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.