MRI: T1, T2, FLAIR and diffusion
- Modality
MRI uses a strong magnet and radio waves, not X-rays. Each sequence lights up different tissues, so reading one starts with knowing which it is.
What it is
Magnetic resonance imaging uses the hydrogen protons in water and fat. Inside a strong static magnetic field the protons line up; a radiofrequency pulse tips them, and as they return to rest (relax) they give off a signal that coils pick up. Two relaxation processes, T1 (recovery of the longitudinal magnetisation) and T2 (loss of the transverse magnetisation), differ between tissues. By choosing the repetition time (TR) and echo time (TE), the scanner weights the image towards one of them. T1-weighted images show anatomy well: fat is bright and fluid is dark. T2-weighted images show disease well: fluid and oedema are bright. FLAIR is a T2 image in which the signal of free fluid (cerebrospinal fluid) is cancelled, so lesions next to the ventricles and in the cortex stand out. Diffusion-weighted imaging (DWI) measures how freely water molecules move; where movement is restricted (acute infarct, abscess, very cellular tumour) the tissue is bright on DWI and dark on its ADC map. Gadolinium contrast shortens T1 and makes enhancing tissue bright on T1 after contrast. MRI gives the best soft-tissue contrast of any modality and uses no ionising radiation.
How the image is made
- A strong static magnet (commonly 1.5 or 3 tesla) aligns the hydrogen protons of the body along the field.
- Radiofrequency pulses at the protons' resonance (Larmor) frequency tip them; receiver coils record the signal as they relax.
- Gradient coils vary the field across the body so each signal can be located in space; their switching causes the loud knocking.
- T1-weighted: short TR and short TE. T2-weighted: long TR and long TE.
- Inversion recovery sequences null one tissue: FLAIR cancels free fluid, STIR cancels fat.
- DWI and its ADC map show water diffusion; susceptibility sequences (gradient echo, SWI) show blood products and calcium as dark blooming.
What it is good for
- Brain: stroke (DWI within minutes), multiple sclerosis (FLAIR), tumours, infection and the posterior fossa, which CT shows poorly.
- Spine: cord compression, disc disease, infection and tumour.
- Joints and soft tissues: ligaments, menisci, tendons, cartilage, marrow oedema and soft-tissue masses.
- Abdomen and pelvis: liver lesions, bile ducts (MRCP), rectal and uterine or prostate cancer staging, and the pelvis in pregnancy.
- Heart and vessels: myocardium, scar and angiography, some without contrast.
| Sequence | Bright | Dark | Best for |
|---|---|---|---|
| T1 | Fat, subacute blood (methaemoglobin), gadolinium, protein, melanin | Fluid, CSF, oedema, cortical bone, air | Anatomy, fat, enhancement after contrast |
| T2 | Fluid, CSF, oedema, most disease | Cortical bone, air, fast-flowing blood, some old blood | Finding pathology |
| FLAIR | Oedema and lesions, but not CSF | CSF | Plaques and lesions near the ventricles and cortex |
| DWI with ADC | Restricted diffusion on DWI (dark on ADC) | Free water | Acute ischaemic stroke, abscess, cellular tumour |
Limitations and risks
- Slow (often 20 to 60 minutes) and motion-sensitive: children and confused or breathless patients may need sedation or cannot be scanned.
- Closed bore, noise and stillness: claustrophobia is a common cause of a failed study.
- Implants, devices and metal can make MRI unsafe or spoil the image: every patient is screened first.
- Poor for cortical bone detail, lung parenchyma and small calcifications, where CT is better.
- In the unstable patient and for acute head bleeding in the emergency room, CT is faster and usually first.
- Bright on DWI but also bright on ADC is T2 shine-through, not true restriction.
Next step
- Write a precise clinical question on the request: the radiologist chooses the sequences, and contrast, from it.
- Complete the MRI safety questionnaire and list any implant, device or metal before booking.
- If gadolinium may be needed, check kidney function in patients at risk of kidney disease.
- Suspected acute stroke: follow the local stroke pathway (CT first in most centres; MRI with DWI where available) and call the stroke team.
- Ask the radiologist when unsure which study answers the question; MRI is costly and its slots are few.
Test yourself
Think of the answer, then tap the question to see it.
On which MRI sequence is cerebrospinal fluid bright, and on which is it dark?
Bright on T2; dark on T1 and on FLAIR, where the signal of free fluid is cancelled.
Which pattern on diffusion imaging means true restricted diffusion?
Bright on DWI and dark on the ADC map; bright on both is T2 shine-through.
Why is FLAIR useful in multiple sclerosis?
It is T2-weighted but suppresses CSF, so bright plaques beside the ventricles are not hidden by the bright fluid next to them.
Which TR and TE give a T1-weighted image?
A short repetition time (TR) and a short echo time (TE); T2 weighting uses long TR and long TE.
Connections
Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.
Sources: Magnetic Resonance Imaging Physics (StatPearls (NCBI Bookshelf), 2023).
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Representative model, for educational purposes only.