We present an iterative transmission reconstruction algorithm based on principles used for block-iterative emission reconstruction. Filtered backprojection (FBP) is adequate for transmission CT image reconstruction under ideal conditions. In cases where insufficient count rates are obtained, FBP suffers from significant biases. Our block-iterative transmission algorithm is developed by analogy to ordered-subset emission CT algorithms. We show similarities between our algorithm and a previously-proposed Ordered-Subset Convex (OSC) algorithm. Under ideal conditions, our block-iterative transmission algorithm generates estimates of attenuation line integrals that are not significantly different from those from FBP or OSC. In the case of very low count-rate data, our block-iterative algorithm produces line integral estimates that are less biased than those from FBP and nearly identical to those from OSC. We show that our algorithm is more computationally efficient than OSC, but that OSC performs slightly better with different subset configurations. We conclude that both BIT and OSC are viable alternatives to FBP, especially under poor imaging conditions.
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