1. A method for processing medical image data, the method comprising: receiving a first three-dimensional image comprising a plurality of voxels; reducing a regional trend within the three-dimensional image; computing, with a processing device, a semivariogram for a region of interest, comprising determining an extent of correlation among at least a subset of the plurality of voxels; defining at least one block of spatially correlated voxels; calculating, with the processing device, voxel weights for each voxel within the at least one block; and determining a block average count and variance for the at least one block.