N-acetylaspartate distribution in proton spectroscopic images of ischemic stroke: relationship to infarct appearance on T2-weighted magnetic resonance imaging.

BACKGROUND AND PURPOSE It is generally considered that tissue that appears abnormal on T2 MRI is already infarcted and that any penumbra lies outside the T2-visible lesion. We investigated the distribution of infarcted tissue using proton spectroscopic MRI. METHODS In patients with symptoms of acute hemispheric ischemic stroke, imaged within a maximum of 3 days of stroke, we explored the distribution of N:-acetylaspartate (NAA), a marker of intact neurons, within and around the abnormal (hyperintense) areas on T2-weighted MR images, using proton spectroscopic MRI. RESULTS In 11 patients, imaged 24 to 72 hours after stroke onset, there was little evidence of damaged neurons (reduced NAA) beyond the margins of hyperintensity on the T2 image. However, within the abnormal T2 area, there were statistically significant differences in the amount of NAA (ie, the proportion of intact neurons) between areas that were obviously abnormal on T2 (very hyperintense) and those that were only slightly abnormal (slightly hyperintense). CONCLUSIONS The extent and degree of hyperintensity of the T2-visible lesion directly reflect the amount of neuronal damage; lack of a T2-visible lesion would suggest predominantly intact neurons at the time of imaging. We hypothesize that once tissue damage has reached a critical (probably irreversible) level, the T2 image quickly becomes abnormal without any significant time lag between the pathological staging of the infarct and its visualization on T2. Further testing in a larger study with information on blood flow levels would be required to confirm this.