Direct volume rendering techniques use semi-transparency to visualize much more important features in the three-dimensional scalar sample volumes than isosurface rendering methods. However, since all voxels participate in the generation of each image,these techniques are computationally expensive and far from interactive manipulation. In fact, on the base of the boundary representation scheme for the volumetric data, the 3D spatial coherence can be fully exploited to drastically reduce the rendering time for arbitrary viewing orientation. This data model only considers the bounding cells pierced by theisosurfaces of specified materials ; therefore, it requires O(kn²) memory cells to represent O(n³) volumetric data, and produces a high degree of data compression. A PC-based rendering algorithm in use of the boundary scheme is proposed in this paper, which producesresults visually similar to the volume rendering techniques, but speedup at least an order of magnitude. Examples from medical CT studies are given in contrast to the results from ray casting method both in image quality and in processing time.