 In addition to showing anatomic structures, functional ultrasound shows color image maps that can indicate the softness or hardness of specific tissues, movement and velocity of tissue or blood, and other physical characteristics. One type of functional ultrasound is Doppler ultrasound. If you've ever listened to how a siren sounds as it approaches, passes and drives away, you are hearing what's called the Doppler effect. The waveform is compressed as the ambulance approaches, making it sound higher pitched, and it's stretched as the ambulance departs, making it sound lower. Doppler ultrasound uses the same principle to determine the speed and direction of the blood inside your arteries. The transducer is held at an angle to the blood vessel, and the altered frequency of the echo returning to the transducer from the blood flow tells the computer the blood speed and direction. This information is displayed in different colors on the final image to show changes in or absence of blood flow, indicating blocked or narrowing blood vessels, decreased circulation, heart valve defects, blood clots, bulging arteries, or even the presence of new blood vessels and flow in tumors. Another form of functional ultrasound, elastography, can be used to differentiate tumors from healthy tissue based on the tissue's relative stiffness. Healthy tissue and benign tumors tend to be compressible, unlike malignant tumors, which are more firm. This compression was originally done manually, but newer elastography systems send out high-pressure pulses that compress target tissues by a predefined amount of force and display the resulting compression levels in various coded colors.