A voxel is a volume element, representing a colour value in three dimensional space. This is analogous to a pixel, which represents 2D image data. Voxels are frequently used in the visualisation and analysis of medical and scientific data. Some true 3D displays use voxels to describe their resolution. For example, a display might be able to show 512×512×512 voxels.

Voxel data

There are two interpretations for a voxel value, depending on usage:

• A tiny cube with particular properties in a larger volume.
• A point sample in a regularly spaced 3D grid.

The value of a voxel may represent various properties. In CT scans, the values are Hounsfield units, giving the opacity of material to X-rays. Different types of value are acquired from MRI or ultrasound.

Voxels can contain multiple scalar values; in the case of ultrasound scans with B-mode and Doppler data, density and flow rate are captured as separate channels of data relating to the same voxel positions.

Other values may be useful for immediate 3D rendering, such as a surface normal vector and colour.

Visualisation

A volume containing voxels can be visualised either by direct volume rendering or by the extraction of polygon iso-surfaces which follow the contours of given threshold values. The marching cubes algorithm is often used for iso-surface extraction.

Height maps

Voxels are said to be used in height maps which are rectangular arrays containing height values. Height maps contain a single value for each point on the 2D grid and so can represent simple landscapes but not complex overlapping surfaces.