A subfield of acoustics dealing in the radio frequency range higher than SOUND waves (beyond 20 kilohertz), but usually restricted to frequencies above 500,000 Hertz. Ultrasonic radiation is used therapeutically (DIATHERMY and ULTRASONIC THERAPY) to generate HEAT and to selectively destroy tissues. It is also used in diagnostics, for example, ULTRASONOGRAPHY; ECHOENCEPHALOGRAPHY; and ECHOCARDIOGRAPHY, to visually display echoes received from irradiated tissues.