The effect of truncation reduction in fan beam transmission for attenuation correction of cardiac SPECT

A limitation of fan beam transmission imaging using a 40 cm field-of-view scintillation camera is the data truncation that occurs when imaging medium to large-sized patients. With filtered backprojection, truncation may cause bright rings in the reconstructed image. The primary objective of this study is to evaluate a method to extrapolate the truncated transmission data under clinically relevant count density conditions. The method involves obtaining the patient contour by processing the scatter and photopeak emission data, filling the contour with the attenuation coefficient for soft tissue, reprojecting the contour image, extrapolating the truncated projection set with the projections. A long focal length (114 cm) fan collimator is used on one head of a triple camera SPECT system to acquire transmission data. The two remaining detectors are equipped with low energy, ultra high resolution parallel hole collimators. A large thorax phantom (38 cm×26 cm) and patient data are used to evaluate the method. For SPECT image reconstruction, non-uniform attenuation correction is performed with a truncated attenuation map, an extrapolated attenuation map and the untruncated attenuation map. The SPECT results indicate that image uniformity changes very little using any of the three different attenuation maps when a long focal length fan beam collimator is used for transmission data acquisition. Truncation artifacts that are apparent in the transmission image can be substantially reduced for objects up to 40 cm wide