String-Averaging Expectation-Maximization for Maximum Likelihood   Estimation in Emission Tomography
  We study the maximum likelihood model in emission tomography and propose a new family of algorithms for its solution, called String-Averaging Expectation-Maximization (SAEM). In the String-Averaging algorithmic regime, the index set of all underlying equations is split into subsets, called "strings," and the algorithm separately proceeds along each string, possibly in parallel. Then, the end-points of all strings are averaged to form the next iterate. SAEM algorithms with several strings presents better practical merits than the classical Row-Action Maximum-Likelihood Algorithm (RAMLA). We present numerical experiments showing the effectiveness of the algorithmic scheme in realistic situations. Performance is evaluated from the computational cost and reconstruction quality viewpoints. A complete convergence theory is also provided. 
E. S. Helou, Y. Censor, T.-B. Chen, I-L. Chern, \'A. R. De Pierro, M.   Jiang, H. H.-S. Lu
physics.med-ph cs.CY math.NA math.OC