In vivo oximetry by a pulsed longitudinally detected ESR spectrometer

By using a narrow single electron spin resonance (ESR) line agent, triarylmethyl, tris(8-carboxy-2,2,6,6-tetrahydroxyethylbenzo1,2-d:4,5-d′] bis(1,3)dithiole-4-yl)methyl sodium salt (TAM OX063), pulsed longitudinally detected ESR (LODESR) measurements of a phantom or the chest of a living mouse at the operating frequency of ca. 300 MHz were taken and the effective longitudinal relaxation time (T ₁^*) was estimated for oximetry. Under irradiation of a pair of π-pulses with a variable interval between pulses (τ), in-phase LODESR signal intensities were obtained from the phantoms containing TAM dissolved in a physiological saline solution at a concentration of 1 mM and various concentrations of oxygen. TheT ₁^* of the phantom was calculated from the plotted curve of the LODESR signal intensity against τ. It was found that the reciprocal ofT ₁^*, i.e., the longitudinal relaxation rate, increased with the concentration of oxygen. In vivo pulsed LODESR measurements of the chest of living mice that had received a TAM injection via the intraperitoneal route were made. While the LODESR measurements were being made, the mice in one group breathed normal air and those in another group breathed 100% oxygen. It was found that the longitudinal relaxation rate of the mice breathing 100% oxygen was significantly greater than that of mice breathing normal air, indicating that breathing 100% oxygen elevates the thoracic longitudinal relaxation rate.