How does ultrasound let us see inside the body?

The answer

Ultrasound imaging works by sending high-frequency sound waves (1–18 MHz, far above human hearing) into the body using a transducer. When these waves hit boundaries between tissues of different densities, they bounce back as echoes. The machine measures the time delay and intensity of returning echoes to construct a real-time image. Unlike X-rays, ultrasound uses no ionizing radiation, making it safe for imaging fetuses. Modern 4D ultrasound can even create real-time three-dimensional video of an unborn baby moving.