Background--Because of rapid changes in myocardial intracellular Na⁺ (Na⁺_i) during ischemia and reperfusion (R), ²³Na magnetic resonance imaging (MRI) appears to be an ideal diagnostic modality for early detection of myocardial ischemia and viability. So far, cardiac ²³Na MRI data are limited and mostly concerned with imaging of total Na⁺. For proper interpretation, imaging of both Na⁺_i and extracellular Na⁺ is essential. In this study, we tested whether Na⁺_i imaging can be used to assess viability after low-flow (LF) ischemia.

Methods and Results--Isolated rat hearts were subjected to LF (1%, 2%, or 3% of control coronary flow) and R. A shift reagent was used to separate Na⁺_i and extracellular Na⁺ resonances. Acquisition-weighted ²³Na chemical shift imaging (CSI) was alternated with ²³Na MR spectroscopy. Already during control perfusion, Na⁺_i could be clearly seen on the images. Na⁺_i image intensity increased with increasing severity of ischemia. During R, Na⁺_i image intensity remained highest in 1% LF hearts. Not only did we find very good correlations between Na⁺_i image intensity at end-R and end-diastolic pressure (R=0.85, P<0.001) and recovery of the rate-pressure product (R=-0.88, P<0.001) at end-R, but most interestingly, also Na⁺_i image intensity at end-LF was well correlated with end-diastolic pressure (R=0.78, P<0.01) and with recovery of the rate-pressure product (R=-0.81, P<0.01) at end-R. Furthermore, Na⁺_i image intensity at end-LF was well correlated with creatine kinase release during R (R=0.79, P<0.05) as well as with infarct size (R=0.77, P<0.05).

Conclusions--These data indicate that ²³Na CSI is a promising tool for the assessment of myocardial viability.