medscape/viewarticle/811594_5 Diffusion Tensor Magnetic Resonance Imaging Diffusion tensor imaging (DTI) is sensitive to the random translational motion of water molecules in tissue. This movement (diffusion) can be quantified by applying magnetic field gradients in different directions, thus enabling calculation of various metrics that measure the interaction of water molecules with cell membranes, myelin sheaths, and macromolecules, and providing information on tissue integrity at a microscopic scale well beyond the typical MRI resolution. Tissue has physical structures that limit diffusion in different directions, so diffusion is typically described as a three-dimensional ellipsoid through a 3 × 3 matrix. In nerve tissue, the directional diffusivity derived from DTI measurements describes microscopic water movement parallel to (λ||, axial diffusivity) and perpendicular to (λ┴, radial diffusivity) axonal tracts. Studies using experimental models of white matter injury have shown that decreased λ|| is associated with acute axonal injury, and increased λ┴ is associated with myelin injury [Budde et al. 2007]. Diffusion ellipsoids in highly organized fiber tracts (e.g. pyramidal tracts and the corpus callosum) are very elongated. Fractional anisotropy (FA) is a common metric to describe the degree of diffusion directionality or elongation. A high FA within a single voxel indicates that diffusion occurs predominantly along a single axis, while a low FA signifies that diffusion occurs along all three cardinal axes. An overall measure of diffusion magnitude is described by the mean diffusivity (MD), which ignores anisotropy and simply describes the overall magnitude of diffusion [Pagani et al. 2007]. DTI has demonstrated an increased MD and decreased FA in areas of normal appearing brain tissue from patients with MS, indicating subtle, diffuse injury with increasing mobility of water molecules and disruption of tissue architecture. DTI has also been used to characterize the pathological substrate of focal demyelinating plaques. Typically, these lesions show increased MD values and decreased FA compared with the contralateral normal-appearing white matter, which are especially high in acute contrast-enhanced lesions and in chronic T1-hypointense lesions [Rovaris et al. 2005]. These abnormalities persist to a variable extent in lesions that have the most severely altered tissue matrix [Castriota-Scanderbeg et al. 2003]. Fox and colleagues serially analyzed the FA of new enhanced MS lesions and observed that after an initial FA decrease, there is a subsequent increase that is most prominent during the first 2 months [Fox et al. 2011]. This increase was mainly driven by changes in radial diffusivity, a feature that may represent remyelination. These authors also found that a higher decrease in radial diffusivity within gadolinium-enhanced lesions at baseline, but not changes in this or other DTI metrics during 1 year of follow up, predicted conversion of these lesions to T1 black holes at 12 months. These findings support the notion that this type of evolution is predominantly influenced by the degree of initial injury and not by the amount of later recovery. All these findings support the use of DTI as a quantitative measure of the degree of brain tissue in MS. Acute MS plaques may show a transient decrease in MD values soon after the onset of new symptoms, with subsequent pseudonormalization and signs of developing vasogenic edema (Figure 8). Although the pathological substrate of this transient, early MD reduction in a subgroup of newly forming MS plaques has not been demonstrated, it could reflect swelling of the myelin sheaths, a decrease in vascular supply leading to cytotoxic edema, or dense inflammatory cell infiltration [Rovira et al. 2002; Eisele et al. 2012].
Posted on: Sun, 20 Oct 2013 07:24:28 +0000
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