A COMBINED HALF-CONE BEAM AND PARALLEL HOLE COLLIMATION SYSTEM FOR SPECT BRAIN IMAGING

Our research group has recently examined new types of collimator desig ns with the goal of improving sensitivity and lesion detection for hea d imaging. One of these collimator designs is a half-cone beam collima tor. However, the sensitivity is reduced as objects are removed from t he focal line and it does not satisfy Tuy's sufficiency condition. Par allel hole collimation does not have this problem with axial blurring, however, values for sensitivity are not as large as for half-cone col limators. A potential configuration is to use one parallel hole collim ator in conjunction with two half-cone beam collimators on a triple ca mera single photon emission computed tomography (SPECT) system. This m ight produce results that represent the best of both collimation syste ms. We acquired projection data with a Defrise disk phantom filled wit h 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 supe r high resolution (LESR) parallel hole collimator was placed on a seco nd head. Different projection data sets were combined to model acquisi tion in the three headed gamma camera with three half-cone beam collim ators, two half-cone beam and one parallel beam collimators, one half- cone beam and two parallel beam collimators and three parallel beam co llimators. Image reconstruction used a modified maximum likelihood max imization-expectation (ML-EM) algorithm. For the reconstruction with t hree half-cone beam collimators, we observe axial blurring. This is la rgely reduced with two half-cone beam and one parallel beam collimator s. Graphs of the image profiles demonstrate that the blurring along th e axial direction is decreased with the addition of parallel hole coll imators.