Digitally reconstructed radiograph (DRR) is vital to 2D/3D image registration, but computationally expensive to generate. Introduced in this paper is a novel fast DRR generation method that reuses building blocks of DRRs previously generated for known poses. The Slab algorithm is presented for illustration of the principle where projections of multiple slices for known poses are combined to form bigger slabs that are subsequently re-projected as unities and added together to compute DRRs for new poses. As there are only a small number of slabs to process, DRRs can be computed for new poses efficiently. Preliminary results showed that on a computer with an Intel Core2 Duo 2.8 GHz CPU, the algorithm generated full content DRRs of size 512×512 in 62 ms, with peak signal-to-noise ratio (PSNR) greater than 50 dB and high sensitivity to pose change, representing speedup in computation by a factor of 60 and demonstrating its potential to be a fast high quality DRR generation algorithm suited for image registration.