Diffusion weighted imaging (DWI) has primarily been discussed for its use in identifying acute infarcts. One of the additional main uses for DWI is in characterizing cystic lesions. The main differential considerations in this case are necrotic tumor versus abscess. Usually, brain abscesses have restricted diffusion and necrotic primary tumors or metastases do not. Magnetic resonance (MR) spectroscopy can supplement the DWI findings in equivocal cases or for a more definitive diagnosis. It has been suggested that high apparent diffusion coefficient (ADC) in abscesses may be due to a response to antibiotics. DWI is also commonly used to differentiate epidermoid cysts (restricted diffusion) from arachnoid cysts (no restricted diffusion). With increased clinical experience, many other causes of restricted diffusion and uses for DWI have been identified.

Causes of Restricted Diffusion

Common Causes

• Epidermoid
  
• Infarct (both arterial and venous) Acute, subacute, generally up to 7 days
  
• Abscess Except cysticercosis and toxoplasmosis

Less Common Causes