Module 6 · Particles and medical physics

Medical Imaging Overview

Specification: OCR A H556 | Section: 6.5 Medical imaging | Focus: X-rays, CAT scans, gamma camera/PET, ultrasound, acoustic impedance, image formation, risks and advantages. Note: MRI is not explicitly listed in OCR A H556 6.5, so it is not required content for this resource.

By the end of this topic you should be able to…

describe the basic structure of an X-ray tube and explain how X-rays are produced
apply X-ray attenuation ideas, including the exponential attenuation equation and contrast media
explain how a CAT scan forms images and why it improves on a single X-ray image
describe gamma camera and PET scanner image formation at overview level
explain ultrasound A-scan and B-scan, acoustic impedance and reflection at boundaries
use pulse-echo timing and impedance equations in worked calculations

Big idea: different imaging methods trade off resolution, risk, speed and diagnostic purpose. The best technique depends on what you are trying to see and how safely you can get the information.

Part 1

How OCR frames medical imaging

OCR’s medical imaging content in Module 6 is a physics comparison exercise as much as a medical one. You are expected to understand how the imaging method works, what quantities control the image, and why one method may be preferred over another.

X-rays / CAT

Ionising radiation. Strong for bone and dense structures. Contrast comes from attenuation differences.

Gamma camera / PET

Uses tracers to reveal function as well as structure. Excellent for metabolic activity and physiology.

Ultrasound

Non-ionising, safe, portable and real-time. Best where acoustic boundaries produce useful echoes.

Scope note

Although MRI is common in real hospitals, OCR A H556 6.5 does not list it explicitly. Do not let general medical-imaging knowledge distract you from the specification content you can actually be examined on.

Part 2

Using X-rays

X-rays are produced when fast electrons strike a metal target and lose energy rapidly. Some of that energy emerges as X-ray photons. A photon is a packet of electromagnetic radiation energy. In this topic, higher photon energy means a more penetrating X-ray beam.

X-ray tube schematic

Electrons are emitted from the heated cathode, accelerated by a high voltage, and collide with the anode target, producing X-ray photons. The anode is the positive electrode. A tungsten target is commonly used because tungsten has a very high melting point, so it can survive intense heating.

Attenuation and image contrast

Different tissues attenuate X-rays by different amounts. Bone attenuates more strongly than soft tissue, so fewer photons reach the detector and stronger contrast appears.

X-ray attenuationI = I₀e^−μx

Here I₀ is the incident intensity, I is the transmitted intensity, μ is the attenuation coefficient, and x is the thickness of the absorber. The attenuation coefficient tells you how strongly a material reduces the X-ray intensity per unit thickness. A larger μ means the beam is weakened more quickly.

Exponential attenuation graph

The graph shows exponential decay: the X-ray intensity drops quickly at first, then levels off as depth increases.

Try the equation immediately

Attenuation coefficient μ (cm⁻¹) 0.20

Thickness x (cm) 5.0

Incident intensity I₀ (arb. units) 100

Transmitted intensity I = 36.8

Fraction transmitted = 0.368

Higher μ or larger x gives stronger attenuation

Use the sliders straight after the equation. Increasing μ or x makes the transmitted beam fall more steeply.

Attenuation mechanisms named by OCR

Simple scatter — changes direction, reducing useful beam intensity
Photoelectric effect — photon energy transferred to an electron and fully absorbed
Compton effect — photon transfers part of its energy and continues with reduced energy
Pair production — only possible at sufficiently high photon energies

Contrast media

Contrast media are substances put into the body to make one region show up more clearly on the image. Barium and iodine are used because they strongly attenuate X-rays, increasing contrast in parts of the body that would otherwise be difficult to distinguish.

Part 3

CAT scans

A CAT scan improves on a single X-ray image by taking many attenuation measurements from different angles, then reconstructing a slice image by computer.

CAT scanner rotation principle

A gantry is the large ring-shaped frame of the scanner. Here the X-ray source sits on one side and the detector arrays sit opposite. The beam spreads out as a fan beam, so many rays pass through the patient at one view angle before the system rotates and repeats the measurement from new angles.

Advantages of a CAT scan over a single X-ray image

much better contrast and localisation of structures inside the body
cross-sectional slices reduce overlap of tissues
can reconstruct three-dimensional information from multiple slices

Trade-off

CAT scanning generally gives more diagnostic information, but also involves more complex equipment, more data processing and often a higher radiation dose than a single plain X-ray image.

Part 4

Gamma camera and PET at overview level

OCR includes two tracer-based methods in 6.5.2: the gamma camera and the PET scanner. These differ from X-rays and ultrasound because the source of the signal is placed inside the patient.

Medical tracers

OCR names technetium-99m and fluorine-18. These tracers accumulate in particular organs or tissues so emitted radiation reveals function.

Gamma camera

Key components: collimator, scintillator, photomultiplier tubes, computer and display. Gamma rays from the tracer are detected to form an image.

PET

Positron emission leads to positron–electron annihilation. The paired gamma photons are detected to reconstruct where the tracer was located.

Exam focus

You are not expected to become a radiographer. Focus on the named components, the basic formation-of-image ideas, and the reason tracer methods are especially useful for showing function as well as structure.

Part 5

Using ultrasound

Ultrasound is a longitudinal wave with frequency above 20 kHz. Medical ultrasound uses frequencies far above that, typically in the MHz range, to achieve useful resolution. Resolution means how well the image can separate two nearby features and show them as distinct.

Pulse-echo timing

The pulse travels to the boundary and back, so the ultrasound covers twice the depth during the measured time. That is why depth = cΔt / 2.

Acoustic impedance and reflection

At a boundary, part of the wave is reflected and part is transmitted. The reflected fraction depends on the difference between the acoustic impedances of the two media.

Acoustic impedanceZ = ρc

Acoustic impedance tells you how difficult it is for sound to travel through a medium. A large impedance mismatch at a boundary means more of the ultrasound is reflected and less is transmitted.

Reflected intensity fractionI_r / I₀ = ((Z₂ − Z₁) / (Z₂ + Z₁))²

A-scan and B-scan

A-scan means amplitude scan. It shows echoes as peaks on a graph against time, so it is useful for finding boundaries and measuring distances.
B-scan means brightness scan. It converts echo strength into brightness, building up a two-dimensional picture.

Why gel is used

The air gap between the transducer and the skin would create a huge impedance mismatch, causing almost complete reflection. Gel improves impedance matching so more ultrasound enters the body.

Part 6

Comparing imaging methods

Method	Main physics	Strengths	Limitations / risks	Best used for
X-ray image	attenuation of ionising radiation	fast, cheap, good for bone	2D overlap, ionising dose	fractures, chest imaging
CAT scan	many X-ray attenuation measurements from different angles	cross-sectional detail, better localisation	higher cost, more complex, more dose than plain X-ray	internal structure, trauma, detailed anatomy
Gamma camera / PET	radiation emitted by tracers	shows function and metabolism	expensive tracers/equipment, ionising radiation	organ function, metabolic activity, oncology
Ultrasound	pulse-echo, acoustic impedance, Doppler	real-time, portable, non-ionising, safe	poor through bone or air, lower detail in some tissues	pregnancy, soft tissue, blood flow

If asked to compare methods, always mention both the image quality / information content and the safety / practicality issues.

Worked Examples

Worked examples

Worked example 1

An X-ray beam enters tissue with intensity 120 W m⁻². The attenuation coefficient is 0.18 cm⁻¹ and the tissue thickness is 6.0 cm. Find the transmitted intensity.

Use I = I₀e^−μx

I = 120 × e^−0.18×6.0

I = 120 × e^−1.08

I = 120 × 0.340 ≈ 40.8 W m⁻²

Worked example 2

An ultrasound pulse is sent into tissue where the speed of sound is 1540 m s⁻¹. The echo returns after 260 μs. Calculate the depth of the boundary.

Convert time: 260 μs = 2.60 × 10⁻⁴ s

The pulse travels to the boundary and back, so depth = cΔt / 2

depth = 1540 × 2.60 × 10⁻⁴ / 2

depth = 0.200 m = 20.0 cm

Worked example 3

Medium 1 has acoustic impedance 1.5 × 10⁶ kg m⁻² s⁻¹ and medium 2 has acoustic impedance 7.8 × 10⁶ kg m⁻² s⁻¹. Find the fraction of ultrasound intensity reflected.

Use I_r/I₀ = ((Z₂ − Z₁)/(Z₂ + Z₁))²

= ((7.8 − 1.5)/(7.8 + 1.5))² in units of 10⁶

= (6.3/9.3)²

= 0.458 ≈ 0.46

So about 46% of the intensity is reflected.

Knowledge

Knowledge Check

Name the main components of an X-ray tube required by OCR.

3 marks

heater / cathode
anode / target metal
high voltage supply

Why is gel used in ultrasound scanning?

2 marks

It reduces the air gap between transducer and skin
It improves acoustic impedance matching so less ultrasound is reflected at the surface

What is one advantage of a CAT scan over a single X-ray image?

1 mark

Cross-sectional detail / less overlap of tissues / better localisation

State the equation for acoustic impedance.

1 mark

Z = ρc

Exam Practice

Exam-Style Questions

An X-ray beam with intensity 90 W m⁻² passes through 4.0 cm of tissue with attenuation coefficient 0.30 cm⁻¹. Calculate the transmitted intensity. generated exam-style

2 marks

I = I₀e^−μx = 90e^−0.30×4.0
I = 90e^−1.2 = 27.1 W m⁻²

Explain two advantages of ultrasound over X-ray imaging in pregnancy. generated exam-style

2 marks

Ultrasound is non-ionising / does not expose the patient or foetus to ionising radiation
It is real-time / safer for repeated monitoring / good for soft tissue imaging

A pulse of ultrasound is sent into tissue where the speed of sound is 1500 m s⁻¹. The echo returns after 80 μs. Find the depth of the reflector. generated exam-style

2 marks

depth = cΔt / 2 = 1500 × 80 × 10⁻⁶ / 2
depth = 0.060 m = 6.0 cm

A student says that a PET scan and an X-ray image are basically the same because both involve radiation. Explain why this is not a good description. generated exam-style

3 marks

In PET, radiation comes from a tracer inside the body
In an X-ray image, radiation comes from an external X-ray tube
PET can show functional / metabolic activity whereas X-rays mainly show attenuation differences in structure

Summary

Topic Summary

Medical Imaging Overview

X-rays

Produced in an X-ray tube when fast electrons strike a metal target. Image contrast depends on attenuation differences.

CAT scan

Uses a rotating X-ray tube and detectors to reconstruct cross-sectional images with better localisation than a plain X-ray.

Gamma camera / PET

Tracer-based methods reveal physiological function as well as structure.

Ultrasound

Non-ionising pulse-echo method. Key ideas: A-scan, B-scan, acoustic impedance, reflection and Doppler.

6.5.1 X-rays + CAT 6.5.2 gamma camera + PET 6.5.3 ultrasound