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Fast Transmission CT for Determining Attenuation Maps Using a Collimated Line Source, Rotatable Air-Copper-Lead Attenuators and Fan-Beam Collimation

Department of Radiology, Duke University Medical Center
Department of Biomedical Engineering and Institute of Statistics and Decision Sciences, Duke University, North Carolina

Correspondence: For correspondence and reprints contact: Ronald J. Jaszczak, PhD, Dept.of Radiology/Nuclear Medicine, Duke University Medical Center, Box 3949, Durham, NC 27710.

ABSTRACT

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 scans. The technique minimizes problems associated with discriminating ^99mTc transmission and ²⁰¹Tl emission photons and requires only a modest increase in total study time. A ^99mTc line source and a stacked foil ("multislat") collimator are placed near the focal line of a fan-beam collimator (114 cm focal length) mounted on one detector of a triple-camera SPECT system. We acquired TCT data of plastic rod and anthropomorphic thorax phantoms to investigate the capability of the line source and rotatable air-copper lead attenuators to determine attenuation maps. The data were acquired with and without 5.4 MBq (145 µCi) of ²⁰¹Tl placed in the myocardial chamber of the thorax phantom. Phantoms also were scanned using a curved transmission slab source mounted to a parallel-hole collimator. Fan-beam TCT images have improved resolution compared with parallel-beam TCT images. Two patient scans also were performed to evaluate the clinical usefulness of fan-beam TCT. The rotatable air-copper-lead attenuator method eliminates contamination of emission data by transmission photons and reduces spill-over of emission data into the transmission energy window for some cases. Results show the feasibility of using fast, sequential or interlaced transmission scans of a line source within a rotatable air-copper-lead attenuator assembly to obtain accurate attenuation maps for SPECT attenuation compensation.

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