In vivo noninvasive observation of acute mesenteric ischemia in rats

Acute intestinal ischemia remains a catastrophic event even with the advent of modern diagnostic and vascular surgical techniques. An early noninvasive test would be valuable since early operation yields better survival rates. We have used an in vivo rat model to study acute intestinal ischemia after occlusion of the superior mesenteric artery (SMA). 31Phosphorus magnetic resonance spectroscopy (MRS), a noninvasive nondestructive technique, can detect the phosphorus metabolites most likely to be altered in ischemia: adenosine triphosphate, phosphocreatine (PCr), inorganic phosphate (Pi) and phosphomonoesters and phosphodiesters. Furthermore, intracellular pH can be estimated from the pH dependent position of the Pi spectral line relative to PCr. A tourniquet was loosely placed around the SMA in five Wistar rats through a transabdominal approach to the retroperitoneum. The abdomen was immediately closed. A 20 millimeter MRS surface coil was placed on the abdomen and 31Phosphorus spectra were accumulated. The SMA was then occluded and additional 31Phosphorus spectra were taken for the next 75 minutes. Significant (p less than 10(-4) changes in the position and magnitude of the spectra lines occurred within 20 minutes; the Pi position indicates severe intracellular acidosis and rapidly increases to three times its original magnitude. The PCr line decreases in magnitude. In a similar experiment, occlusion of the superior mesenteric vein (SMV) produced equivalent results. Occlusion of vessels other than the SMA or SMV not accompanied by transmural ischemia resulted in spectra unaltered from control. These findings support the application of phosphorus MRS to clinical studies.