How Does Ultrasound Imaging Work?

A handheld probe called a transducer contains piezoelectric crystals that vibrate to produce sound far above human hearing, typically 2 to 18 megahertz. A gel is applied to the skin to remove air, which would otherwise block the sound. When the waves meet boundaries between tissues of different density, part of the sound reflects back to the probe. A computer measures how long each echo took to return and how strong it was, then converts this into a real-time image. Because it uses no ionising radiation, ultrasound is considered safe for monitoring pregnancies.