Note that this reference documentation is identical to the help that is displayed in MATLAB when you type “help ft_datatype_segmentation”.

  FT_DATATYPE_SEGMENTATION describes the FieldTrip MATLAB structure for segmented
  voxel-based data and atlasses. A segmentation can either be indexed or probabilistic
  (see below).
 
  A segmentation is a volumetric description which is usually derived from an anatomical
  MRI, which describes for each voxel the tissue type. It for example distinguishes
  between white matter, grey matter, csf, skull and skin. It is mainly used for masking
  in visualization, construction of volume conduction models and for construction of
  cortical sheets. An volume-based atlas is basically a very detailed segmentation with
  an anatomical label for each voxel.
 
  For example, the AFNI TTatlas+tlrc segmented brain atlas (which can be created
  with FT_READ_ATLAS) looks like this
 
               dim: [161 191 141]        the size of the 3D volume in voxels
         transform: [4x4 double]         affine transformation matrix for mapping the voxel coordinates to head coordinate system
          coordsys: 'tal'                the transformation matrix maps the voxels into this (head) coordinate system
              unit: 'mm'                 the units in which the coordinate system is expressed
            brick0: [161x191x141 uint8]  integer values from 1 to N, the value 0 means unknown
            brick1: [161x191x141 uint8]  integer values from 1 to M, the value 0 means unknown
       brick0label: {Nx1 cell}
       brick1label: {Mx1 cell}
 
 
  An example segmentation with binary values that can be used for construction of a
  BEM volume conduction model of the head looks like this
 
            dim: [256 256 256]         the dimensionality of the 3D volume
      transform: [4x4 double]          affine transformation matrix for mapping the voxel coordinates to head coordinate system
       coordsys: 'ctf'                 the transformation matrix maps the voxels into this (head) coordinate system
           unit: 'mm'                  the units in which the coordinate system is expressed
          brain: [256x256x256 logical] binary map representing the voxels which belong to the brain
          scalp: [256x256x256 logical] binary map representing the voxels which belong to the scalp
          skull: [256x256x256 logical] binary map representing the voxels which belong to the skull
 
  An example of a whole-brain anatomical MRI that was segmented using FT_VOLUMESEGMENT
  looks like this
 
          dim: [256 256 256]         the size of the 3D volume in voxels
    transform: [4x4 double]          affine transformation matrix for mapping the voxel coordinates to head coordinate system
     coordsys: 'ctf'                 the transformation matrix maps the voxels into this (head) coordinate system
         unit: 'mm'                  the units in which the coordinate system is expressed
         gray: [256x256x256 double]  probabilistic map of the gray matter
        white: [256x256x256 double]  probabilistic map of the white matter
          csf: [256x256x256 double]  probabilistic map of the cerebrospinal fluid
 
  The examples above demonstrate that a segmentation can be indexed, i.e. consisting of
  subsequent integer numbers (1, 2, ...) or probabilistic, consisting of real numbers
  ranging from 0 to 1 that represent probabilities between 0% and 100%. An extreme case
  is one where the probability is either 0 or 1, in which case the probability can be
  represented as a binary or logical array.
 
  The only difference to the volume data representation is that the segmentation
  structure contains the additional fields xxx and xxxlabel. See FT_DATATYPE_VOLUME for
  further details.
 
  Required fields:
    - dim, transform
 
  Optional fields:
    - coordsys, unit
 
  Deprecated fields:
    - none
 
  Obsoleted fields:
    - none
 
  Revision history:
  (2012/latest) The explicit distunction between the indexed and probabilistic
  representation was made. For the indexed representation the additional
  xxxlabel cell-array was introduced.
 
  (2005) The initial version was defined.
 
  See also FT_DATATYPE, FT_DATATYPE_VOLUME, FT_DATATYPE_PARCELLATION