A combined half-cone beam and parallel hole collimation system for SPECT brain imaging

摘要

The authors' research group has recently examined new types of collimator designs with the goal of improving sensitivity and lesion detection for head imaging. One of these collimator designs is a half-cone beam collimator. However, the sensitivity is reduced as objects are removed from the focal line and it does not satisfy Tuy's sufficiency condition. Parallel hole collimation does not have this problem with axial blurring, however, values for sensitivity are not as large as for half-cone collimators. A potential configuration is to use one parallel hole collimator in conjunction with two half-cone beam collimators on a triple camera single photon emission computed tomography (SPECT) system. This might produce results that represent the best of both collimation systems. The authors acquired projection data with a Defrise disk phantom filled with Tc-99m. A half-cone beam collimator with a focal length of 50 cm was placed on one head of a triple camera SPECT system. A low energy super high resolution (LESR) parallel hole collimator was placed on a second head. Different projection data sets were combined to model acquisition in the three headed gamma camera with three half-cone beam collimators, two half-cone beam and one parallel beam collimators, one half-cone beam and two parallel beam collimators and three parallel beam collimators. Image reconstruction used a modified maximum likelihood maximization-expectation (ML-EM) algorithm. For the reconstruction with three half-cone beam collimators, the authors observe axial blurring. This is largely reduced with two half-cone beam and one parallel beam collimators. Graphs of the image profiles demonstrate that the blurring along the axial direction is decreased with the addition of parallel hole collimators.