Attenuation maps for SPECT determined using cone beam transmission computed tomography

A novel method for measuring nonuniform attenuation maps is presented. A cone beam geometry CT (computed tomography) scan acquired on a standard rotating gamma camera normally used for SPECT (single-photon-emission computed tomography) imaging is used. 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 can be used on the gamma camera, but the cone beam CT scans can also be acquired without the 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 were used to test 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