Quantitative and simultaneous measurement of oxygen consumption rates in rat brain and skeletal muscle using ¹⁷O MRS imaging at 16.4T

Purpose: Oxygen-17 (¹⁷O) MRS imaging, successfully used in the brain, is extended by imaging the oxygen metabolic rate in the resting skeletal muscle and used to determine the total whole-body oxygen metabolic rate in the rat. Methods: During and after inhalations of ¹⁷O₂ gas, dynamic ¹⁷O MRSI was performed in rats (n = 8) ventilated with N₂O or N₂ at 16.4 T. Time courses of the H₂¹⁷O concentration from regions of interest located in brain and muscle tissue were examined and used to fit an animal-adapted 3-phase metabolic model of oxygen consumption. CBF was determined with an independent washout method. Finally, body oxygen metabolic rate was calculated using a global steady-state approach. Results: Cerebral metabolic rate of oxygen consumption was 1.97 ± 0.19 μmol/g/min on average. The resting metabolic rate of oxygen consumption in skeletal muscle was 0.32 ± 0.12 μmol/g/min and >6 times lower than cerebral metabolic rate of oxygen consumption. Global oxygen consumed by the body was 24.2 ± 3.6 mL O₂/kg body weight/min. CBF was estimated to be 0.28 ± 0.02 mL/g/min and 0.34 ± 0.06 mL/g/min for the N₂ and N₂O ventilation condition, respectively. Conclusion: We have evaluated the feasibility of ¹⁷O MRSI for imaging and quantifying the oxygen consumption rate in low metabolizing organs such as the skeletal muscle at rest. Additionally, we have shown that CBF is slightly increased in the case of ventilation with N₂O. We expect this study to be beneficial to the application of ¹⁷O MRSI to a wider range of organs, although further validation is advised.