Why are hibernomas FDG avid?
The brown fat in hibernomas has a large number of mitochondria to facilitate thermogenesis (the physiologic role of brown fat). Although hibernomas are FDG avid, PET imaging would not be able to exclude a sarcoma, which would also be avid.
Similar to the CT study, there is a well-marginated mass between the right gluteus maximus and medius muscles. There are prominent signal voids at the lateral aspect of the mass, compatible with feeding vessels. Although slightly hypointense relative to fat on both the T1 and T2-weighted sequences, it is slightly hyperintense on the T1 fat sat sequence (it's about the same intensity as the adjacent muscle).
The mass shows moderate enhancement on the postcontrast images, greater than that of muscle.