Trouble Thinking

December 6, 2010

What Exactly is an fMRI?

Filed under: Science — Tags: , , — Durandal @ 4:02 pm

It's really quite simple.

For a while, people figured that one of the downsides to an MRI was that all it could do was take a picture of the physical state of whatever you were looking at. Your body, after all, rarely changes on a gross physical level very fast unless you’re a shapeshifter. An MRI couldn’t tell you whether or not some interesting things were happening in your brain because it couldn’t look at neuronal transmissions or anything correlated with brain activity.

But then someone got very clever. See, your brain is constantly full of blood.

First think about that, actually. Ha, now it’s even more full of blood.

See, the hypothesis was basically: cells need blood, specifically oxygenated blood, in order to perform their functions. A part of the body that is currently active, then, should need more blood than a part of the body that is currently inactive.

This was actually one of the very first psychological experiments, a man was laid down on a carefully weighted see-saw, then asked to do sums in his head. The see-saw was observed to tilt slightly toward the head, and forever enshrined children’s playgrounds as impromptu neuroscience laboratories. Though parents do stubbornly object to my experiments in trepanning.

So if a part of the body gets more blood when it’s active, what about a part of a part of the body? Specifically, scientists hypothesized that different bits of your brain would receive different amounts of blood during different tasks. After all, if you’re lying still and solving puzzles, your Somatosensory Cortex (the bit in charge of movement/touch sensation) shouldn’t be quite as active as your Frontal Lobe (the bit in charge of executive function).

So, how do you measure bloodflow with some actual accuracy? Well, it turns out that MRI is surprisingly perfect for it. You see, your blood contains hemoglobin that carries oxygen. Hemoglobin contains iron, and iron is magnetic. You see where this is going. Due to the way the hemoglobin molecule is shaped, iron “hides” when hemoglobin is full of oxygen and “pops out” when there’s none left.That makes hemoglobin diamagnetic when it’s got oxygen, and paramagnetic when it doesn’t. The important part isn’t really what diamagnetic and paramagnetic mean (I think one is the evil twin or something) so much as the fact that this difference allows you to observe the magnetic properties of blood using an MRI and tell how highly oxygenated it is.

You might expect that when an area of the brain is using up oxygen, you’d have less oxygenated blood nearby, and that’s what fMRI would notice. Prepare to have your expectations shattered! See, your brain is so important, that any time there’s any request for oxygen from it at all, your body goes crazy. So there’s a tiny initial dip in blood oxygen levels, but it’s quickly overshadowed by a massive increase about 5 seconds later. That massive increase is what imaging with fMRI measures.

fMRI has most of the advantages of MRI going for it. It’s safe, there’s no radiation, and it takes a pretty clear picture, 2-3 mm resolution. Plus it can see what we presume is brain activity.

That, of course, is also the downside. It can see what we presume is brain activity because so far it seems to hang together. But there’s no getting around the fact that you’re actually observing the blood-oxygen level of an area of the brain about 5 seconds after something occurred. As most neuronal signals seem to take on the order of milliseconds, this is a bit of a significant time lag. There’s also the fact that blood flow itself is significantly different between for instance a young man and a 90 year old heart patient, which can lead to difficult to interpret data.

So there you have it! All those pretty color images of generically termed “brain activity” are charts of statistically significant changes in blood flow gathered using a technique to discern the relative level of blood oxygenation 5 seconds after deoxygenation of the blood in a certain area of the brain that we believe is brought on by neuronal activity. You can understand why it’s usually shortened.

1 Comment »

  1. Is the seesaw story true?

    Great little write up, by the way. I enjoyed it more than Neuroskeptic’s “fMRI in 1000 words”, which is the thing to beat when it comes to blog-length fMRI explanations.

    Comment by Curio — July 17, 2014 @ 11:53 pm

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