The role of diffusion tensor imaging in the evaluation of ischemic brain injury - a review

• Published in: NMR in biomedicine 2002-11, Vol.15 (7-8), p.561-569
• Main Author: Sotak, Christopher H.
• Format: Article
• Language: English
• Subjects: Animals; Anisotropy; Biological and medical sciences; Brain - metabolism; Brain - pathology; Brain Chemistry; Brain Ischemia - metabolism; Brain Ischemia - pathology; cerebral ischemia; Diffusion; diffusion anisotropy; diffusion imaging; Diffusion Magnetic Resonance Imaging - methods; diffusion tensor imaging; Humans; Investigative techniques, diagnostic techniques (general aspects); magnetic resonance imaging; Medical sciences; multiparametric magnetic resonance imaging; Nervous system; Neurology; Radiodiagnosis. Nmr imagery. Nmr spectrometry; Vascular diseases and vascular malformations of the nervous system; Water - chemistry; Water - metabolism; water apparent diffusion coefficient
• ISSN: 0952-3480; 1099-1492
• DOI: 10.1002/nbm.786

Water diffusion in brain tissue is affected by the presence of barriers to translational motion such as cell membranes and myelin fibers. The measured water apparent diffusion coefficient (ADC) value is therefore frequently anisotropic and varies depending upon the orientation of restricting barriers (such as white matter tracts) relative to the diffusion‐sensitive‐gradient direction. Anisotropic water diffusion can be specified using indices of diffusion anisotropy [e.g. standard deviation of the individual ADC values, fractional anisotropy (FA), lattice index (LI)], which are derived from measurements of the full diffusion tensor. The rotationally invariant nature of particular diffusion anisotropy indices (e.g. FA, LI) allows orientation‐independent comparisons of these parameters between different subjects. Pathophysiological processes (such as cerebral ischemia) that modify the integrity of the tissue microstructure result in significant alterations in tissue anisotropy and make this metric a useful endpoint for characterizing the temporal evolution of the disease. Diffusion‐tensor imaging (DTI) studies of both experimental and human stroke suggest that DTI may provide additional information about the evolution of the disease that is not available from diffusion‐weighted MRI (DWI) alone. Acute reductions in the average diffusivity [  = (λ1 + λ2 + λ3)/3 where λ1, λ2, and λ3 are the eigenvalues of the diffusion tensor] following the onset of cerebral ischemia are often accompanied by increases in diffusion anisotropy. In the transition from acute to sub‐acute and chronic stroke, renormalizes and subsequently increases whereas diffusion anisotropy measures (e.g. FA) decline and remained reduced in chronic infarcts. Overall isotropic ADC changes during infarct evolution have been observed to be greater in white matter (WM) than in gray matter (GM) lesions (although there have been conflicting reports on this issue) and GM lesions tend to renormalize prior to WM lesions as the infarct evolves. Ischemic WM exhibits a significant decrease in diffusion anisotropy (relative to normal WM) during ischemic evolution whereas that of ischemic GM remains statistically unchanged. Furthermore, the percentage decrease in ischemic WM is largely determined by reductions in λ1, the eigenvalue that coincides with the long axis of the WM fiber tract. Variations in unidirectional ADC or over the ischemic time course limit the usefulness of this parameter alone as a predictor