ATTENUATION MAPS FOR SPECT DETERMINED USING CONE BEAM TRANSMISSION COMPUTED-TOMOGRAPHY

This paper presents a new method for measuring non-uniform attenuation maps, using a cone beam geometry CT scan acquired on a standard rotating gamma camera normally used for SPECT imaging. The resulting map is intended for use in nonuniform attenuation compensation of SPECT images. The method was implemented using a light-weight point source holder attached to the camera. A cone beam collimator may be used on the gamma camera, but the cone beam CT scans may also be acquired without collimator. In either implementation, the advantages include very high efficiency and resolution limited not by the collimator but by the intrinsic camera resolution (about 4 mm). Several phantoms tested the spatial uniformity, noise, linearity as a function of attenuation coefficient, and spatial resolution. Good quality attenuation maps were obtained, at least for the central slices where no truncation was present. Uniform spheres and cylinders gave uniform reconstructions, except for a decrease due to scatter in the image center. Plots of reconstructed attenuation value versus known attenuation coefficient in the linearity phantom were linear, with correlation coefficient R2 = 0.995. The resolution phantom tests were limited by noise, but structures at least as small as 11 mm were readily resolved. Due to the efficient geometry, only 200 µCi of Tc99m was used. This is about 100 times less than necessary for sheet source measurements of the attenuation map. The corresponding dose to a patient would be truly negligible. These first tests were successful, and encourage further work on this technique. Copyright © 1990 by The Institute of Electrical and Electronics Engineers, Inc.