Sodium MRI with coated magnetite: measurement of extravascular lung water in rats.

This study was done to see if signal intensity in sodium images of edematous rat lungs made after iv administration of a negative intravascular contrast agent could serve as a measure of the edema fluid present. First, a method to produce a stable condition of hydrostatic pulmonary edema was developed and verified by CT. Second, dose-response curves for coated magnetite preparations were constructed by giving edematous rats varied doses of these preparations and measuring signal intensity changes of various organs by sodium MRI in a 31-cm-bore 1.9-T magnet. Third, rats were given varied levels of pulmonary edema followed by a constant dose of coated magnetite to eliminate the plasma sodium signal. Finally, coated magnetite particles of two sizes were administered to rats, and the differences in effects on signal from various organs were measured. Signal intensity of the lungs after magnetite correlated (r = 0.86) with extravascular lung water measured gravimetrically, suggesting that sodium MRI may be useful for measuring pulmonary edema fluid. Smaller particles appear to remain in the blood longer than larger particles.