OK, so you actually have cross-sectional images. Not projections. I guess you have those kind of volumes where you have high-resolution slices with huge gaps between them (spacing between slices is a magnitude larger than pixel spacing within the slice).
Unfortunately, it is a very difficult problem to create high-resolution data from these sparse slices, since in general you have no way of knowing what is between slices. If you have a priori information about the image content then you may be able to reconstruct super-resolution images, but I am not aware of such algorithms in Slicer (or in libraries that Slicer includes). For a similar question, somebody recommended NiftyMIC toolkit.