International A Level Biology / Coordination in animals and plants
Imaging the brain: MRI, fMRI, PET and CT revision notes
Edexcel International A Level Biology (YBI11), chapter 24: Coordination in animals and plants, specification point 8.15. Free lesson.
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CT and MRI show brain structure, while fMRI and PET show brain activity. Knowing how each scan works, what it shows and its risks helps you choose the right scan for a patient.
What each brain scan shows
The four scans are divided into two groups. CT and MRI show the brain’s structure. fMRI and PET show its activity, but they detect different signals. CT (computed tomography) uses X-rays to show structure. It is quick and useful when a stroke is suspected. MRI (magnetic resonance imaging) uses a strong magnetic field and radio waves to show structure in fine detail. fMRI (functional MRI) detects changes in blood oxygen to show areas active during a task. PET (positron emission tomography) follows an injected radioactive tracer to show metabolic activity, such…
What the scan images look like
The appearance of an image reflects what the scan detects. CT and MRI produce slices through the head. An fMRI activation map can be shown on the brain’s surface, with colours marking activity during a task. A PET scan uses colours to show where the tracer has collected. CT shows bone clearly in an X-ray slice. MRI shows finer detail in soft tissues. On an fMRI map, warm colours mark areas that were more active and blue marks areas that were less active. PET colours show tracer uptake, which indicates metabolic activity.
How CT and MRI show brain structure
Both scans use a computer to build cross-sectional slices, which can be stacked to make a 3D image. CT measures how much X-ray each tissue absorbs. MRI detects signals from hydrogen nuclei, mostly in water, which vary between tissues. CT sends X-rays through the head from many angles. Bone absorbs a lot of the X-rays, so bone and bleeding show clearly, but soft tissue detail is lower. MRI uses a strong magnetic field and pulses of radio waves. It has higher resolution for soft tissue, so it can distinguish grey matter from white matter and show the size and position of a small tumour.…
How fMRI and PET show brain activity
When an area of the brain is active, its cells respire faster and more oxygenated blood flows there. fMRI detects this blood oxygen change. PET detects a radioactive tracer taken up by active cells, often a radioactive form of glucose. Oxygenated and deoxygenated haemoglobin respond differently in the magnetic field, allowing fMRI to show where oxygenated blood has gone. In PET, the tracer gives off positrons, which produce gamma rays that the scanner detects. fMRI can make a new image every few seconds, so activity can be followed during a task. PET is slower because the tracer builds up…
Choosing a scan and describing it accurately
Choose the scan that answers the patient’s particular question, then explain why. Do not describe an MRI image simply as a picture or photo. Say what kind of image it is and what it shows. Suspected stroke in an emergency: choose CT because it is quick and shows a bleed straight away. Finding the size and position of a small tumour: choose MRI for the finest soft tissue detail. Finding areas used for speech: choose fMRI because it shows active areas during a task, without radiation. Investigating brain activity in dementia: choose PET because it can show areas of reduced metabolic activity.
Comparing PET and CT
A comparison needs a clear similarity and differences that say what each scan detects and shows. Both use a computer to produce cross-sectional images and both expose the patient to ionising radiation. CT passes X-rays through the head and shows structure. PET uses an injected radioactive tracer and shows metabolic activity. PET has lower resolution than CT.