Brain imagingEdexcel A-Level Biology A: Revision notes
Section 1
Why we image the brain
Brain imaging lets doctors and researchers see inside the skull without surgery. Scans are used in diagnosis (for example tumours, strokes, bleeding and Alzheimer's disease) and in investigating brain function (which areas are active during a task).
The four techniques divide into two groups:
- Structural: CT and MRI show what the brain looks like.
- Functional: fMRI and PET show which areas are active.
Section 2
CT scans
A computed tomography (CT) scan passes X-rays through the head from many angles. Tissues of different density absorb X-rays to different extents (dense bone absorbs most), detectors record the X-rays that emerge and a computer builds up cross-sectional images.
- Advantages: quick, relatively cheap, shows bone and recent bleeding well, so ideal in an emergency
- Disadvantages: ionising radiation (small cancer risk), less detail of soft tissue
Section 3
MRI scans
A magnetic resonance imaging (MRI) scan uses a strong magnetic field and radio waves. Hydrogen nuclei in water molecules in tissues give off signals that differ between tissues, and a computer builds up detailed images.
- Advantages: no ionising radiation, excellent detail of soft tissue, so good for small tumours and grey and white matter
- Disadvantages: slow, noisy and expensive; patient must stay still; unsuitable for people with pacemakers or some metal implants
MRI does not use X-rays. Do not describe it as using ionising radiation.
Section 4
fMRI scans
Functional MRI (fMRI) uses an MRI scanner to detect changes in the oxygen content of blood. Active neurones respire faster and use more oxygen, so blood flow to active areas increases, which changes the signal.
fMRI therefore shows which areas are active during a task, such as problem solving or speaking. It uses no radioactive tracer, so it can be repeated and used on healthy volunteers.
Section 5
PET scans
In positron emission tomography (PET), a radioactive tracer (often a glucose analogue labelled with a radioactive isotope) is injected into the blood. More active cells take up more glucose, so more tracer collects there. The tracer emits positrons, which lead to gamma rays that are detected, and a computer builds an image of activity.
- Advantages: shows metabolic activity; useful for tumours and Alzheimer's disease
- Disadvantages: ionising radiation, expensive, short-lived tracers must be made nearby
For evaluation questions, give an advantage and a limitation of each scan and finish with a judgement about which suits the situation.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Brain imaging
- A 68-year-old woman arrives at an accident and emergency department after a sudden, severe headache and weakness down one side of her body. Doctors need to find out quickly whether she has bleeding inside her skull.Explain how a CT scan can show the difference between brain tissue, skull bone and a collection of blood.2 marks
- A research team wants to find out which areas of a volunteer's brain are most active while she solves arithmetic problems. The team compares two functional imaging techniques, fMRI and PET.Explain why an active area of the brain shows up on an fMRI scan.2 marks
- A patient with suspected Alzheimer's disease is injected with a glucose analogue labelled with a radioactive isotope that emits positrons. A PET scan shows much less uptake of the tracer in some regions of the cerebral cortex than in a healthy brain.Explain how a PET scan produces an image of activity in the brain.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).