Me, Myself and MRI

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Seeing inside the human body

Before doctors can treat a patient they have to find out what the problem is. X-rays are good at showing damage to hard tissues like bones, but they pass straight through softer tissues such as muscles, blood vessels and nerves. An MRI scanner can see details of these types of soft tissue. In fact it is even possible to observe how our brain responds when we are listening to music or looking at pictures.

What is MRI?

Magnetic Resonance Imagining (MRI) is a medical scanning technique. It does not need any surgery and yet can produce the most amazing and highly detailed images of the inner structures of the human body. MRI relies on magnetism, rather than radiation, and so it is even safer than X-rays.

MRI works because 60% of the human body is made up of water. Each molecule of water, or H20, contains two hydrogen atoms and one oxygen atom. The MRI scanner can detect these hydrogen atoms and uses them to build up a detailed image of organs such as the brain.

How does MRI work?

The MRI scanner contains very powerful magnets that can be switched on and off independently. These magnets produce two strong magnetic fields at right angles to each other. The person in the scanner does not feel any effects of the magnetic fields but they do affect the hydrogen atoms inside their body.

When the person lies in the MRI scanner, one magnetic field causes the centres of the hydrogen atoms in the water molecules inside their body to become aligned with the magnetic field. The hydrogen atoms are now all aligned in the same direction and turning like spinning tops. The patient feels absolutely nothing.

The second magnetic field is turned on and off in a series of quick pulses. Each pulse causes the hydrogen atoms to change their alignment and then relax back to the original state when it is switched off. This is called resonance and is a little like twanging a stretched rubber band. These changes cannot be felt by the patient but they do cause tiny changes in the magnetic field passing through their body. The MRI scanner detects these tiny changes and turns them into an image.

The scanner contains large, sensitive wire coils placed around the machine. They are sensitive enough to detect the changes in the magnetic field that are caused by the hydrogen atoms resonating in the pulsed magnetic field. This allows the scanner to build up a picture of where the water molecules are in the body and so show the details of the internal soft tissues. The scanner takes many readings and uses the information to build up a computer-generated image of a slice through the part of the body that is being scanned.

For example, sixty of the on-off pulses might be used to produce twenty slices that give a complete image of the brain. This whole process would only take about three minutes and it would show muscles, blood vessels and other tissues in remarkable detail.

The information can be used to create 2-dimensional, or 'flat' images of each slice or processed further to build a complete 3-dimensional view that can be explored and examined from all angles. Many of these 2-D and 3-D images have been used in the portraits that make up this exhibition.

Next section: What's it like when you have an MRI scan?