Diagnostic Medical Sonography (ultrasonography) is an imaging technique whereby high-frequency acoustic energy is transmitted into the human body using transducers in contact with the skin. The ultrasound waves produced by the transducer reflect from boundaries between organs and surrounding fluid, and between regions of differing tissue density and are used to visualise subcutaneous body structures including tendons, muscles, joints, vessels and internal organs for possible pathology.
Obstetric sonography is routinely employed during pregnancy to observe the condition and behavior of fetuses prior to birth and there are a numerous diagnostic and therapeutic applications practiced in medicine.
In physics the term 'ultrasound' applies to all acoustic energy with a frequency above human hearing (20 kilohertz). Typical diagnostic sonographic scanners operate in the frequency range of 2 to 18 megahertz, hundreds of times greater than the limit of human hearing.