A doctor uses ultrasound waves to scan a patient for a kidney stone. The ultrasound pulse travels into the body and is partially reflected at the boundary between soft tissue and the kidney stone, returning to a receiver on the skin. Explain how the doctor is able to detect the kidney stone using this method, and explain why ultrasound is preferred over X-rays for this type of repeated scanning.

Ultrasound waves are generated by a transducer and travel through the body. When the wave reaches the interface between soft tissue and a kidney stone, part of the wave is reflected back because the acoustic impedance of the stone is different from that of the tissue.

The speed of the ultrasound in soft tissue is essentially constant (≈ 1540 m s⁻¹). By measuring the time interval between the emission of the pulse and the detection of the reflected echo, the doctor can calculate the distance to the stone using d = ½ v t.

The receiver records many such echoes from different depths and angles; these signals are processed to form a two‑dimensional image, allowing the stone’s position and size to be visualised.

X‑rays are ionising radiation; their high frequency and energy can break chemical bonds in DNA, causing mutations and increasing cancer risk. Ultrasound, being non‑ionising, does not damage cells, making it safe for repeated examinations.

Thus the doctor detects the stone by measuring reflected ultrasound echoes and prefers ultrasound because it is harmless and can be used repeatedly without radiation risk.