Explain why ultrasound is used to image a developing fetus rather than X-rays.

AQA A-Level Physics (7408) — 3.10.4 Non-ionising Imaging · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Ultrasound is non‑ionising, so it does not damage cells, DNA or tissue.
X‑rays are ionising radiation, which can damage cells, cause mutations and increase cancer risk.
Ultrasound waves are reflected at boundaries between tissues of different densities.
The reflected pulses (echoes) are detected and used to build up an image.
Ultrasound can distinguish between soft tissues, whereas X‑rays are only absorbed by denser materials such as bone.

Examiner tips

Use the word "non‑ionising" to justify safety. Mention that X‑rays are ionising and harmful. Explain reflection at density boundaries. State that echoes form the image. Highlight soft‑tissue contrast advantage.

Common mistakes

Confusing ionising with non‑ionising. Saying X‑rays are better for soft tissue. Omitting the reflection/echo mechanism.

Mark scheme (5 marks)

Ultrasound is non-ionising (so does not damage cells / DNA / tissue)
X-rays are ionising radiation (which can damage cells / cause mutations / increase cancer risk)
Ultrasound is reflected at boundaries between tissues of different densities
The reflected pulses / echoes are detected and used to build up an image
Ultrasound can distinguish between soft tissues (whereas X-rays cannot / X-rays are only absorbed by denser materials such as bone)

Key terms in this question

ultrasound