False aneurysm. Colour Doppler ultrasound scan of a false (or pseudo) aneurysm (blue and red) in a 78-year-old man. The false aneurysm is seen here as a circular eddy of blood, with blood moving towards (red) and away (blue) from the detector. It is circulating in a false cavity outside the blood vessel (not seen). This phenomenon usually occurs after trauma to an artery, such as surgery. A true aneurysm is a swelling in the artery wall. Colour Doppler ultrasound scanning colour-codes moving fluids, such as blood, by detecting the frequency change they cause when reflecting high- frequency sound waves.