Variations in T2* and fat content of murine brown and white adipose,A tissues by chemical-shift MRI

Purpose: The purpose was to compare T-2* relaxation times and proton density fat-fraction (PDFF) values between brown (BAT) and white (WAT) adipose tissue in lean and oblob mice. Materials and Methods: A group of lean male mice (n=6) and two groups of oblob male mice placed on similar 4-week (n=6) and 8-week (n=8) ad libitum diets were utilized. The animals were imaged at 3 T using a T-2*-corrected chemical-shift-based water fat magnetic resonance imaging (MRI) method that provides simultaneous estimation of T-2* and PDFF on a voxel-wise basis. Regions of interest were drawn within the interscapular BAT and gonadal WAT depots on co-registered T-2* and PDFF maps. Measurements were assessed using analysis of variance, Bonferroni-adjusted t test for multigroup comparisons and the Tukey post hoc test. Results: Significant differences (P<.01) in BAT T-2* and PDFF were observed between the lean and oblob groups. The ob/ob animals exhibited longer BAT T2* and greater PDFF than lean animals. However, only BAT PDFF was significantly different (P<.01) between the two oh/oh groups. When comparing BAT to WAT within each group, T-2* and PDFF values were consistently lower in BAT than WAT (P<.01). The difference was most prominent in the lean animals. In both oblob groups, BAT exhibited very WAT-like appearances and properties on the MRI images. Conclusion: T-2* and PDFF are lower in BAT than WAT. This is likely due to variations in tissue composition. The values were consistently lower in lean mice than in oblob mice, suggestive of the former's greater demand for BAT thermogenesis and reflective of leptin hormone deficiencies and diminished BAT metabolic activity in the latter. (C) 2012 Elsevier Inc. All rights reserved.