A doctor explains to a patient that an ultrasound scan will be used to check for a suspected abnormality in their liver. Explain how ultrasound waves are used to produce an image of the liver and detect any abnormality within it.
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 imaging, a probe placed against the skin both emits ultrasound waves into the body and detects any waves that return.
Model answer (5 marks)
Ultrasound waves are emitted from a probe placed on the skin and travel into the body. At the interface between two tissues with different acoustic impedance, part of the wave is reflected back while the rest is transmitted further. The probe contains a receiver that detects the reflected waves. By measuring the time taken for a reflected wave to return, the distance to the reflecting interface is calculated using the speed of sound in tissue. A series of such reflections from different depths is displayed as a two‑dimensional image of the liver. If an abnormality such as a tumour or cyst is present, it creates an interface at an unexpected depth or position, producing a distinct echo pattern that differs from normal liver tissue, allowing the abnormality to be identified.
Examiner tips
- Use the word ‘reflection’ and ‘transmission’ to show understanding of wave behaviour.
- Show the time‑distance calculation (t/2 × speed of sound).
- Explain that an abnormality changes the echo pattern at a specific depth.
- Mention that the probe both emits and receives the waves.
Mark scheme (5 marks)
- Ultrasound waves are partially reflected at the boundary between two different media / tissues / organs
- The remainder of the waves continue to pass through (are transmitted) into the next medium
- The reflected waves are detected by the probe / receiver next to the emitter
- The time between emission and detection of reflected waves is used to determine the depth / distance of each boundary
- An abnormality causes a reflection at an unexpected depth / position, producing a different pattern of reflected waves compared to healthy tissue, allowing it to be detected
Key terms in this question
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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