[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Radiology](https://durgical.com/en/radiology/) › [Neuroradiology](https://durgical.com/en/radiology/areas/neuro/) › Brain MRI: when it beats CT

# Brain MRI: when it beats CT

- Reading

MRI uses a magnet, not X-rays. It is slower than CT but shows brain tissue in far more detail, especially early stroke, the brainstem and small lesions.

Updated Nov 13, 2026

## Key points

- Name the sequence by the CSF: bright on T2, dark on T1 and FLAIR.
- Acute infarct: bright DWI and dark ADC, visible within minutes when CT may be normal.
- SWI is the most sensitive sequence for microbleeds and haemorrhagic diffuse axonal injury.
- MRI beats CT in the posterior fossa, early ischaemia, demyelination, infection and tumours; CT wins for unstable patients and acute trauma.

## What it is

Brain MRI builds several images of the same slices, each weighted to show a different tissue property. Reading it starts by naming each sequence (the colour of the CSF is the quickest clue) and knowing what each one is for: T1 for anatomy and contrast enhancement, T2 and FLAIR for oedema and most lesions, diffusion for acute infarct and pus, and susceptibility sequences for blood. CT stays first for the unstable patient, acute trauma and a quick look for haemorrhage. MRI wins when the question is early or small ischaemia, the posterior fossa, demyelination, infection, tumour, diffuse axonal injury or a normal CT that does not fit the clinical picture.

## What to check

- T1: CSF dark, fat bright, white matter brighter than grey; best for anatomy and, after gadolinium, for enhancing lesions.
- T2: CSF bright; oedema, most tumours, infection and demyelination are bright.
- FLAIR: a T2 image with CSF turned dark, so lesions next to the ventricles and cortex stand out.
- Diffusion (DWI with its ADC map): restricted diffusion is bright on DWI and dark on ADC; seen in acute infarct within minutes, abscess pus and very cellular tumours.
- Susceptibility (SWI or gradient echo): blood products and calcium look dark and larger than they are (blooming); shows microbleeds.
- MRI beats CT in the posterior fossa, where bone artefact on CT hides the brainstem and cerebellum.
- Diffuse axonal injury: small lesions at the grey-white junction, corpus callosum (often the splenium) and dorsolateral brainstem, with CT often normal.

## Reading it step by step (and why)

- Check name, date and the sequences done — you need to know what you are looking at before reading it.
- Name each sequence by the CSF: dark on T1 and FLAIR, bright on T2 — this tells you what bright and dark mean on each image.
- Acute deficit: read DWI and ADC first — bright DWI with dark ADC in a vascular territory is acute infarct.
- Compare DWI with FLAIR in stroke — DWI positive but FLAIR still normal suggests onset within a few hours (useful when onset is unknown).
- Go through FLAIR and T2 for oedema, plaques and masses — most disease is bright here; note location and pattern.
- Look at SWI or gradient echo for dark blooming foci — microbleeds, diffuse axonal injury, old blood, calcium.
- Review post-contrast T1 if given — enhancement marks a broken blood-brain barrier: tumour, abscess wall, active inflammation.
- Check the posterior fossa, brainstem, pituitary and the venous sinuses — CT misses these areas most often.
- Compare with previous CT and MRI and match findings to the clinical question — a lesion's change over time often decides the diagnosis.

| Sequence | CSF looks | Bright means | Best for |
| --- | --- | --- | --- |
| T1 | Dark | Fat, some blood stages, gadolinium enhancement | Anatomy, contrast studies |
| T2 | Bright | Water: oedema, most lesions | General survey |
| FLAIR | Dark | Oedema and lesions, with CSF suppressed | Periventricular and cortical lesions |
| DWI / ADC | Dark on DWI | Restricted diffusion (bright DWI, dark ADC) | Acute infarct, abscess |
| SWI / gradient echo | Variable | Blood and calcium appear dark, not bright | Microbleeds, axonal injury |

## Common mistakes

- Calling a bright T2 lesion acute without checking diffusion: old infarcts are bright on T2 but not restricted on ADC.
- Reading bright DWI alone as restriction: T2 shine-through also looks bright; the ADC map must be dark too.
- Sending an unstable patient to MRI: scans are long, the room is far from the resuscitation area and only MRI-safe equipment can enter.
- Forgetting the safety screen: pacemakers, some implants, metal fragments and pumps need checking before the scan.
- Expecting MRI to show fractures and acute blood as easily as CT: for trauma, CT first.

## Next step

- Suspected stroke: follow the local stroke pathway; CT and CT angiography are usually first, MRI when the diagnosis or onset time is unclear.
- Unexplained deficit, seizure, suspected encephalitis, demyelination or tumour: request MRI with and without contrast, stating the clinical question.
- Head trauma with a poor Glasgow score but a near-normal CT: discuss MRI with SWI and diffusion to look for diffuse axonal injury.
- Before requesting: complete the MRI safety checklist and check kidney function if gadolinium may be given.
- Unsure which sequences or whether contrast is needed: call the radiologist before the scan, not after.

## Test yourself

Think of the answer, then tap the question to see it.

## How can you tell a T2 image from a FLAIR image at a glance?

CSF is bright on T2 and dark on FLAIR; FLAIR is a T2-weighted image with the CSF signal suppressed.

## What combination on diffusion imaging confirms true restricted diffusion?

Bright on DWI and dark on the ADC map; bright DWI with a bright ADC is T2 shine-through, not restriction.

## Where does diffuse axonal injury typically appear on MRI?

At the grey-white matter junction, the corpus callosum (often the splenium) and the dorsolateral brainstem; SWI shows small haemorrhagic foci that CT usually misses.

## Why is MRI better than CT for a suspected brainstem stroke?

Bone around the posterior fossa causes streak artefact on CT, and small infarcts are hard to see; diffusion MRI shows them within minutes.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/radiology/brain-mri-when-it-beats-ct/)

## Elsewhere in DurgiPedia

- [The neurological examination: order and screening exam](https://durgical.com/en/clinical-exam/the-neurological-examination-order-and-screening-exam/)
- [Recognising a stroke: BE-FAST and the NIH Stroke Scale](https://durgical.com/en/clinical-exam/recognising-a-stroke-be-fast-and-the-nih-stroke-scale/)

## Connections

## Related 7

- [MRI: T1, T2, FLAIR and diffusion](https://durgical.com/en/radiology/mri-t1-t2-flair-and-diffusion/)
- [MRI safety: implants, metal and gadolinium](https://durgical.com/en/radiology/mri-safety-implants-metal-and-gadolinium/)
- [Gadolinium and the kidney](https://durgical.com/en/radiology/gadolinium-and-the-kidney/)
- [Reading a head CT step by step](https://durgical.com/en/radiology/reading-a-head-ct-step-by-step/)
- [Imaging in acute stroke: CT first, then CT angiography](https://durgical.com/en/radiology/imaging-in-acute-stroke-ct-first-then-ct-angiography/)
- [Head trauma: who needs a CT (Canadian CT head rule)](https://durgical.com/en/radiology/head-trauma-who-needs-a-ct/)
- [Spine MRI: cord compression, disc and infection](https://durgical.com/en/radiology/spine-mri-cord-compression-disc-and-infection/)

## Related conditions 6

- [Ischaemic stroke](https://durgical.com/en/conditions/ischaemic-stroke/)
- [Traumatic brain injury](https://durgical.com/en/conditions/traumatic-brain-injury/)
- [Multiple sclerosis](https://durgical.com/en/conditions/multiple-sclerosis/)
- [Viral encephalitis (herpes simplex encephalitis)](https://durgical.com/en/conditions/viral-encephalitis/)
- [Brain abscess](https://durgical.com/en/conditions/brain-abscess/)
- [Brain metastases](https://durgical.com/en/conditions/brain-metastases/)

[← All of radiology](https://durgical.com/en/radiology/)

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](https://www.ncbi.nlm.nih.gov/books/NBK564320/) (StatPearls (NCBI Bookshelf), 2023); [Stroke Imaging](https://www.ncbi.nlm.nih.gov/books/NBK546635/) (StatPearls (NCBI Bookshelf), 2026); [Diffuse Axonal Injury](https://www.ncbi.nlm.nih.gov/books/NBK448102/) (StatPearls (NCBI Bookshelf), 2025).

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