Sodium-23-nuclear magnetic resonance imaging of the rabbit kidney in vivo

The mechanism by which the mammalian kidney generates a concentration gradient of sodium from cortex to papilla is still not entirely understood. Studies of how the kidney as an organ generates this gradient have been hampered by the lack of a noninvasive method for monitoring the intrarenal sodium distribution. Herein, we demonstrate the value of sodium-23 nuclear magnetic resonance (²³Na-NMR) imaging to nondestructively assess the intrarenal sodium distribution. ²³Na-NMR images were obtained from a surgically exposed kidney preparation that showed the two-dimensional distribution of sodium in the rabbit kidney. In the antidiuretic kidney this gradient resulted in papillary sodium concentrations that were approximately threefold higher than cortical values. Serial ²³Na-NMR images obtained during saline infusion demonstrated the kinetics by which the sodium gradient increases with diuresis. The half-time for ²³Na washout of the medulla of the kidney was ~6 min with this protocol. In addition, a three-dimensional data set of the sodium distribution of the kidney was obtained with voxel dimensions of 1.5 mm³ by use of a three-dimensional ²³Na-NMR imaging technique. Without surgical exposure, ²³Na-NMR images of the rabbit kidney were collected under completely noninvasive conditions by use of a surface coil. The ²³Na-NMR signal from the kidney was easily detected; however, to obtain images of comparable signal-to-noise ratio to the surgically exposed kidney, spatial and temporal resolution were significantly reduced.