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.

More recently, magnetic resonance imaging (MRI) has become popular. It can detect the presence of a particular substance in the brain by scanning for its unique chemical “signature”. The images produced are amazingly detailed, but it’s an expensive means of getting them.

How do the widgets in beer cans work?

Widgets – devices in the bottom of beer cans to give a frothy head – consist of a plastic container of nitrogen with a closed valve. The widget is placed in the can; then beer is added along with a small amount of nitrogen to fill the small gap between the top of the beer and the can. The beer is pasteurised, and the heat expands the widget and increases pressure in the can. When it’s opened, the pressure bursts the valve in the widget and lets out the nitrogen. This gives smaller bubbles that last longer and look whiter.

Does a bullet still accelerate once it has left the barrel of a gun?

No. It begins to decelerate as soon as it leaves the barrel, or even before if the barrel is long, because there is no new impulse applied. A bullet fired horizontally from a gun held one metre off the ground will hit the ground at the same time as a pebble dropped from someone’s hand at the same time and height – about half a second later.