Modular 64x64 CdZnTe Arrays With Multiplexer Readout for High-Resolution Nuclear Medicine Imaging
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ABSTRACT We are developing modular arrays of CdZnTe radiation detectors for high-resolution nuclear medicine imaging. Each detector is delineated into a 64x64 array of pixels; the pixel pitch is 380 ptm. Each pixel is connected to a corresponding pad on a multiplexer readout circuit. The imaging system is controlled by a personal computer. We obtained images of standard nuclear medicine phantoms in which the spatial resolution of approximately 1.5 mm was limited by the collimator that was used. Significant improvements in spatial resolution should be possible with different collimator designs. These results are promising for high-resolution nuclear medicine imaging. INTRODUCTION Gamma-ray imaging in nuclear medicine demonstrates the biodistribution of radionuclides in the body. Spatial resolution of gamma-ray imaging devices has improved greatly in the nearly 50 years since rectilinear scanners were introduced. The intrinsic spatial resolution at present for gamma cameras with sodium iodide scintillation detectors is about 3-4 mm for planar images; the spatial resolution for tomographic images is about 10 mm [1]. Since spatial resolution is degraded by the collimator, by increasing distance from the collimator, and by scatter of gamma rays in the body, the resolution in clinical images seldom achieves these values. Better spatial resolution would be useful for many imaging applications in general and for single-photon emission computed tomography (SPECT) of the brain in particular. Radiopharmaceuticals are being developed that target specific receptors in brain, and the ability to image abnormalities in receptor binding that are in the range of 1-2 mm in size would be expected to have important clinical benefits. When such abnormalities do not have accompanying structural abnormalities, they are unlikely to be apparent on x-ray computed tomography (CT) or magnetic resonance imaging (MRI), and so the radionuclide images will contain unique information. Semiconductor detectors for gamma-ray imaging are attractive alternatives to NaI(TI) and other scintillators for imaging in nuclear medicine. The good energy resolution of semiconductors such as germanium (Ge) permits identification of photons that have undergone Compton scattering and that may therefore give misleading information about the site of radioactive decay in the body. Disadvantages of Ge include its relatively low atomic number, which results in poor absorption of medium and high-energy gamma rays, and its small band gap, which requires operation at cryogenic temperatures. Other semiconductors with higher effective atomic numbers and larger band gaps that permit room-temperature operation include mercuric iodide (HgI 2), cadmium telluride (CdTe), and cadmium zinc telluride (CdZnTe). An advantage of CdZnTe is its production in multi-kilogram boules by the high-pressure vertical Bridgman method, which may permit economies of scale and lower prices compared to other semiconductors. A disadvantage of Hg92, CdTe and CdZnTe is lower 267 Mat. Res. Soc. Symp. Proc
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