How an MRI scan uses magnets and radio waves to peer inside your body

With the advance of technology, doctors can learn a huge amount about what’s going on inside our bodies without cutting us open or taking blood. One of these methods is an MRI scan, but how does it actually work?

A magnetic resonance imaging (MRI) scan uses a combination of radio waves and strong magnetic fields to create an image of inside the body.

MRI scans can see much more detail than a traditional X-ray, including joints, muscles, tendons and ligaments. It can be used to diagnose a huge variety of conditions, including strokes, tumours and spinal chord injuries. Recently, it’s been used to find structural abnormalities in the brain of people with autism and to monitor the eyesight of astronauts after long stints on the ISS.

 MRI works using the fact our bodies are made mostly of water containing hydrogen nuclei, made up of a single proton. When water molecules are placed in a magnetic field, the proton spins become aligned. They ‘point’ in the same direction like a tiny compass.

Inside an MRI scanner, a strong magnetic field is used to align all the protons in the water molecules in your body. 

Short bursts of radio waves are then directed to different areas of the body to create a varying magnetic field which flips the protons out of alignment. When the signals stop, the protons slowly move back into alignment, this sends out a signal that is picked up by the detectors.

Through this process, the MRI scan can build a picture of exactly where all the protons are in the body. This can be used to distinguish between lots of tissue types.

What is a functional MRI (fMRI) scan?

Another kind of MRI scan is called a functional MRI or fMRI scan. This is used to look into the brain and monitors changes in blood flow. It works using a similar technique to standard MRI but tracks oxygen levels instead of hydrogen nuclei.

What is a CAT or CT scan?

Instead of magnetic fields and radio waves, a computerised axial tomography (CT or CAT) scan uses X-rays to detail the structures inside the body. Like an MRI, the machine is a donut-shape and the patient is placed inside.

The bed slowly moves through the machine while the tube rotates. Unlike traditional X-rays, which use a film to capture the image, CT scans use X-ray detectors placed on the opposite side of the tube from the X-ray source. The images can be stacked together to create a 3D picture. 

Sometimes a patient is given a contrast dye, which has to be swallowed or injected, to improve the quality of the images.

CT scans can be used to diagnose heart disease, tumours, clots and damage to internal organs. CT scans could potentially do more harm than MRI scans, as the patient is subjected to a small dose of radiation.