Development of a high-resolution gamma-ray imaging system with synthetic collimation.

Abstract

This work outlines the development of a multi-pinhole single-photon emission computed tomography (SPECT) system designed to produce a synthetic-collimator image of a small field of view. The synthetic-collimator image was formed by estimating the activity distribution within the FOV and summing the activities in the voxels along the axis perpendicular to the detector face. A large-area, CCD-based gamma-ray detector, called BazookaSPECT, was used to detect gamma rays. A focused multi-pinhole collimator was constructed using rapid-prototyping and casting techniques. The collimator projected the centre of the field of view (FOV) through forty-six pinholes when the detector was adjacent to the collimator. Synthetic collimation requires that data are acquired using several system configurations. The object-collimator distance remained constant and the magnification was controlled by changing the collimator-detector distance. The amount of pinhole-projection overlap (multiplexing) increased as the system magnification increased. An advantage of synthetic collimation is that multiplexed data can be used, thereby improving the efficiency of data acquisition. The first system configuration acquires non-multiplexed projection data and each subsequent system configuration will make more efficient use of the detector by acquiring multiplexed data that can be demultiplexed. Synthetic collimation enables the use of multiplexing to produce high-resolution, artefact-free reconstructions. There was no rotation in the system; a single tomographic angle was used in each system configuration. Image reconstruction was performed using maximum-likelihood expectation-maximization (MLEM) and an experimentally measured system matrix. The system matrix was measured for each system configuration by translating a point source through a sparsely-sampled grid encompassing the FOV. An algorithm for multi-pinhole identification and tracking was developed. A 2D elliptical Gaussian distribution was fitted to each pinhole projection. The system matrix was interpolated using the coefficients of the distributions. Simulations with a hot-rod phantom demonstrated the efficacy of combining low-resolution non-multiplexed data with high-resolution multiplexed data to produce high-resolution reconstructions. The system was validated experimentally with a hot-rod phantom, and a small-animal imaging study was also performed.