Hybrid PET/MRI as a tool to detect brown adipose tissue: Proof of principle

The purpose of this study was to assess the performance of (18)F-FDG hybrid PET/MRI to detect and localise the presence of metabolically active brown adipose tissue (BAT).

We retrospectively analyzed 197 consecutive (18)F-flurodeoxyglucose ((18)F-FDG) positron-emission tomographic (PET) and magnetic resonance imaging (MRI) images performed with a hybrid whole-body PET-MRI tomography in 192 patients. These patients were originally investigated mainly for oncological staging, in the absence of a cooling protocol. The presence of BAT was defined as a soft tissue structure that was larger than 4mm in diameter, had the characteristics of fat tissue on MRI and had a maximal standardised uptake value (SUV) of (18)F-FDG of at least 2.0. No specific MRI sequences for BAT detection were acquired.

PET/MRI identified the presence of metabolically active BAT in 5 out of 192 patients (2.6%). BAT positive subjects were all female, significantly younger and with significantly lower body weight than BAT negative subjects.

Whole body hybrid PET/MRI allowed for the identification of BAT, with a low prevalence, comparable to previous retrospective PET/CT studies realised in the absence of a cooling protocol. The main advantages of the PET/MRI hybrid technique, as compared with PET/CT, includes a lower radiation burden, and the possibility to combine a multiparameter fat characterization by dedicated MRI sequences. Hybrid PET/MRI might represent the ideal tool for BAT evaluation.