1. A method of processing an image data set of a 3D PET scanner to correct for scattered coincidence events, wherein the 3D PET scanner includes a detector ring assembly having a central axis, the method comprising: (a) providing an image data set including a first plurality of data subsets respectively corresponding to a plurality of native slices; (b) axially downsampling the first plurality of data subsets to generate a second plurality of data subsets respectively corresponding to a plurality of super-slices; and (c) calculating scatter in the second plurality of data subsets by way of a model-based scatter correction algorithm.