Imaging comparisons (A-level only) — AQA A-Level Physics

This topic explores the physics behind major medical imaging techniques: X-ray radiography, computed tomography (CT), magnetic resonance imaging (MRI), ultrasound, and positron emission tomography (PET). You will compare how each modality generates images, their resolution, safety profiles, and clinical applications, building on your understanding of waves, photons, and nuclear physics.

Understanding imaging comparisons is crucial for appreciating how physics directly impacts healthcare and technology. It also develops evaluative skills, as you must weigh advantages and disadvantages to recommend the most appropriate imaging method for a given scenario, a common requirement in A-level exams and beyond.

• →X-rays and CT use ionising radiation and differential attenuation to form images; CT provides 3D cross-sections via multiple X-ray projections.
• →MRI relies on nuclear magnetic resonance: strong magnetic fields align proton spins, radio waves excite them, and relaxation signals create images with excellent soft tissue contrast.
• →Ultrasound uses high-frequency sound waves (1-20 MHz) reflected at tissue boundaries; resolution improves with frequency but penetration decreases.
• →PET detects gamma rays from positron-electron annihilation, using radiotracers like fluorine-18 to map metabolic activity; often combined with CT for anatomical context.
• →Key comparison criteria: spatial resolution, contrast, safety (ionising vs non-ionising), cost, speed, and suitability for different tissues.

• Identify the physical principle of each technique, such as differential attenuation for X-rays, annihilation photon detection in PET, or reflection of ultrasound at tissue boundaries.
• Compare spatial resolution and contrast, explaining why one technique shows particular tissues or functional information better than another.
• Compare patient dose and risk, noting that techniques using ionising radiation deliver a dose whereas ultrasound and MRI do not.
• Compare practical factors such as cost, portability, availability, scan time and patient comfort or safety.
• Use consistent criteria across the techniques being compared so that the comparison is fair and leads to a justified choice.
• Relate the comparison to a specific clinical purpose, such as imaging a suspected fracture, a soft-tissue injury or metabolically active tissue.
• Identifies image resolution as the ability to distinguish small, closely spaced features in the resulting image, linking it to the imaging wavelength or reconstruction method where relevant.
• Discusses convenience in terms of scan duration, patient comfort, need to remain still, portability, availability and whether the procedure is invasive.
• Discusses safety in terms of ionising radiation dose, heating or cavitation effects, contrast agent risks and suitability for particular patients such as pregnant women.
• Compares two or more techniques by weighing resolution against convenience and safety for a stated clinical situation rather than listing techniques separately.
• Reaches a justified conclusion that identifies the most suitable technique for the context and acknowledges a trade-off, for example lower resolution but no ionising radiation.

• 💡Choose three or four comparison criteria and use them consistently for every technique discussed.
• 💡Include at least one quantitative or semi-quantitative point, such as relative dose or typical resolution, where you can do so accurately.
• 💡Finish with a justified recommendation for a stated clinical scenario rather than a neutral list.
• 💡Read the clinical context carefully and let it drive your judgement, because the best technique depends on the patient and the diagnostic question.
• 💡Use comparative connectives such as whereas and however to show you are weighing criteria rather than describing techniques in isolation.
• 💡State a clear overall recommendation and support it with at least one resolution point, one convenience point and one safety point.
• 💡Always structure comparison answers in paragraphs or bullet points that address specific criteria: physical principle, safety, resolution, and clinical use. This ensures you cover all mark scheme points.
• 💡Use correct terminology: 'ionising radiation', 'attenuation', 'proton precession', 'annihilation', 'acoustic impedance'. Avoid vague terms like 'rays' or 'waves' without qualification.
• 💡When asked to 'evaluate' or 'compare', include both advantages and limitations, and conclude with a justified recommendation based on the scenario.

• Listing features of each technique separately without comparing them: correct this by applying the same criteria to each technique in turn.
• Claiming that MRI uses ionising radiation: correct this by stating that MRI uses strong magnetic fields and radio waves, not ionising radiation.
• Treating higher resolution as automatically better in every case: correct this by linking the choice of technique to the clinical question and the information required.
• Treating resolution as brightness or image size rather than the ability to distinguish fine detail; correct this by defining resolution as the smallest separable feature and linking it to the imaging method.
• Listing advantages and disadvantages of each technique without comparing them; correct this by explicitly weighing the criteria against each other for the clinical task given.
• Assuming all radiation-based techniques carry equal risk regardless of dose; correct this by distinguishing dose levels and noting that some techniques avoid ionising radiation altogether.
• MRI uses ionising radiation: Actually, MRI uses non-ionising radio waves and magnetic fields, making it safer for repeated use, though strong magnetic fields pose risks with metal implants.
• Ultrasound can image any part of the body with equal resolution: In reality, ultrasound resolution and penetration are trade-offs; high frequency gives better resolution but limited depth, so it is unsuitable for deep structures like lungs or bones.
• CT and X-ray are the same: CT uses multiple X-ray projections to create cross-sectional images, while plain X-ray gives a 2D superposition image; CT involves higher radiation dose but provides 3D detail.

1. 1Week 1: Create a comparison table for X-ray, CT, MRI, ultrasound, and PET, listing physical principle, radiation type, resolution, safety, cost, and typical uses. Use your textbook and revision guide to fill it in.
2. 2Week 1: Practice explaining each modality's image formation in your own words, focusing on key physics concepts like attenuation, resonance, and reflection.
3. 3Week 2: Attempt past paper questions on imaging comparisons, paying attention to command words like 'compare', 'evaluate', and 'explain'. Mark your answers using the mark scheme and note recurring themes.
4. 4Week 2: Use flashcards for active recall of key facts, such as half-life calculations for PET tracers or typical frequencies for ultrasound.
5. 5Ongoing: Watch educational videos or animations of imaging techniques to visualise the physics, then summarise each in 3-4 bullet points.

• 📋Comparison questions: 'Compare the use of CT and MRI for imaging soft tissue.' Advice: Structure your answer around criteria like resolution, safety, and contrast; use comparative language.
• 📋Calculation questions: 'Calculate the activity of a PET tracer after a given time.' Advice: Remember the half-life equation A = A0 / 2^n and convert units carefully.
• 📋Explanation questions: 'Explain how ultrasound produces an image of a fetus.' Advice: Describe pulse-echo technique, reflection at boundaries, and conversion to electrical signals.
• 📋Evaluation questions: 'Evaluate the suitability of PET for diagnosing cancer.' Advice: Discuss sensitivity, specificity, cost, and availability, and conclude with a justified judgement.