Computed Tomography brain scan

What a CT brain scan is

A Computed Tomography brain scan uses X-rays from multiple angles, reconstructed by computer into cross-sectional images of the brain. It takes 5 minutes from start to finish. There is a small radiation dose, but the risk is well below the benefits of the test in older adults.

UK NICE NG97 recommends offering structural neuroimaging in dementia assessment, with Magnetic Resonance Imaging preferred where available. CT is used as the alternative when MRI is not possible.

When CT is used instead of MRI

• Pacemaker, defibrillator or some other implanted device incompatible with MRI;
• Some metallic implants (cochlear implants, certain stents, intracranial clips);
• Severe claustrophobia not amenable to wider-bore MRI or sedation;
• Acute assessment where speed matters (although less relevant in dementia work-up);
• Where MRI is not locally available.

In our 476-letter audit, around 9.5 per cent of patients had CT rather than MRI, predominantly because of cardiac implants and claustrophobia.

What CT shows in dementia

CT is excellent at ruling out the major structural causes of cognitive symptoms:

• Tumours and other space-occupying lesions;
• Hydrocephalus (enlarged ventricles, sometimes with characteristic patterns);
• Large strokes (chronic infarcts and recent ischaemia);
• Subdural haemorrhage or collection;
• Cerebral atrophy (less precise than MRI);
• White matter changes suggesting Small Vessel Disease (less sensitive than MRI).

What CT does not show as well as MRI

CT is less sensitive than MRI for:

• Subtle medial temporal lobe atrophy (the typical Alzheimer's pattern);
• Small white matter lesions (the early signature of Small Vessel Disease);
• Small infarcts and microbleeds;
• Posterior fossa structures (brainstem and cerebellum);
• Acute ischaemic changes in the first few hours after a stroke.

For these reasons, NICE NG97 prefers MRI in dementia assessment. Where CT is performed, the report uses similar visual rating principles but with less detail.

What to expect on the day

You lie on a flat table that slides into the scanner. The scanner is a wide ring (less enclosed than an MRI machine, often more comfortable for claustrophobia). The scan itself takes a few seconds; positioning and machine setup take a few minutes. No injection is usually needed for a routine dementia work-up CT.

Reading the CT report

A typical CT brain report mentions:

• Ventricles: normal size or enlarged;
• Sulci: normal or widened (suggesting atrophy);
• White matter: any low-attenuation areas suggesting Small Vessel Disease;
• Infarcts: any chronic infarcts or evidence of recent ischaemia;
• Mass lesions: tumours, collections or haemorrhage (usually noted to exclude);
• Bones and sinuses: usually a brief comment.

Common terminology: "age-related involutional change" means atrophy in keeping with the person's age; "chronic ischaemic changes" or "leucoaraiosis" suggests Small Vessel Disease; "no acute intracranial findings" means no recent stroke or haemorrhage; "incidental findings" are findings unrelated to memory (often sinus disease, small skull lesions).

Where to have a CT brain scan

NHS CT scans are arranged through your memory clinic or GP. Private CT scans are widely available in most UK cities, typically costing £200 to £400, with same-week appointments. The Dementia Service can arrange private imaging as part of an assessment package.