Three-dimensional SPECT reconstruction of combined cone beam and parallel beam data

Single photon emission computed tomography (SPECT) using cone beam (CB) collimation exhibits increased sensitivity compared with acquisition geometries using parallel (P) hole collimation. However, CB collimation has a smaller field-of-view which may result in truncated projections and image artifacts. A primary objective of this work is to investigate maximum likelihood-expectation maximization (ML-EM) methods to reconstruct simultaneously acquired parallel and cone beam (P&CB) SPECT data. Simultaneous P&CB acquisition can be performed with commercially available triple camera systems by using two cone-beam collimators and a single parallel-hole collimator. The loss in overall sensitivity (relative to the use of three CB collimators) is about 15 to 20%. We have developed three methods to combine P&CB data using modified ML-EM algorithms. The first method consists of using both data sets to reconstruct a single intermediate image after each iteration using the ML-EM algorithm. The other two iterative algorithms combine intermediate parallel beam (PB) and CB source estimates to enhance image quality. For these methods, a PB estimate and a CB estimate are obtained for the first iteration. The second method consists of summing the PB and CB estimates for each subsequent iteration to obtain new PB and CB estimates. The third method is similar to the second method, with the exception that the new PB estimate simply is set equal to the PB estimate after each iteration. The combined source estimate is used in each subsequent iteration step of the EM algorithm. These algorithms are evaluated using projection data simulated using a Monte Carlo SPECT model. The P&CB SPECT images demonstrate marked improvement as compared with the CB-only reconstruction, particularly when the projections are truncated.