Magnetic resonance imaging (MRI) [explained]

Magnetic resonance imaging (MRI) can be used to detect things an X-ray can’t. MRI can look for disease, bleeding, Parkinson’s disease and head injuries.

For example, we can use MRI to image water movement in the brain (called diffusion MRI) and also to learn more about disorders such as dyslexia or attention deficit hyperactivity disorder (ADHD).

MRI is also very useful for looking at blood flow in our bodies. Arteries are responsible for carrying oxygen around our bodies to our tissues and organs, and the MRI can tell us how the blood is flowing and the concentration of the oxygen in it. The inner lining of arterial blood vessels is normally smooth, allowing blood to flow easily. In smokers, the lining of the arteries can become damaged, leading to build-up of cholesterol and other lipids. This causes the arterial wall inner lining to become rough and thickened, leading to a high risk of a heart attack or stroke. The MRI can be used to diagnose this, and patients can undergo bypass surgery or have stents inserted that widen the affected arteries.

Because MRI doesn't use ionising radiation, it doesn't cause any lasting damage to the patient, so a person can have numerous MRI scans, although they are expensive – the machines required to do them are very expensive (in New Zealand, they cost over one million dollars) and skilled staff need to run the equipment and interpret the results.

How does MRI work?

How many atoms do you think are inside your body? Billions and billions. The important thing for MRI is that about 63% of them are hydrogen.

Atoms never stop moving, as long as the temperature remains above absolute zero –273°C, so these billions of hydrogen atoms in your body are constantly moving, like spinning tops, in all directions.

If you put these spinning atoms in very strong magnetic field, like in an MRI, they act like little bar magnets. Some atoms align with the field, and the rest align against it. It is not quite 50/50 though, and this is very important. Aligning with the field required slightly less energy and so for every two million hydrogen atoms, about nine more will line up with the field. This doesn’t sound like many, but since you have billions of them, it is enough.

Radio waves are then pulsed into the magnetic field, causing the extra nine atoms lined up with the magnetic field to align in a different direction. This different alignment is detected and converted to an image by a computer.

So MRI relies on the spin of hydrogen atoms being affected by a strong magnetic field. To get it to work, the magnetic field must be very large.

MRI scanners are basically large superconducting magnets – the coils have no resistance, so current flows forever. This means that, once the magnetic field is running, it cannot just be switched off. For the wires to remain superconducting, they have to be kept very cold, so it is bathed in liquid helium, which keeps it at -269°C, only 4°C above absolute zero.

As the MRI contains a very large and powerful magnet, it is important to keep metals, even those that aren’t magnetic, away from it. People with pacemakers, aneurism clips, artificial limbs or plates have to be very careful around the MRI. Also, people employed working around metals, such as engineers, have to be very careful as they are prone to having small shards in their eyes that normally cause no problems, but in the MRI, they can move around causing permanent eye damage.

The work of US chemist Dr Paul Lauterbur and the British physicist Dr Peter Mansfield in 1973 made the development of MRI possible. They were awarded the Nobel prize for Medicine in 2003.

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