The Way Things Work: fMRI

Here is a bit from an excellent short discussion of how fMRI works from a post on the Neuroskeptic blog site:

So blood oxygen is a proxy for activation. How do you measure it? Oxygen in blood binds to haemoglobin, a protein that contains iron (which is why blood is red, like rust, and tastes metallic...like iron). By a nice coincidence, haemoglobin with oxygen is red; haemoglobin without oxygen is blueish or purple. This is why your veins, containing deoxygenated blood, are blue and why you turn blue if you're suffocating.

You could measure neural activity by literally looking to see how red the brain is. This is actually possible, but obviously it's a bit impractical. Luckily, as well as being blue, deoxygenated haemoglobin acts as a magnet. So blood is magnetic, and the strength of its magnetic field depends on how oxygenated it is. That's really useful, but how do you measure those magnetic fields?

Using an extremely strong magnet - like the liquid-helium-cooled superconducting coil at the heart of every MRI scanner, for example - you can make some of the protons in the body align in a special way. If you then fire some radio waves at these aligned protons, they can absorb them ("resonate"). As soon as you stop the radio waves, they'll release them back at you, like an echo - which is why the most common form of fMRI scan is called Echo-Planar Imaging (EPI). All matter contains protons; in the human body, most of them are found in water.

Each proton only responds to a specific frequency of radio waves. This frequency is determined by the strength of the magnetic field in which it sits - stronger fields, higher frequencies. Crucially, the magnetic fields surrounding deoxygenated blood therefore shift the radio frequency at which nearby protons respond. Suppose a certain bit of the brain resonates at frequency X. If some deoxygenated blood appears nearby, it will stop them from responding to that frequency - by making them respond to a different one.

fMRI is essentially a way of measuring the degree to which protons in each part of the brain don't respond at the "expected" resonant frequency X, due to interference from nearby deoxygenated haemoglobin.

I chopped out the key bit, but the real story is more complicated, so go read the original post. It has nice illustrations too.

I'm impressed with the Neuroskeptic's blog. It looks quite good. It is narrow in breadth but deep in content. That's the complete reverse of my world view, but I love to use things like this for source material.