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
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
1. In the absence of an external magnetic field, the spins of protons are randomly oriented.
- S2
2. When placed in a strong magnetic field, the protons align themselves with the magnetic field lines.
- S3
3. Most protons will align parallel to the magnetic field due to the lower energy state.
- S4
4. This alignment creates a net magnetization vector in the direction of the magnetic field.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
1. In the absence of an external magnetic field, the spins of protons are randomly oriented.
- S2
2. When placed in a strong magnetic field, the protons align themselves with the magnetic field lines.
- S3
3. Most protons will align parallel to the magnetic field due to the lower energy state.
- S4
4. This alignment creates a net magnetization vector in the direction of the magnetic field.
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.
Common mistakes
- Confusing the alignment of protons with random orientation in the magnetic field. — 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. — 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. — 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. — 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. — 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. — Emphasize that MRI is non-invasive and does not use ionizing radiation, making it safer for repeated use.
Where the marks go
- Full worked solution (all marking points)4 marks