Noninvasive in vivo determination of intracerebral oxygen concentration in rats by a longitudinally detected ESR (LODESR) technique.

We have developed a noninvasive method to determine oxygen concentration in the brain tissue of rats in vivo. The method is based upon measuring the fundamental harmonic-to-secondary harmonic ratio (FSR) of longitudinal magnetization changes of a blood-brain barrier (BBB)-permeable nitroxide radical, 3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl (hydroxymethyl-PROXYL), by employing a longitudinally detected ESR (LODESR) spectrometer operating at an ESR frequency of 280 MHz. FSRs of phantoms, including a hydroxymethyl-PROXYL solution and various concentrations of oxygen, were measured. We found that the FSRs of the phantom increased as the oxygen concentration increased. In vivo FSRs in the brains of rats that had received a hydroxymethyl-PROXYL injection were measured without the use of any surgical procedures. It was found that when the rats breathed 100% oxygen, rather than normal air, the FSR was significantly greater.