Dissolved oxygen minimally affects magnetic susceptibility in biologically relevant conditions

Purpose To investigate the potential of quantitative susceptibility mapping (QSM) with MRI as a biomarker for tissue oxygenation in fat-water mixture. Oxygen molecules (O 2 ) are paramagnetic. This suggests that dissolved O 2 in tissue should affect the measured magnetic susceptibility. However, direct measurements of dissolved O 2 in tissues is challenging with QSM as the induced change in susceptibility is below the sensitivity of existing algorithms. QSM in regions that contain fat could be sensitive enough to be used as a marker of tissue oxygenation as oxygen has a larger solubility in fat than in water. Methods The relationship between dissolved O 2 concentration and magnetic susceptibility was investigated based on MRI measurements using phantoms made of fat-water emulsions. Dairy cream was used to approximate fat-containing biological tissues. Phantoms based on dairy cream with 35 % fat were designed with controlled concentrations of dissolved O 2 . O 2 was bubbled into the dairy cream to reach O 2 concentrations above the concentration at atmospheric pressure, while nitrogen was bubbled in cream to obtain O 2 concentrations below atmospheric pressure. Magnetic susceptibility was expected to increase, becoming more paramagnetic, as O 2 concentration was increased. Results Magnetic susceptibility from MRI-based QSM measurements did not reveal a dependence on O 2 concentration in fat-water mixture phantoms. The relationship between susceptibility and O 2 was weak and inconsistent among the various phantom experiments. Conclusion QSM in fat-water mixture appears to be minimally sensitive to dissolved O 2 based on phantom experiments. This suggests that QSM is not likely to be sensitive enough to be proposed as a marker for tissue oxygenation, as the change in magnetic susceptibility induced by the change in dissolved O 2 concentration is below the current detection limit, even in the presence of fat.