Imaging appearances are non-specific, differentials are broad and include: 

• inflammatory myopathies
  • dermatomyositis
  • polymyositis
  • inclusion body myositis
  • eosinophilic myositis
  • proliferative myositis 
• myositis associated with connective tissue diseases
  • systemic lupus erythematosus (SLE)
  • Sjögren syndrome
  • overlap syndrome
  • scleroderma
  • mixed connective tissue disease

This MRI was performed prior to muscle biopsy, which revealed: 

MICROSCOPIC DESCRIPTION: Paraffin and Cryostat sections show a moderate variation in muscle fiber diameters. There is multifocal endomysial and perimysial infiltration by mononuclear inflammatory cells with multiple foci of myophagia. Inflammatory cells are a mixture of T lymphocytes (CD8>CD4) and monocyte-macrophages. There are no actively degenerating or necrotic fibers. Several regenerating fibers are seen. There is no fiber vacuolation. Three COX negative fibers are noted. There is no evidence of myofibrillar disorganization. ATP'ase staining shows a normal random distribution of fiber types. There is no lipid stasis or excess accumulation of glycogen in muscle fibers and the activity of glycolytic enzymes is normal.

DIAGNOSIS: "Left thigh muscle biopsy": Inflammatory immune myopathy (IIM) with endomysial and perimysial pathology most consistent with polymyositis.

COMMENT: It is noted that the patient has high titers of rheumatoid factor and antinuclear antibodies but no anti-Ro or anti-Jo antibodies. Testing for antibodies against signal recognition particle (SRP) is recommended. Immunohistochemistry for MHC Class I and II and C5b-9 (MAC) is pending.