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