DTI Processing - Voxel-based versus tract-based diffusion imaging

Sunday, October 21, 2012 Do Tromp 0 Comments

The
 development
 of
 diffusion
 magnetic resonance imaging (dMRI)

 enabled
 the
 research
 of
 white 
matter 
micro- and macro-structure in 
vivo. 
DMRI 
measures 
the 
magnitude 
and
 orientation
 of 
water
 diffusion.
 This
 is

 done
 in
 multiple
 directions

 to
 calculate
 the
 
 three
 dimensional
 representation
 of
 the
 water
 diffusion
 profile.
  Gray
 matter
 has
 predominantly
 isotropic (soccer ball shaped)
 water
 diffusion, while dense
 white
 matter 
tracks 
have 
highly 
anisotropic 
(rugby ball shaped) diffusion 
of
 water 
pointing 
in 
the
 direction
 of
 the
 fiber
 bundle.


The
 measure
 most 
commonly 
used 
to
 characterize 
directional 
diffusion 
is
 fractional
 anisotropy
 (FA).
 This
 measure
 gives
 a
 value
 between
 0
 and
 1
 to
 indicate
 the
 fraction
 of
 diffusion
 that
 is
 in
 the
 longitudinal
 direction
 compared
 to
 the
 proportion
 of 
diffusion 
in 
both 
transverse
 directions.

 Other measures that can be used are axial diffusivity (AD), radial diffusivity (RD) and mean diffusivity (MD).

Voxel-Based Morphometry

There 
are 
two 
main 
methods 
of 
analyzing diffusion 
images.
 The 
first 
is
 voxel‐based
 analyses
 (VBA)
,
 which
 is specifically suited for whole
 brain
 analysis.
 It
 is 
a
 voxel wise 
method 
to
 statistically 
compare 
local 
anisotropy 
values
 for 
the 
whole
 brain 
between 
different 
subjects. It has to be kept in mind that this method should correct for multiple comparisons.
 One way to reduce the number or comparisons is to use an atlas based segmentation methods to selectively investigate white matter areas of interest.

Tract-Based Analysis

The
 second
 method
 is
 called
 tract‐based
 analysis.
 It
 uses
 the
 more
 anisotropic
 tensors
 to
 form
 streamlines
 of
 tensors
 leading
 to
 estimations
 of
 white
 matter
 fiber
 tracts.
 A
 region
 of
 interest
 is
 used
 as
 seed
 region
 from
 where
 the
 fibers
 are
 traced.
 For
 each
 tract
 mean
 FA
 values
 can be
 calculated.
 These
 values
 per
 tract
 can
 be
 compared
 across
 groups
 to
 investigate
 structural
 connectivity.
 

VBA
 and
 fiber
 tractography
 are
 two
 methods
 using
 a
 fairly
 different
 approach in dMRI.
 In
 VBA
 the
 whole
 brain
 is
 investigated,
 but
 the
 method
 relies
 heavily
 on
 effective
 registration
 between
 subjects.
 When
 regions
 of
 abnormal
 FA
 values
 do
 not
 map
 onto
 each
 other
 correctly
 this
 will
 greatly
 reduce
 the
 likelihood to
 find
 significant
 results.
 In
 tract‐based
 analyses
 tracts can be delineated without relying on subject registration. Although specific
 a
 priori
 regions
 of
 interest
 or
 specific
 tracts
 need to be
 selected
 for
 comparison.