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

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