MRI brain scan: what we look for

What an MRI brain scan is

Magnetic Resonance Imaging (MRI) uses a strong magnetic field and radio waves, with no ionising radiation, to produce detailed images of the brain. A standard memory clinic MRI includes T1, T2 and FLAIR sequences (which show structure and fluid changes), diffusion-weighted imaging (which detects acute infarcts) and sometimes a haem-sensitive sequence (which shows old micro-bleeds). The scan takes 20 to 40 minutes.

UK NICE NG97 recommends offering structural imaging to anyone undergoing a dementia work-up, with MRI preferred over CT where available. CT is used when MRI is contraindicated (for example, certain pacemakers or metallic implants) or unavailable.

Why MRI matters for diagnosis

MRI serves three purposes:

1. To rule out other causes of cognitive symptoms: tumours, hydrocephalus, large infarcts, demyelinating disease, subdural haematoma, and other structural lesions.
2. To support the diagnostic subtype: medial temporal atrophy in Alzheimer's Disease, vascular changes in Vascular Dementia, asymmetric or lobar atrophy in Frontotemporal Dementia, and relatively preserved structure in early Dementia with Lewy Bodies.
3. To provide a baseline for future comparison. A second scan at 12 to 24 months, when clinical change is suspected, gives much more useful information than a single scan in isolation.

Crucially, NICE NG97 1.2.17 explicitly states that Alzheimer's should not be ruled out on imaging alone. Structural imaging supports rather than determines the diagnosis.

The visual rating scales, explained

UK neuroradiologists increasingly use standardised visual rating scales in their reports. The three you will most commonly see are MTA, GCA and Fazekas.

MTA (medial temporal lobe atrophy), Scheltens Scale

This grades the volume of the medial temporal lobe (which contains the hippocampus, central to memory formation). Grades 0 to 4:

• MTA 0: no atrophy.
• MTA 1: mild widening of the choroid fissure.
• MTA 2: clear hippocampal volume loss; common at age 75+ and consistent with Alzheimer's Pathology in the right clinical context.
• MTA 3 to 4: more pronounced atrophy, generally seen in more advanced dementia.

GCA (global cortical atrophy), Pasquier Scale

This grades atrophy in the cortex as a whole. Grades 0 to 3:

• GCA 0: no atrophy.
• GCA 1: minimal sulcal widening.
• GCA 2: moderate widening, with some volume loss.
• GCA 3: marked atrophy.

Fazekas Scale (white matter hyperintensities)

This grades Small Vessel Disease. Grades 0 to 3:

• Fazekas 0: absent or punctate changes.
• Fazekas 1: a few focal lesions.
• Fazekas 2: moderate, beginning to coalesce.
• Fazekas 3: extensive, confluent disease, consistent with significant Small Vessel Disease.

Other scales sometimes used include the Koedam Scale (parietal atrophy, helpful for Posterior Cortical Atrophy and atypical Alzheimer's) and the Wahlund Scale (an alternative for white matter changes).

How the report relates to your diagnosis

Common patterns in our cohort:

• Alzheimer's Disease. Mean MTA grade 1.8 in our 476 letter audit, with relatively low Fazekas (mean 1.1). Hippocampal volume loss is typical.
• Mixed Alzheimer's and vascular. Mean MTA 1.9, mean Fazekas 2.0. Both temporal atrophy and vascular changes are present.
• Vascular Dementia. Mean Fazekas 2.2 with chronic infarcts or lacunes. MTA may be normal or only mildly raised.
• Dementia with Lewy Bodies. Structural MRI is often relatively unremarkable; the diagnosis is supported by other features (visual hallucinations, REM Sleep Behaviour Disorder, parkinsonism) and may be confirmed with a DAT Scan.
• Frontotemporal Dementia. Asymmetric or lobar (frontal or temporal) atrophy is suggestive; MRI may be normal in early disease.
• MCI. Mild atrophy or age-related involutional changes are common; the diagnostic threshold is functional, not radiological.

What to expect on the day

You will be asked about pacemakers, surgical implants, metal fragments and claustrophobia. You will change into a gown. The scan lasts 20 to 40 minutes. You will hear loud knocking and buzzing sounds; ear protection is provided. You will lie still on a sliding table inside a tunnel-shaped magnet. A handheld buzzer lets you contact the radiographer at any time.

Contrast injections are rarely needed for memory work-up. If they are, the radiographer will explain.

Tips for claustrophobia

• Ask in advance whether a wider-bore scanner is available locally;
• Ask whether a brief pre-scan visit is possible;
• Consider taking a low-dose sedative (your GP can advise), arranging a lift home;
• Keep your eyes closed and use a breathing technique (4-4-4: in 4 seconds, hold 4, out 4);
• Bring a piece of music; many scanners can play it through headphones.

When MRI is not possible: CT

If MRI is not possible, CT (Computed Tomography) provides a reasonable alternative. CT is quick (5 minutes), well tolerated, and excellent at ruling out the major reversible causes. It is less sensitive than MRI for fine atrophy patterns and for small white matter disease. About one in ten people in our audit had CT rather than MRI, predominantly because of metallic implants, claustrophobia or mobility issues.

Functional imaging: SPECT, FDG-PET and DAT

If the structural scan is inconclusive, your clinician may request functional imaging:

• FDG-PET (Fluorodeoxyglucose Positron Emission Tomography) shows brain glucose metabolism and is helpful in early or atypical Alzheimer's and in Frontotemporal Dementia.
• 123I-FP-CIT SPECT (DAT Scan) shows dopamine transporters and is the recommended test if Lewy Body Dementia is uncertain.
• Perfusion SPECT shows cerebral blood flow and supports specific diagnostic patterns.

Access to these is variable across the UK and may be quicker via a private route. The Dementia Service can support arrangement and interpretation where indicated.

Reading the report at home

If you receive a copy of your MRI report, the following terms are common and worth recognising:

• Involutional change: age-related volume reduction;
• Periventricular hyperintensities: white matter changes near the ventricles, usually a vascular sign;
• Microvascular disease or Small Vessel Disease: similar meaning;
• Chronic ischaemic changes: long-standing reduced blood flow;
• Lacunes: small old infarcts (under 1.5 cm);
• Microbleeds: tiny old haemorrhages, often related to Small Vessel Disease or amyloid angiopathy;
• Virchow-Robin spaces: small fluid-filled spaces around blood vessels, usually a normal finding;
• Incidental findings: things noted that are unrelated to memory, such as sinus disease.

Your clinician will translate the report in plain English when discussing the diagnosis.