What brain scans can reveal, and why they don't always explain symptoms

In 2007, researchers in the Netherlands scanned the brains of 2,000 individuals aged 45 and above who were leading typical lives. Among them, seven out of every 100 showed signs of a stroke they were unaware they had experienced. Approximately two out of every 100 individuals possessed a small bulge in a blood vessel, and a comparable number had a benign tumor, indicating it was not cancerous.

Nevertheless, only two of these 2,000 people reported symptoms that could be associated with the findings from the brain scans. Both of these individuals exhibited hearing loss.

This highlights the central question: how brain scans are utilized in medicine. A scan provides an image of the brain, yet it cannot independently determine the cause of someone's health issues. Connecting this image to a specific symptom is the challenging part, and it is predominantly accomplished by humans rather than machines.

The most detailed brain structures are often captured through MRI, which employs a potent magnet and radio waves instead of X-rays. MRI can reveal tumors, bleeding, the scar left by a stroke or head injury, or an area of the brain that did not develop correctly before birth. Some of these changes are as minuscule as a grain of rice.

However, a standard MRI cannot demonstrate how the brain's electrical activity fluctuates over time. Migraine and various forms of epilepsy, for instance, often leave the brain appearing completely normal even on a clear scan. This suggests that a clear scan is reassuring, but it does not necessarily mean there is no underlying issue.

The level of detail shown by a scan also relies on the method employed and the individual interpreting it. In my specific field of epilepsy, a German study of 123 people undergoing evaluation for epilepsy surgery discovered that routine scans reported by radiologists lacking specialized epilepsy expertise identified only 39% of abnormalities linked to each person's seizures.

In contrast, scans conducted using an epilepsy-specific approach and read by specialists identified 91%. Scars in the hippocampus, a seahorse-shaped structure deep within the brain that can be a source of seizures, were missed in 86% of the routine reports.

Many scan reports employ the term "lesion," which can evoke fear when encountered online. However, it should not be alarming on its own. A lesion is a deliberately vague term that merely signifies an area appearing different from the surrounding tissue. This could represent a tumor, but it could also be an old scar or a pocket of fluid.

Another frequent finding is small bright spots in the brain's wiring, known as white matter lesions. In an Australian study involving healthy individuals in their 40s, around half had at least one. These become more common with age, and small ones often cause no noticeable problems. Nonetheless, some are associated with changes in the brain's small blood vessels and may warrant a check on risk factors such as high blood pressure.

Thus, a lesion is merely a description, not a diagnosis. It indicates where the doctor should look, not what they have found.

So, how do specialists decide whether something observed on a scan actually explains someone's symptoms? The first consideration is location. Different areas of the brain perform distinct functions. A scar in the area responsible for vision aligns with a seizure beginning with flashing lights. The same scar in someone with tension headaches is less likely to explain their symptoms.

The second consideration is whether other evidence supports the same conclusion. Doctors pay close attention to how the symptoms start and progress. They may also record the brain's electrical activity using small sensors on the scalp or repeat the scan months later to observe any changes. Anything that grows is treated differently from something that has remained unchanged for years.

In epilepsy surgery, this correlation is established formally. Neurologists, radiologists, neurosurgeons, and psychologists gather to determine if the scan, symptoms, and electrical recordings all point to the same brain area. If they agree, removing that small area of tissue can halt seizures entirely in some people. If they do not, the team may implant electrodes inside the brain to pinpoint the source before contemplating surgery.

This process can also work in reverse. A team may decide that a striking-looking abnormality is merely a bystander and leave it alone, thus avoiding an unnecessary operation that would not have benefited the individual. This is why a scan report should be viewed as one piece of evidence rather than a verdict. The overall picture matters, but only after it has been matched to the person it originated from.