Abnormal signal intensity in skeletal muscle at MR imaging: Patterns, pearls, and pitfalls

Abnormal signal intensity within skeletal muscle is frequently encountered at magnetic resonance (MR) imaging. Potential causes are diverse, including traumatic, infectious, autoimmune, inflammatory, neoplastic, neurologic, a nd iatrogenic conditions. Alterations in muscle signal intensity seen in pa thologic conditions usually fall into one of three recognizable patterns: m uscle edema, fatty infiltration, and mass lesion. Muscle edema may be seen in polymyositis and dermatomyositis, mild injuries, infectious myositis, ra diation therapy, subacute denervation, compartment syndrome, early myositis ossificans, rhabdomyolysis, and sickle cell crisis. Fatty infiltration may be seen in chronic denervation, in chronic disuse, as a late finding after a severe muscle injury or chronic tendon tear, and in corticosteroid use. The mass lesion pattern may be seen in neoplasms, intramuscular abscess, my onecrosis, traumatic injury, myositis ossificans, muscular sarcoidosis, and parasitic infection. Some of these conditions require prompt medical or su rgical management, whereas others do not benefit from medical intervention. The ability to accurately diagnose these conditions is therefore necessary , and biopsy may be required to establish the correct diagnosis. Clues to t he correct diagnosis and whether biopsy is necessary or appropriate are oft en present on the MR images, especially when they are correlated with clini cal features and the findings from other imaging modalities.