Directional diffusivity as a magnetic resonance (MR) biomarker in demyelinating disease

Directional diffusivities derived from diffusion tensor magnetic resonance imaging (DTI) measurements describe water movement parallel to (λ_∥, axial diffusivity) and perpendicular to (λ_⊥, radial diffusivity) axonal tracts. λ_∥ and λ_⊥ have been shown to differentially detect axon and myelin abnormalities in several mouse models of central nervous system white matter pathology in our laboratory. These models include experimental autoimmune encephalomyelitis (EAE), (1) myelin basic protein mutant mice with dysmyelination and intact axons, (2) cuprizone-induced demyelination, and remyelination, with reversible axon injury (2, 3) and a model of retinal ischemia in which retinal ganglion cell death is followed by Wallerian degeneration of optic nerve, with axonal injury preceding demyelination. (4) Decreased λ_∥ correlates with acute axonal injury and increased λ_⊥ indicates myelin damage. (4) More recently, we have translated this approach to human MR, investigating acute and chronic optic neuritis in adults with multiple sclerosis, brain lesions in adults with multiple sclerosis, and acute disseminated encephalomyelitis (ADEM) in children. We are also investigating the use of this technique to probe the underlying structural change of the cervical spinal cord in acute and chronic T2-hyperintense lesions in spinal stenosis, trauma, and transverse myelitis. In each of these demyelinating diseases, the discrimination between axonal and myelin injury which we can achieve has important prognostic and therapeutic implications. For those patients with myelin injury but intact axons, early, directed drug therapy has the potential to prevent progression to axonal loss and permanent disability.