In-vivo oxygen mapping using LODESR imaging
A novel imaging modality is introduced, using Radio-Frequency Longitudinally Detected Electron Spin Resonance (RF-LODESR), capable of providing qualitative and semi-quantitative information on a variety of parameters reflecting physiological function, the most significant being tissue oxygenation. Effective spin-lattice (T1e*) and spin-spin (T2e*) electronic relaxation time maps of the abdomen of living 200 g rats were generated after intravenous administration of a triarylmethyl free radical (TAM). These maps were used to evaluate oxygen distribution. Differences between the liver, kidneys and bladder were noted. Conclusions were made regarding distribution, perfusion and excretion rate of the contrast medium. Ligature induced anoxia in the kidney was also visualised. LODESR involves transverse ESR irradiation with a modulated excitation and observing oscillations in the spin magnetisation parallel to the main magnetic field. The T1e* and T2e* maps were calculated from a set of LODESR signal phase images collected at different detection frequencies. Each phase image also provides qualitative information on tissue oxygen level without any further processing. Additionally, a new method is introduced for measuring T1e, valid for inhomogeneous spectra, based on measuring the spectral area as a function of detection frequency. These methods present alternatives to the conventional transverse ESR linewidth-based oximetry methods, particularly for animal whole body imaging applications.