Three-dimensional imaging has become more accessible with the continued development of instrumentation for generating 3D image data (tomographic equipment of various kinds, confocal microscopes, etc.) and more complete and versatile software for increasingly powerful computers. Just as the pixel is the unit of brightness measurement for a twodimensional image, the voxel (volume element, the three-dimensional analog of the pixel or picture element) is the unit for three-dimensional imaging. And just as processing and analysis in 2D are much simpler if the pixels are square, so the use of cubic voxels is preferred for three dimensions, although it is not as often achieved.