Triarylmethyl Radical OX063d24 Oximetry: Electron Spin Relaxation at 250 MHz and RF Frequency Dependence of Relaxation and Signal-to-Noise

The triarylmethyl radical OX063d24 is currently used for pulsed electron paramagnetic resonance oximetry at 250 MHz. Both 1/T-1 and 1/T-2 increase with increasing oxygen concentration. The dependence of 1/T-1 on probe concentration is smaller than for 1/T-2. To inform the selection of the optimum frequency for in vivo oximetry 1/T-1, 1/T-2 and signal-to-noise were measured as a function of frequency between 400 and 1000 MHz on a variable-frequency spectrometer with an adjustable-frequency cross-loop resonator. 1/T-1 and 1/T-2 decrease with increasing frequency and signal-to-noise increases with increasing frequency, which are all favourable for imaging at higher frequencies. However, depth of penetration of the radio frequency (RF) into an animal decreases with increasing frequency. Assuming that the RF loss in the animal to be studied determines the resonator Q, our results indicate that the optimum frequency for in vivo imaging will be determined by the desired depth of penetration in the tissue.