1. A method for creating a plurality of virtual projection images, the method comprising: defining a plurality of sub-volumes within an image volume represented by volumetric image data, wherein at least one sub-volume includes a sub-object of interest and an unwanted object, and wherein each sub-volume partially overlaps another sub-volume; and simulating transmission of x-rays through each of the plurality of sub-volumes using volumetric image data for each sub-volume to generate a plurality of virtual projection images, wherein a simulated x-ray transmission direction is different for each of the plurality of sub- volumes, and wherein for the at least one sub-volume that includes the unwanted object, volumetric image data corresponding to the unwanted object is weighted less than volumetric image data corresponding to the sub-object of interest, wherein the plurality of sub-volumes include respective rectangular basal surfaces with respective widths from 15 mm to 30 mm and respective heights from 25 mm to 40 mm.