Nearly Raw Raster Data (NRRD) is a data format and toolset for describing volume data.
The NRRD website includes extensive details, examples, conversion utilities, resources, and references.
NRRD and the corresponding teem software library facilitates the pre-processing tasks (cropping, quantizing, resampling) of volume data. It also helps with the basic analysis tasks which help to understand the structure of a dataset (histograms, maximum intensity projections, connected components).
The X3D Volume Rendering component describes how to create volumetric scenes. These capabilities were approved in X3D version 3.3. Support for the NRRD format is optional for X3D players.
The X3D Basic Examples - Volume Visualization provides example X3D scenes utilizing NRRD. These scenes have been prepared by Virginia Tech (VT) Visionarium research group and published under an open-source license.