Hibernomas

• rare, benign tumor composed of brown (immature) fat
• slow growing, painless soft tissue mass
• 4 subtypes: typical, myxoid, lipoma-like, spindle cell
• 3rd-4th decade of life
• slight female predominance
• US: well circumscribed, hyperechoic, hypervascular
• CT: intermuscular mass with density between that of fat and muscle
• MR: appearance varies based on subtype; branching vessels are often seen; T1WI - usually hyperintense to muscle but hypointense to subcutaneous fat, prominent septa; T2WI - variable
• NM: moderate uptake on blood pool images in bone scintigraphy
• PET: increased FDG uptake 
• Angiography: hypervascular, intense blush, arteriovenous shunting; do not do a core biopsy
• treatment: surgical resection

REFERENCES

Murphey MD, Carroll JF, Flemming DF, et al. Benign musculoskeletal lipomatous lesions. Radiographics 2004;24:1433-66.