# A medical team uses ultrasound waves to carry out a foetal scan on a pregnant patient. Explain how ultrasound waves are used to produce an image of the foetus, and why ultrasound is chosen for this purpose rather than X-rays.

> OCR A-Level Physics B: Advancing Physics (H557) — 5.2 Circular motion and gravitational fields · Explain · 5 marks

> Ultrasound waves have a frequency above 20 kHz, which is beyond the upper limit of human hearing. When ultrasound reaches a boundary between two different media, the waves are partially reflected back towards the source.

## Mark scheme (5 marks)

1. Ultrasound waves are emitted from a device placed against the patient's body and travel into the womb
2. At each boundary between different media / tissues, some of the ultrasound is partially reflected back
3. A receiver / detector detects the reflected ultrasound waves and measures the time between emission and detection
4. Because the speed of ultrasound is constant, the time taken allows the distance to each boundary to be calculated, building up an image
5. Ultrasound is chosen instead of X-rays because ultrasound is non-ionising / does not damage cells, whereas X-rays can cause cell damage or mutations

## Key terms

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

## Related

- [Revision notes for OCR A-Level Physics B: Advancing Physics (H557)](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-medical-team-uses-ultrasound-waves-e2116eba) · Published by Druglandscape Ltd.