Reduction of Truncation Artifacts in Fan Beam Transmission Imaging Using a Spatially Varying Gamma Prior

Using fan beam transmission imaging to obtain an attenuation estimate of an object for attenuation correction is limited because the object may not be completely in the field of view of the camera at all angles. This truncation of object data creates artifacts in the attenuation map during reconstruction which may result in problems in the attenuation correction of the emission image. To improve the quality of the attenuation map, we have used information from the scatter window reconstruction of projections from a parallel hole collimator on one of the heads of the SPECT system. This scatter window reconstruction is used to create a segmentation-based attenuation map of the object. We use this map as a pixel-by-pixel gamma prior to aid in the iterative reconstruction of the final attenuation map. In the fully sampled central region, use of the segmentation map information is unnecessary; but in the poorly sampled, outer region, the use of the segmentation map is essential to obtain an accurate reconstruction. We use a spatially varying weight function for the gamma prior to reflect our confidence in the transmission data in the central region while emphasizing the prior in the outer region. The result is a high-quality density map over both the fully sampled and poorly sampled regions. Furthermore, since the line integrals for the transmission data are preserved in the iterative reconstruction, the algorithm does not require precise determinations of the attenuation coefficients in the segmentation maps to create an accurate attenuation map.