Results suggest that quality-guided unwrapping should be preferred, with PDF or LBV for background removal, for QSM in regions with large Δχ and negligible signal. This reduces the susceptibility inaccuracy introduced by phase processing. Accurate background removal remains an open question. Magn Reson Med 79:3103-3113, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

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Purpose: To investigate the potential of quantitative susceptibility mapping (QSM) with MRI as a biomarker for tissue oxygenation in fat-water mixture. Oxygen molecules (O2) are paramagnetic. This suggests that dissolved O2 in tissue should affect the measured magnetic susceptibility. However, direct measurements of dissolved O2 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 O2 concentration and magnetic susceptibility was investigated using phantoms made of fat-water emulsions in MRI measurements. 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 O2. O2 was bubbled into the dairy cream to reach O2 concentrations above the concentration at atmospheric pressure, while nitrogen was bubbled in cream to obtain O2 concentrations below atmospheric pressure. Magnetic susceptibility was expected to increase, becoming more paramagnetic, as O2 concentration was increased. Results: Magnetic susceptibility from MRI-based QSM measurements did not reveal a dependence on O2 concentration in fat-water mixture phantoms. The relationship between susceptibility and O2 was weak and inconsistent among the various phantom experiments. Conclusion: QSM in fat-water mixture appears to be minimally sensitive to dissolved O2 based on phantom experiments. This suggests that QSM is not likely sensitive enough to be proposed as a marker for tissue oxygenation, as the change in magnetic susceptibility induced by the change in dissolved O2 concentration is below the current detection limit, even in the presence of fat.

Commercial methods for single breath-hold proton density fat fraction (PDFF) quantification in liver can suffer from bias due to the presence of iron. In this work, the PDFF accuracy of a high-speed T2-corrected multi-echo (HISTO) sequence was evaluated at 3T in phantoms at variable R2*, mimicking different iron levels. PDFF errors up to 70% for R2* larger than 150 s-1 were obtained, suggesting that HISTO is unreliable at large liver R2*, as seen with moderate to high iron overload. A Dixon sequence was more accurate at large R2*. Both techniques agreed in patients and phantoms at low PDFF and R2*.

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