With the NIfTI-1 header extensions the world is your oyster.  We decided that one use for such a capability is to keep an audit trail of all operations performed on the image data volume.  This is a key feature in the dcemriS4 package.  There are three main components to a NIfTI-1 header extension: esize, ecode and edata.  At first we arbitrarily set ecode = 1002 and further developed the software.  Turns out, FSLView is quite strict about the numbers used in ecode and it failed to accept our NIfTI-1 header extensions saved from R.  Looking at the file nifti1_io.h, the following ecodes have been defined:

Advice from the FSL team (thanks: Matt Webster and Dave Flitney) was for us to use ecode = 6 (ASCII) since the audit trail is text in XML format.  Now one can produce an example data volume

We now have a file “img_nifti.nii.gz” that is available to us and using a utility in FSL we can investigate the attributes of this file

% $FSLDIR/bin/fslhd img_nifti | grep 'vox_offset'

vox_offset     1168

and reading the file back into R allows us to interrogate the ecode via its S4 slot