Variations in T(2)* and fat content of murine brown and white adipose tissues by chemical-shift MRI

Abstract

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 ob/ob mice.

Materials and methods: A group of lean male mice (n=6) and two groups of ob/ob 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 ob/ob groups. The ob/ob animals exhibited longer BAT T(2)* and greater PDFF than lean animals. However, only BAT PDFF was significantly different (P<.01) between the two ob/ob 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 ob/ob 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 ob/ob 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.

Figure 1

Coronal, sagittal, and axial water (w)-only and fat (f)-only images in a lean mouse. Solid arrows highlight the interscapular BAT depot that resides on the dorsal side of the animal, inferior to the shoulder and fore limbs. In the coronal and sagittal views, the depot appears triangular in shape. In the axial view, the unique trapezoid shape is also unmistakable along the posterior (dorsal, top edge of image) side of the animal. Dotted arrow points to the gonadal WAT depot located on the ventral side of the animal.

Figure 2

Scatter plots of T₂* and PDFF for interscapular BAT (left) and gonadal WAT (right) for the lean, obese-4, and obese-8 mice groups. Note the vertical PDFF scale of the WAT plot.

Figure 3

Coronal single slices illustrating differences in T₂* and PDFF between the three mice groups. Arrows in each pair of images point to the same BAT location. Note that the triangular shape of the interscapular BAT depot is clearly visible in all three PDFF maps. However, based on the signal contrast in the T₂* maps, the depot is only visually identifiable in the lean and obese-4 examples. It is indistinguishable from surrounding tissues in the obese-8 example. Note the near absence of WAT in the lean mouse. Enlargements of the BAT depot are also shown along the bottom row, with a white outline drawn about the perimeter on the T₂* maps.