MRI revision
MRI for FRCR Part 1 Physics.
MRI accounts for six of the forty questions in the current RCR Physics distribution — thirty independently marked statements. This is a concept map for revision rather than a textbook chapter: work through each block, then test yourself with the checklist at the end.
Precession and resonance
- Hydrogen nuclei possess spin and a magnetic moment. In a static field B₀, their magnetic moments precess at the Larmor frequency. For a given nucleus the Larmor frequency is proportional to B₀, so proton precession frequency is higher at higher field strength.
- At equilibrium, a small excess of spins occupies the lower-energy state, producing a net longitudinal magnetisation aligned with B₀. The available equilibrium magnetisation generally increases with field strength and contributes to the higher potential SNR of higher-field systems.
- An RF pulse applied at the resonance frequency transfers energy to the spin system, tips the net magnetisation away from the longitudinal axis and creates coherent transverse magnetisation. The rotating transverse magnetisation induces the received MR signal.
Relaxation and contrast
- T1 describes recovery of longitudinal magnetisation after excitation. T1 varies between tissues and generally lengthens as field strength increases, although the exact value is tissue- and field-dependent.
- T2 describes loss of transverse phase coherence through spin–spin interactions. T2* also includes dephasing caused by static field inhomogeneity, so T2* is shorter than or equal to T2. A spin-echo refocusing pulse compensates for much of the reversible static-field dephasing.
- In conventional spin-echo imaging, a short TR accentuates T1 weighting and a long TE accentuates T2 weighting. Short TR/short TE is therefore typically T1 weighted, long TR/long TE T2 weighted, and long TR/short TE reduces both T1 and T2 weighting towards proton-density contrast.
- Gradient-echo contrast also depends on flip angle and sequence timing because there is no 180° refocusing pulse. Inversion-recovery sequences add an inversion time (TI) that can be selected to null a particular tissue, as in STIR and FLAIR.
Spatial encoding and k-space
- Slice selection. Applying a magnetic-field gradient during a frequency-selective RF pulse makes resonance frequency depend on position. Slice thickness is determined by the RF excitation bandwidth together with the strength of the slice-select gradient.
- Phase encoding. A brief gradient imposes a position-dependent phase shift. In conventional Cartesian 2D imaging, different phase-encoding amplitudes are acquired over repeated excitations, making the number of phase-encoding steps an important determinant of scan time.
- Frequency encoding. A gradient applied during signal readout makes precessional frequency vary with position along the readout axis.
- k-space. K-space stores spatial-frequency data rather than a miniature version of the image. Central k-space is dominated by low spatial frequencies and strongly influences overall signal and contrast; the periphery increasingly contains high spatial frequencies that define edges and fine detail. All sampled k-space contributes to the reconstructed image.
SNR, resolution and time trade-offs
- Larger voxels collect more signal. Reducing slice thickness or increasing the matrix at unchanged field of view therefore improves spatial resolution but reduces SNR per voxel unless another parameter compensates.
- SNR increases approximately with the square root of the number of signal averages, while scan time increases approximately in direct proportion to the number of averages.
- Wider receiver bandwidth reduces chemical-shift displacement and shortens readout, but increases the amount of noise admitted per voxel and therefore lowers SNR.
- Conventional acquisition time depends on sequence architecture as well as TR, phase-encoding steps and averages. Fast spin echo, parallel imaging and other acceleration methods shorten acquisition with their own image-quality trade-offs.
- Receive-coil geometry matters: a coil closely matched to the anatomy can improve SNR over its intended field of sensitivity.
Artefact families
| Artefact | Typical appearance | Useful controls |
|---|---|---|
| Motion and ghosting | In conventional Cartesian imaging, periodic motion commonly produces replicated signal along the phase-encoding direction. | Immobilisation, shorter acquisitions and appropriate respiratory or cardiac synchronisation can help. Changing the phase-encoding direction changes where Cartesian ghosts appear. |
| Wrap / aliasing | Tissue outside the field of view folds onto the opposite side of the image. | Increase the field of view or use oversampling/no-phase-wrap. Saturation bands can also suppress unwanted signal outside the region of interest. |
| Chemical shift (first kind) | Fat–water misregistration occurs along the frequency-encoding direction at tissue interfaces. | Increase receiver bandwidth or use fat suppression when appropriate. The displacement increases with field strength and decreases as receiver bandwidth increases. |
| Susceptibility | Signal loss and geometric distortion occur near metal, air–tissue interfaces or blood products and are usually more conspicuous on gradient-echo imaging. | Spin-echo or fast-spin-echo techniques, shorter TE, higher receiver bandwidth and thinner slices can reduce susceptibility artefact. |
| Magic angle | Ordered collagenous tissue near 55° to B₀ can show artificially increased signal on short-TE sequences. | Confirm the finding on a longer-TE sequence rather than diagnosing disease from it alone. |
| Truncation (Gibbs) | Parallel ripples appear next to a sharp high-contrast boundary. | Increasing the number of encoding steps reduces truncation; filtering can suppress the ripples at a cost in sharpness. |
Safety categories
Learn MRI hazards by their physical source. Local access rules and implant-screening procedures remain essential, but they should sit on top of the underlying physics.
- Static field. B₀ is present continuously while a superconducting magnet is energised. Ferromagnetic objects can experience translational force and torque, creating projectile and implant hazards. The spatial extent of the fringe field is magnet-specific and is one basis for controlled access and zoning.
- Gradient fields. Rapidly switched gradients induce electric fields in tissue and can cause peripheral nerve stimulation. Switching gradient currents also produces mechanical forces and vibration of the gradient assembly within B₀, generating the high acoustic noise of MRI; hearing protection is therefore required.
- RF fields. Absorbed RF energy can heat tissue and is monitored using quantities such as specific absorption rate (SAR). RF deposition depends on the sequence, field strength, flip angles/duty cycle, patient and setup rather than on one parameter alone. Conductive leads or loops and skin contact points can create local heating hazards.
- Implants and devices. MR Conditional devices are safe only under the manufacturer-specified conditions. Implant assessment must therefore use the exact device information and the planned scan conditions. Cryogen release during a quench is a separate environmental hazard, including asphyxiation and cold injury if venting fails.
Test yourself
Answer these aloud before moving on. They are conceptual revision prompts, not copied examination or question-bank items.
- Why does resonance occur at the Larmor frequency, and how does that frequency change with B₀?
- What physically distinguishes T1 recovery from T2 decay, and why is T2* shorter than or equal to T2?
- How do TR and TE influence spin-echo contrast at a conceptual level?
- How do the slice-select gradient and RF bandwidth determine which slice is excited?
- Why does conventional Cartesian phase encoding contribute strongly to scan time?
- What information is concentrated near the centre of k-space and what is represented increasingly towards its periphery?
- If voxel volume is reduced, what happens to SNR and what trade-offs can recover it?
- Which common MRI artefacts depend on phase- or frequency-encoding direction?
- Which hazards arise from the static field, switched gradients and RF energy?
Test yourself
One FRCR Physics-style sample stem
This is newly authored public sample content and is not taken from the protected question bank. Mark each statement True or False, then check the explanations.
Regarding MRI signal, encoding and safety:
True
The Larmor relationship links precessional frequency directly to B0 for a given gyromagnetic ratio.
False
T2* includes true T2 dephasing plus additional dephasing from static-field inhomogeneity, so T2* is shorter than or equal to T2.
True
Low spatial frequencies are concentrated near the centre of k-space, while progressively higher spatial frequencies contribute increasingly to fine detail and edges.
False
A wider receiver bandwidth admits more noise and therefore generally reduces SNR per voxel, although it can reduce chemical-shift displacement and shorten readout.
True
Time-varying gradient fields induce electric fields in tissue and can stimulate peripheral nerves.
Physics revision guides
Choose your next guide
Core candidate guides
Exam format and syllabus distribution
The 40-stem, 200-item paper, timing, marking, current topic distribution and official candidate guidance.
Pass mark and standard setting
How modified Angoff and Hofstee standard setting work, and why there is no permanent fixed percentage.
Revision strategy
A practical route from concept learning and topic blocks to mixed practice, error repair and full mocks.
True/False exam technique
Statement-level pacing, absolutes, units, proportionality, trade-offs and a disciplined review method.
Mock exams and question-bank practice
Use targeted stems and exam-length mocks for different jobs, then convert each result into focused recovery work.
Topic revision guides
Matter and Radiation
FRCR Physics matter and radiation revision: atomic structure, photon energy, attenuation, interactions, inverse square law and common True/False traps.
Radiography and Fluoroscopy
FRCR Physics radiography and fluoroscopy revision: X-ray production, detectors, grids, AEC, image quality, fluoroscopic dose and common exam traps.
Radionuclide Imaging
FRCR Physics radionuclide revision: decay, gamma cameras, collimators, SPECT, PET, counting statistics, corrections, dose and common True/False traps.
Radiation Safety
FRCR Physics radiation safety revision: dose quantities, biological effects, IRR17, IR(ME)R 2017, duty holders, optimisation, DRLs and common traps.
Ultrasound
FRCR Physics ultrasound revision: acoustics, transducers, pulse and beam properties, resolution, Doppler, artefacts, safety and common True/False traps.
Computed Tomography
Attenuation, Hounsfield units, helical acquisition, reconstruction, dose indices, artefacts and dual-energy CT.
Practise what you have revised
Work five complete stems with no account, or sit an exam-length mock with a free account.
Further reading
The Physics Bank is independent examination preparation. It is not affiliated with or endorsed by the Royal College of Radiologists. Exam rules change: always check the current RCR candidate guidance for your own sitting.