Virtual Endoscopy is an interactive exploration inside human organs to detect polyps by combining medical imaging and computer graphics technologies. In order to realize rendering acceleration, we present a novel two-steps visibility algorithm, Enclosure Sphere Based Cell Visibility (ESBCV), which performs visibility computation between cells assisted by Z-buffer in preprocessing and eye-to-cell visibility through a simple numerical calculation of the intersection of a circle with the rendered image in navigation. Experimental results demonstrated virtual navigation with high image quality and interactive rendering speed.