说明：  This code is as per SPECT reconstruction By: Martin Š ámal Charles @ Regional Training Workshop on Advanced Image Processing of SPECT Studies 19-23 April 2004. The principle of the iterative algorithms is to reconstruct an image of a tomographic slice projections by successive estimates. The projections corresponding to the current estimate are compared with the measured projections. The result of the comparison is used to modify the current estimate, thereby creating a new estimate. The algorithms differ in the way the measured and estimated projections are compared and the kind of correction applied to the current estimate. The process is initiated by arbitrarily creating a first estimate - for example, a uniform image (all pixels equal zero, one, or a mean pixel value,…). Corrections are carried out either as addition of differences or multiplication by quotients between measured and estimated projections.