MRI: Alignment and Precession Revision Notes | A Level AQA Physics

1. Why MRI Matters: Seeing Inside Without Radiation

X-rays and CT scans use ionising radiation — energy high enough to strip electrons from atoms and potentially damage DNA. MRI takes a completely different approach: it uses magnetic fields and the quantum properties of hydrogen nuclei to build detailed cross-sectional images of the body, with no ionising radiation involved.

The human body is roughly 60% water, and every water molecule contains two hydrogen atoms. Each hydrogen nucleus is a single proton — and protons behave like tiny spinning magnets. MRI exploits this fact on a grand scale, aligning billions of protons simultaneously using a powerful superconducting magnet.

Understanding MRI at A-level means grasping two core physical ideas: alignment (what happens to proton spins when a magnetic field is switched on) and precession (how those aligned protons wobble in a characteristic way). These two ideas underpin everything else in the scanner.

2. The Superconducting Magnet and the Patient Cross-Section

At the heart of every MRI scanner is a superconducting magnet — a coil of wire cooled to near absolute zero (typically using liquid helium) so that its electrical resistance drops to zero. Because there is no resistance, a current can circulate indefinitely without any power input, sustaining a magnetic field that is far stronger and more stable than any conventional electromagnet could produce.

The patient lies inside the bore of this magnet, immersed in a magnetic field that typically ranges from 1.5 T to 3 T in clinical use. The scanner does not image the whole body at once; instead, it selects a thin cross-sectional slice of the patient by applying additional gradient magnetic fields that vary in strength along the body's length. Protons in different slices experience slightly different field strengths, which means they precess at slightly different frequencies — allowing the scanner to distinguish signals from one slice versus another.

The AQA specification does not require you to explain how these gradient fields are produced or how the image is reconstructed mathematically. What you do need to understand is the physical principle: spatial variation in field strength encodes positional information, enabling a cross-sectional map of the patient to be built up from the proton signals detected.

3. Proton Alignment: Spins Parallel to the Field

Picture a compass needle in the Earth's magnetic field: it settles along the field direction because that is the lowest-energy orientation. Hydrogen protons behave similarly, but with a quantum twist — they can only adopt one of two orientations relative to an external field: parallel (spin aligned with the field, lower energy) or anti-parallel (spin opposing the field, higher energy).

When a patient enters the scanner and the superconducting magnet is active, the vast majority of hydrogen protons settle into the parallel alignment. The energy difference between the two states is tiny, so a small excess — perhaps a few protons per million — ends up in the parallel state rather than the anti-parallel state. However, because the body contains an enormous number of hydrogen nuclei, this small excess produces a measurable net magnetisation aligned with the field.

A common point of confusion here is the word 'aligned'. It does not mean the proton's spin axis points perfectly along the field like a rigid rod. The proton is still spinning, and its axis is tilted slightly away from the field direction — which leads directly to the next key phenomenon: precession.

4. Precession: The Wobbling Proton

Think of a spinning top that is not perfectly upright. Gravity pulls it downward, but because it is spinning, it does not fall over — instead, its axis traces out a cone shape around the vertical. This wobbling motion is called precession. A hydrogen proton in a magnetic field does exactly the same thing: the magnetic force on its spinning charge causes its spin axis to trace a cone around the direction of the magnetic field lines.

The rate at which the proton precesses is called the Larmor frequency. Crucially, the Larmor frequency is directly proportional to the strength of the magnetic field: a stronger field causes faster precession. For the field strengths used in clinical MRI (1.5 T to 3 T), the Larmor frequency falls in the radiofrequency (RF) range — around 64 MHz at 1.5 T and 128 MHz at 3 T.

The Larmor frequency relationship is the key to how MRI achieves spatial encoding. Because the gradient coils create a field that varies slightly from one end of the patient to the other, protons at different positions along the body precess at slightly different frequencies. The scanner can therefore identify where a signal comes from by analysing its frequency — this is the physical basis for building a cross-sectional image.

5. Putting It Together: Basic Principles of the MR Scanner

Now that alignment and precession are clear, the overall sequence of events in an MR scanner follows a logical chain. The superconducting magnet creates a strong, uniform field. Hydrogen protons throughout the patient's body align with spins parallel to this field. Each proton's spin axis precesses about the field lines at the Larmor frequency. Gradient coils then impose a small, controlled variation in field strength across the patient, so that protons at different positions precess at slightly different frequencies.

A radiofrequency (RF) pulse is then applied at the Larmor frequency. This is a resonance condition: the RF energy matches the precession frequency exactly, so it is absorbed by the precessing protons and tips their net magnetisation away from the field direction. When the RF pulse is switched off, the protons return towards their equilibrium alignment, emitting RF signals as they do so. These emitted signals are detected by receiver coils surrounding the patient.

The AQA specification explicitly states that you will not be asked about de-excitation relaxation times (the T1 and T2 times that describe how quickly protons return to equilibrium). You should, however, be able to describe the full sequence from alignment through precession to signal detection, and explain how cross-sectional images are formed from the spatially encoded frequency information.

• Superconducting magnet switched on — strong, stable magnetic field created through the patient
• Hydrogen protons align with spins parallel to the field (small net excess in lower-energy parallel state)
• Each proton precesses about the field lines at the Larmor frequency ($f \propto B$)
• Gradient coils vary field strength along the patient — protons at different positions precess at different frequencies
• RF pulse at the Larmor frequency excites the precessing protons, tipping their net magnetisation
• Protons return to equilibrium, emitting RF signals that are detected and used to reconstruct a cross-sectional image

6. Comparing MRI with Other Imaging Techniques

MRI is classified as a non-ionising imaging technique. The radiofrequency radiation used to excite protons has photon energies far too low to ionise atoms or break chemical bonds — in stark contrast to X-rays and gamma rays used in CT scanning and PET scanning. This makes MRI particularly suitable for repeated use and for imaging sensitive tissues such as the brain and spinal cord.

MRI also provides excellent soft-tissue contrast. Because different tissues (muscle, fat, tumour, cerebrospinal fluid) contain different concentrations of hydrogen and different local chemical environments, the proton signals they produce differ in strength and character. This gives MRI a significant advantage over X-ray-based techniques for imaging soft structures, where X-ray contrast is poor.

7. Exam Technique: Scoring Full Marks on MRI Questions

AQA questions on MRI alignment and precession tend to use the command words 'describe' and 'explain'. These require different responses. A 'describe' answer states what happens in sequence, without needing to give reasons. An 'explain' answer must include the physical principle — the 'because' — that links cause to effect.

When asked to explain precession, many students write that protons 'spin around the field lines' — this is imprecise and will not score the mark. The correct language is that the proton's spin axis traces a cone about the direction of the magnetic field lines. Equally, when describing alignment, state explicitly that spins are parallel to the field and that this is the lower-energy state — both details are mark-worthy.

Worked example: Answering a 4-Mark 'Explain' Question on MRI Alignment and Precession

Explain how hydrogen protons in a patient's body behave when the patient is placed inside an MRI scanner. [4 marks]

1. Command word: 'Explain' — requires cause and physical principle, not just description. Plan: alignment → precession → Larmor frequency → link to imaging. — Spotting 'explain' tells you that stating facts alone is insufficient. Each point needs a 'because' or a physical reason to earn the mark. [M1 available for each correct explained point]
2. The hydrogen protons (spinning nuclei) align with their spins parallel to the magnetic field lines of the superconducting magnet, because this is the lower-energy orientation. — Two mark-worthy elements here: 'spins parallel to the field' and 'lower-energy state'. Either earns [B1]. Saying 'protons line up' without specifying parallel orientation or energy is too vague.
3. The spin axis of each proton precesses (traces a cone) about the direction of the magnetic field lines, because the magnetic force on the spinning charge causes the axis to wobble rather than remain stationary. — 'Spin axis precesses about the field lines' is the precise mark-scheme phrase. [B1] Avoid writing 'the proton rotates around the field line' — this conflates orbital motion with precession.
4. The proton precesses at the Larmor frequency, which is proportional to the magnetic field strength: $f \propto B$. — [B1] for stating the proportionality. This is also the link to spatial encoding — gradient fields vary $B$, so protons at different positions precess at different frequencies.
5. Because gradient coils vary the field strength across the patient, protons at different positions precess at different Larmor frequencies. The scanner detects these frequency differences to build a cross-sectional image. — [B1] for the link between frequency variation and positional encoding. This completes the four-mark chain: alignment → precession → Larmor frequency → cross-sectional image.

Answer: Hydrogen protons align with spins parallel to the magnetic field (lower-energy state). Their spin axes precess about the field lines at the Larmor frequency, which is proportional to the local field strength. Gradient coils vary the field across the patient, so protons at different positions precess at different frequencies. The scanner uses these frequency differences to build a cross-sectional image. [4 marks]

The core of this topic comes down to two physical ideas working together. Alignment gives the scanner a net magnetisation to work with — the small excess of parallel-spin protons across the patient's body. Precession at the Larmor frequency gives the scanner a way to encode position — because gradient fields make that frequency vary with location, turning the patient's body into a frequency map that can be decoded into a cross-sectional image.