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MRI physics and sequences
Protons in the B0 field are tipped by an RF pulse at the Larmor frequency, and signal comes from longitudinal recovery (T1) and transverse decay (T2): short TR and TE give T1, long give T2, and an inversion pulse nulls a chosen tissue (FLAIR for fluid, STIR for fat).
Why it matters in the exam
In a magnetic field, protons precess at the Larmor frequency, 42.58 MHz per tesla — about 64 MHz at 1.5 T. An RF pulse at that frequency tips the magnetisation (excitation); that is the role of RF. After the pulse the magnetisation recovers longitudinally — T1 (spin-lattice) — and decays transversely — T2 (spin-spin); T2* also includes field inhomogeneity. Fat is bright on T1 because its T1 is short; fluid is bright on T2 because its T2 is long. Gadolinium is a paramagnetic agent that shortens the T1 of surrounding tissue, so it brightens on T1.
Weighting: short TR and short TE → T1; long TR and long TE → T2; long TR and short TE → proton density (PD). A very long TE gives an image in which only static fluid is bright — this is how MRCP and MR myelography are made. In spin echo, a 180° pulse refocuses the spins and cancels static field inhomogeneity; gradient echo has no 180° pulse, so it is T2*-sensitive — to old blood (haemosiderin), calcium and iron (SWI). Flowing blood is black on spin echo (flow void) because it leaves the slice between the two pulses. In time of flight, blood entering the slice unsaturated is bright — this gives MRA without contrast.
In inversion recovery a 180° pulse inverts the magnetisation, and TI is the time at which the magnetisation of the tissue to be nulled crosses zero. STIR nulls fat (short TI, about 150 ms at 1.5 T) and FLAIR nulls cerebrospinal fluid (long TI, above 2000 ms). Gradients: slice-selection gradient (in axial imaging the longitudinal, Z gradient), frequency-encoding gradient and phase-encoding gradient. DWI: restricted diffusion is bright on DWI and dark on ADC — in acute infarction, abscess, epidermoid cyst and highly cellular tumour; bright on ADC as well is T2 shine-through. Artefacts: chemical shift at fat–water boundaries, susceptibility around metal and air, wrap-around when the FOV is smaller than the body, and magic angle in a tendon at about 55° with short TE.
Key points
- T1 — short TR and TE; T2 — long.
- FLAIR nulls fluid; STIR nulls fat.
- Gradient echo is sensitive to old blood and calcium.
- Bright DWI and dark ADC — restricted diffusion.
- Gadolinium shortens T1.
Summary table
| Sequence | TR and TE or TI | What is bright | Use |
|---|---|---|---|
| T1 | Short TR, short TE | Fat, gadolinium, methaemoglobin, protein | Anatomy, enhancement |
| T2 | Long TR, long TE | Fluid, oedema | Pathology |
| FLAIR | Inversion with long TI | Periventricular oedema; CSF nulled | White matter, multiple sclerosis |
| STIR | Inversion with short TI | Oedema; fat nulled | Bone marrow, musculoskeletal |
| DWI and ADC | Diffusion gradients | Restriction: bright on DWI, dark on ADC | Acute infarct, abscess |
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Sources
- Bushberg et al., The Essential Physics of Medical Imaging, 4th edition (2020) — Magnetic resonance basics; Magnetic resonance imaging
- McRobbie et al., MRI from Picture to Proton, 3rd edition (2017) — Contrast, sequences and artefacts
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.