Differentiating supraclavicular from gluteal adipose tissue based on simultaneous PDFF and T₂* mapping using a 20-echo gradient-echo acquisition

Background: Adipose tissue (AT) can be classified into white and brown/beige subtypes. Chemical shift encoding-based water–fat MRI-techniques allowing simultaneous mapping of proton density fat fraction (PDFF) and T₂* result in a lower PDFF and a shorter T₂* in brown compared with white AT. However, AT T₂* values vary widely in the literature and are primarily based on 6-echo data. Increasing the number of echoes in a multiecho gradient-echo acquisition is expected to increase the precision of AT T₂* mapping. Purpose: 1) To mitigate issues of current T₂*-measurement techniques through experimental design, and 2) to investigate gluteal and supraclavicular AT T₂* and PDFF and their relationship using a 20-echo gradient-echo acquisition. Study Type: Prospective. Subjects: Twenty-one healthy subjects. Field Strength/Sequence Assessment: First, a ground truth signal evolution was simulated from a single-T₂* water–fat model. Second, a time-interleaved 20-echo gradient-echo sequence with monopolar gradients of neck and abdomen/pelvis at 3 T was performed in vivo to determine supraclavicular and gluteal PDFF and T₂*. Complex-based water–fat separation was performed for the first 6 echoes and the full 20 echoes. AT depots were segmented. Statistical Tests: Mann-Whitney test, Wilcoxon signed-rank test and simple linear regression analysis. Results: Both PDFF and T₂* differed significantly between supraclavicular and gluteal AT with 6 and 20 echoes (PDFF: P < 0.0001 each, T₂*: P = 0.03 / P < 0.0001 for 6/20 echoes). 6-echo T₂* demonstrated higher standard deviations and broader ranges than 20-echo T₂*. Regression analyses revealed a strong relationship between PDFF and T₂* values per AT compartment (R² = 0.63 supraclavicular, R² = 0.86 gluteal, P < 0.0001 each). Data Conclusion: The present findings suggest that an increase in the number of sampled echoes beyond 6 does not affect AT PDFF quantification, whereas AT T₂* is considerably affected. Thus, a 20-echo gradient-echo acquisition enables a multiparametric analysis of both AT PDFF and T₂* and may therefore improve MR-based differentiation between white and brown fat. Level of Evidence: 2. Technical Efficacy: Stage 2. J. Magn. Reson. Imaging 2019;50:424–434.