Liver tumour. Three-dimensional power Doppler ultrasound scan of blood vessels (orange) in a liver tumour in a 29-year-old woman. The tumour, a focal nodular hyperplasia (FNH), contains the network of thin blood vessels at centre. An FNH is a rare and benign type of liver tumour that is not easily distinguished from malignant tumours. It tends to affect young women. The orange flare at lower right is an artefact of gas in the bowels. 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.