DTI Measurements in Multiple Sclerosis: Evaluation of Brain Damage and Clinical Implications

Table 1

Schematic description of the main DTI parameters.

	Unit of measure	Formula	Object measured
FA	Scalar value ranging between 0-1	12𝜆1−𝜆2^2+𝜆2−𝜆3^2+𝜆3−𝜆1^2𝜆^21+𝜆^22+𝜆^23	Fibers directionality/axonal loss
MD	mm^2/sec	(𝜆1+𝜆2+𝜆3)/3	Amount of water diffusion/myelin loss
AD	mm^2/sec	𝜆1	Diffusivity parallel to the fibers/myelin and axonal content
RD	mm^2/sec	(𝜆2+𝜆3)/2	Diffusivity perpendicular to the fibers/myelin content

FA: fractional anisotropy; MD: mean diffusivity; AD: axial diffusivity; RD: radial diffusivity; mm: millimeters; sec: second.