Direct, longitudinal comparison of (1)H and (23)Na MRI after transient focal cerebral ischemia.

Abstract

(23)Na MRI may offer new insight into the evaluation of tissue injury. We performed a direct, longitudinal, morphological comparison of (1)H T2 relaxation, (1)H apparent diffusion coefficient (ADC), (23)Na content, and histopathology after cerebral ischemia to address the hypotheses that (a) (23)Na MRI is unique in comparison to (1)H MRI, and (b) accumulation of (23)Na is an unambiguous marker for dead tissue. Rats underwent 30 minutes of focal ischemia. MRIs of (1)H T2, (1)H ADC, and (23)Na content were acquired from 12 hours up to 1, 2, or 14 days after reperfusion. On excision, brains were stained with triphenyltetrazolium chloride (TTC). In all cases, the region of abnormality increased in size for 2 days. On day 5, both (1)H T2 and ADC temporarily appeared normal despite the presence of TTC-defined infarction. By comparison, the volume of tissue exhibiting abnormally intense (23)Na signal mirrored the TTC-defined infarct at all time points. Regions of high (23)Na content correlate well with the TTC-defined infarct and may be a quantitative in vivo marker for dead tissue. In contrast, the dynamics of the (1)H T2 and ADC make it difficult to interpret these images without additional information because they may appear normal despite infarction. Neither type of (1)H image delineates dead tissue, and none of these methods predicts the potential infarct size at early time points.