Activation of brown adipose tissue (BAT) by cold exposure. BAT thermogenesis and BAT volume of metabolic activity will be assessed by Positron-Emitting-Tomography (PET/CT) and MRI/MRS imaging and new pharmacological methods to modulate BAT thermogenesis. All previous data on the functioning of Brown Adipose Tissue (BAT) were obtained by Positron-Emitting-Tomography (PET) imaging studies using fluorodeoxyglucose F18 ( \[18F\]- FDG). This approach underestimates the actual activity of the BAT. In this study, the investigator is going to use a new PET tracer (C11-palmitate) which is a fat molecule. This will allow to quantify more accurately the activity of brown fat.