1. A method of volume rendering two digital image datasets comprising the steps of: providing at least a first digitized image volume and a second digitized image volume, each image volume comprising a plurality of intensities corresponding to a domain of points on a 3-dimensional grid; providing a volume-rendering computing sub-system, said sub-system adapted to function as part of a personal computer workstation, said system comprising memory for storing said 3-dimensional image volumes, an image buffer for storing 2-dimensional renderings of said 3-dimensional images, and a depth buffer for storing depth values for said 2-dimensional renderings; loading a first image volume into a memory of said volume-rendering sub-system; rendering said first image volume, wherein a 2-dimensional image is output into an image buffer and a set of depth values are output into a depth buffer; loading a second image volume into a memory of said volume-rendering sub-system; rendering said second image volume up to the depth values output from the rendering of said first image volume, wherein values from said rendering of said second image volume are merged with non-zero values of said image buffer; and rendering the remainder of said second image volume to include second image volume points beyond the depth values output from the rendering of said first image volume, wherein values from said rendering of said remainder of said second image volume are merged with non-zero values of said image buffer.