Abstract:
Objective To investigate and analyze the imaging manifestations and rules of brown adipose tissue (BAT) uptake in 18F-fluorodeoxyglucose (FDG) PET/MRI.
Methods The imaging and clinical data of 1 529 subjects (836 males and 693 females, aged 14–93(53.6±13.2) years) who underwent 18F-FDG PET/MRI examination in the Second Affiliated Hospital of Air Force Medical University from July 2017 to January 2020 were analyzed retrospectively. Thirty-one subjects, including 13 males and 18 females aged 16–61 (33.3±11.6) years, had positive BAT uptake of 18F-FDG. The PET/MRI imaging features of 18F-FDG uptake of BAT were analyzed, and three-dimensional sketching was adopted to select sites with increased 18F-FDG uptake to delineate the region of interest (with the maximum standardized uptake value (SUVmax) of surrounding muscles as the boundary value). SUVmax and the mean standardized uptake value (SUVmean) were calculated automatically by the software program. The correlation between SUVmax and SUVmean with variable factors(eg. gender, age, body mass index (BMI), mean temperature on the examination day and number of ingestion sites) were evaluated by Spearman or Pearson correlation analysis. Count data between groups was compared by χ2 test. Measurement data were compared by independent sample t test.
Results (1) The PET/MRI images of subjects with positive BAT uptake of 18F-FDG showed symmetrical flaky, nodular and bead uptake in sucravicula, cervical regions, and paravertebral, mediastinal, and perirenal mediastinum. T1-weighted imaging (WI) and T2WI of the MRI images showed high signal. The frequency attenuation inversion recovery sequence showed low signal. The diffusion weighted imaging showed no high signal. (2) Most BAT uptake occurred in cold periods, and no significant difference was found in the positive rate of BAT uptake between males and females (χ2=2.07, P=0.15). The age, BMI, and the temperature on the day of examination of the subjects with positive uptake of BAT were significantly lower than those with negative uptake ((33.3±11.7) years vs. (54.1±13.5) years, (21.89±2.79) kg/m2 vs. (24.01±3.26) kg/m2, (7.5±6.5)℃ vs. (16.5±11.9)℃), and the differences were statistically significant (t=−12.03, −5.15, −8.97; all P<0.001). (3) SUVmax and SUVmean of BAT were negatively correlated with age and BMI (r=−0.45 to −0.36; all P<0.05) but positively correlated with the number of BAT depots (r=0.61, 0.59; both P<0.001). SUVmax and SUVmean of BAT had no significant correlation with gender or the average temperature on the day of examination (r=0.01−0.29; all P>0.05).
Conclusion In 18F-FDG PET/MRI imaging, BAT uptake has specific imaging manifestations and regularity, which mostly occurs in cold season and is more likely to occur in young and low-BMI subjects.