Atheroma plaque. Power Doppler ultrasound scan of reduced blood flow (orange) around an atheroma plaque (black) on the wall of the internal carotid artery of a neck. An atheroma plaque is a deposit of fatty material on an artery wall. It can be caused by a high-cholesterol diet, as well as smoking, and increases the risk of a thrombus (clot) forming. Treatment is with drugs or a graft to bypass the affected area. 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.