Chemical shift‐based water/fat separation in the presence of susceptibility‐induced fat resonance shift

Abstract

Chemical shift-based water/fat separation methods have been emerging due to the growing clinical need for fat quantification in different body organs. Accurate quantification of proton-density fat fraction requires the assessment of many confounding factors, including the need of modeling the presence of multiple peaks in the fat spectrum. Most recent quantitative chemical shift-based water/fat separation approaches rely on a multi-peak fat spectrum with pre-calibrated peak locations and pre-calibrated or self-calibrated peak relative amplitudes. However, water/fat susceptibility differences can induce fat spectrum resonance shifts depending on the shape and orientation of the fatty inclusions. The effect is of particular interest in the skeletal muscle due to the anisotropic arrangement of extracellular lipids. In the present work, the effect of susceptibility-induced fat resonance shift on the fat fraction is characterized in a conventional complex-based chemical shift-based water/fat separation approach that does not model the susceptibility-induced fat resonance shift. A novel algorithm is then proposed in order to quantify the resonance shift in a complex-based chemical shift-based water/fat separation approach that considers the fat resonance shift in the signal model, aiming to extract information about the orientation/geometry of lipids. The technique is validated in a phantom and preliminary in vivo results are shown in the calf musculature of healthy and diabetic subjects.

Comparison of dixon and T1‐weighted MR methods to assess the degree of fat infiltration in duchenne muscular dystrophy patients

“…Accurate fat quantification in chemical shift‐based water‐fat separation can also be influenced by other factors such as the difference in T1 values between fat and water (which itself is dependent on the fat/water fraction in a particular voxel) if the images are T1‐weighted (). By choosing a low flip angle and a long TR in this study, T1 effects were minimized ().…”

Chemical shift-based MRI to measure fat fractions in dystrophic skeletal muscle

“…A limitation of this study was the use of a long initial TE and only two subsequent echoes acquired for fat–water discrimination. Consequently, the analysis of the imaging data is complicated by the effects of susceptibility on lipid chemical shift , T 2 * , and noise bias . This could potentially explain the larger variability at low FFs, especially in the 5‐degree MRI acquisitions.…”

Quantitative MRI and spectroscopy of bone marrow

“…Mean bone marrow fat spectra have been also derived from constrained peak fitting of single‐voxel MRS measurements in bone marrow of the spine and the proximal femur . Susceptibility‐induced fat resonance shift effects have not been reported in bone marrow applications. Second, T 1 bias effects can be corrected using estimated T 1 values and flip angle mapping techniques .…”