Anesthetics change tissue proton NMR relaxation.

Previous studies have shown the treatment of experimental animals with therapeutic doses of pharmaceuticals in acute and chronic regimens has resulted in extensive (up to 33%) change in tissue proton NMR relaxation times. In this study, the effects of injectable anesthetics (Innovar, pentobarbital, ketamine and alpha-chloralose) were compared with inhalational anesthetics (halothane, enflurane and isoflurane). Proton NMR T1 and T2 relaxation times were obtained from excised tissues of anesthetized rats (6 each treatment). Sodium pentobarbital and alpha-chloralose showed no alterations, whereas Innovar produced increases in brain and kidney relaxation times and ketamine showed increases in the heart. More widespread and extensive changes were observed after 1.5 hours of inhalation anesthesia: liver, muscle and fat showed relaxation time decrease; heart, spleen and lung were increased. No change in water content, as measured by wet/dry ratios, was observed for any tissue. Therefore, intravenous anesthesia may be preferred to inhalational anesthesia for magnetic resonance imaging based on potential changes in tissue relaxation properties. In addition, the absence of a measurable effect on brain suggests that the mechanism for anesthesia does not involve major changes in brain water.