Transmission CT reconstruction for offset fan beam collimation

Offset fan beam collimation has shown to be an effective means of obtaining transmission CT (TCT) data for SPECT attenuation correction. This paper compared three methods for handling the truncation problem and reconstructing 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 sinogram interpolation, and the second (FBP2) uses parallel beam rebinning. With a TCT spatial resolution phantom, FBP2 demonstrated a small degree of resolution loss which was equalized across methods after post-reconstruction filtering. With a thorax phantom, all methods performed adequately at high count level; however, at low count level, FBP1 showed degraded image quality due to its handling of zero count projection bins. This finding was corroborated in patient images. We conclude that at practical count levels, the iterative OS-EM method offers a clear advantage in TCT image quality compared to FBP methods