Changes in the proton T₂ relaxation times of cerebral water and metabolites during forebrain ischemia in rat at 9.4 T

Proton T₂ relaxation times of cerebral water and metabolites were measured before, during, and after transient forebrain ischemia in rat at 9.4 T using localized proton magnetic resonance spectroscopy (¹H-MRS) with Hahn echoes formed at different echo times (TEs). It was found that the T₂ values of water and N-acetyl aspartate (NAA) methyl, but not total creatine (tCr) methyl, decrease significantly (∼10%) during ischemia, and this T₂ reduction is reversed by reperfusion. The T₂ reduction observed for NAA was most likely caused by the extravascular component of the blood oxygenation level-dependent (BOLD) effect induced by a drastically increased deoxyhemoglobin content during ischemia. The absence of T₂ changes for tCr can probably be explained by the fact that the BOLD-related T₂ decrease was counterbalanced by the conversion of phosphocreatine (PCr) to creatine (Cr), which has a longer T₂ than PCr, during ischemia. The changes in T₂ should be taken into account for the quantification of metabolite concentrations during ischemia.