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Transmission Imaging for Nonuniform Attenuation Correction Using a Three-Headed SPECT Camera

Department of Radiology, Division in Nuclear Medicine, Duke University Medical Center

Correspondence: For correspondence or reprints contact: David R. Gilland, PhD, DUMC-3949, Durham, NC 27710.

ABSTRACT

Our objective was to build and test a new system for transmission CT (TCT) imaging on a three-headed SPECT camera. The TCT images are intended for use in nonuniform attenuation correction of cardiac SPECT data. Methods: The system consists of a transmission line source mounted to the camera gantry at the focal line of a long focal length, asymmetric fanbeam collimator. The focal line is 114 cm from the collimator surface and shifted 20 cm from the detector midline. This asymmetric fanbeam geometry is used to reduce truncation artifacts in the reconstructed TCT image. The line source fixture accommodates a 25-cm long source and contains removable, variable thickness attenuator plates (copper or lead) to modulate the photon flux density and a slat collimator to collimate the TCT source beam in the axial direction. For the TCT reconstruction, an iterative maximum likelihood-expectation maximization algorithm is used that models the asymmetric fanbeam geometry. Our initial studies with this system used a 1850 MBq (50 mCi) ^123mTe line source. The evaluation included TCT scans of a resolution phantom, an anthropomorphic thorax phantom and a human subject. For the thorax phantom and human subject, short (2-min) and long (14-min) scans were performed. The SPECT imaging performance of the fanbeam collimator was also characterized. Results: For both phantom and human data, high quality TCT reconstructions were obtained with linear attenuation coefficients closely matching narrow beam values. In the images of the resolution phantom, the smallest rods (4.8-mm diam) were resolved. The long scan images of the thorax phantom and human subject demonstrated the high resolution nature of the system and contained no evidence of truncation artifacts. With smoothing to control noise, the short scan images generally retained the attenuation features of the lung and of soft tissue and may provide a practical approach for clinical application. The fanbeam collimator demonstrated high resolution SPECT performance. Conclusion: These results suggest this system may provide an effective and practical approach to TCT imaging for nonuniform attenuation correction on a three-headed SPECT camera.

Key Words: SPECT • attenuation correction • transmission CT • maximum likelihood-expectation maximization reconstruction • fanbeam collimation

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