Magnetic Resonance Imaging (MRI) is a form of medical imaging. Whereas X-rays look at hard tissue like bone, MRI actually images the water in your body. This allows it to image pretty much every single organ in your body. It also manages to do this in 3D, or even over time, tracking changes and metabolism.

X-rays use what is known as ionising radiation and thus too much of them can be bad for you, just like too much time in the sun. MRI uses a very powerful magnetic field and radio waves, and thus is (as far as we know) harmless to humans.

The Magnet

The magnetic field is created by a huge coil of super-conducting wire surrounded by liquid helium. The field is fine-tuned, and altered for imaging by coils of 'regular' wire. The patient area looks like a cylindrical, hollow space about two feet across within the magnet. The strength of the field means that it is very dangerous to take any ferromagnetic objects into the scanner room. Any non-magnetic metals will also affect the scans as they will conduct a current in the changing field.

The loud noises made by MRIs are caused by vibrations in these regular-wire coils as the current through them changes. Being inside the magnet can be uncomfortable as you have to remain very, very still for the duration of the scan, and you are placed in a fairly confined space, which is no good if you are claustrophobic.

How It Works

MRI works by aligning all of the protons which are in hydrogen (H₂) atoms in the H₂O in your body (any magnetic species would do but H₂ outnumbers the others by millions to one). In a very, very strong field (about 1.5 Tesla for most clinical magnets, which is 30,000 times stronger than the Earth's magnetic field) slightly more than half of the protons will have their magnetic fields aligned with the external field, and slightly less than half will be aligned against it. The reason for this has a lot to do with quantum mechanics. The number of 'extra' protons aligned with the field is much less than 1% at 1.5T, which is why the field needs to be so large.

Once they are lined up, they precess¹ around the direction of the field, much like a spinning top does as it begins to fall over. The rate at which they precess is dependent on the field strength and the situation of the proton. If a radio-frequency pulse is emitted at the same frequency as this precession, the proton will absorb the energy and then re-emit it. A receiver antenna picks up these emissions and based on the strength and location obtains information about the protons.

Do this a few million times with a big computer and some fancy code, and you can build up a detailed picture of the whole body. By looking at oxygen usage in the brain over time (deoxyhaemoglobin is paramagnetic ² and thus messes up some of the signal), you can even see which parts of the brain are being used at any time. This is called functional MRI. Tap your fingers during one of these scans and the motor cortex of the brain will light up. Watch a film and the visual cortex will be active.

MRI is a very safe way of viewing the inside of the human body. It is a very powerful medical tool that is helping to improve the health and quality of life of millions of people by improving the level of the diagnosis.