A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Comparing Imaging Techniques in Medical Physics — mark scheme explained
The short answer
In medical physics, the ability to compare different imaging techniques is crucial for selecting the most appropriate method for a given clinical scenario. This involves evaluating three key factors: image resolution, convenience, and safety issues. Understanding these aspects helps healthcare professionals make informed decisions that balance diagnostic accuracy with patient comfort and risk.
The question
Compare the image resolution of X-ray and MRI imaging techniques. Which technique would be more suitable for diagnosing a brain tumor?
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
Marks are typically awarded for identifying key points (e.g., image resolution, convenience, safety) and providing clear, concise explanations. Make sure to address all parts of the question and provide specific examples where relevant.
What the command words demand
- Compare
- Identify similarities and differences between two or more imaging techniques.
- Evaluate
- Assess the advantages and disadvantages of a particular imaging technique in a given context.
- Discuss
- Provide a detailed examination of the factors involved, including image resolution, convenience, and safety issues.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate about 5-7 minutes per question to ensure you have enough time to thoroughly compare imaging techniques and discuss all required factors.
- X-ray imaging provides good resolution for dense structures like bones but has limited resolution for soft tissues.1 mark
- MRI offers excellent resolution for soft tissues, making it ideal for detailed images of the brain and spinal cord.1 mark
- For diagnosing a brain tumor, MRI would be more suitable due to its superior soft tissue resolution.1 mark
Final answer: MRI is more suitable for diagnosing a brain tumor due to its excellent soft tissue resolution.
Work through every step correctly and you earn all 3 marks.
Another worked example
Discuss the convenience factors of CT and ultrasound imaging. Which technique would be more appropriate for an emergency room setting?
- CT scans are relatively quick (a few minutes) and provide detailed images, making them convenient for many diagnostic purposes.1 mark
- Ultrasound is non-invasive, portable, and can be performed at the bedside, making it very convenient for both patients and healthcare providers.1 mark
- In an emergency room setting, ultrasound would be more appropriate due to its portability and speed.1 mark
Final answer: Ultrasound is more appropriate for an emergency room setting due to its portability and speed.
Work through every step correctly and you earn all 3 marks.
Common mistakes
Confusing image resolution with image contrast
Why it happens: Students sometimes mix up these two concepts, thinking that higher contrast always means better resolution. However, they are distinct: resolution is about the level of detail, while contrast is about the difference in brightness between different parts of the image.
Fix: Understand and clearly differentiate between image resolution (level of detail) and image contrast (difference in brightness).
Overlooking the safety issues associated with ionizing radiation
Why it happens: Students may focus on the diagnostic benefits of X-ray and CT imaging without considering the potential risks, especially for high-risk groups like pregnant women and young children.
Fix: Always consider the safety implications of using ionizing radiation in medical imaging, particularly for vulnerable patient groups.
Failing to discuss the time required for MRI scans
Why it happens: Students might not realize that MRI scans can take a long time and require patients to remain still, which can be uncomfortable and inconvenient.
Fix: Include the time factor and patient comfort in discussions about MRI convenience.
Neglecting the portability of ultrasound
Why it happens: Students might not appreciate the practical advantages of ultrasound, such as its portability and ease of use at the bedside.
Fix: Highlight the convenience factors of ultrasound, including its portability and real-time imaging capabilities.
Not considering the impact of metal implants on MRI safety
Why it happens: Students might overlook the fact that certain metal implants can pose a risk in MRI due to the strong magnetic fields involved.
Fix: Always mention the contraindications for MRI, particularly the presence of metal implants or pacemakers.
Confusing CT and X-ray imaging in terms of radiation dose
Why it happens: Students might think that CT scans have a similar radiation dose to single X-rays, but CT scans often involve higher doses due to the multiple X-ray images taken.
Fix: Understand that while both CT and X-ray use ionizing radiation, CT scans typically involve higher radiation doses.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Compare Imaging Techniques | Compare X-ray and MRI resolution and justify the best technique for brain tumour diagnosis. | 3 |
| Compare Imaging Techniques | Compare the convenience of CT and ultrasound and justify the better choice for emergencies. | 3 |
| Evaluate Imaging Safety | Compare the safety risks of X-ray and MRI, then choose the safer option for pregnancy. | 3 |
| Compare Imaging Techniques | Compare CT and ultrasound resolution and safety, then choose the safer option for fetal monitoring. | 3 |
| MRI Safety Evaluation | Discuss the convenience and safety of MRI, explaining why some patients cannot use it. | 4 |
| Total across these question types | 16 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.