Repeatability of brown adipose tissue measurements on FDG PET/CT following a simple cooling procedure for BAT activation.

Brown Adipose Tissue (BAT) is present in a significant number of adult humans and can be activated by exposure to cold. Measurement of active BAT presence, activity, and volume are desirable for determining the efficacy of potential treatments intended to activate BAT. The repeatability of 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) measurements of BAT presence, activity, and volume under controlled conditions has not been extensively studied. Eleven female volunteers underwent double baseline FDG PET imaging performed following a simple, regional cold intervention intended to activate brown fat. The cold intervention involved the lightly-clothed participants intermittently placing their feet on a block of ice while sitting in a cooled room. A repeat study was performed under the same conditions within a target of two weeks. FDG scans were obtained and maximum standardized uptake value adjusted for lean body mass (SULmax), CT Hounsfield units (HU), BAT metabolic volume (BMV), and total BAT glycolysis (TBG) were determined according to the Brown Adipose Reporting Criteria in Imaging STudies (BARCIST) 1.0. A Lin's concordance correlation (CCC) of 0.80 was found for BMV between test and retest imaging. Intersession BAT SULmax was significantly correlated (r = 0.54; p < 0.05). The session #1 mean SULmax of 4.92 ± 4.49 g/mL was not significantly different from that of session #2 with a mean SULmax of 7.19 ± 7.34 g/mL (p = 0.16). BAT SULmax was highly correlated with BMV in test and retest studies (r ≥ 0.96, p < 0.001). Using a simplified ice-block cooling method, BAT was activated in the majority (9/11) of a group of young, lean female participants. Quantitative assessments of BAT SUL and BMV were not substantially different between test and retest imaging, but individual BMV could vary considerably. Intrasession BMV and SULmax were strongly correlated. The variability in estimates of BAT activity and volume on test-retest with FDG should inform sample size choice in studies quantifying BAT physiology and support the dynamic metabolic characteristics of this tissue. A more sophisticated cooling method potentially may reduce variations in test-retest BAT studies.

Measurement of Brown Adipose Tissue Activity Using Microwave Radiometry and 18F-FDG PET/CT.

The aim of this study was to evaluate the operating characteristics of a microwave radiometry system in the noninvasive assessment of activated and nonactivated brown adipose tissue (BAT) and normal-tissue temperatures, reflecting metabolic activity in healthy human subjects. The radiometry data were compared with 18F-FDG PET/CT images in the same subjects. Methods: Microwave radiometry and 18F-FDG PET/CT were sequentially performed on 19 participants who underwent a cold intervention to maximize BAT activation. The cold intervention involved the participants' intermittently placing their feet on an ice block while sitting in a cool room. Participants exhibiting BAT activity qualitatively on PET/CT were scanned again with both modalities after undergoing a BAT minimization protocol (exposure to a warm room and a 20-mg dose of propranolol). Radiometry was performed every 5 min for 2 h before PET/CT imaging during both the warm and the cold interventions. A grid of 15-20 points was drawn on the participant's upper body (data were collected at each point), and a photograph was taken for comparison with PET/CT images. Results: PET/CT identified increased signal consistent with BAT activity in 11 of 19 participants. In 10 of 11 participants with active BAT, radiometry measurements collected during the cold study were modestly, but significantly, higher on points located over areas of active BAT on PET/CT than on points not exhibiting BAT activity (P < 0.01). This difference lessened during the warm studies: 7 of 11 participants showed radiometry measurements that did not differ significantly between the same set of points. The mean radiometry result collected during BAT maximization was 33.2°C ± 1.5°C at points designated as active and 32.7°C ± 1.3°C at points designated as inactive (P < 0.01). Conclusion: Passive microwave radiometry was shown to be feasible and, with substantial improvements, has the potential to noninvasively detect active brown adipose tissue without a radiotracer injection.