FT_DATATYPE_VOLUME describes the FieldTrip MATLAB structure for volumetric data.

 The volume data structure represents data on a regular volumetric
 3-D grid, like an anatomical MRI, a functional MRI, etc. It can
 also represent a source reconstructed estimate of the activity
 measured with MEG. In this case the source reconstruction is estimated
 or interpolated on the regular 3-D dipole grid (like a box).

 An example volume structure is
       anatomy: [181x217x181 double]  the numeric data, in this case anatomical information
           dim: [181 217 181]         the dimensionality of the 3D volume
     transform: [4x4 double]          affine transformation matrix for mapping the voxel coordinates to the head coordinate system
          unit: 'mm'                  geometrical units of the coordinate system
      coordsys: 'ctf'                 description of the coordinate system

 Required fields:
   - transform, dim

 Optional fields:
   - anatomy, prob, stat, grey, white, csf, or any other field with dimensions that are consistent with dim
   - unit, size, coordsys

 Deprecated fields:
   - dimord

 Obsoleted fields:
   - none

 Revision history:

 (2014) The subfields in the avg and trial fields are now present in the
 main structure, e.g. source.avg.pow is now source.pow. Furthermore, the
 inside is always represented as logical array.

 (2012b) Ensure that the anatomy-field (if present) does not contain
 infinite values.

 (2012) A placeholder 2012 version was created that ensured the axes
 of the coordinate system to be right-handed. This actually never
 has made it to the default version. An executive decision regarding
 this has not been made as far as I (JM) am aware, and probably it's
 a more principled approach to keep the handedness free, so don't mess
 with it here. However, keep this snippet of code for reference.

 (2011) The dimord field was deprecated and we agreed that volume
 data should be 3-dimensional and not N-dimensional with arbitary
 dimensions. In case time-frequency recolved data has to be represented
 on a 3-d grid, the source representation should be used.

 (2010) The dimord field was added by some functions, but not by all

 (2003) The initial version was defined