Abscess: make it swirl

Answer b. Cobblestoning is oedema between fat lobules in inflamed tissue, the typical pattern of cellulitis. An abscess shows a discrete collection. A lymph node is oval with a bright fatty centre. A haematoma is a collection, often after trauma or anticoagulation.

Answer a. Movement under gentle pressure shows liquid contents, even when the echoes make the collection look grey. Solid tissue, lymph nodes and lipomas do not swirl.

Answer c. A femoral pseudoaneurysm or a vessel mimics an abscess in the groin. Colour Doppler shows flow inside a vascular lesion. Needling one causes major bleeding. In the groin, many departments ask for specialist input before drainage.

Answer d. An oval structure with a bright fatty hilum and vessels entering through it is a lymph node. An abscess has no flow inside. A pseudoaneurysm shows swirling flow in a sac connected to an artery. A sebaceous cyst sits in the skin and shows no internal flow.

Answer a. The high-frequency linear probe gives the best detail of skin and subcutaneous tissue. Switch to a curvilinear probe only for deep collections, such as in the buttock or a large thigh.