Ultrasound or ultrasonography is a medical imaging technique that uses high frequency sound waves and their echoes. The technique is similar to the echolocation used by bats, whales and dolphins, as well as SONAR used by submarines. In ultrasound, the following events happen:
The ultrasound machine transmits high-frequency (1 to 5 megahertz) sound pulses into your body using a probe.
The sound waves travel into your body and hit a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone).
Some of the sound waves get reflected back to the probe, while some travel on further until they reach another boundary and get reflected.
The reflected waves are picked up by the probe and relayed to the machine.