AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Basic principles of MR scanner; cross-section of patient scanned using…
Full specification wording
Magnetic resonance (MR) scanner (A-level only): Basic principles of MR scanner; cross-section of patient scanned using magnetic fields protons initially aligned with spins parallel spinning hydrogen nuclei (protons) precess about the magnetic field lines of a superconducting magnet 'gradient' field coils used to scan cross-section short radio frequency (RF) pulses cause excitation and change of spin state in successive small regions protons excited during the scan emit RF signals as they de-excite RF signals detected and the resulting signals are processed by a computer to produce a visual image.
An MR scanner images a patient's cross-section by exploiting hydrogen nuclei, single protons, in body tissue.
Checked against the AQA 7408 specification (opens in a new tab) on .
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
An MR scanner images a patient's cross-section by exploiting hydrogen nuclei, single protons, in body tissue. In a strong uniform field from a superconducting magnet, proton spins align parallel to the field and precess about the field lines at the Larmor frequency. Gradient field coils add small position-dependent fields so the precession frequency varies with location, labelling each small region. A short radio frequency pulse at the matching frequency excites protons in one region, flipping their spin state. When they de-excite, those protons emit RF signals. A detector coil picks up the signals, and a computer processes them to build a visual image. Scanning successive small regions builds the whole cross-section.
What earns the marks
- 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.
Common mistakes and how to put them right
Thinking the scanner uses X-rays or ionising radiation.
Put it right 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.
Put it right by explaining that gradient fields make the precession frequency position-dependent.
Saying protons absorb energy continuously rather than in short RF pulses.
Put it right by describing brief pulses that excite protons in successive small regions.
Exam tips
- 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.
Try the lesson
The start of the Magnetic resonance (MR) scanner (A-level only): Basic principles of MR scanner; cross-section of patient scanned using magnetic fields protons initially aligned with spins parallel spinning hydrogen nuclei (protons) precess about the magnetic field lines of a superconducting magnet 'gradient' field coils used to scan cross-section short radio frequency (RF) pulses cause excitation and change of spin state in successive small regions protons excited during the scan emit RF signals as they de-excite RF signals detected and the resulting signals are processed by a computer to produce a visual image. lesson, made from checked teaching notes for this specification point.
Have a go first
In an MR scanner, which nuclei in the patient's body are the ones that precess about the magnetic field lines and emit the detected RF signals?
Listen
An MR scanner images a patient's cross-section by exploiting hydrogen nuclei, single protons, in body tissue. In a strong uniform field from a superconducting magnet, proton spins align parallel to the field and precess about the field lines at the Larmor frequency. Gradient field coils add small position-dependent fields so the precession frequency varies with location, labelling each small region. A short radio frequency pulse at the matching frequency excites protons in one region, flipping their spin state. When they de-excite, those protons emit RF signals. A detector coil picks up the signals, and a computer processes them to build a visual image. Scanning successive small regions builds the whole cross-section. A strong answer earns marks for these points. 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. A common slip: Thinking the scanner uses X-rays or ionising radiation. Instead, stating that MR uses a strong magnetic field and radio frequency pulses with no ionising radiation. Exam tip: Link each stage in order: alignment and precession, gradient encoding, RF excitation, RF emission on de-excitation, detection, computer image reconstruction.
Key words
Say what each word means before you turn the card.
Precession in a magnetic field
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Spatial encoding using gradient fields
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Signal detection and computer image reconstruction
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See every definition
- Precession in a magnetic field
- Protons in the strong uniform field of a superconducting magnet initially align parallel to the field and precess about the magnetic field lines.
- Spatial encoding using gradient fields
- Gradient field coils superimpose small position-dependent fields so that the precession frequency differs between small regions of the patient.
- Signal detection and computer image reconstruction
- Excited protons emit RF signals as they de-excite, these signals are detected by a receiver coil, and a computer processes them to reconstruct a visual image.
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Test your understanding of Magnetic resonance (MR) scanner (A-level only): Basic principles of MR scanner; cross-section of patient scanned using magnetic fields protons initially aligned with spins parallel spinning hydrogen nuclei (protons) precess about the magnetic field lines of a superconducting magnet 'gradient' field coils used to scan cross-section short radio frequency (RF) pulses cause excitation and change of spin state in successive small regions protons excited during the scan emit RF signals as they de-excite RF signals detected and the resulting signals are processed by a computer to produce a visual image.
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