Digitally enhanced MRI scan of the human head

The science of medical imaging

How do we see inside the body?

MRI and CT are important ways to see inside the body. They are used, alongside other imaging techniques, to diagnose disease and injury or to screen for conditions in people without symptoms.

CT (computed tomography) is commonly used to look inside the body. It uses X-rays to build up cross-sectional views of the body, which can be used in several ways, including to diagnose bone injuries, look at the chest and lungs, and detect tumours.

Advantages of CT

  • Non-invasive (does not require cutting into the body).
  • Generally quicker than MRI.
  • Suitable for patients with metal (implants etc) in their body.
  • Less expensive than MRI.
  • Good for looking at bones.

Disadvantages of CT

  • Requires expensive, specialised equipment.
  • Some people are allergic to the ‘contrast’ that is drunk before a scan.
  • Uses radiation (more than a typical chest X-ray), and exposure to certain levels of radiation can lead to cancer.
  • Images are restricted to horizontal slices of the body.

MRI (magnetic resonance imaging) is commonly used in hospitals to look at the structures and tissues inside the body – for example, to monitor blood flow and to diagnose or determine how far advanced cancers are. It uses magnets and radio waves to build up cross-sectional images of the body. MRI (and variations of it, such as functional MRI) is also used extensively in biomedical research.

Advantages of MRI

  • Non-invasive (does not involve cutting into the body).
  • Does not use potentially harmful radiation.
  • Can provide three-dimensional information.
  • Good for capturing a variety of soft and hard tissue types (in contrast to CT, which is mainly good for looking at bones).
  • Able to capture functional information (eg fMRI in neuroimaging).

Disadvantages of MRI

  • Requires expensive, specialised equipment.
  • Can be claustrophobic for patients.
  • No metal (including implants and pacemakers) is allowed anywhere near the powerful magnets.

You can find lots more information on medical imaging in our ‘Inside the Brain’ issue.

Lead image:

Digitally enhanced MRI scan of the human head.

Mark Lythgoe and Chloe Hutton/Wellcome Images CC BY NC ND


Further reading

About this resource

This resource was first published in ‘Careers From Biology’ in June 2012 and reviewed and updated in November 2017.

Neuroscience, Careers
Careers From Biology
Education levels:
16–19, Undergraduate, Continuing professional development