A diagnostic procedure that uses ultrasound at a frequency of 2.5–10 MHz to provide an image of the heart. It is based on the principle that the interface between tissues of different acoustical impedance causes the ultrasound to be reflected to the transducer, which spends a fraction of each second receiving these echoes. There are many interfaces between blood and the various structures in the heart that contact blood, such as the heart walls, valves, and great vessels. Also, the surface of the heart reflects ultrasound because it is surrounded by the lungs, which are filled with air.