A brief introduction to fMRI

First, a common question is: what is MRI and fMRI, and how do they differ?

MRI (Magnetic Resonance Imaging) is fMRI's older brother. It is simply a way to use the magnetic fields to look at the brain's structure, using the density of water as it's measuring tool.

fMRI (functional Magnetic Resonance Imaging) is the use of magnetic fields to measure activity in the brain as subjects do set tasks.

These imaging methods are used on other parts of the body too, most notably the spine, for which special magnet arrays have been developed.

fMRI uses the ratio of oxygenated:deoxygenated blood in different brain regions as an indication of brain activity. This works because the magnetic properties of heamoglobin change when it is bound to oxygen.

Deoxygenated blood is paramagnetic (attracting he magnetic field)
Oxygenated blood is dimagnetic (repelling the magnetic field)

All this works because of the hydrogen atoms involved (H1) which act like small magnetic dipoles. These may be in high-energy or low-energy states, depending on whether the dipole is aligned against the magnetic field, or with it, respectively. In moving between these states, energy is released (or absorbed) in the radiofrequencybandwidth, and this is what can be detected in the fMRI paradigm. This last step requires a small, fast-fluctuating magnetic field to be present alongside the whopper big magnet that will be referred to below. However the mechanics of this smaller magnet are interesting in the following way: for better detection of disturbances in the magnetic field, we would like this small magnet to provide a magnetic field that alternates quickly. However, too fast an alternating magnetic field will cause a practical problem: an electric current may be generated in the axons of people's cells (remember this from GCSE Physics?). This electric current creates involuntary muscle spasms in some participants, especially across the shoulders.

The units of measurement for a magnetic field are the Tesla (T) or the Gauss (G)

The Oxford magnet is a 3.5 T. This is pretty big. The most advanced MRI machines in the world are perhaps 7 T, but these are few and far between. Such powerful magnet are not always so useful, because they have an decreased signal:noise ratio.

I would approximate the value of Oxford's machine to be around £1 million. It was unveiled in 1997. We have a smaller machine too, which is not the usual full-body scanner that you see in pictures. It scans only the head, and is good if the experimenter wants the participant to do anything that might involve looking around, being touched, etc. Also, of course, claustrophobia can be a serious problem in the full-body scanner.

The major theoretical problem with fMRI is that it does not measure synaptic activity directly, but rather the sluggish blood response that follows brain activity. I won't get into the details of that debate here, but suffice to say other methodologies will hopefully replace fMRI in the future, perhaps methods that can somehow sidestep the formidable barrier of the skull in order to directly tap the electrical maze below. So far, no ideas. Damn skull.

The major practical problem with fMRI is how the magnet interacts with anything containing ferromagnetic material (see below).

The Oxford magnet has never in living history been turned off. The magnetic field is generated using electricity and superconducting coils, but the field is propagated by keeping the coils cold using a cucoon of liquid helium.

In emergencies, the magnet can be turned off, but only by warming the superconducting material. Employees or research scientists use the 'Quench' button to do this, which you hit with your fist. The Quench button causes the helium to quickly boil off, the magnet warms, and the magnetic field dies. This creates quite a spectacle.

What emergencies, you might ask?

The most serious problem is that metal is everywhere. MRI machines are used in hospitals where not all members of staff understand protocols regarding the magnet. Sissors are perhaps the biggest problem. Entering the magnet room with sissors will cause them to hurtle towards the centre of the magnetic field, perhaps where your participant is currently in residence. However the magnet can lift more than just sissors; oxygen canisters are a classic weighty example. Once an object like this is in the centre of the magnet, it would take a huge force to remove without switching the magnet off.

In 2001, a boy in an American hospital was killed when a member of hospital staff entered the magnet room with an oxygen canister, which flew though the air and hit the boy on the head. He died several days later in hospital.

Other major problems include cases where people have had surgical staples under their skulls, but forget or don't tell the experimenter. Any ferromagnetic metal object, when in a magnetic field, will try to align itself to the field depending on the structure and dimensions of the object. A metal staple will try to swivel around, in a way reminiscent to how early labotomies were administered using a tool somewhat like a knitting needle.

But! Never fear, Most of the time the magnet is perfectly safe for everybody involved. For example people with metal braces on their teeth will be unaffected, because the amount of ferromagnetic material is low, and the magnet is not strong enough to lift you up by your gold filling. Likewise, metal buttons or zips on clothing are kosher. However people with pacemakers should get the heck away from the MRI machine, because the device powering their heart also relies on a small magnetic signal to regulate the heart.