Diffusion Tractography-Made Simple

Teaching points
 Diffusion(Brownian Movement)denotes random motion of molecules at temperatures more than absolute zero. If it is equal in all directions called isotropic (as in grey matter). However in white matter diffusion is restricted to perpendicular to the long axis axonsand diffuses faster along the Z axis and is called ANISOTROPY. By acquiring multiple images each sensitive to diffusion to different orientation, fit the model (diffusion tensor)to measurements , can quantify mean diffusion and its orientation dependant of each voxel(Fractional anisotropy). Submitted by Dr MGK MURTHY.

Estimate mean diffusion, fractional anisotropy and maximum diffusion orientation to construct white matter paths. Mapping principal eigen vector, in each voxel ,forms basics of tractography. Sequence used is Single shot EPI. Disadvantage is no spatial /directional anisotropy. To get over this, we give color coding to tell directions.
Blue=superior/inferior
Green=Anterior/Posterior
Red=Left /Right

On 2D wherever fibers cross, mixture of colors are formed

Yellow=Green+Red
Magenta=Red+Blue
Cyan=Green+Blue

Other disadvantages include, inability to differentiate anterograde or retrograde connectivity, presence of synapses, and whether the path is functional.

There are 4 basic types of fibers

1. Projection fibers

–corticobulbar (cortex to brainstem)
-corticospinal(from pre/post central sulcus to spinal)
-Thalamic projections (post thalamic projections consist of optic radiation)
All these pass through the internal capsule



2.Association fibers
-superior longitudinal fasciculus (goes to most lateral of temporal region with C shape)
-Inferior longitudinal fasciculus
-inferior fronto-occipital fasciculus (ILF and IFOF go to post temporal/occipital regions)
-Uncinate fasciculus(IFOFand UF go to frontal lobe )
-superior fronto occipital fasciculus (only one to project medially along thalamus and ventricle)

3.Limbic system fibers
-cingulum(from cingulate gyrus to temporal lobes)
-Fornix(goes to hypothalamus)
-StriaTerminalis (goes to temporal lobe)
-Hippocampus
-Amygadala

4.Callosal fibers

-Genu(projections from this form forceps minor)
-Splenium(projections from this form forceps major)
-from splenium to temporal lobe along lateral ventricle forms Tapetum

Clinical uses

a) Temporal Lobe Epilepsy(TLE)
b) Multiple sclerosis (MS)
c) Amyotrophic lateral sclerosis (ALS)
d) Neuropsychiatry particularly schizophrenia
e) Cerebral palsy
f) Early acquired blindness





 

Fiber Tracking-MRI

Diffusion tensor imaging is unique in its ability to non-invasively visualize white matter fiber tracts in the human brain in vivo. Diffusion is the incoherent motion of water molecules on a microscopic scale. This motion is itself dependent on the micro-structural environment that restricts the movement of the water molecules. In white matter fibers there is a pronounced directional dependence on diffusion. With white matter fiber tracking projections among brain regions can be detected in the three dimensional diffusion tensor dataset according to the directionality of the fibers. This is a case of glioma shown with fiber tracking. 

Diffusion Tensor Imaging-Anatomy

Few of the images of DTI showing some white matter tracts.  Reported by- Teleradiology Providers

Diffusion Tensor Imaging in Obsessive-Compulsive Disorder

Yukiko Saito et al in Radiology 2008;246:536-542 have examined the examine microstructural white matter abnormalities in the corpus callosum (CC) of patients with obsessive-compulsive disorder (OCD). A significant reduction in FA was observed in the rostrum of the CC in patients with OCD compared with the rostral FA in the control subjects. (FA-Fractional anisotropy). Study results support the widely held view that the orbital prefrontal region is involved in the pathophysiology of OCD and indicate that the orbitofrontal circuit influences symptom severity in patients with OCD.