Magnetic resonance (MR) scanner (A-level only) — AQA A-Level Physics

Magnetic resonance (MR) scanning is a medical imaging technique that uses nuclear magnetic resonance (NMR) of hydrogen nuclei to produce detailed images of soft tissue. It relies on the fact that hydrogen nuclei possess a magnetic moment and, when placed in a strong uniform magnetic field, precess at a characteristic Larmor frequency. A radio frequency pulse at this frequency excites the nuclei, and as they relax they emit a weak RF signal that is detected and processed to form an image.

This topic is A-level only because it requires understanding of fundamental physics concepts such as magnetic moments, precession, resonance, and relaxation times, which go beyond the GCSE curriculum. It also has important applications in medicine, offering non-ionising, high-contrast imaging of soft tissues. In the AQA A-level Physics specification, it appears under the section on medical physics, linking to electromagnetic induction, resonance, and the behaviour of nuclei in magnetic fields.

• →Hydrogen nuclei (protons) have a magnetic moment and in a strong uniform magnetic field B0 they align either parallel or anti-parallel, creating a net magnetisation along B0.
• →The nuclei precess about B0 at the Larmor frequency, f = (gamma/2pi)B0, where gamma is the gyromagnetic ratio.
• →A radio frequency pulse at the Larmor frequency causes resonance, tipping the net magnetisation into the transverse plane; when the pulse is removed, the nuclei relax and emit a detectable RF signal.
• →T1 (spin-lattice) relaxation is the recovery of longitudinal magnetisation; T2 (spin-spin) relaxation is the decay of transverse magnetisation due to dephasing.
• →Spatial encoding is achieved using linear magnetic field gradients that make the Larmor frequency position-dependent, allowing 3D image reconstruction via Fourier transformation.

• Hydrogen nuclei in the body are single protons, so they possess spin and behave as tiny magnetic dipoles.
• In the strong uniform field of a superconducting magnet, proton spins initially align parallel to the field and precess about the magnetic field lines.
• Gradient field coils superimpose small position-dependent fields so that the precession frequency differs between small regions of the patient.
• Short RF pulses at the matching frequency excite protons in a chosen small region, changing their spin state.
• Excited protons emit RF signals as they de-excite, and these signals are detected by a receiver coil.
• A computer processes the detected RF signals to reconstruct a visual image of the scanned cross-section.

• 💡Link each stage in order: alignment and precession, gradient encoding, RF excitation, RF emission on de-excitation, detection, computer image reconstruction.
• 💡Use precise terms such as precess, spin state, excite, de-excite and gradient field rather than vague wording like 'the magnet takes a picture'.
• 💡When explaining why hydrogen is used, state that its nucleus is a single proton and is abundant in body tissue.
• 💡Always use precise terminology: 'precession', 'Larmor frequency', 'resonance', 'spin-lattice' and 'spin-spin' relaxation. Avoid vague phrases like 'spinning back'.
• 💡When explaining image formation, explicitly state that field gradients make the Larmor frequency vary with position, and that Fourier analysis converts the detected signal into an image.
• 💡For calculation questions, remember to convert gamma from rad s^-1 T^-1 to Hz T^-1 by dividing by 2pi, and always give units (Hz or MHz).

• Thinking the scanner uses X-rays or ionising radiation; correct this by stating that MR uses a strong magnetic field and radio frequency pulses with no ionising radiation.
• Believing all protons in the body precess at exactly the same frequency; correct this by explaining that gradient fields make the precession frequency position-dependent.
• Saying protons absorb energy continuously rather than in short RF pulses; correct this by describing brief pulses that excite protons in successive small regions.
• Students often think the RF pulse aligns the protons; in fact, the static magnetic field B0 aligns them, while the RF pulse excites them into the transverse plane.
• Many believe MR uses ionising radiation like X-rays or CT; MR uses non-ionising radio waves and a strong magnetic field, making it safer for repeated use.
• Students sometimes confuse T1 and T2 relaxation: T1 is the time for longitudinal magnetisation to recover (energy transfer to lattice), while T2 is the time for transverse magnetisation to decay (dephasing).

1. 1Start by reviewing the fundamental physics of magnetic moments and precession, ensuring you can define Larmor frequency and calculate it using f = (gamma/2pi)B0.
2. 2Learn the sequence of events in an MR scan: alignment, excitation by RF pulse, relaxation, signal detection, and spatial encoding using gradients.
3. 3Create a table comparing T1 and T2 relaxation times, including their definitions, what they measure, and typical values for different tissues.
4. 4Practice exam-style questions, especially 6-mark explanations of image formation and calculations of Larmor frequency for different field strengths.
5. 5Use flashcards for key terms and active recall to test your understanding of the whole process without notes.

• 📋Calculation of Larmor frequency given B0 and gamma, often requiring conversion between rad s^-1 and Hz. Advice: always divide gamma by 2pi and check units.
• 📋6-mark structured question explaining how an MR image is formed, including roles of B0, RF pulse, gradients, and relaxation. Advice: use a logical sequence and precise terminology.
• 📋Comparison question between MRI and CT or X-ray, focusing on safety, contrast, and physical principles. Advice: emphasise non-ionising nature and soft tissue contrast.
• 📋Explanation of T1 and T2 relaxation and their use in producing contrast between tissues. Advice: clearly distinguish spin-lattice and spin-spin mechanisms.