Magnetic resonance diffusion tensor imaging in stroke

Introduction Diffusion-weighted imaging (DWI) is recognized as one of the most reliable methods for the early detection and evaluation of cerebral ischemia. In acute ischemia, cytotoxic edema occurs from disruption of energy metabolism through failure of Na⁺/K⁺ ATPase and other ionic pumps. This, in turn, leads to a loss of ionic gradients and the osmotic transfer of water from the extracellular compartment into the intracellular compartment, where water mobility is relatively decreased [1-3]. With cellular swelling and decreased extracellular space volume, water diffusion in the extracellular compartment is also reduced through the increased tortuosity of the extracellular pathways [4-6]. This leads to an abrupt reduction in apparent diffusion coefficient (ADC) by 33-60% below that of normal tissue within minutes following the onset of ischemia is highly conspicuous on DWI; this change.[7,8] Indeed, few cellular processes exhibit larger tissue contrast in MRI than acute ischemia with DWI, which allows the diagnosis, localization, and extent of densely ischemic tissue to be identified within 3 to 6 h of stroke onset, when intravenous (up to 3 h) or intra-arterial (up to 6 h) thrombolysis may be effective treatment options. Although DWI has been useful in research and clinical management of stroke, diffusion tensor imaging (DTI) may offer additional diagnostic information on the microstructural status of tissue. This is because diffusion in tissue is affected by the presence of semipermeable membranes and oriented microstructures in the intracellular, extracellular, and vascular compartments, which result in preferential movement of water parallel to these obstacles. This directional dependence of diffusion is known as anisotropy. In the brain, white matter (WM) has relatively high anisotropy since diffusion is much greater parallel than perpendicular to major WM tracts. Gray matter (GM), by comparison, has relatively low anisotropy. The purpose of this chapter is to report on progress in the development and applications of DTI in stroke.