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
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
X-ray imaging provides good resolution for dense structures like bones but has limited resolution for soft tissues.
- S2
MRI offers excellent resolution for soft tissues, making it ideal for detailed images of the brain and spinal cord.
- S3
For diagnosing a brain tumor, MRI would be more suitable due to its superior soft tissue resolution.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
X-ray imaging provides good resolution for dense structures like bones but has limited resolution for soft tissues.
- S2
MRI offers excellent resolution for soft tissues, making it ideal for detailed images of the brain and spinal cord.
- S3
For diagnosing a brain tumor, MRI would be more suitable due to its superior soft tissue resolution.
Final answer: MRI is more suitable for diagnosing a brain tumor due to its excellent soft tissue resolution.
Common mistakes
- Confusing image resolution with image contrast — Understand and clearly differentiate between image resolution (level of detail) and image contrast (difference in brightness).
- Overlooking the safety issues associated with ionizing radiation — 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 — Include the time factor and patient comfort in discussions about MRI convenience.
- Neglecting the portability of ultrasound — Highlight the convenience factors of ultrasound, including its portability and real-time imaging capabilities.
- Not considering the impact of metal implants on MRI safety — 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 — Understand that while both CT and X-ray use ionizing radiation, CT scans typically involve higher radiation doses.
Where the marks go
- Full worked solution (all marking points)3 marks