It’s the voxel that makes an MRI or fMRI accessible in 3 dimensions.

Image: The voxel, a 3D counterpart of a pixel, is what enables the merged stack of microscopic images to stand out as layers. The photographer, By Howard Vindin via Wikimedia Commons, describes the photograph as:

A merged stack of confocal images showing actin filaments within a cell. The image has been colour coded in the z axis to show in a 2D image the heights at which filaments can be found within cells.

What is a voxel?

Computer users are quite familiar with the pixel, the number of image elements that make up a computer image. Every computer user may select the resolution of the image on the computer screen. Just as in a photograph, the more pixels, the finer the resolution of the image, and the more colors that can be part of the image rendering.

In scientific visualization and computer graphics, volume rendering is a set of techniques used to display a 2D projection of a 3D discretely sampled data set, typically a 3D scalar field.

So begins a wikipedia article on Volume Rendering which discusses, in detail, many technical aspects of employing the voxel in computer image rendering.

Once the voxel has been employed in Functional Magnetic Resonance Imaging, analysis of the internal workings of the human body have become much more useful in the medical field.

A voxel is a unit of graphic information that defines a point in three-dimensional space. Since a pixel (picture element) defines a point in two dimensional space with its x and y coordinates , a third z coordinate is needed. In 3-D space, each of the coordinates is defined in terms of its position, color, and density. Think of a cube where any point on an outer side is expressed with an x , y coordinate and the third, z coordinate defines a location into the cube from that side, its density, and its color. With this information and 3-D rendering software, a two-dimensional view from various angles of an image can be obtained and viewed at your computer.

To see the second portion of the above short description of voxel, visit techtarget.com

To see a brief definition of MRI technology with an extremely short history, the next page.