The following antibodies were used: a rabbit polyclonal antibody against fibrinogen (a gift from Dr J Degen), MBP (Chemicon, Temecula, CA, USA), Olig2 (Chemicon), MAP2 (Sigma, St Louis, MO, USA), and NeuN (Chemicon). We found that HI caused a rapid reduction of axial and radial diffusivities in all three axonal bundles. A large decrease in fractional anisotropy, but not in axial diffusivityper se, was associated with structural breakdown of axons. Furthermore, the decrease in radial diffusivity correlated with swelling of myelin sheaths and compression of the axoplasma. The gray matter of the hippocampus also exhibited GNF 2 a high level of diffusion anisotropy, and its reduction signified dendritic degeneration. Taken together, these results suggest that cross-evaluation of multiple DTI parameters may provide a fuller picture of axonal and dendritic injury in acute ischemic stroke. Keywords:axial diffusivity, fractional anisotropy, leukoaraiosis, oligodendrocyte, radial diffusivity, white matter == Introduction == White matter (WM), occupying 50% of total brain volume in humans, has a metabolic rate similar to that of gray matter (GM) (Zhang and Sejnowski, 2000;Goldberg and Ransom, 2003). Yet, WM receives a disproportionately small blood supply and little collateral circulation, making it particularly susceptible to hypoxic-ischemic insults (Dewaret al, 1999;Arai and Lo, 2009). However, although the importance of WM injury in cerebrovascular diseases is recognized, it is difficult to distinguish it from GM damage and determine the point of irreversible WM injury in acute stroke using traditional imaging methods. Diffusion tensor imaging (DTI) is a relatively new magnetic resonance imaging (MRI) technique to visualize axonal organization and to measure directional water diffusion in the nervous system (Basseret al, 1994). In DTI, measurements of water diffusion along multiple axes are fitted into a three-dimensional (3D) model to determine three eigenvalues in the principal (1) and two shorter, perpendicular axes (2and3) of the diffusion ellipsoid’. The three eigenvalues are used to calculate several DTI indexes, including fractional anisotropy (FA), axial/longitudinal diffusivity (II), and radial/transverse diffusivity () (Mori and Zhang, 2006). Fractional anisotropy, scaled from 0 (isotropic) to 1 1 (anisotropic), is a measurement of the relative difference of the three eigenvalues. Axial diffusivity, equivalent to1, represents water motion along the length of axons, which can be reduced by structural breakdown or focal enlargementconstriction of the neurites according to theoretical modeling (Beaulieu, 2002;Budde and Frank, 2010). Radial diffusivity, the average of2and3, indicates perpendicular water diffusion across the axon, which is hindered by barriers such as myelin sheaths (Songet al, 2002). In trauma- or axotomy-induced Wallerian degeneration, axial diffusivity steadily declines and radial diffusivity gradually increases, resulting in a progressive reduction of FA (MacDonaldet al, 2007;Zhanget al, 2009). In contrast, patients with acute ischemic stroke often exhibit a rapid reduction of both axial and radial diffusivities, accompanied by little to no change of initial FA (Yanget al, 1999;Bhagatet al, 2008;Sakaiet al, 2009). A sizable reduction of FA only occurred 24 hours after stroke onset, which was postulated to indicate irreversible axonal injury (Bhagatet al, 2008). However, this hypothesis of important clinical application is yet to be validated, because to date, there are no DTIneuropathology correlation data in patients with acute ischemic stroke (<24 hours). In this study, we describe DTI measurements and corresponding histologic changes in a rodent model of thrombotic stroke induced by cerebral hypoxiaischemia (HI) (Adhamiet al, 2006). We show that a combined hypoxic-ischemic insult triggers a rapid reduction of both axial and radial diffusivities in multiple axonal tracts, similar to the GNF 2 pattern observed in acute stroke patients. In contrast, a large reduction of FA only occurs slowly and is restricted to the nerve tracts with severe axonal destruction. FA-H Furthermore, the rapid reduction of radial diffusivity correlates with oligodendrocyte swelling and compression GNF 2 of the axoplasma. Finally, the reduction of anisotropy contrast inside the GM of the hippocampus signifies dendritic degeneration. Taken together, these results suggest that DTI can be used to detect and distinguish multiple aspects of axonal and dendritic injuries in acute ischemic stroke. == Materials and methods == == Animal Surgery == Male CD-1 mice (Charles River, Wilmington, MA, USA) and Thy1-YFP mice (Jackson Laboratories, Bar Harbor, ME, USA; stock number 003782;Fenget al(2000)), aged 8 to 12 weeks, were used in this study. Details of the HI-induced thrombotic model of stroke have been described previously (Adhamiet al, 2006). In brief, animals were anesthetized using 1% to 2% isoflurane while maintaining respiration at 80 to 120 breaths per minute. Partial cerebral ischemia was established by permanent unilateral (right) common carotid artery occlusion. After carotid ligation, GNF 2 hypoxia was initiated by administering.