Monitoring blood oxygen state in muscle microcirculation with transverse relaxation

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Abstract
Oxygen uptake from the microcirculation is a direct measure of tissue function. Magnetic resonance is capable of detecting differences between oxygenated and deoxygenated blood due to the paramagnetic properties of deoxyhemoglobin. At the level of the microcirculation, however, imaging methods cannot directly visualize the vessels. Instead, bulk MR parameters are investigated for their ability to monitor blood oxygen saturation (%O2) changes in the microcirculation of tissue, specifically skeletal muscle. Experiments in an in vitro model verified the feasibility of detecting changes in exponential decay signals, and also verified the prediction of only two distinct decay components. Experiments in a rabbit model demonstrate that T2′ and monoexponential T2 decay are not sensitive to blood oxygen changes, but that the long‐T2 component in a biexponential fit is correlated to the blood oxygen state. Assuming a two‐pool model for water protons in muscle, and with knowledge of the T2‐%O2 relation, estimates of the microcirculation blood oxygen state can be made with some reasonable assumptions. Magn Reson Med 45:662–672, 2001.