A voxel is the 3D equivalent of a pixel, that is a small rectangular (usually cubic) patch of space. For example, if a room is 5 metres long, by 4 metres wide, by 3metres high (5mx4mx3m), we might divide it into 5000x4000x3000 1mm cubes for the purposes of indexing objects or performing 3D rendering. While photographs are usually 2D, various forms of medical scanners, such as ultrasound or MRI generate 3D views of the patient. However, these may have non-cubic voxels as they create a series of frames or slices and the gap between slices (usually deterimined by mechanical movement of the device or patient) may be different from the resolution within a slice.
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