A doctor uses ultrasound waves to scan a patient's abdomen. Explain how ultrasound waves are used to produce an image of internal organs, and why ultrasound is preferred over X-rays for this type of scan.

OCR A-Level Physics B: Advancing Physics (H557) — 5.3 Astrophysics and cosmology · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Ultrasound waves have a frequency higher than 20 kHz, above the upper limit of human hearing. When used in medical scanning, a device placed on the skin both emits and detects ultrasound waves.

Model answer (5 marks)

Ultrasound waves are emitted from a transducer placed on the skin. At each interface between tissues of different acoustic impedance, part of the wave is reflected and part is transmitted. The reflected waves return to the transducer, which acts as a receiver. The time interval between emission and reception is measured. Because the speed of sound in soft tissue is essentially constant (~1540 m s⁻¹), this time gives the distance to the reflecting boundary (distance = ½ speed × time). By scanning across the body and recording many such distances, a two‑ or three‑dimensional image of the internal organs is constructed. Ultrasound is preferred to X‑rays for abdominal scans because it does not involve ionising radiation; it is non‑invasive and does not damage cells or increase cancer risk, whereas X‑rays can cause cell damage and mutations.

Examiner tips

  • Mention reflection at tissue boundaries and transmission to next boundary
  • Explain time‑of‑flight calculation to give depth
  • State safety advantage: no ionising radiation, no cell damage

Common mistakes

  • Forgetting that the transducer acts as both emitter and receiver
  • Confusing speed of sound with speed of light
  • Overlooking that ultrasound is safe because it is non‑ionising

Mark scheme (5 marks)

  1. Ultrasound waves are partially reflected at a boundary between two different media (e.g. between different tissues or organs)
  2. The remainder of the waves continue to pass through (are transmitted) to the next boundary
  3. A receiver (detector) next to the emitter detects the reflected waves and measures the time between emission and detection
  4. Because the speed of ultrasound is constant, the time taken allows the depth/distance of each boundary to be calculated, building up an image
  5. X-rays can cause cell damage and mutations (potentially causing cancer), whereas ultrasound is non-invasive and does not damage cells, making it safer for the patient

Key terms in this question

ultrasound waves · X-rays

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