ULTRA-HIGH-RESOLUTION BRAIN SPECT IMAGING - SIMULATION RESULTS

The spatial resolution in a reconstructed SPECT image is obviously inf luenced by the intrinsic resolution of the detector, but it is not gen erally recognized that the photon-counting efficiency of SPECT systems is also determined by the intrinsic resolution. In fact, it is often stated that increased intrinsic detector resolution is of little use s ince the overall resolution is limited by the collimator rather than t he detector, and that collimator resolution cannot be increased withou t an unacceptable sacrifice in efficiency. In this paper we attempt to demonstrate that improvements in detector resolution can lead to both improved spatial resolution in the image and improved counting effici ency compared to conventional systems. This paradoxical conclusion res ults from optimizing the geometry of a multiple-pinhole coded-aperture system when detectors of very high resolution are available. Suitable semiconductor detectors, with sub-millimeter pixels, are currently un der development in our laboratory and are discussed elsewhere in this volume. In this paper we report simulation studies that demonstrate th e image quality that is attainable with such detectors. Reconstruction s were performed using an iterative search algorithm on a custom-desig ned parallel computer. The imaging system was described by a calculate d system matrix relating all voxels in the object space to all pixels on the detector. We found a resolution close to 2 mm on the reconstruc ted images obtained from these computer simulations with clinically re asonable exposure times. This resolution may be even further improved by optimization of the multiple-pinhole aperture. Thus the novel semic onductor modular gamma-camera design should provide a large improvemen t not only in detector resolution but also in reconstructed resolution .