What does a PET scan show that a CT scan does not?

Which parts of the brain are active

Why do active brain areas show up on a PET scan using a glucose tracer?

Active cells use more glucose for respiration

Investigating the brain: CT and PET scans

The brain is hidden inside the skull, so doctors use scans to see inside it. Damage to the brain and spinal cord is still very hard to treat.

GCSE · 1BI0 · 2.11B, 2.12B (Biology only)
About 13 minutes

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Why the brain is hard to study

The brain is protected by the skull, so it is hard to reach.
Operating on the brain is risky: removing or damaging tissue can affect memory, movement or speech.
Scans let doctors look inside without opening the skull.

CT and PET scans

	CT scan	PET scan
How it works	X-rays pass through the head from many angles; a computer builds up images of slices	A radioactive tracer (such as a form of glucose) is injected; the scanner detects where it collects
What it shows	The structure of the brain: tumours, bleeding or damaged areas	Which areas are most active, because active cells use more glucose
Drawbacks	Uses X-rays, which are ionising	Uses a radioactive tracer, so it is not repeated often

Limits to treatment

Neurones in the brain and spinal cord do not easily repair themselves, so damage is often permanent.
A spinal injury can cut the pathway between the brain and the body, causing paralysis below the injury.
Brain tumours can be hard to remove without damaging healthy tissue around them.
Many drugs cannot pass from the blood into the brain, which makes some conditions hard to treat with medicines.

Worked example

A doctor wants to find out which part of a patient's brain is less active than normal. Which scan should be used, and why?

A CT scan shows structure, not activity.
A PET scan shows where a glucose tracer is used.
Active areas use more glucose, so they show up brightly.

Answer: A PET scan, because it shows which areas are using the most glucose, so it shows activity.

Key idea

CT scans use X-rays to show the structure of the brain. PET scans use a radioactive tracer to show which areas are active. Brain and spinal cord damage is hard to treat because neurones there rarely repair.

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Extra practice

2 more questions

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What does an MRI scan use to make images?
1. Strong magnetic fields and radio waves
2. A radioactive tracer
3. X-rays
4. Sound waves
Show the answer
Answer: Strong magnetic fields and radio waves
MRI uses magnets.
What does a CT scan show?
1. The structure of the brain
2. The blood glucose level
3. Which parts of the brain are active
4. The brain's electrical signals only
Show the answer
Answer: The structure of the brain
CT shows structure.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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