Inaudible sound in the frequency range of approx. 20,000 to 10 billion (109) cycles/sec. Ultrasound has different velocities that differ in density and elasticity from one kind of tissue to the next. This property permits the use of ultrasound in outlining the shape of various tissues and organs in the body. In obstetrics, for example, identifying the size and position of the fetus, placenta, and umbilical cord enables estimation of gestational age, detects some fetal anomalies and fetal death, and facilitates other diagnostic procedures, e.g., amniocentesis. In physical therapy, the thermal effects of ultrasound are used to treat musculoskeletal injuries by warming tissue, increasing tissue extensibility, and improving local blood flow. Ultrasound is used to facilitate movement of certain medications, e.g., pain relievers, into tissue (phonophoresis). Ultrasound is also used with electric current for muscular stimulation.

Resources used:

For Farlex Medical Dictionary:
ultrasound. (n.d.) Medical Dictionary. (2009). Retrieved March 4 2022 from Link