1. A method of automatically identifying bone voxels in a medical image data set of voxels that includes at least a portion of the large bones of the legs and at least a portion of the bones outside the legs, the method comprising: a) identifying at least a portion of the voxels belonging to large bones of the legs by applying a first procedure to the voxels of the data set, comprising finding first connected sets of volume greater than a minimum volume, of voxels having density greater than a first minimum density; b) identifying at least a portion of the voxels belonging to bones of the body outside the legs, by applying a second procedure to the voxels of the data set, comprising finding second connected sets of voxels having density greater than a second minimum density; and c) excluding those voxels from the second connected sets of voxels that satisfy a criterion involving one or more of average density, uniformity of density, and thickness, thereby excluding voxels belonging to the aorta exceeding a threshold level of contrast agent; wherein the second minimum density and the criterion are chosen so that voxels belonging to the large leg bones would also be excluded if the second procedure and the criterion were applied to them.