Inflamed lymph node. Three-dimensional power Doppler ultrasound scan of blood vessels (orange) in an inflamed lymph node in the neck of a 13- year-old boy. The node contains a network of thin blood vessels (centre right). These drain into the thicker jugular vein at left. Inflammation of a lymph node (lymphadenitis) is usually a response to an infection. The increased lymph node activity produces more infection-fighting lymphocyte cells. Doppler ultrasound scanning detects moving fluids, such as blood, by the frequency change they cause when reflecting high-frequency sound waves. Power Doppler scans use colour to show the moving fluid, but do not show the direction of the flow.