Crohn's disease. Power Doppler ultrasound scan of abnormally increased blood flow (orange) in a 26- year-old man's inflamed colon (large intestine). The colon is seen in cross-section. The black circle is the thickened walls of this tube, which is about 4 centimetres across. These are symptoms of Crohn's disease, inflammation of the digestive tract for unknown reasons. Treatment includes changes in diet and use of corticosteroid drugs. 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.