Endocrine Abstracts

Background: Imaging of brown adipose tissue (BAT) is currently performed with combined positron emission tomography and x-ray computed tomography (PET-CT). Recent studies showed promising imaging features of BAT with magnetic resonance imaging (MRI) through modified 2-point-dixon (mDixon) water-fat separation method. The aim of this study was to establish a reliable MRI-procedure for quantification of BAT volumes and weighted fat-fraction (WFF) and to correlate them to cold-induced thermogenesis (CIT).

Methods: 23 healthy volunteers (13 males, 10 females, age 18–47 years) were recruited for this prospective, observational study. Energy expenditure (EE) was measured in supine position by indirect calorimetry during warm conditions and after a mild cold stimulus of 120 minutes. CIT was defined as the difference between EE during cold (EE_cold) and warm (EE_warm) conditions. MRI was performed on a Siemens MAGNETOM Prisma 3T using mDixon water-fat separation method and T2* relaxation time. MRI was performed twice, once during warm conditions and once during mild cold exposure with a cooling vest. Volumes of supraclavicular BAT-depots (_scBAT) and WFF were calculated by manual segmentation using ITK-Snap. Primary Endpoint was correlation of _scBAT volume with CIT. Data were analysed in R/R-Studio and are given as mean ± standard deviation.

Results: Mean Volume of both _scBAT was 71.4 ml ±39.4 ml. Mean WFF was 74.2%±6%. Mean EE_warm was 1638 kcal/day ±362 kcal/day and EE_cold 1794 kcal/day ±408 kcal/day. Mean CIT was 156 kcal/day ±133 kcal/day respectively 8.5%±9.6% of EE_warm. Contrary to our assumption, we found no correlation between the volume of _scBAT and CIT (r=0.05, P=0.87) or between WFF and CIT (r=0.13, P=0.66). _scBAT volume correlated however significantly with increasing age (r=0.59, P=0.003), body mass index (r=0.53, P < 0.0001) and WFF (r=0.39, P=0.0016).

Conclusion: Despite the correlation between MRI and glucose uptake of human brown adipose tissue in recent studies, our study found no correlation between the volume and WFF of BAT to cold induced, non-shivering thermogenesis. Thus, MRI without tracer-based molecular imaging is currently not a promising tool for determining the metabolic activity of human brown adipose tissue.