Originally Posted by Geof "A voxel is a three dimensional pixel!"
No, not quite, a pixel is a picture element and has two dimensions, a voxel is a volume element and has three dimensions. A three dimensional pixel would have a total of six dimensions. I can barely bend my brain around curved space-time and the concept of four dimensions; six leaves me spinning my wheels.   |
In the traditional usage a voxel is characterized by three spatial dimensions x,y,z, a unit volume scale factor in each axis ie mm/voxel, um/voxel, etc, and an average volumetric value ie density, intensity, etc. In addition the origin of the volume may be offset from 0,0,0 by some known distance for reference to the source of the data or in the case of processing limitations to allow the correlation of multiple sub-volumes. Since virtually all 3D data is organized into slices that can be displayed as 2D images containing pixels it is commonly understood that voxels are 3D pixels.
Presenting volumetric data in 3D on a 2D display device requires the projection of the volumetric data to a plane. Seeing a voxel's position within the volume is accomplished using either a sequential slice projection over time or transparency. The brain is fooled into thinking it is seeing 3D by the geometry of the presentation. When using a 2D display device voxels are displayed and seen as pixels.
As computers and scanners became more capable time was added to the data structure and multiple volumes were taken at intervals dependent upon the sampling device ie CT scanner, MRI scanner, confocal microscope, seismic survey, etc.
I am now retired from this field but during the time I worked at Vital Images, Inc. (
www.vitalimages.com) I taught short courses on the subject at Purdue, University of Wisconsin, UC Davis, Texas Tech, OSHA, MUSC, Wright Patterson AFB, etc.