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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We have designed a novel QSM algorithm that addresses some of the limitations of existing techniques that combine the background removal and dipole inversion steps in a single step. We propose that the solution to the direct inversion problem can be aided by an iterative k-space algorithm and the inclusion of a priori information that represents feature-based and voxel-fidelity-based constraints. The considered approach, when compared with other techniques, resulted in a more accurate depiction of the susceptibility in high susceptibility deep gray matter (dGM) structures without sacrificing performance in regions like the cortex of the brain.

The impact of the fat fraction in fat-water mixtures on the fat and water T1s is an open question, with variable observations reported in phantoms, in vivo in bone marrow, and in liver. This work investigated variations of fat and water T1 as a function of fat content in phantoms in the presence of a water relaxation enhancing agent (MnCl2). Fat fraction and MnCl2 impacted both the water and fat T1s, in different ways. Our observations may shed light on variable trends of fat and water T1 as a function of fat content reported in the literature.

Longitudinal (T 1 ) relaxation of triglyceride molecules and water is of interest for fatwater separation and fat quantification. A better understanding of T 1 relaxation could benefit modeling for applications in fat quantification and relaxation mapping. This work investigated T 1 relaxation of spectral resonances of triglyceride molecules and water in liquid fat-water mixtures and its dependence on the fat fraction. Dairy cream and a safflower oil emulsion were used. These were diluted with distilled water to produce a variety of fat mass fractions (4.4% to 35% in dairy cream and 6.3% to 52.3% in safflower oil emulsion). T 1 was measured at room temperature at 3 T using an inversion recovery STimulated Echo Acquisition Mode (STEAM) MR spectroscopy method with a series of inversion times. T 1 variations as a function of fat fraction were investigated for various resonances. A two-component model was developed to describe the relaxation in a fat-water mixture as a function of the fat fraction. The T 1 of water and of all fat resonances studied in this work decreased as the fat fraction increased. The relative variation in T 1 was different for each fat resonance. The T 1 of the methylene resonance showed the least variation as a function of the fat fraction. The proposed two-component model closely fits the observed T 1 variations. In conclusion, this work clarifies how the T 1 of major and minor fat resonances and of the water resonance varies as a function of the fat fraction in fat-water mixtures. Knowledge of these variations could serve modeling, analysis of MRI measurements in fatwater mixtures, and phantom preparation.