Application of 23Na MRI to Examine Corticomedullary Sodium Gradient during Ischemia and Reperfusion in the in vivo Rat Kidney
From Iibis
Bharath Atthe, graduate student (Dr. Navin Bansal’s lab)
Abstract
Homeostasis of renal fluids and electrolytes is dependent on the corticomedullary sodium concentration gradient in the kidney. Alteration in this gradient is apparently observed early in the course of acute tubular necrosis (ATN). 23Na magnetic resonance imaging (MRI) has been applied to study the alterations in renal sodium distribution in the rat kidney during ischemia and reperfusion which induces ATN.
All 23Na and 1H MRI experiments were performed on a Varian 9.4-Tesla, 31-cm horizontal bore system. In-magnet ischemia was induced for 0 (control), 10, 20, 30 or 50 minutes on Wistar rats. 23Na images were collected every 10 minute during baseline, ischemia and 60 min reperfusion periods. T2-weighted 1H images were obtained for drawing regions of interest (ROI) around the medulla and cortex. The longitudinal and transversal (T1 and T2) relaxation time measurements were conducted on separate cohort of animals during 50 min ischemia and 60 min reperfusion using 23Na MRI and magnetic resonance spectroscopy (MRS). Changes in tissue sodium concentration (TSC) due to IR injury were quantified by applying T1 and T2 corrections.
Renal 23Na MRI revealed a marked decrease in medulla and cortex 23Na MRI signal intensity (SI) during the early evolution of ATN caused by ischemia-reperfusion injury. The sodium reabsorption function of the kidney appears to be irreversibly damaged after 50 min of ischemia and clearly demonstrated the inability to maintain the corticomedullary sodium contrast. 23Na relaxation time measurements by MRI and MRS showed that sodium relaxation characteristics were similar in renal medulla and cortex in normal kidney, and ischemia causes a significant decrease in the relaxation times which affects the calculation of medulla and cortex TSC from MRI SI data. The changes in relaxation times in both the compartments are identical, thus the medulla to cortex sodium SI ratio represent the TSC ratio of the two compartments.
Noninvasive 23Na MRI may have great potential in clinical diagnosis of evolving ATN in the setup of acute renal failure and in differentiating ATN from other causes of renal failure where tubular function is maintained.
