A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Basic Principles of MRI Scanning — mark scheme explained
The short answer
Magnetic Resonance Imaging (MRI) is a powerful diagnostic tool used in medical physics to produce detailed images of the body's internal structures. The basic principles of an MR scanner involve the use of strong magnetic fields, radio frequency (RF) pulses, and gradient coils to create cross-sectional images of the patient.
The question
Explain how protons align with a magnetic field in an MRI scanner.
[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
For explanation questions, ensure you cover all key points such as magnetic field alignment, precession, gradient coils, RF pulses, and signal detection. Use clear and concise language, and provide step-by-step reasoning where appropriate.
What the command words demand
- Explain
- Provide a detailed account of the process or concept.
- Describe
- Give a detailed description of the features or characteristics.
- Discuss
- Consider and write about the topic in detail, including different aspects and viewpoints.
- Identify
- Recognize and name specific elements or components.
- State
- Clearly express the facts or information without elaboration.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate about 5-7 minutes for a 4-mark question to ensure you have enough time to provide a detailed and accurate answer.
- 1. In the absence of an external magnetic field, the spins of protons are randomly oriented.1 mark
- 2. When placed in a strong magnetic field, the protons align themselves with the magnetic field lines.1 mark
- 3. Most protons will align parallel to the magnetic field due to the lower energy state.1 mark
- 4. This alignment creates a net magnetization vector in the direction of the magnetic field.1 mark
Final answer: Protons align with the magnetic field lines, with most protons aligning parallel to the field due to the lower energy state. This creates a net magnetization vector in the direction of the magnetic field.
Work through every step correctly and you earn all 4 marks.
Another worked example
Describe the concept of precession and Larmor frequency in MRI.
- 1. Precession is the wobbling motion of aligned protons around the magnetic field lines.1 mark
- 2. The Larmor frequency is the specific frequency at which protons precess, determined by the strength of the magnetic field and the gyromagnetic ratio of the proton.1 mark
- 3. This precession creates a small but detectable magnetic field.1 mark
Final answer: Precession is the wobbling motion of aligned protons around the magnetic field lines at the Larmor frequency, which is determined by the strength of the magnetic field and the gyromagnetic ratio of the proton.
Work through every step correctly and you earn all 3 marks.
Common mistakes
Confusing the alignment of protons with random orientation in the magnetic field.
Why it happens: Students may not fully understand that protons align parallel to the magnetic field due to the lower energy state, rather than remaining randomly oriented.
Fix: Emphasize that in a strong magnetic field, most protons will align parallel to the field lines because this is the lower energy state.
Misunderstanding the concept of precession and Larmor frequency.
Why it happens: Students might think that protons remain stationary in the magnetic field, rather than precessing around it.
Fix: Clarify that protons wobble (precess) around the magnetic field lines at a specific frequency called the Larmor frequency.
Forgetting the role of gradient coils in MRI scanning.
Why it happens: Students may overlook the importance of gradient coils in selecting specific slices for imaging.
Fix: Explain that gradient coils vary the magnetic field strength across different regions, allowing the scanner to select specific slices by varying the Larmor frequency.
Confusing RF pulses with continuous signals in MRI.
Why it happens: Students might think that RF pulses are continuously applied rather than being short and intermittent.
Fix: Clarify that RF pulses are short and used to excite protons, after which they de-excite and emit signals.
Misunderstanding the process of signal detection and image reconstruction.
Why it happens: Students may not grasp that the emitted RF signals are detected by receiver coils and processed to create an image.
Fix: Explain that the emitted RF signals contain spatial information, which is processed by a computer to produce a visual image of the selected slice.
Thinking MRI uses ionizing radiation like X-rays.
Why it happens: Students might confuse MRI with other imaging techniques that use ionizing radiation, such as X-rays or CT scans.
Fix: Emphasize that MRI is non-invasive and does not use ionizing radiation, making it safer for repeated use.
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 |
|---|---|---|
| Explain Proton Alignment | Describe how protons behave and align when placed in a strong external magnetic field. | 4 |
| Explain MRI Physics | Describe proton precession and define Larmor frequency in the context of MRI. | 3 |
| Explain MRI Component | Describe how gradient coils vary the magnetic field to enable spatial location in MRI. | 4 |
| MRI RF Pulses | Explain how RF pulses excite protons and describe the relaxation and signal emission when switched off. | 5 |
| MRI Imaging Explanation | Explain how emitted RF signals are detected and processed to form an MRI image. | 4 |
| Total across these question types | 20 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.