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

<bold>Background: </bold>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.<bold>Purpose: </bold>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.<bold>Study Type: </bold>Prospective.<bold>Subjects: </bold>Twenty-one healthy subjects.<bold>Field Strength/sequence Assessment: </bold>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.<bold>Statistical Tests: </bold>Mann-Whitney test, Wilcoxon signed-rank test and simple linear regression analysis.<bold>Results: </bold>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).<bold>Data Conclusion: </bold>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.<bold>Level Of Evidence: </bold>2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;50:424-434. [ABSTRACT FROM AUTHOR]