Abstract
Quantifying the amount of brown adipose tissue (BAT) within white adipose tissue (WAT) in human depots may serve as a target to combat obesity. We aimed to quantify proton density fat fraction (PDFF) of BAT and WAT in relatively pure and in mixed preparation using water-fat imaging. Three ex-vivo experiments were performed at 3 T using excised interscapular BAT and inguinal/subcutaneous WAT from mice. The first two experiments consisted of BAT and WAT in separate tubes, and the third used mixed preparation with graded quantities of BAT and WAT. To investigate the influence of partial volume on PDFF metrics, low ( 2.66mm32.66mm3<math><mrow><mn>2.66</mn><mtext> </mtext><msup><mrow><mi>mm</mi></mrow><mrow><mn>3</mn></mrow></msup></mrow></math> ) and high spatial resolution ( 0.55mm30.55mm3<math><mrow><mn>0.55</mn><mtext> </mtext><msup><mrow><mi>mm</mi></mrow><mrow><mn>3</mn></mrow></msup></mrow></math> acquired voxels) in two orthogonal three-dimensional sections were compared. The low-resolution acquisitions are corrected for T2* and multipeak lipid spectrum, thus considered "quantitative," whereas the high-resolution acquisitions are not corrected but were performed to better spatially segment BAT from WAT zones. As potential BAT metrics, we quantified the average PDFF and the volume of tissue having PDFF ≤50%≤50%<math><mrow><mo>≤</mo><mn>50</mn><mo>%</mo></mrow></math> ( VOLPDFF≤50%VOLPDFF≤50%<math><mrow><msub><mrow><mi>VOL</mi></mrow><mrow><mi>PDFF</mi><mo>≤</mo><mn>50</mn><mo>%</mo></mrow></msub></mrow></math> ) based on the PDFF histogram. In the first experiment, the average PDFF of BAT was 23±6%23±6%<math><mrow><mn>23</mn><mo>±</mo><mn>6</mn><mo>%</mo></mrow></math> and 21±7.6%21±7.6%<math><mrow><mn>21</mn><mo>±</mo><mn>7.6</mn><mo>%</mo></mrow></math> and the average PDFF of WAT was 76±7%76±7%<math><mrow><mn>76</mn><mo>±</mo><mn>7</mn><mo>%</mo></mrow></math> and 87±7%87±7%<math><mrow><mn>87</mn><mo>±</mo><mn>7</mn><mo>%</mo></mrow></math> using high- and low-resolution techniques, respectively. A similar trend with excellent reproducibility in average PDFF of BAT and WAT was observed in the second experiment. In the third experiment over the four acquisitions, the BAT-dominant tube demonstrated lower PDFF (mean ± SD) of 55±2%55±2%<math><mrow><mn>55</mn><mo>±</mo><mn>2</mn><mo>%</mo></mrow></math> than WAT-dominant (69±4%)(69±4%)<math><mrow><mo>(</mo><mn>69</mn><mo>±</mo><mn>4</mn><mo>%</mo><mo>)</mo></mrow></math> and WAT-only tubes (88±4%)(88±4%)<math><mrow><mo>(</mo><mn>88</mn><mo>±</mo><mn>4</mn><mo>%</mo><mo>)</mo></mrow></math> . Estimating VOLPDFF≤50%VOLPDFF≤50%<math><mrow><msub><mrow><mi>VOL</mi></mrow><mrow><mi>PDFF</mi><mo>≤</mo><mn>50</mn><mo>%</mo></mrow></msub></mrow></math> , the BAT-dominant tube demonstrated higher volume of 0.26cm30.26cm3<math><mrow><mn>0.26</mn><mtext> </mtext><msup><mrow><mi>cm</mi></mrow><mrow><mn>3</mn></mrow></msup></mrow></math> than WAT-dominant ( 0.16cm30.16cm3<math><mrow><mn>0.16</mn><mtext> </mtext><msup><mrow><mi>cm</mi></mrow><mrow><mn>3</mn></mrow></msup></mrow></math> ) and WAT-only tubes ( 0.01cm30.01cm3<math><mrow><mn>0.01</mn><mtext> </mtext><msup><mrow><mi>cm</mi></mrow><mrow><mn>3</mn></mrow></msup></mrow></math> ). The presence of BAT exhibits a lower PDFF relative to WAT, thus allowing segmentation of low PDFF tissue for quantification of volume representative of BAT. Future studies will determine the clinical relevance of BAT volume within human depots.