# Medical imaging

> CIE A-Level Physics · CIE 9702
> Source: https://www.owlsprep.com/study/cie-9702-u28-overview/
> Weight: n/a

This unit covers the core physics principles behind four common non-invasive medical imaging techniques used to diagnose internal body conditions. You will learn how radiation and sound generate clear, usable diagnostic images.

**Prerequisites:** Basic wave properties and radiation interaction with matter

## Learning objectives

- Explain the core physics principles behind common medical imaging techniques for non-invasive diagnosis
- Compare advantages, limitations and use cases for different imaging modalities
- Calculate key parameters for attenuation, signal reflection and image resolution
- Distinguish between structural and functional imaging approaches

## Unit at a glance

This unit follows a consistent structure for each imaging modality: we cover how the signal is produced, how it interacts with body tissue, how the final image is formed, and the relative advantages and limitations for clinical use. Topics progress from ionizing radiation-based techniques (X-rays, CT scans) to sound-based ultrasound, then to radiopharmaceutical-based nuclear medicine imaging.

Throughout the unit, you will connect core physics concepts you have already learned (attenuation, wave reflection, radioactive decay) to real-world clinical applications that are regularly tested in exams.

This unit is split into four sub-topics, each covering a major imaging modality:
- [X-ray imaging](https://www.owlsprep.com/study/cie-9702-u28-x-ray-imaging/) — Covers X-ray production, attenuation, image detection and factors affecting image quality.
- [CT scanning](https://www.owlsprep.com/study/cie-9702-u28-ct-scanning/) — Explains how computed tomography builds 3D cross-sectional images from multiple X-ray projections.
- [Ultrasound imaging](https://www.owlsprep.com/study/cie-9702-u28-ultrasound-imaging/) — Covers ultrasound generation, acoustic impedance, reflection, A-scans and B-scans for diagnosis.
- [Nuclear medicine imaging](https://www.owlsprep.com/study/cie-9702-u28-nuclear-medicine-imaging/) — Explains gamma camera and PET scanning using radiopharmaceuticals to image organ function.

## Common pitfalls

- **Wrong:** Confusing X-ray attenuation with ultrasound contrast generation
  - Why it fails: X-ray contrast comes from differing absorption rates, while ultrasound contrast comes from impedance differences at boundaries
  - Correct: Remember X-rays rely on attenuation, while ultrasound relies on reflection from impedance mismatches
- **Wrong:** Mixing up CT scanning and nuclear medicine imaging
  - Why it fails: CT images body structure using an external X-ray source, while nuclear medicine images function using internal radiopharmaceuticals
  - Correct: CT = external X-ray = structure; nuclear medicine = internal emitter = function

## Cheatsheet

| Concept / Formula | Key Description |
| --- | --- |
| $I = I_0 e^{-\mu x}$ | X-ray intensity after attenuation through thickness $x$ with coefficient $\mu$ |
| $x_{1/2} = \frac{\ln 2}{\mu}$ | Half-value thickness for X-ray attenuation |
| $Z = \rho c$ | Acoustic impedance equals density multiplied by speed of sound |
| $\frac{I_r}{I_0} = \frac{(Z_2 - Z_1)^2}{(Z_2 + Z_1)^2}$ | Fraction of ultrasound intensity reflected at a tissue boundary |
| PET imaging | Uses positron-emitting radionuclides, detects 511 keV gamma photons from annihilation |

## What's next

Begin this unit by learning the fundamentals of X-ray imaging, the oldest and most widely used medical imaging technique. Once you complete all four sub-topics in this unit, you can move on to the next unit in your CIE A-Level Physics syllabus.

- [X-ray imaging](https://www.owlsprep.com/study/cie-9702-u28-x-ray-imaging/)
- [Next unit: Astrophysics](https://www.owlsprep.com/study/cie-9702-u29-overview/)
- [CT scanning](https://www.owlsprep.com/study/cie-9702-u28-ct-scanning/)

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