NIFTI
NIFTI (*.nii and *nii.gz) is standard file format for volumetric data that is widely used in neuroimaging.  The original NIFTI-1 file format was based on 16-bit signed integers and was limited to 32,767 in each dimension.  The NIFTI-2 format (http://www.nitrc.org/forum/message.php?msg_id=3738) is based on 64-bit integers and can handle very large volumes and matrices.