# 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.

> AQA A-Level Physics (7408) — 3.3.1 Progressive and Stationary Waves · Explain · 5 marks

## Mark scheme (5 marks)

1. Ultrasound waves are partially reflected at a boundary between two different media/tissues
2. The reflected waves are detected by a receiver/detector (placed next to the emitter)
3. The time between emission and detection of the reflected wave is measured
4. Since the speed of ultrasound is known/constant, the distance to the boundary/structure can be calculated
5. X-rays are ionising radiation which can cause mutation of genes/cancer, whereas ultrasound is non-ionising/does not carry this risk

## Key terms

- [ultrasound waves](https://www.gradenine.co.uk/glossary/ultrasound-waves)
- [partially reflected](https://www.gradenine.co.uk/glossary/partially-reflected)

## Related

- [Revision notes for AQA A-Level Physics (7408)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-doctor-uses-ultrasound-to-scan-5710a88a) · Published by Druglandscape Ltd.