Acute astrocyte activation in brain detected by MRI: new insights into T1 hypointensity

Increases in the T1 of brain tissue, which give rise to dark or hypointense areas on T1-weighted images using magnetic resonance imaging (MRI), are common to a number of neuropathologies including multiple sclerosis (MS) and ischaemia. However, the biologic significance of T1 increases remains unclear. Using a multiparametric MRI approach and well-defined experimental models, we have experimentally induced increases in tissue T1 to determine the underlying cellular basis of such changes. We have shown that a rapid acute increase in T1 relaxation in the brain occurs in experimental models of both low-flow ischaemia induced by intrastriatal injection of endothelin-1 (ET-1), and excitotoxicity induced by intrastriatal injection of N-methyl-d-aspartate (NMDA). However, there appears to be no consistent correlation between increases in T1 relaxation and changes in other MRI parameters (apparent diffusion coefficient, T2 relaxation, or magnetisation transfer ratio of tissue water). Immunohistochemically, one co...