DTI/BrainSeg

Question 1

Explain how DTI may be used for fibre trackin in the human brain

Answer 1

DTI tractography provides 3D reconstruction of the trajectory of white matter pathways.

The white matter is made of bundles/tracts of myelinated axons which are significantly anisotropic. Fractional anisotropy index is used for imaging the tracts.

Question 2

Explain what dissusion is and how it is affected by tissue microstructure.

Answer 2

Diffusion is the brownian movement of molecules in a medium (solvent). The movement of the molecules is described by a tensor, mathematical entity, which describes the direction of the diffusion. The human body is made of 60 % water which moves among tissues. Some structures such as cellular membranes can limit the diffusion of the water making the movement of molecules anisotropic, i.e, the diffusion varies when measured in different directions. In clear fluid and some homogenous gels, the diffusion is considered isotropic.

Question 3

Explain ellipsoidal mode of diffusion tensor

Answer 3

The diffusion tensor D in the voxel (I,J,K) can be
visualized as an ellipsoid, with the eigenvectors indicating
the directions of the principal axes, and the square root of
the eigenvalues defining the ellipsoidal radii.

Scalar maps can be derived from the
rotationally invariant eigenvalues λ1, λ2, λ3 to characterize the size
and shape of the diffusion tensor

Question 4

Are the following media anisotropic or isotropic?

1. Gray matter
2. White matter
3. Cerebrospinal fluid

Answer 4

1. Gray matter - Isotropic media
2. White matter - Anisotropic media
3. Cerebrospinal fluid - Isotropic media

Question 5

Explain Trace in DTI setting.

Answer 5

Characterizing the Size of the tensor: Trace

Trace(D) = λ1+ λ2+λ3

• Trace(D) is intrinsic to the tissue and is independent of fiber orientation, and diffusion sensitizing gradient directions
• Trace(D) is a clinically relevant parameter for monitoring stroke and neurological condition ( degree of structural coherence in tissue)
• Trace(D) is useful to characterize the size of the diffusion ellipsoid

Question 6

Explain Fractional Anisotropy.

Answer 6

Scalar Maps: Fractional Anisotropy

• FA(D) is intrinsic to the tissue and is independentof fiber orientation, and diffusion sensitizing gradient directions.
• FA(D) is useful to characterize the shape (degree of ‘out-of-roundness’) of the diffusion ellipsoid’
• Low FA => Isotropic, High FA => Anisotropic