X-ray imaging

CIE A-Level Physics· Unit 28: Medical imaging· 35 min read

1. Production of Medical X-rays★★☆☆☆⏱ 10 min

Medical X-rays for imaging are high-energy photons (30 keV to 150 keV) produced in a vacuum X-ray tube. Electrons are emitted from a heated cathode, accelerated through a high potential difference towards a tungsten anode target.

When electrons collide with the target, ~99% of their kinetic energy is lost as heat, and only ~1% is converted to X-ray photons. The maximum energy of an X-ray photon equals the maximum kinetic energy of the accelerated electron.

📘 Definition

X-ray tube

Vacuum device that produces X-rays by accelerating electrons to a high-density metal anode

Example:

Tungsten is used as an anode because it has a high melting point and high atomic number, improving X-ray production efficiency.

📐 Worked Example

An X-ray tube operates at 80 kV. Calculate the maximum energy of an emitted X-ray photon, in joules.

1
Maximum photon energy equals the kinetic energy gained by an electron across the accelerating potential difference:
2
$E_{\text{max}} = eV$
3
Substitute $e = 1.6 \times 1 0^{- 19}$ C and $V = 80 \times 1 0^{3}$ V:
4
$E_{\text{max}} = (1.6 \times 10^{-19}) \times (80 \times 10^3) = 1.28 \times 10^{-14} \text{ J}$

Exam tip:

A 1-mark question often asks what percentage of energy becomes X-rays: remember only 1% is X-rays, 99% is heat.

2. X-ray Attenuation★★★☆☆⏱ 15 min

When X-rays pass through body tissue, photons are absorbed or scattered, reducing intensity: this process is called attenuation. Attenuation follows an exponential relationship that depends on the type of tissue and X-ray energy.

📘 Definition

Linear attenuation coefficient

\mu

Measure of how quickly X-ray intensity decreases per unit thickness of material. Higher values mean more attenuation.

I = I_0 e^{-\mu x}

Where $I_{0}$ = incident intensity, $I$ = transmitted intensity, $x$ = material thickness. Half-value thickness ( $x_{1/2}$ ), the thickness that reduces intensity by half, is related to $μ$ by $x_{1/2} = \frac{l n 2}{μ}$ .

📐 Worked Example

Bone has $μ = 0.35$ cm⁻¹, soft tissue has $μ = 0.10$ cm⁻¹. Calculate the ratio $I_{bone} / I_{soft}$ for 2 cm thickness of each.

1
Cancel $I_{0}$ from the intensity ratio for both materials:
2
$\frac{I_{\text{bone}}}{I_{\text{soft}}} = \frac{I_0 e^{-\mu_{\text{bone}} x}}{I_0 e^{-\mu_{\text{soft}} x}} = e^{-(\mu_{\text{bone}} - \mu_{\text{soft}})x}$
3
Substitute values: $μ_{bone} - μ_{soft} = 0.25$ cm⁻¹, $x = 2$ cm:
4
$\frac{I_{\text{bone}}}{I_{\text{soft}}} = e^{-(0.25)(2)} = e^{-0.5} \approx 0.61$

3. X-ray Image Quality★★★☆☆⏱ 12 min

Two key metrics describe X-ray image quality: contrast and sharpness (resolution). Both are controlled by practical choices when taking an X-ray.

Contrast: The difference in brightness between different regions of the image. Improved by lower X-ray energy, contrast media (like barium), and anti-scatter grids.
Resolution (Sharpness): The ability to distinguish small adjacent objects. Improved by using a small focal spot on the anode, and minimizing patient movement.

📐 Worked Example

Explain why barium is used to image the intestine.

1
Intestinal wall tissue has a similar attenuation coefficient to surrounding soft tissue, so it produces very little contrast on a standard X-ray.
2
Barium has a high atomic number, so it has a much higher attenuation coefficient than body tissue.
3
When the intestine is filled with barium, it strongly attenuates X-rays, creating a high-contrast outline that allows radiologists to see abnormalities like tumors.

4. Patient Dose and Safety★★☆☆☆⏱ 8 min

X-rays are ionizing radiation, so they can damage living cells and increase long-term cancer risk. All exposure must be kept as low as reasonably achievable (the ALARA principle).

Intensifying screens are used to reduce patient dose. These screens contain fluorescent material that converts each absorbed X-ray photon into many visible light photons, which expose the film. This means far fewer X-ray photons are needed for the same image exposure, cutting patient dose significantly.

Key safety rules: Shield sensitive tissue (reproductive organs) with lead aprons
Staff must stand behind lead shielding during exposure
Keep exposure time as short as possible
Avoid X-rays for pregnant patients where possible

5. Common Pitfalls

Wrong move:

Using mismatched units for $μ$ and $x$ in the attenuation formula.

Why:

If $μ$ is in m⁻¹ and $x$ is in cm, the exponent will be wrong and give an incorrect intensity.

Correct move:

Always convert thickness to match the length unit of $μ$ (e.g. convert x to metres if $μ$ is in m⁻¹).

Wrong move:

Stating most energy in an X-ray tube becomes X-rays.

Why:

This is a common factual recall question that students often get backwards.

Correct move:

Remember: 1% of energy becomes X-rays, 99% becomes heat.

Wrong move:

Confusing contrast and resolution.

Why:

Exam questions often ask to define or compare these two, and marks are lost for mixing them up.

Correct move:

Contrast = difference in brightness between tissues; Resolution = ability to see small separate objects.

Wrong move:

Claiming intensifying screens increase patient dose.

Why:

Students incorrectly think more photons mean higher patient dose, but the extra photons are visible light, not X-rays.

Correct move:

Intensifying screens reduce the number of X-rays needed, so they decrease patient dose.

6. Quick Reference Cheatsheet

Concept	Formula / Key Fact
Max X-ray energy	$E_{ma x} = e V$
Exponential attenuation	$I = I_{0} e^{- μx}$
Half-value thickness	$x_{1/2} = \frac{l n 2}{μ}$
X-ray tube energy split	1% → X-rays, 99% → heat
Image contrast	Difference in intensity from different attenuation
Image resolution	Ability to distinguish adjacent small objects
Intensifying screen purpose	Reduce patient X-ray dose
Contrast media purpose	Increase contrast for soft tissue imaging

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 2
X-ray attenuation calculation
2023 · 4
X-ray image quality factors
2021 · 2
X-ray production in a tube

Going deeper

syllabusCIE 9702 A-Level Physics Syllabus 2025-2027

What's Next

X-ray imaging is the foundation of modern diagnostic radiology, and the principles of attenuation and image quality you learned here apply to all other medical imaging techniques covered in this unit. Understanding how X-ray production and attenuation work is critical for tackling more advanced topics like computed tomography (CT) scanning, which is a frequent extended response question in CIE A-Level Paper 4. These concepts also connect to radiation safety principles you learned earlier in the radioactivity unit, which often are combined with medical imaging questions in exams.