WJEC Eduqas Eduqas 2019-style practice: A parallel beam of X-rays of intensity I_0 passes through 4.0\ cm of tissue with a linear attenuation coefficient of 0.20\ cm^-1. Calculate the fraction of the intensity transmitted.

X-ray attenuation follows I = I_0 e^-μ x, where μ is the linear attenuation coefficient and x the thickness. The transmitted fraction is II_0 = e^-μ x = e^-(0.20)(4.0) = e^-0.80. e^-0.80 = 0.449, so about 45\% of the intensity is transmitted. Markers reward I = I_0 e^-μ x, substituting μ x = 0.80, and the transmitted fraction about 0.45 (45%).

EnglandPhysicsSyllabus dot point

How do X-rays, ultrasound and PET scanning use physics to image the body, and how is radiation dose measured?

Medical physics option: the production and attenuation of X-rays, ultrasound imaging and acoustic impedance, PET scanning and positron annihilation, and radiation dose and its biological effect.

A focused answer to the Eduqas A-Level Physics Component 3 medical physics option, covering the production and attenuation of X-rays, ultrasound imaging and acoustic impedance, PET scanning and positron annihilation, and the measurement of radiation dose and its biological effect.

Generated by Claude Opus 4.813 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Quick answer

X-rays are produced when fast electrons strike a metal target and decelerate; they are attenuated exponentially through tissue, $I = I_0 e^{-\mu x}$ , where $\mu$ is the linear attenuation coefficient, so dense bone absorbs more than soft tissue and shows up on the image. Ultrasound imaging sends high-frequency sound pulses into the body and times the reflections from boundaries between tissues of different acoustic impedance. PET scanning uses a positron-emitting tracer: each positron annihilates with an electron, producing two gamma photons in opposite directions that are detected to locate the activity. Radiation dose is measured as absorbed dose (gray, energy per kilogram) and equivalent dose (sievert, weighted for the type of radiation's biological harm).

Jump to a section

What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

This page covers Option B Medical physics, the most widely taught Component 3 option. Eduqas wants you to explain the production and attenuation of X-rays, describe ultrasound imaging and acoustic impedance, explain PET scanning and positron annihilation, and describe radiation dose and its biological effect. (Study this only if your school has chosen Option B.)

The answer

Production and attenuation of X-rays

Ultrasound and acoustic impedance

PET scanning and annihilation

Radiation dose and biological effect

Examples in context

Medical physics is the basis of modern diagnostic imaging: X-ray radiography and CT scanning for bone and dense tissue, ultrasound for obstetrics and soft-tissue imaging, and PET (often combined with CT) for cancer detection and brain studies. The same radiation physics underlies radiotherapy, where carefully targeted ionising radiation destroys tumours, and the dosimetry that keeps both patients and staff safe.

Try this

Q1. State the equation for the attenuation of an X-ray beam through tissue. [1 mark]

Cue. $I = I_0 e^{-\mu x}$ , where $\mu$ is the linear attenuation coefficient.

Q2. Explain why a coupling gel is used in ultrasound scanning. [2 marks]

Cue. It removes the air gap between the transducer and skin, whose large acoustic impedance mismatch would otherwise reflect almost all the sound.

Q3. State the unit of absorbed dose and what it measures. [2 marks]

Cue. The gray (Gy); the energy absorbed per unit mass of tissue ( $1\ \text{Gy} = 1\ \text{J kg}^{-1}$ ).

Exam-style practice questions

Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Eduqas 20194 marksA parallel beam of X-rays of intensity

I_0

passes through

4.0\ \text{cm}

of tissue with a linear attenuation coefficient of

0.20\ \text{cm}^{-1}

. Calculate the fraction of the intensity transmitted.

Show worked answer →

X-ray attenuation follows $I = I_0 e^{-\mu x}$ , where $\mu$ is the linear attenuation coefficient and $x$ the thickness.

The transmitted fraction is $\dfrac{I}{I_0} = e^{-\mu x} = e^{-(0.20)(4.0)} = e^{-0.80}$ .

$e^{-0.80} = 0.449$ , so about $45\%$ of the intensity is transmitted.

Markers reward $I = I_0 e^{-\mu x}$ , substituting $\mu x = 0.80$ , and the transmitted fraction about $0.45$ (45%).

Eduqas 20214 marksIn PET scanning a tracer emits positrons. Explain what happens when an emitted positron meets an electron, and state why two gamma photons are produced travelling in opposite directions.

Show worked answer →

When the emitted positron meets an electron, the two annihilate: their entire mass is converted into energy as electromagnetic radiation (gamma photons).

Two photons are produced, not one, and they travel in opposite directions, because momentum must be conserved. The electron and positron are almost at rest, so the total momentum is nearly zero; two photons moving in opposite directions carry equal and opposite momenta that sum to zero. Each photon has energy $0.51\ \text{MeV}$ (the rest energy of an electron).

Markers reward the annihilation of the positron and electron converting mass to energy, and explaining the two opposite photons by conservation of momentum.

Related dot points

Sources & how we know this

Eduqas GCE AS/A Level Physics specification (A720QS) — WJEC Eduqas (2015)