Explain how an MRI scanner produces an image of soft tissues inside the body, and state one advantage of MRI over X-ray imaging when examining soft tissue organs.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
MRI (magnetic resonance imaging) is a non-ionising imaging technique used widely in hospitals to examine internal soft tissue structures such as the brain and spinal cord.
Model answer (5 marks)
MRI uses a strong magnetic field to align the hydrogen nuclei (protons) in the body. A radio‑frequency pulse is then applied; when it is switched off the protons relax and emit radio signals. The signals differ in strength and timing for different tissues, so a computer can construct a detailed image of the soft tissues.
One advantage of MRI over X‑ray imaging is that MRI gives much better contrast for soft tissues, allowing clearer images of organs such as the brain and spinal cord.
One advantage of MRI over X‑ray imaging is that MRI gives much better contrast for soft tissues, allowing clearer images of organs such as the brain and spinal cord.
Examiner tips
- Show the sequence: magnetic field → proton alignment → RF pulse → relaxation signals → image construction
- Mention the key advantage: superior soft‑tissue contrast
Common mistakes
- Confusing ionising radiation with the magnetic field; forgetting to mention the RF pulse
- Failing to state that MRI provides better soft‑tissue contrast
Mark scheme (5 marks)
- MRI uses a strong magnetic field (and radio waves) applied to the body
- The magnetic field causes hydrogen nuclei (protons) in the body to align / respond to the field
- When the radio waves are switched off, the nuclei emit signals / release energy as they return to their original alignment
- Signals from different tissues differ / the strength or timing of signals depends on tissue type, allowing a detailed image to be constructed
- MRI does not use ionising radiation (so there is no risk of cell/DNA damage) / MRI gives much better contrast / detail for soft tissue than X-rays
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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