⚡ How it works
An X-ray machine moves around the head and takes X-rays from many different angles. A computer puts them together into slices, like slices of bread, to build a picture of the brain.
An Introduction to Neuropsychology » Brain Scanning: CT, PET and fMRI
What you'll learn this session
Study time: 30 minutes
AQA spec: 3.2.3.4
- Why scanning techniques are used to study the brain
- How CT, PET and fMRI scans work in simple terms
- What each scan shows and what it is used for
- The strengths and weaknesses of each scan, and how to compare them
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Unlock This CourseWhy scan the brain?
You cannot open up a healthy person's head to see what their brain is doing. Instead, psychologists and doctors use scanning techniques to look inside the brain safely. Some scans show the brain's structure, which means what it looks like and where things are. Others show its function, which means which areas are working hardest while a person does something.
This is how cognitive neuroscientists identify brain functioning. Three scans are on the spec: CT, PET and fMRI.
Key terms:
- Scanning technique: a way of making a picture of the inside of the brain without opening the head.
- CT scan: a scan that uses X-rays taken from many angles to make a picture of the brain's structure.
- PET scan: a scan that uses a radioactive tracer to show which areas of the brain are active.
- fMRI scan: a scan that uses magnetic fields to detect changes in blood oxygen and blood flow in active areas of the brain.
- Tracer: a tiny amount of a radioactive substance that is injected so a scanner can follow it.
CT scan: a picture of structure
📷 What it shows
It shows structure only. You can see the shape and size of the brain and any damage or change, but you cannot see which areas are active.
What it is used for: finding physical damage or problems, such as the damage caused by a stroke or a head injury.
- Strengths: it is quick, and it costs less than PET and fMRI. It is widely available in hospitals.
- Weaknesses: it uses a small dose of radiation from X-rays. It shows less detail than an fMRI scan, and it cannot show the brain at work.
PET scan: a picture of activity
☢ How it works
A radioactive tracer is injected into the blood. It travels to the brain. Areas that are more active use more of it, so they give off more radiation. The scanner detects this and a computer makes a colour map of the active areas.
🎨 What it shows
It shows activity. The brightest colours mark the areas that are working hardest while the person does a task, such as remembering.
What it is used for: finding out which brain areas are involved in a mental process, and spotting problems in how the brain is working.
- Strengths: it shows brain activity, not just structure, so it links mental processes to brain areas.
- Weaknesses: the person is exposed to radiation from the tracer, so it is used with care. It is expensive. It is slow, because the tracer takes time to build up, so it cannot show very quick changes (it has poor time resolution). The picture is also less detailed than an fMRI scan.
fMRI scan: activity without radiation
🧲 How it works
A strong magnetic field is used to detect changes in blood oxygen and blood flow. When an area of the brain is active, more oxygen-rich blood flows to it. The scanner picks up this change and the computer shows the active areas.
💡 What it shows
It shows activity, laid over a clear picture of the brain's structure. You can see which areas are active during a task.
What it is used for: studying which parts of the brain are used for thinking, memory, language and other mental processes.
- Strengths: it gives very detailed pictures. It uses no radiation, so it is safe. It has better time resolution than PET, so it follows changes more closely.
- Weaknesses: it is expensive. The person must lie very still in a noisy, narrow tube, which is an artificial setting, so they may not behave as they normally would. It measures blood flow, which is only an indirect sign of activity, and the blood flow change is slightly delayed.
Safety note
Because fMRI uses no radiation, it is the safest of the three. CT uses X-rays and PET uses a radioactive tracer, so both involve some radiation.
Comparing the three scans
| CT | PET | fMRI | |
|---|---|---|---|
| How it works | X-rays from many angles | Radioactive tracer shows active areas | Magnetic fields detect blood oxygen and flow |
| Shows | Structure | Activity | Activity |
| Detail | Lower | Lower than fMRI | Very high |
| Radiation | Yes, X-rays | Yes, tracer | None |
| Cost | Lower | High | High |
| Time resolution | Not used for activity | Poor, slow | Better than PET |
Common mistakes
- Saying a CT scan shows brain activity. It only shows structure.
- Saying fMRI uses radiation. It does not. PET uses the radioactive tracer.
- Writing "it is expensive" as the only point. Say which scan, and add a second point, such as detail or safety.
- Mixing up what is detected: PET detects a tracer, fMRI detects changes in blood oxygen and flow.
Exam-style question
(a) Describe how an fMRI scan identifies which areas of the brain are active. [2 marks]
(b) Give one strength and one weakness of PET scans compared with fMRI scans. [2 marks]
(c) Explain one way a CT scan is different from a PET scan. [2 marks]
Model answer
(a) It uses magnetic fields to detect changes in blood oxygen and blood flow (1). More oxygen-rich blood goes to active areas, so these show up on the scan (1).
(b) Strength: it shows which areas are active, so it links mental processes to brain areas (1). Weakness: it is less detailed, or it uses a radioactive tracer, unlike fMRI (1).
(c) A CT scan shows only structure, such as damage (1), but a PET scan shows activity, because the tracer collects in the areas that are working hardest (1).
Exam tip
For "compare" and "explain" questions, name both scans and say what is different about each. A point about only one scan does not earn the second mark.