Development of an ultra-high resolution SPECT system with a CdTe semiconductor detector

Objective: The aim of this work was to evaluate an ultra-high spatial resolution SPECT system with a semiconductor detector and a high-resolution parallel-hole collimator or a pinhole collimator for small animal imaging. Methods: We evaluated an ultra-high spatial resolution SPECT system with a high-resolution parallel-hole collimator attached to a cadmium telluride (CdTe) semiconductor detector for small animal imaging. The sizes of an active area and a pixel in the semiconductor detector were 44 × 44 and 0.5 × 0.5 mm², respectively. In the high-resolution parallel-hole collimator the size of a hole was 0.4 × 0.4 mm², the thickness of a septum 0.1 mm, and the hole-length 30 mm. We also used a high-resolution pinhole collimator with a hole size of 0.3 or 0.5 mm φ. The physical performance of this SPECT system was evaluated with some experiments with phantoms filled with ^99mTc-pertechnatate solution. In addition ideal performance and limitations of the system were evaluated with Monte Carlo simulations under the same geometrical conditions as in the experiments. In the evaluation for small animal imaging, we used mice that were administered with ^99mTc-MDP. We also conducted an ultra-high resolution X-ray CT of the mice to verify the accumulated location of ^99mTc-MDP using the bone CT images of the mice. Results: The results of the phantom experiments showed that we could resolve 1 mm φ hot-channels and 1.6 mm φ cold-rods with the high-resolution parallel-hole collimator and pinhole collimators. We could image 0.3 mm φ hot-channels with the high-resolution pinhole collimators. The results of the simulations showed that the resolution limit in the pinhole imaging was about 0.6 mm FWHM. And the results of experiments with mice showed that we could reconstruct high-resolution images of ^99mTc-MDP. Furthermore, the distribution of ^99mTc-MDP in a mouse was found to correspond closely to the location of the bones of the mouse in reconstructions made with the ultra-high resolution X-ray CT system. Conclusions: Our results demonstrated that the ultra-high spatial resolution SPECT system was feasible for small animal imaging allowing a relatively long data acquisition time.