MRI physics scares most residents because it is usually taught as equations. You do not need the equations to read scans or to pass FRCR physics: you need about ten ideas, explained simply. Here they are.
How an MR image is made, in five steps
- Protons line up. Hydrogen nuclei in water and fat behave like tiny magnets and align with the scanner's main field (B0), creating net magnetisation along the field.
- They precess at the Larmor frequency, which is proportional to field strength: about 42.58 MHz per tesla, so about 64 MHz at 1.5 T and 128 MHz at 3 T.
- An RF pulse tips them over into the transverse plane, where their rotating magnetisation induces a signal in the receiver coil.
- They relax. Longitudinal magnetisation recovers (T1 recovery) and transverse magnetisation decays as spins dephase (T2 decay).
- Gradients encode position (slice selection, phase encoding and frequency encoding), filling k-space, which a Fourier transform turns into the image.
T1, T2 and PD weighting
| Weighting | TR | TE | Fluid looks | Good for |
|---|---|---|---|---|
| T1 | Short | Short | Dark | Anatomy, fat, contrast enhancement, subacute blood |
| T2 | Long | Long | Bright | Oedema, fluid, most pathology |
| Proton density | Long | Short | Intermediate to bright | Joints, menisci and cartilage |
Memory aid: TR controls T1 weighting, TE controls T2 weighting. Short TR exaggerates T1 differences; long TE exaggerates T2 differences.
| Tissue | T1 | T2 |
|---|---|---|
| Water/CSF | Dark | Bright |
| Fat | Bright | Bright on fast spin echo |
| Cortical bone, air, calcification | Dark | Dark |
| Gadolinium-enhancing tissue | Bright | Little effect at clinical doses |
| Melanin, protein-rich fluid, methaemoglobin | Bright | Variable |
Blood on MRI: the age table
| Stage | Time | Main product | T1 | T2 |
|---|---|---|---|---|
| Hyperacute | Under 24 hours | Oxyhaemoglobin | Iso | Bright |
| Acute | 1–3 days | Deoxyhaemoglobin | Iso | Dark |
| Early subacute | 3–7 days | Intracellular methaemoglobin | Bright | Dark |
| Late subacute | 1–4 weeks | Extracellular methaemoglobin | Bright | Bright |
| Chronic | Months | Haemosiderin | Dark | Dark (rim) |
The classic mnemonic reads the T1 then T2 pattern: "It Be, Iddy Biddy, Baby, Doo Doo" (I = iso, B = bright, D = dark).
The sequences you will meet
- Spin echo and fast spin echo: the workhorses. A 180° refocusing pulse cancels static field inhomogeneity, so they resist susceptibility artifact.
- Gradient echo: fast, no 180° pulse, so it is sensitive to T2* effects. Susceptibility-weighted imaging (SWI) uses this to show microbleeds and calcification.
- Inversion recovery: STIR nulls fat (short inversion time, works at any field strength and with metal); FLAIR nulls CSF so periventricular lesions stand out.
- Diffusion-weighted imaging: restricted diffusion is bright on DWI and dark on the ADC map; acute infarcts, abscesses, epidermoids and highly cellular tumours restrict. Bright on DWI but bright on ADC is "T2 shine-through".
- Fat suppression options: spectral (chemical) fat saturation, STIR, and Dixon (in-phase and opposed-phase, which also detects microscopic fat, as in adrenal adenomas).
MRI artifacts: cause and fix
| Artifact | Cause | Fix |
|---|---|---|
| Motion/ghosting | Movement, breathing, pulsation; appears along the phase-encoding direction | Breath-hold, gating, swap phase and frequency directions, saturation bands |
| Aliasing (wrap-around) | Anatomy outside the field of view in the phase direction | Larger field of view, oversampling, swap directions |
| Chemical shift | Fat and water precess at slightly different frequencies; dark and bright bands at fat–water edges along the frequency direction | Increase receiver bandwidth, fat suppression |
| Susceptibility | Metal, air or blood products distort the local field | Spin echo instead of gradient echo, higher bandwidth, metal artifact reduction sequences |
| Gibbs (truncation) | Too few phase-encoding steps at sharp boundaries; fine parallel lines (can mimic a syrinx) | Increase matrix size |
| Zipper | Radiofrequency leak, for example a door left open | Close the door, check the RF shielding |
| Magic angle | Tendons at about 55° to B0 look bright on short TE sequences | Check on a long TE (T2) sequence |
MRI safety in one minute
- The magnet is always on. Ferromagnetic objects become projectiles; screen every person entering the scanner room.
- Implants are MR safe, MR conditional or MR unsafe. Check the exact device and its conditions (field strength, SAR, spatial gradient).
- RF heating is monitored as SAR; it rises with field strength, so 3 T has tighter limits. Avoid skin-to-skin loops and cable loops that can cause burns.
- Acoustic noise needs hearing protection for every patient.
- Gadolinium: use macrocyclic agents and check renal function in at-risk patients because of nephrogenic systemic fibrosis.
- Quench: rapid boil-off of helium; staff must know the evacuation procedure.
1.5 T versus 3 T
3 T roughly doubles the signal-to-noise ratio, which buys thinner slices or faster scans, and it helps neuro, MSK and prostate imaging. The trade-offs are more susceptibility and chemical shift artifact, higher SAR, more standing-wave (dielectric) artifact in the abdomen and more implants with restrictions. Many body and cardiac studies are still done well at 1.5 T.
Ready to test yourself? Try our 20 free FRCR physics true/false questions, which include MRI.
Frequently asked questions
What is the Larmor frequency at 1.5 T and 3 T?
Hydrogen precesses at about 42.58 MHz per tesla, so about 64 MHz at 1.5 T and about 128 MHz at 3 T.
How do I tell T1 from T2 images quickly?
Look at fluid such as CSF, urine or joint fluid: dark on T1, bright on T2. Fat is bright on both T1 and fast spin echo T2, so use fluid, not fat.
What is the difference between STIR and FLAIR?
Both are inversion recovery sequences. STIR uses a short inversion time to null fat; FLAIR uses a long inversion time to null CSF.
What is T2 shine-through?
A lesion that is bright on DWI because it is very bright on T2, not because diffusion is restricted. The ADC map is bright, not dark, which tells you diffusion is not restricted.
Written by the Radiology Boards team from standard MRI physics teaching. Safety rules summarised; always follow your department's MR safety policy.