Evaluation of a Large Pixellated Cadmium Zinc Telluride Detector for Small Animal Radionuclide Imaging

Small animal microSPECT systems require high resolution and efficiency to faithfully image biodistributions of molecular reporters and radiopharmaceuticals in a short time. Because of their variable magnification, pinhole and multipinhole cameras are particularly well suited for small animal imaging systems. These cameras nonetheless require a high degree of sampling to achieve high resolution in tomographic images. In order to construct a high resolution pinhole and multipinhole gamma camera for small animal imaging, we developed a large cadmium zinc telluride (CZT) detector array. This detector, having 128times128 1.5times1.5 mm
²
pixels, is one of the largest of its kind in terms of number of pixels and readout channels. The CZT detector crystal array and application-specific integrate circuits (ASICs) are embedded in an aluminum enclosure to form a compact cassette unit. The signals generated by gamma interactions in the CZT crystal are amplified, shaped and multiplexed within the detector unit, and thereafter read by a computer-based data acquisition system. A high-energy keel edge pinhole collimator was coupled to this detector and used to image photons with energies up to 250 keV. This new CZT gamma detector was characterized using Tc-99m (140 keV) and In-111 (171 keV and 245 keV). Specifically, we measured the dead pixel fraction, the uniformity, the intrinsic spatial resolution, and the energy resolution of this detector. Furthermore, we assessed the sensitivity of the pinhole camera. The detector was shown to have fewer than 1% of dead pixels, and also demonstrated energy resolutions of 6.8 % at 140keV (Tc-99m), 6.2 % at 171 keV, and 6.0 % at 245 keV (In-111). Using a microsphere phantom at 3 cm from a 0.5 mm pinhole, a sensitivity of 20 cps/MBq (Tc-99m) was achieved. This new detector will be integrated into our recently developed microSPECT/microCT small animal scanner to increase the overall system sensitivity and image resolution.