Diffusion tensor magnetic resonance imaging (DT-MRI) was applied for in vivo quantification of myelin loss and regeneration. A transgenic mouse line (Oligo-TTK) expressing a truncated form of the herpes simplex virus 1 thymidine kinase gene (hsv1-tk) in oligodendrocytes was studied along with two induced phenotypes of myelin pathology. Myelin loss and axonal abnormalities differentially affect values of DT-MRI parameters in the brain of transgenic mice. Changes in the anisotropy of the white matter were assessed by calculating and mapping the radial (D perpendicular) and axial (D parallel) water diffusion to axonal tracts and fractional anisotropy (FA). A significant increase in D perpendicular attributed to the lack of myelin was observed in all selected brain white matter tracts in dysmyelinated mice. Lower D parallel values were consistent with the histological observation of axonal modifications, including reduced axonal caliber and overexpression of neurofilaments and III beta-tubulin. We show clearly that myelination and axonal changes play a role in the degree of diffusion anisotropy, because FA was significantly decreased in dysmyelinated brain. Importantly, myelin reparation during brain postnatal development induced a decrease in the magnitude of D( perpendicular) and an increase in FA compared with the same brain before recovery. The progressive increase in D parallel values was attributed to the gain in normal axonal morphology. This regeneration was confirmed by the detection of enlarged oligodendrocyte population, newly formed myelin sheaths around additional axons, and a gradual increase in axonal caliber.

The failure of the remyelination processes in multiple sclerosis contributes to the formation of chronic demyelinated plaques that lead to severe neurological deficits. Long-term cuprizone treatment of C57BL/6 mice resulted in pronounced white matter pathology characterized by oligodendrocyte depletion, irreversible demyelination and persistent functional deficits after cuprizone withdrawal. The use of a combination of in vivo diffusion tensor magnetic resonance imaging (DT-MRI) and histological analyses allowed for an accurate longitudinal assessment of demyelination. Injection of triiodothyronine (T 3 ) hormone over a 3 week interval after cuprizone withdrawal progressively restored the normal DT-MRI phenotype accompanied by an improvement of clinical signs and remyelination. The effects of T 3 were not restricted to the later stages of remyelination but increased the expression of sonic hedgehog and the numbers of Olig2 ϩ and PSA-NCAM ϩ precursors and proliferative cells. Our findings establish a role for T 3 as an inducer of oligodendrocyte progenitor cells in adult mouse brain following chronic demyelination.

A linear relationship between the proton relaxation rates and the fraction of deoxyhemoglobin of circulating whole blood (fHb) has been established in vitro at 4.7 T. These results have been interpreted on the basis of the additivity of relaxation rates. The slope of the lines of transverse relaxation rates (R2) versus fHb was found to increase with interpulse delay in Carr-Purcell Meiboom-Gill (CPMG) experiments. The Luz-Meiboom relation applied to this interpulse delay dependence of R2 suggests a two-site chemical exchange rather than a diffusion mechanism. The 1-ms water proton exchange time derived from these observations has been interpreted in terms of exchange between exchangeable protons close to the paramagnetic center of hemoglobin and protons of bulk water.

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Proton dynamical polarization is a method which unites the advantages of continuous wave ESR sensitivity and pulsed NMR imaging in order to localize free radicals. Injected nitroxide has been imaged for the first time in living rats. The high sensitivity of this method to oxygen is demonstrated.