SHIFT-REAGENT ENHANCED CONCURRENT NA-23 AND H-1 MAGNETIC-RESONANCE SPECTROSCOPIC STUDIES OF TRANSCELLULAR SODIUM DISTRIBUTION IN THE DOG BRAIN IN-VIVO

The intracellular to extracellular sodium distribution is one of the p rimary determinants of action potentials necessary for the electrical function of organs such as brain, heart and skeletal muscle. The abili ty of shift reagent enhanced Na-23 MRS to directly measure the intrace llular and extracellular sodium distribution in brain is controversial and centers on the relative contributions of bulk magnetic susceptibi lity and hyperfine interactions to the observed chemical shifts. In th is study, infusion of dysprosium (III) triethylenetetraminehexacetate (Dy(TTHA)-3), resulted in a Na-23 MRS spectrum of dog brain with two w ell resolved peaks at 9 and 0.4 ppm. The 9 ppm peak corresponded to th e resonance seen in aspirated blood. After disruption of the blood bra in barrier, the single peak at 0.4 ppm split into two peaks at 3 and 0 ppm. The ability of Dy(TTHA)-3 enhanced Na-23 MRS to follow global ch anges in brain sodium distribution was tested during cardiac arrest. T he expected rapid Na influx into the intracellular space produced a ma rked decrease in the 3 ppm signal and a parallel increase in the 0 ppm peak. This is consistent with the assignment of the 3 ppm peak as int erstitial sodium and the 0 ppm peak as intracellular sodium.