Inge Edler and Carl Hellmuth Hertz at Lund University in Sweden obtained the first recording of the ultrasound echo from the heart. This was the beginning of echocardiography from which diagnostic sonography, or medical ultrasonography, evolved.
"The principle for echocardiography is as follows. The vibrations in a piezoelectric crystal create a beam of high frequency sound waves that are transmitted into the chest. When the waves pass an interface, such as between the heart wall and the surrounding area or the surface of a cardiac valve, some of the sound is reflected, creating an echo. The crystal is reset, enabling it to receive the echo. The longer it took for the echo to return to the crystal, the longer the distance between the crystal and the surface that was the source of the echo. The principle was the same as for sonar, used to measure the depth of water under a vessel, only in this case you measure the distance from the structure that is the source of the echo to the chest wall."
Edler, Inge & Hertz, Carl Hellmuth. The Use of the Ultrasonic Reflectoscope for Continuous Recording of the Movements of Heart Walls. K. Fysiogr. Sellsk. Lund. Foresch., 24 (1954) 1-19.