MRI TRACTOGRAPHY IMAGING AND ITS IMPORTANCE IN DIAGNOSIS

1. MAAJID MOHI UD DIN MALIK
LECTURER COPMS AU,
BATHIDA PUNJAB

2. Tractography
 In neuroscience, tractography is a 3D
modeling technique used to visually
represent nerve tracts using data collected
by diffusion MRI. It uses special techniques
of magnetic resonance imaging (MRI) and
computer-based diffusion MRI. The results
are presented in two- and three-dimensional
images called tractograms.

3.  In addition to the long tracts that connect
the brain to the rest of the body, there are
complicated neural circuits formed by short
connections among
different cortical and subcortical regions. The
existence of these tracts and circuits has been
revealed by histochemistry and biological techniques
on post-mortem specimens. Nerve tracts are not
identifiable by direct exam, CT, or MRI scans.

4.  This difficulty explains the paucity of
their description in neuroanatomy atlases
and the poor understanding of their
functions.

5. MRI technique
 Tractography is performed using data
from diffusion MRI. The free water diffusion
is termed "isotropic" diffusion. If the water
diffuses in a medium with barriers, the
diffusion will be uneven, which is
termed anisotropic diffusion. In such a case,
the relative mobility of the molecules from
the origin has a shape different from
a sphere.

6.  This shape is often modeled as
an ellipsoid, and the technique is then
called diffusion tensor imaging. Barriers
can be many things: cell membranes,
axons, myelin, etc.; but in white
matter the principal barrier is
the myelin sheath of axons. Bundles of
axons provide a barrier to perpendicular
diffusion and a path for parallel diffusion
along the orientation of the fibers.

7.  Anisotropic diffusion is expected to be
increased in areas of high mature axonal
order. Conditions where the myelin or the
structure of the axon are disrupted, such
as trauma, tumors,
and inflammation reduce anisotropy, as the
barriers are affected by destruction or
disorganization.
 Anisotropy is measured in several ways.
One way is by a ratio called fractional
anisotropy (FA).

8.  This additional information is difficult to represent
on 2D grey-scaled images. To overcome this
problem, a color code is introduced. Basic colors can
tell the observer how the fibers are oriented in a 3D
coordinate system, this is termed an "anisotropic
map". The software could encode the colors in this
way:
 Red indicates directions in the X axis: right to left or
left to right.
 Green indicates directions in the Y axis: posterior to
anterior or from anterior to posterior.
 Blue indicates directions in the Z axis: foot-to-head
direction or vice versa.
 The technique is unable to discriminate the
"positive" or "negative" direction in the same axis.

9. Introduction
 Diffusion-tensor magnetic resonance (MR)
imaging (DTI) and fiber tractography (FT)
are new methods that can demonstrate the
orientation and integrity of white matter
fibers in vivo.
 Developmental central nervous system
(CNS) diseases, both congenital and
postnatal, can be a spotlighted field of DTI
due to the potential for generating a fiber
pathway and aberrant connections in the
case of a blockage of normal white matter
formation.

10. Imaging Protocol
 A short acquisition time and instant processing are
essential for the clinical feasibility of a certain
procedure. The authors applied single-shot spin-echo
echo-planar imaging (EPI) and parallel imaging
techniques to achieve motion-free and higher signal-to-
noise ratio (SNR) DTI. The total imaging time for DTI
and FT was 7–9 minutes according to the section
numbers, which was added to the routine MR imaging
examinations.

11. Tractogram

12. Whole-brain tractography Whole-brain tractography shows coronal (A), right
sagittal (B), and left sagittal (C) views. Directionally-encoded color-coded map (D)
shows green regions of interest (arrow) marked for left arcuate fasciculus. 3-D fiber
tractography (E, F) of the blue-colored bilateral corticospinal tract (arrow) shows
red-colored corpus callosum (arrow) and green-colored arcuate fasciculus fibers.

14. Diffusion MRI & Tractography
DTI

15. Healthy human brain from a
young adult, tractography

16. DTI Tractography