In suspected septic arthritis, which imaging modality is used to guide drainage?

• A. X-ray to identify effusion and guide drainage.
• B. CT scan to identify effusion and guide drainage.
• C. MRI to identify soft tissue involvement.
• D. Ultrasound to identify effusion and guide aspiration.

Answer: (D)

The main idea is using real-time visualization to guide needle placement for drainage. Ultrasound lets you see the joint effusion as you advance the needle, so you can aspirate accurately and safely while avoiding surrounding structures. It’s fast, can be done at the bedside, and doesn’t expose the patient to radiation, making it ideal for urgent septic arthritis drainage.

X-ray can show an effusion but lacks soft-tissue detail and cannot reliably guide needle entry. CT provides good anatomy and can guide drainage, but involves radiation and is less convenient for quick bedside procedures. MRI offers excellent soft-tissue detail but is not practical for emergent drainage due to time, cost, and availability.