Evaluation of (18)F-FDG Uptake Pattern in Brown Adipose Tissue Over Extended Time Period as Assessed by Multiple Time Point (18)F-FDG-PET.

Purpose: To study the (18)F-FDG uptake pattern in brown adipose tissue (BAT) over an extended time period, by multiple-time-point fluorodeoxyglucose positron emission tomography (FDG-PET) imaging. The primary objective for this kind of research was that it could form a basis and may have further implications for obesity research, metabolic diseases and for cachexia of both malignant and benign origin.

Methods: A total of 12 patients who had undergone routine FDG-PET for disease evaluation and had demonstrated prominent BAT uptake in their baseline scans were selected. The patients with the diagnosis of neuroendocrine tumors were excluded. Maximum standardized uptake values (SUVmax) were calculated in the BAT of the supraclavicular and paravertebral areas of either side, and were analyzed separately to examine their behavior individually. Time activity curves (TACs) were generated for [A] BAT SUVmax values and [B] SUVmax ratio of BAT/lung (B/L SUVmax ratio) at various time points.

Results: Ten out of the 12 patients were imaged at four time points, and two patients were imaged for two time points. Amongst a total of n = 30 sites, 23 were imaged at four time points and seven were imaged at two time points. Seventeen out of 30 area sites (56.67 %) demonstrated a peak value at 60 min and a falling trend of SUVmax afterwards; the remaining showed a peak uptake value between 85 and 300 min after the first scan (i.e. 145-360 min after injection), and falling values thereafter. With regard to the B/L SUVmax ratio, ten out of 30 sites (33.33 %) demonstrated peak uptake at 60 min, and the remaining showed a rise, with peak uptake at times between 85 and 300 min after the first scan (i.e. 145-360 min after injection) and falling values thereafter. No additional area of BAT uptake was observed over the extended time period in this study.

Conclusion: Wide variability was observed in the BAT FDG uptake over an extended period of time. Nearly half of the sites demonstrated an increase in FDG uptake until 360 min (i.e. 6 h) after injection, while the remaining half showed peak uptake at 1 h and subsequent fall of uptake. In the future, it will be worthwhile to study whether there exists any difference in time course of FDG uptake in brown fat between patients with cancer and those scanned for benign etiologies, or between obese and non-obese individuals.