Questions: Diffusion Tensor Imaging and White Matter Microstructure

Question 1

A radiologist reviewing a DTI scan notes significantly decreased FA in a patient's corticospinal tract following a suspected stroke. A medical student suggests this indicates increased neural activity in the motor cortex. What does decreased FA actually indicate?

• A. Increased neural firing rate in the affected motor region
• B. Disruption to the axonal organization or myelination of white matter fibers
• C. Higher blood oxygenation in the cortex overlying the tract
• D. Enhanced synaptic connectivity between motor neurons

Answer: (B)

FA measures the directionality of water diffusion, not neural activity. Decreased FA indicates that water is diffusing more isotropically — which happens when axonal structure is disrupted (demyelination, axonal damage, edema). DTI is completely distinct from fMRI (which measures blood oxygenation) and EEG (which measures electrical activity). The medical student's confusion conflates the technique with one it was specifically designed to complement, not duplicate.

Question 2

A researcher notices that a region where the arcuate fasciculus and the superior longitudinal fasciculus cross shows unexpectedly low FA values despite both tracts being intact on more advanced imaging. What explains this finding?

• A. Crossing fiber regions always have thinner myelin, genuinely reducing anisotropy
• B. The single-tensor model averages two competing diffusion directions, making diffusion appear more isotropic than either tract actually is
• C. Water diffuses faster in crossing fiber regions due to the extra interstitial space at intersections
• D. Tractography algorithms artificially deflate FA values in regions with many connections

Answer: (B)

This is the central limitation of the single-tensor model. When two fiber populations cross in one voxel, the tensor must fit a single ellipsoid to data that actually has two dominant directions. The result is a flatter, more spherical tensor — spuriously low FA — even though each tract individually would show high anisotropy. Roughly 90% of white matter contains crossing fibers, making this limitation very widespread. Advanced models like constrained spherical deconvolution can resolve this.

Question 3

DTI measures the physical movement of water molecules in the brain rather than neural electrical activity or blood flow.

• T. True
• F. False

Answer: True

DTI's signal comes entirely from the Brownian motion of water molecules. It applies diffusion-sensitizing gradients to detect how far and in which direction water moves within each voxel. This is physically and conceptually distinct from BOLD fMRI (blood oxygenation) or EEG/MEG (electrical fields). The insight that water movement is directionally constrained by myelin-wrapped axon bundles is what makes DTI informative about white matter structure.

Question 4

Higher fractional anisotropy values indicate more disrupted or damaged white matter, while lower values reflect intact, well-organized fiber tracts.

• T. True
• F. False

Answer: False

This reverses the relationship. FA ranges from 0 (perfectly isotropic, like a glass of water or gray matter) to 1 (perfectly anisotropic, like a tightly packed, well-myelinated fiber bundle). High FA indicates dense, well-organized, well-myelinated fibers — the axons are strongly constraining diffusion to one direction. Low FA indicates disruption: demyelination, axonal loss, edema, or developmental immaturity all cause diffusion to become more isotropic, reducing FA.

Question 5

Why does white matter show anisotropic diffusion, and what specifically does fractional anisotropy (FA) measure about the condition of a fiber tract?

Model answer: White matter axons run in organized bundles wrapped in myelin, which physically constrains water molecules to move much more freely along the fiber axis than perpendicular to it. FA quantifies how strongly directional this diffusion is — a value near 1 means diffusion is almost entirely along one axis (dense, well-myelinated, coherent fibers), while a value near 0 means diffusion is equally spread in all directions (disrupted or absent fiber organization).

The key insight is that FA is an indirect structural measure: it does not image myelin or axons directly, but instead uses water's physical behavior as a proxy for fiber integrity. This is why DTI can detect white matter abnormalities — demyelinating diseases like multiple sclerosis, traumatic axonal injury, developmental changes — that are completely invisible to conventional structural MRI, which only distinguishes tissue types by their T1 or T2 relaxation properties.