Quantification of Human and Rodent Brown Adipose Tissue Function Using 99mTc-Methoxyisobutylisonitrile SPECT/CT and 18F-FDG PET/CT

For brown adipose tissue (BAT) to be effective at consuming calories, its blood flow must increase enough to provide sufficient fuel to sustain energy expenditure and also transfer the heat created to avoid thermal injury. Here we used a combination of human and rodent models to assess changes in BAT blood flow and glucose utilization. Methods 99mTc-methoxyisobutylisonitrile (MIBI) SPECT (n=7) and SPECT/CT (n=74) scans done in adult humans for parathyroid imaging were reviewed for uptake in regions consistent with human BAT. Site-directed biopsies of subcutaneous and deep neck fat were obtained for electron microscopy and gene expression profiling. In mice, tissue perfusion was measured with 99mTc-MIBI (n=16) and glucose uptake with 18F-FDG (n=16). Animals were fasted overnight, anesthetized with pentobarbital, and given intraperitoneally either the β3-adrenergic receptor agonist CL-316,243 (CL) 1mg/kg (n=8) or saline (n=8) followed by radiotracer injection 5 minutes later. After 120 minutes, mice were imaged using SPECT/CT or PET/CT. Vital signs were recorded over 30 minutes during the imaging. BAT, white adipose tissue (WAT), muscle, liver, and heart were resected, and tissue uptake of both 99mTc-MIBI and 18F-FDG were quantified by percent injected dose (%ID) per gram of tissue and normalized to per kilogram of total body weight (%ID/g*kg).