Fast transmission CT for determining attenuation maps using a collimated, shuttered line source and fan beam collimation

The authors describe a technique using a shuttered line source to acquire gamma transmission computed tomographic (TCT) data for determining attenuation maps to compensate for nonuniform attenuation in single photon emission computed tomography (SPECT) emission scans. A ^99mTc line source and a stacked foil (slit) collimator were placed near the focal line of a fan beam collimator (114 cm focal length) mounted opposite one detector of a triple-camera SPECT system. To investigate the capability of shuttered line sources to determine accurately attenuation maps, an anthropomorphic thorax phantom was scanned using total TCT acquisition times of 120 and 720 s. The phantom was also scanned with a parallel hole collimator using a collimated slab source. The fan beam and parallel beam TCT images were very similar. The shutter method eliminates contamination of emission data by transmission photons, and reduces the spill-over of emission data into the transmission energy window. These results demonstrate the feasibility of using fast, sequential (or interlaced) transmission scans of high-activity shuttered sources to obtain accurate attenuation maps for SPECT attenuation compensation