keV and the price is significantly higher than a PMT- crystal system. Several recent efforts have described the use of small gamma cameras based on PSPMTs [4-lo]. Our goal is to develop a cost effective high resolution imaging system for a broad energy range (20 to 5 11 keV). 11. EXPERIMENTAL SETUP A. PMT and Electronics Hamamatsu's R3292 has a proximity focused parallel dynode mesh structure with crossed wire anodes. The x and y coordinates consist of 28 wires apiece and the active area of the PMT is 11 cm in diameter. To reduce the number of individual analogue readout channels while still retaining the advantages of a local readout (as opposed to integral readout methods such as current division or delay lines), we grouped the anode wires into sectors with two to four wires per sector (fig. 1). The advantages of local readout include high rate operation, less edge distortion, and the ability to apply a special truncated center of gravity (COG) technique, mentioned below, to maximize the active imaging area Each anode sector was amplified before being sent over a coaxial ribbon cable to the data acquisition system. The data acquisition system was based on LeCroy's FERA ADC's and a Macintosh Power PC running Kmax data acquisition software [ 111. The last dynode was used to trigger the discriminator and provide a gate to the ADCs. Phillips Scientific 1.5 usec Discriminator Generator -- -- - -- - - - -- n 730 Discriminator Generator I- 4301 driver 4302 memory 4300B ADC BufferILlne dnver Figure 1: PMT readout electronics. B. Crystal arrays and Energies Through previous work we found that, from a selection of the most popular crystal scintillators (YAP, CsI(Na), BGC), NaI(Tl), GSO, and LSO), only CsI(Na), NaI(Tl), and GSO arc prime candidates for nuclear medicine gamma cameras [12]. YAP and BGO are ruled out due to relatively low light output and LSO is naturally radioactive and, at this time, not readily commercially available. All of the arrays are made up of small square scintillator pixels arranged in an approximately 11 cm diameter circle. We also tiested a NaI(T1) slab for comparison. Crystal arrays are preferable over slabs of scintillators because of the spatial non-uniformities found in most PSPMTs (fig. 2). There is, however, a small loss in energy resolution. By being able to identify the particular pixel light came from, image distortion correction is easily implemented. The size of the pixel is important iin optimizing the trade-off between spatial resolution and having enough light delivered to the PMT for crystal identification and energy determination. The thickness of the array w,as selected to optimize the tradeoff between stopping power and efficient light transfer to the PMT, again effecting energy resolution. In general, energy resolution is important for rejecting scattered radiation which blurs the image. All of the arrays except for the GSO array were manufactured by various companies [13-161 by taking a slab of the material and sectioning it with a fine diamond saw. The crystals are separated by approximately .25 mm with the region between the cryistals filled with an opaque white reflective material. The GSO array is hand assembled from pixels which only have thcir output face polished. The other 5 sides are left rough cut and no optical separation is used. The NaI(T1) arrays and slab are enclosed in .5 mm thick A1 with a 1 mm quartz window. US. Government Work Not Protected by US. Copyright