In vivo longitudinally detected ESR measurements at microwave regions of 300, 700, and 900 MHz in rats treated with a nitroxide radical.

A signal detector of longitudinally detected ESR (LODESR) is independent of the resonant frequency. We developed an in vivo LODESR spectrometer operating in the regions of 300, 700, and 900 MHz. Using this apparatus, we estimated signal intensities at different operating frequencies obtained from non- or high-dielectric loss phantoms that contained nitroxide radical solutions and from live rats that had received a nitroxide radical. Our result, higher signal intensities in the high-dielectric loss samples (such as physiological saline solution and animals) at a lower frequency, shows that the influence of a decrease in dielectric loss dominates over the signal reduction caused by smaller Zeeman splitting. We believe that this finding strongly supports an in vivo ESR resonant frequency that tends to be low.