PET/MRI of glucose metabolic rate, lipid content and perfusion in human brown adipose tissue

Abstract

This study evaluated the MRI-derived fat fraction (FF), from a Cooling-reheating protocol, for estimating the cold-induced brown adipose tissue (BAT) metabolic rate of glucose (MR_glu) and changes in lipid content, perfusion and arterial blood volume (V_A) within cervical-supraclavicular fat (sBAT). Twelve volunteers underwent PET/MRI at baseline, during cold exposure and reheating. For each temperature condition, perfusion and V_A were quantified with dynamic [¹⁵O]water-PET, and FF, with water-fat MRI. MR_glu was assessed with dynamic [¹⁸F]fluorodeoxyglucose-PET during cold exposure. sBAT was defined using anatomical criteria, and its subregion sBAT_HI, by MR_glu > 11 μmol/100 cm³/min. For all temperature conditions, sBAT-FF correlated negatively with sBAT-MR_glu (ρ ≤ - 0.87). After 3 h of cold, sBAT-FF decreased (- 2.13 percentage points) but tended to normalize during reheating although sBAT_HI-FF remained low. sBAT-perfusion and sBAT-V_A increased during cold exposure (perfusion: + 5.2 ml/100 cm³/min, V_A: + 4.0 ml/100 cm³). sBAT-perfusion remained elevated and sBAT-V_A normalized during reheating. Regardless of temperature condition during the Cooling-reheating protocol, sBAT-FF could predict the cold-induced sBAT-MR_glu. The FF decreases observed after reheating were mainly due to lipid consumption, but could potentially be underestimated due to intracellular lipid replenishment. The influence of perfusion and V_A, on the changes in FF observed during cold exposure, could not be ruled out.