Magnetic Resonance Imaging Techniques for Brown Adipose Tissue Detection

Figure 1. (A) Conceptual schematic highlighting morphological differences between BAT and WAT detectable with MR contrast. (B) Conceptual schematic of MR contrast mechanisms for detecting BAT function. A “+” sign indicates an increase a “–” sign indicates a decrease.

Figure 2. Schematic showing morphological differences between BAT and WAT cells. BAT is composed of adipocytes much smaller in size than white adipocytes, and with a cytoplasm occupied by multiple small lipid droplets (multilocular adipocytes), often of less than few micrometers in diameter, distributed uniformly throughout the entire depot. As a result, compared to classical white adipose tissue, where a single fat droplet occupies most of the cell cytoplasm, brown adipocytes appear much more hydrated, with a water content typically ranging between 50 and 70%. The presence of numerous mitochondria inside the BAT cell with UCP1 in the mitochondrial membrane (Figure 3) allows for its vivid metabolism.

Figure 3. Histology images with UCP1 staining for C57BL/6 and obese phenotype mouse. This figure is original and based on data from (41).

Figure 4. Schematic displaying major BAT depots in (A) rodent and (B) human anatomy.

Figure 5. Axial PDFF and T2* maps of human supraclavicular fat. (A) PDFF map calculated using 6 echoes. (B) The T2* map estimated from the 6-echo data set is noisy and includes severely overestimated values in both the subcutaneous region and the supraclavicular fossa. (C) The T2* map estimated by using 20 echoes yields a more robust and smooth fitting result. This figure is original and based on data from (125).

Figure 6. DW-MRS of water component in human supraclavicular fat using a both respiratory and cardiac triggered acquisition scheme. (A) Example of spectral fitting results for a voxel within the supraclavicular fossa. (B) Fitted spectral peak areas for eight fat peaks and water peak for a range of different b-values. (C) ADC fitting results for the water peak. In pink: Data points and fitted mono-exponential decay without fat referencing. In blue: Data points and fitted mono-exponential decay after referencing to the fat peaks, under the assumption that the decay in fat is due only to motion effects at these low b-values. This figure is original and based on data from (141).

Figure 7. Overlay of PET images and MR-based PDFF maps. The red mark indicates regions with SUV > 2.5. This figure is original and based on data from (92).

Figure 8. Schematic figure illustrating different metabolic pathways detectable with NMR. The uptake in glucose can be traced via PET using [18F]-FDG. 31P can be used to monitor ATP production and phosphocreatine metabolism. By injecting [1-13C]-pyruvate, we can follow anaerobic glycolysis via the conversion of pyruvate into lactate or aerobic glycolysis via the detection of a bicarbonate signal. Alternatively, [6,6-2H]-glucose can discern anaerobic from aerobic glycolysis by detecting lactate and the glutamate/glutamine peak. The heat generated in BAT can be detected as a shift in resonance frequency of both water 1H and lipid-dissolved HP-129Xe.

Figure 9. Xenon uptake in BAT of an ob/ob mouse detected by hyperpolarized 129Xe MRI and xenon-enhanced CT. (A) 2D HP 129Xe MRI (yellow) overlaid onto an anatomical 1H image. The HP 129Xe image shows background free maps of iBAT in an ob/ob mouse. This figure is original and based on data from (41). (B) (Non-enhanced) and (xenon-enhanced) coronal CT image of a different ob/ob mouse. Selective xenon uptake in iBAT leads to a remarkable change in tissue radiodensity. This figure is original and based on data from (153).

Figure 10. Temperature change in iBAT as detected by hyperpolarized 129Xe MR thermometry in a C57BL/6 mouse during norepinephrine stimulation of BAT thermogenesis. Right after the injection of norepinephrine, an up-field drift of the temperature-sensitive resonance frequency of xenon dissolved in the lipid droplets of BAT is observed. The frequency drift reflects a change in iBAT temperature of almost 6°C. The change in rectal temperature follows, with some delay, the change in BAT temperature, indicating that BAT is the main driver for the increase in body temperature. This figure is original and based on data from (41).