# Medical physics

> X-ray, ultrasound, MRI and radionuclide imaging: how each modality forms contrast and the safety trade-offs behind a diagnosis.

Canonical: https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.medical-physics
Last updated: 2026-08-01

Flagship subject: topics and specification statements are authored to the WJEC specification. No question in it has been through human review yet.

- Subject: [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- Specification reference: Unit 4 Option B
- Difficulty: 4/5
- Assessment objectives: AO1, AO2, AO3
- Status: authored · checked · 2026-08-01

## Specification coverage

| Reference | Statement | AOs |
| --- | --- | --- |
| Unit 4 Option B(a) | explain X-ray production attenuation radiography and CT contrast | AO1, AO2 |
| Unit 4 Option B(b) | explain ultrasound reflection Doppler shift and diagnostic use | AO1, AO2 |
| Unit 4 Option B(c) | describe the principles of MRI and tissue contrast | AO1, AO2 |
| Unit 4 Option B(d) | evaluate radionuclide imaging with reference to half-life dose and safety | AO2, AO3 |

## Summary

X-ray, ultrasound, MRI and radionuclide imaging: how each modality forms contrast and the safety trade-offs behind a diagnosis.

## What earns marks

- X-rays are produced when fast electrons decelerate at a target; attenuation depends on thickness, density and atomic number, enabling contrast between tissues.
- Ultrasound pulses reflect at boundaries with different acoustic impedances; time-of-flight and Doppler shifts provide structural and blood-flow information without ionising radiation.
- MRI uses nuclear magnetic resonance in a strong field; radiofrequency pulses and gradients encode the positions and relaxation properties of tissues.
- Radiotracers emit detectable radiation for functional imaging such as gamma cameras or PET, so half-life, dose, shielding and benefit must be balanced.

## Where marks are lost

- Calling ultrasound electromagnetic radiation; it is a mechanical longitudinal wave.
- Saying MRI uses X-rays; its signal comes from nuclear spins and radiofrequency pulses.
- Assuming higher image contrast is always better when it may require a higher patient dose.

## Related

- [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- [Medical physics lesson](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.medical-physics)