FAST TRANSMISSION CT FOR DETERMINING ATTENUATION MAPS USING A COLLIMATED LINE SOURCE, ROTATABLE AIR-COPPER-LEAD ATTENUATORS AND FAN-BEAM COLLIMATION

We describe a technique using a line source and a rotatable air-copper -lead assembly to acquire gamma transmission computed tomographic (TCT ) data for determining attenuation maps to compensate SPECT emission s cans. The technique minimizes problems associated with discriminating Tc-99m transmission and Tl-201 emission photons and requires only a mo dest increase in total study time. A Tc-99m line source and a stacked foil (''multislat'') collimator are placed near the focal line of a fa n-beam collimator (114 cm focal length) mounted on one detector of a t riple-camera SPECT system. We acquired TCT data of plastic rod and ant hropomorphic thorax phantoms to investigate the capability of the line source and rotatable air-copper-lead attenuators to determine attenua tion maps. The data were acquired with and without 5.4 MBq (145 muCi) of Tl-201 placed in the myocardial chamber of the thorax phantom. Phan toms also were scanned using a curved transmission slab source mounted to a parallel-hole collimator. Fan-beam TCT images have improved reso lution compared with parallel-beam TCT images. Two patient scans also were performed to evaluate the clinical usefulness of fan-beam TCT. Th e rotatable air-copper-lead attenuator method eliminates contamination of emission data by transmission photons and reduces spill-over of em ission data into the transmission energy window for some oases. Result s show the feasibility of using fast, sequential or interlaced transmi ssion scans of a line source within a rotatable air-copper-lead attenu ator assembly to obtain accurate attenuation maps for SPECT attenuatio n compensation.