A doctor uses ultrasound to scan a patient's liver. The ultrasound waves travel through the body and are partially reflected at boundaries between different tissues. Explain how this technique produces information about the position of structures inside the body, and explain why ultrasound is preferred over X-rays for this type of soft-tissue imaging.

Ultrasound waves are emitted by a transducer and travel through the body. When they reach a boundary between two tissues with different acoustic impedances, part of the wave is reflected back to the transducer. The reflected waves are detected by the same transducer acting as a receiver. The time interval between the emission of the pulse and the detection of the echo is measured. Because the speed of sound in soft tissue is known (≈1540 m s⁻¹), the distance to the reflecting structure can be calculated using d=½vt. By recording echoes from many points, a picture of the internal structures is constructed.

Ultrasound is preferred to X‑rays for soft‑tissue imaging because X‑rays are ionising radiation that can damage DNA and increase cancer risk, whereas ultrasound uses non‑ionising mechanical waves and poses no such risk.