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In 2007, researchers in the Netherlands scanned the brains of 2,000 people aged 45 and over who were going about their ordinary lives. Seven in every 100 had signs of a stroke they never knew they’d had. Nearly two in 100 had a small bulge in a blood vessel, and a similar number had a tumour that appeared to be benign, meaning it was not cancerous.
Yet only two of those 2,000 people reported symptoms that could be linked to what the brain scanner found. Both had hearing loss.
That finding sits at the heart of how brain scans are used in medicine. A scan shows what the brain looks like but it cannot, on its own, tell you why someone is unwell. Joining those two things up is the hard part, and it is still mostly done by people rather than machines.
The most detailed pictures of the brain’s structure often come from MRI, which uses a powerful magnet and radio waves rather than X-rays. It can reveal a tumour, bleeding, the scar left by a stroke or head injury, or a patch of brain that didn’t develop quite as it should have before birth. Some of these changes are smaller than a grain of rice.
What a standard MRI can’t show is how the brain’s electrical activity changes from moment to moment. Migraine and many types of epilepsy, for example, often leave the brain looking completely normal. A clear scan is reassuring, but it doesn’t mean nothing is wrong.
Specialist interpretation
How much a scan shows also depends on how it is done and who reads it. In my own field, epilepsy, a German study of 123 people being assessed for epilepsy surgery found that routine scans reported by radiologists without specialist epilepsy experience spotted only 39% of the abnormalities linked to people’s seizures. Scans done using an epilepsy-specific approach and read by specialists found 91%. Scarring in the hippocampus, a seahorse-shaped structure deep in the brain that can be a source of seizures, was missed in 86% of the routine reports.
Many scan reports use the word “lesion”, and people reading their own results online often find it frightening. On its own, it shouldn’t be. Lesion is a deliberately vague word that simply means an area looking different from the tissue around it.
That could be a tumour, but it could equally be an old scar or a pocket of fluid. Another common finding is small bright spots in the brain’s wiring, known as white matter lesions. In an Australian study of healthy people in their 40s, about half had at least one.
They become more common with age, and small ones often cause no obvious problems. But some are associated with changes in the brain’s small blood vessels and can be a reason to check risk factors such as high blood pressure.
So a lesion is a description, not a diagnosis. It tells a doctor where to look, not what they have found.
Location and context
So how do specialists decide whether something on a scan actually explains someone’s symptoms? The first question is location. Different parts of the brain do different jobs. A scar in the area that handles vision fits with someone whose seizures start with flashing lights. The same scar in someone with tension headaches is less likely to explain their symptoms.
The second question is whether other evidence points the same way. Doctors listen closely to how the symptoms begin and unfold. They may also record the brain’s electrical activity with small sensors on the scalp or repeat the scan months later to see whether anything has changed. Something that grows is treated very differently from something that has sat still for years.
In epilepsy surgery, this matching is done formally. Neurologists, radiologists, neurosurgeons and psychologists meet to ask whether the scan, the symptoms and the electrical recordings all point to the same part of the brain. When they agree, removing that small area of tissue can stop seizures altogether in some people. When they don’t, the team may place electrodes inside the brain to pin down the source before considering an operation.
The process also works in reverse. A team may decide that a striking-looking abnormality is just a bystander and leave it alone, sparing someone an operation that would not have helped.
That is why a scan report is best read as one piece of evidence rather than a verdict. The picture matters, but only once it has been matched to the person it came from.
Zoom in: Here is what progressives most want to see go to a House vote in 2027, according to lawmakers, candidates and operatives who spoke to Axios:
