Apparent diffusion tensor measurements in myelin-deficient rat spinal cords.

The apparent diffusion tensor (ADT) was measured in excised and fixed spinal cords from myelin-deficient (md) rats and age-matched controls. These data were used to obtain the principal diffusivities of the ADT, and also the scalar invariant parameters _D (averaged principal diffusivity) and A(sigma) (anisotropy index) for four white matter and two gray matter regions. The results for white matter regions showed that the principal diffusivities were significantly higher for md animals, and while the _D was increased in tissue from md animals, the A(sigma) was found to be decreased. Grey matter _D was measured to be between those of white matter from control and md animals, and the A(sigma) was much smaller than that of white matter from both sets of animals, indicating that diffusion in md white matter is more anisotropic than in gray matter. The results show that while myelination is not a prerequisite for diffusion anisotropy, it does influence the magnitude of the observed anisotropy. Magn Reson Med 45:191-195, 2001. Copyright 2001 Wiley-Liss, Inc.