Hyperfast parallel-beam and cone-beam backprojection using the cell general purpose hardware



Tomographic image reconstruction, such as the reconstruction of computed tomography projection values, of tomosynthesis data, positron emission tomography or SPECT events, and of magnetic resonance imaging data is computationally very demanding. One of the most time-consuming steps is the backprojection. Recently, a novel general purpose architecture optimized for distributed computing became available: the cell broadband engine (CBE). To maximize image reconstruction speed we modified our parallel-beam backprojection algorithm [two dimensional (2D)] and our perspective backprojection algorithm [three dimensional (3D), cone beam for flat–panel detectors] and optimized the code for the CBE. The algorithms are pixel or voxel driven, run with floating point accuracy and use linear (LI) or nearest neighbor (NN) interpolation between detector elements. For the parallel-beam case, 512 projections per half rotation, 1024 detector channels, and an image of size 5122 was used. The cone-beam backprojection performance was assessed by backprojecting a full circle scan of 512 projections of size 10242 into a volume of size 5123 voxels. The field of view was chosen to completely lie within the field of measurement and the pixel or voxel size was set to correspond to the detector element size projected to the center of rotation divided by 2. Both the PC and the CBE were clocked at 3GHz. For the parallel backprojection of 512 projections into a 5122 image, a throughput of 11fps (LI) and 15fps (NN) was measured on the PC, whereas the CBE achieved 126fps (LI) and 165fps (NN), respectively. The cone-beam backprojection of 512 projections into the 5123 volume took 3.2min on the PC and is as fast as 13.6s on the cell. Thereby, the cell greatly outperforms today's top-notch backprojections based on graphical processing units. Using both CBEs of our dual cell-based blade (Mercury Computer Systems) allows to 2D backproject 330 images/s and one can complete the 3D cone-beam backprojection in 6.8s.