Transmission CT reconstruction for offset fan beam collimation

Offset fan beam collimation has shown to be an effective means of obtaining transmission CIT (TCT) data for SPECT attenuation correction. This paper c ompared three methods for handling the truncation problem and reconstructin g this TCT data: iterative OS-EM and filtered backprojection (FBP) with two different pre-processing approaches. The first approach (FBP1) extends the fan beam projection by sinogam interpolation, and the second (FBP) uses pa rallel beam rebinning. With a TCT spatial resolution phantom, FBP2 demonstr ated a small degree of resolution loss which was equalized across methods a fter post-reconstruction filtering. With a thorax phantom, all methods perf ormed adequately at high count level; however, at low count level, FBP1 sho wed degraded image quality due to its handling of zero count projection bin s. This finding was corroborated in patient images. We conclude that at pra ctical count levels, the iterative OS-EM method offers a clear advantage in TCT image quality compared to FBP methods.