BRIK/HEAD

General description
BRIK and HEAD files are medical imaging (generally, brain imaging) data files generated and used by the AFNI (Analysis of Functional NeuroImaging) analysis package. The files are multi-dimensional raster data, or "voxel" data -- in essence, pixels with a value in three dimensions. The AFNI format also supports time-series data, or multiple values at each voxel over time, per participant, or with different meaning (for example, a beta value and a t-statistic).

The .BRIK portion of the file contains the actual voxel data, while the .HEAD file contains the metadata needed to interpret the data.

Software
The AFNI software suite is open-source, written in C, and available here.

Other considerations
Note that (nearly?) all AFNI programs support the much more widely-used .nii file format; converting data to that format may be a good move. A caveat is that the .nii header supports much less information than .head files.