Parallel programming for OSEM reconstruction with MPI, OpenMP, and hybrid MPI-OpenMP

To improve the parallel efficiency (PE) of the ordered-subsets expectation-maximization (OSEM) algorithm for 3D PET image reconstruction, we implemented the algorithm with 1) an OpenMP and 2) a hybrid message passing interface (MPI)-OpenMP model on the basis of a standard MPI implementation. The motivation was to reduce the inter-processor data exchange time which was the dominant PE limiting factor of the MPI model when large number of processors was used. The OpenMP model used a fine-grained approach and showed significant speedup only up to 2-3 processors for both the true shared memory and the single system image (SSI) distributed shared memory architectures. The hybrid MPI-OpenMP model achieved a consistent improvement of ~10% in terms of speedup factor on a large number of parallel processors compared to the pure MPI approach. As clusters of larger symmetric multiprocessor (SMP) machines continue to become more cost effective, we expect this hybrid MPI-OpenMP approach to be increasingly valuable to accelerate 3D PET reconstructions, and other applications with similar computational characteristics