Longitudinal relaxation in fat-water mixtures and its dependence on fat content at 3 T

Longitudinal (T-1) relaxation of triglyceride molecules and water is of interest for fat-water 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 fat-water mixtures, and phantom preparation.