Effect of microscopic susceptibility gradients on chemical-shift-based fat fraction quantification in supraclavicular fat

Abstract

Background: Susceptibility differences between fat and water can cause changes in the water-fat frequency separation that can negatively affect the accuracy of fat fraction techniques. This may be especially relevant for brown adipose tissue, as MRI fat fraction techniques have been proposed for its detection.

Purpose: To assess the effect of microscopic magnetic susceptibility gradients on the water-fat frequency separation and its impact on chemical-shift-based fat fraction quantification techniques in the supraclavicular fat, where brown adipose tissue is commonly found in humans.

Study type: Prospective.

Population/subjects/phantom/specimen/animal model: Subjects: 11 healthy volunteers, mean age of 26 and mean BMI of 23, three overweight volunteers, mean age of 38 and mean BMI of 33. Phantoms: bovine phantom and intralipid fat emulsion. Simulations: various water-fat distributions.

Field strength/sequence: Six-echo gradient echo chemical-shift-encoded sequence at 3T.

Assessment: Fat fraction values as obtained from a water-fat spectral model accounting for susceptibility-induced water-fat frequency variations were directly compared to traditional spectral models that assume constant water-fat frequency separation.

Statistical tests: Two-tail t-tests were used for significance testing (p < 0.05.) A Bayesian Information Criterion difference of 6 between fits was taken as strong evidence of an improved model.

Results: Phantom experiments and simulation results showed variations of the water-fat frequency separation up to 0.4 ppm and 0.6 ppm, respectively. In the supraclavicular area, the water-fat frequency separation produced by magnetic susceptibility gradients varied by as much as ±0.4 ppm, with a mean of 0.08 ± 0.14 ppm, producing a mean difference in fat fraction of -1.26 ± 5.26%.

Data conclusion: In the supraclavicular fat depot, microscopic susceptibility gradients that exist within a voxel between water and fat compartments can produce variations in the water-fat frequency separation. These variations may produce fat fraction quantification errors of 5% when a spectral model with a fixed water-fat frequency separation is applied, which could impact MR brown fat techniques.

Level of evidence: 1 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;49:141-151.

Keywords: brown adipose tissue; chemical shift encoding; fat fraction; imaging; magnetic susceptibility.