title: Accelerated 3D-OSEM image reconstruction using a Beowulf PC cluster for pinhole SPECT.
abstract: OBJECTIVE
  A conventional pinhole single-photon emission computed tomography (SPECT) with a single circular orbit has limitations associated with non-uniform spatial resolution or axial blurring. Recently, we demonstrated that three-dimensional (3D) images with uniform spatial resolution and no blurring can be obtained by complete data acquired using two-circular orbit, combined with the 3D ordered subsets expectation maximization (OSEM) reconstruction method. However, a long computation time is required to obtain the reconstruction image, because of the fact that 3D-OSEM is an iterative method and two-orbit acquisition doubles the size of the projection data. To reduce the long reconstruction time, we parallelized the two-orbit pinhole 3D-OSEM reconstruction process by using a Beowulf personal computer (PC) cluster.
  
  
  METHODS
  The Beowulf PC cluster consists of seven PCs connected to Gbit Ethernet switches. Message passing interface protocol was utilized for parallelizing the reconstruction process. The projection data in a subset are distributed to each PC. The partial image forward- and back-projected in each PC is transferred to all PCs. The current image estimate on each PC is updated after summing the partial images. The performance of parallelization on the PC cluster was evaluated using two independent projection data sets acquired by a pinhole SPECT system with two different circular orbits.
  
  
  RESULTS
  Parallelization using the PC cluster improved the reconstruction time with increasing number of PCs. The reconstruction time of 54 min by the single PC was decreased to 10 min when six or seven PCs were used. The speed-up factor was 5.4. The reconstruction image by the PC cluster was virtually identical with that by the single PC.
  
  
  CONCLUSIONS
  Parallelization of 3D-OSEM reconstruction for pinhole SPECT using the PC cluster can significantly reduce the computation time, whereas its implementation is simple and inexpensive.
authors.affiliation: Department of Investigative Radiology, Advanced Medical Engineering Center, National Cardiovascular Center Research Institute, 5-7-1 Fujishiro-dai, Suita, 565-8565, Japan. zeniya@ri.ncvc.go.jp
authors.forename: Tsutomu
authors.initials: T
authors.lastname: Zeniya
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Watabe
authors.forename: Antti
authors.initials: A
authors.lastname: Sohlberg
authors.forename: Hidehiro
authors.initials: H
authors.lastname: Iida
mesh_terms: D000465:Algorithms; D016000:Cluster Analysis; D003198:Computer Simulation; D003199:Computer Systems; D004867:Equipment Design / Q000295:instrumentation; D019146:Hypermedia; D007089:Image Enhancement; D007091:Image Processing, Computer-Assisted / Q000295:instrumentation / Q000379:methods / Q000706:statistics & numerical data; D021621:Imaging, Three-Dimensional / Q000379:methods* / Q000706:statistics & numerical data*; D016247:Information Storage and Retrieval; D016013:Likelihood Functions; D015233:Models, Statistical; D019047:Phantoms, Imaging; D012680:Sensitivity and Specificity; D017748:Time Management / Q000379:methods*; D015899:Tomography, Emission-Computed, Single-Photon / Q000379:methods*