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.
- 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.
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.
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.
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
Attenuation and image contrast
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
Try the equation immediately
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 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.
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
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
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.
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.
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.
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
Acoustic impedance and reflection
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.
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.
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.
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
Knowledge Check
- heater / cathode
- anode / target metal
- high voltage supply
- It reduces the air gap between transducer and skin
- It improves acoustic impedance matching so less ultrasound is reflected at the surface
- Cross-sectional detail / less overlap of tissues / better localisation
- Z = ρc
Exam-Style Questions
- I = I₀e−μx = 90e−0.30×4.0
- I = 90e−1.2 = 27.1 W m−2
- 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
- depth = cΔt / 2 = 1500 × 80 × 10−6 / 2
- depth = 0.060 m = 6.0 cm
- 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
Topic Summary
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.