How do doctors look inside brains without surgery?

We explore the curious questions that science can answer

MRI scans detect the presence of a particular substance in the brain by scanning for its unique chemical ‘signature’ (Getty/iStock)

How do doctors look inside brains without surgery?

The oldest technique is X-ray imaging. X-rays are good for examining skull fractures, but reveal little about the fine structure of soft brain tissues. An improvement came with computerised tomography (CT), which uses a series of small X-ray beams at different angles to one another to give a more detailed picture. CT gives pictures of a slice of brain (tomos is the Greek word for “cut”) which help diagnose many brain diseases.

In positron emission tomography (PET), the patient is injected with weakly radioactive compounds. A scanner picks up the radioactivity emitted, indicating where the blood is being used – ie, where the brain is working hardest. Such scans are useful not just for diagnosing brain problems, but also to understand how a healthy brain works.