Which imaging modality is most sensitive for early detection of avascular necrosis of the femoral head?

• A. X-ray
• B. CT
• C. MRI
• D. Ultrasound

Answer: (C)

Magnetic resonance imaging is the most sensitive way to detect avascular necrosis of the femoral head at an early stage. Early AVN involves bone marrow and subchondral changes long before the bone damage is visible on X-rays. MRI picks up these subtle alterations, such as bone marrow edema and the characteristic necrotic interface, often described as the double-line sign on T2 or a serpiginous low-signal line on T1, which signal the presence of necrosis before any collapse occurs. This allows detection before radiographic changes like the crescent sign appear. In comparison, X-ray often looks normal early and only shows later-stage changes; CT can reveal structural detail and help assess collapse but is less sensitive for initial marrow changes; ultrasound doesn’t visualize intraosseous AVN well.