Unit Overview

Medical imaging

CIE A-Level Physics· 5 min read 📊 n/a

1. 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

Covers X-ray production, attenuation, image detection and factors affecting image quality.

★★⏱ 12 min

CT scanning

Explains how computed tomography builds 3D cross-sectional images from multiple X-ray projections.

★★★⏱ 10 min

Ultrasound imaging

Covers ultrasound generation, acoustic impedance, reflection, A-scans and B-scans for diagnosis.

★★★⏱ 15 min

Nuclear medicine imaging

Explains gamma camera and PET scanning using radiopharmaceuticals to image organ function.

★★★★⏱ 12 min

2. Common Pitfalls

Wrong move:

Confusing X-ray attenuation with ultrasound contrast generation

Why:

X-ray contrast comes from differing absorption rates, while ultrasound contrast comes from impedance differences at boundaries

Correct move:

Remember X-rays rely on attenuation, while ultrasound relies on reflection from impedance mismatches

Wrong move:

Mixing up CT scanning and nuclear medicine imaging

Why:

CT images body structure using an external X-ray source, while nuclear medicine images function using internal radiopharmaceuticals

Correct move:

CT = external X-ray = structure; nuclear medicine = internal emitter = function

3. Quick Reference Cheatsheet

Concept / Formula	Key Description
$I = I_{0} e^{- μx}$	X-ray intensity after attenuation through thickness $x$ with coefficient $μ$
$x_{1/2} = \frac{l n 2}{μ}$	Half-value thickness for X-ray attenuation
$Z = ρ c$	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.