1. A method for constructing from volumetric CT or MRI scan data of a patient region, a virtual two-dimensional fluoroscopic or shadowgraph image of the patient region as seen from a selected point in space, comprising (a) selecting a point in space representing a virtual fluoroscopic irradiation source, (b) defining a plurality of rays extending between the source point and each of a plurality of points in a two-dimensional XY array of points, where (i) the array points correspond to pixels in an XY pixel array in a display screen, (ii) each pixel in the display screen has multiple N-bit registers for receiving digitized values for each of multiple colors, (iii) the scan data is composed of voxels having defined coordinates and associated M-bit tissue-density values, and (iv) at least some of the rays pass through a plurality of such voxels in the patient target region, (c) for each ray, distributing the M bits of the density values of voxels along that ray among the multiple N-bit registers of the associated pixel, such that one or more bit positions of each M-bit value is assigned to a selected register of that pixel, (d) for each ray, summing the M-bit values of the voxels along that ray by (i) individually summing the one or more bit-position values in each associated pixel register, and (ii) determining the sum of the M-bit values along the associated ray from the values in the individual registers, and (e) displaying the image constructed of gray-scale values representing the summed M-bit density values at each pixel in said display screen.