Distribution of Brown Adipose Tissue Radiodensity in Young Adults: Implications for Cold [F]FDG-PET/CT Analyses.

Purpose: Nowadays, 2-deoxy-2-[F]fluoro-D-glucose ([F]FDG) positron emission tomography (PET)/X-ray computed tomography (CT) is considered the best available technique to in vivo determination of human BAT volume. The most used Hounsfield unit (HU) threshold for BAT quantification is from - 250 to - 50 HU. Therefore, the main objective of the present study is (i) to examine the influence of SUV and HU thresholds on BAT quantification by [F]FDG-PET/CT scan, (ii) to identify the proportion of BAT which is not detected by [F]FDG-PET/CT scan when limiting the range between - 10 and - 50 HU, and (iii) to describe the distribution of BAT radiodensity by weight status and sex in young healthy individuals.

Procedures: We measured 125 individuals after a personalized cooling protocol with a static [F]FDG-PET/CT scan. We quantified BAT using different combination of threshold in every single HU for all participants.

Results: We observed that the SUV threshold influences BAT quantification by [F]FDG-PET/CT scans more than the HU range. We found that the range from - 50 to - 10 HU had the highest proportion of total BAT volume (43.2 %), which represents 41.4 % of the total BAT metabolic activity in our cohort. We also observed that BAT volume was not different between categories of body mass index, as well as BAT activity (SUV). In addition, BAT was less dense in women than in men, although the BAT activity (SUV) was higher in all ranges of HU. We also observed that the radiodensity of BAT located in the cervical area was mainly in the range from - 50 to - 10 HU.

Conclusion: Therefore, all future human studies using static [F]FDG-PET/CT scans should include BAT in the radiodensity range from - 50 to - 10 HU.