Two dimensional objects can be scanned in a raster pattern into an array of pixels.
The equivalent can be done in 3 dimensions. This is often used in applications like medical scanners.
There are many ways to code voxel data, for instance:
DVR (direct volume rendering)
DSR (direct surface rendering)
MIP (maximum intensity projection)
vizmed sites (check out Volrend, Volpack, etc).
surfels" and surfel rendering techniques (effectively floating voxels).