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

Abstract

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