Twenty years of fMRI

Functional magnetic resonance imaging, better known as fMRI, is 20 years old this week. October’s NeuroPod marks the celebrations by looking back at the brain scanning technology, it’s successes, and its troublesome teenage years.

The imaging technique was first announced in a 1991 study published in Science that announced how a standard MRI scanner could be used to used to track where oxygenated and deoxygenated blood flowed in the brain.

The technique takes advantage of the fact that haemoglobin, the iron containing protein that carries oxygen to essential tissues in the body, is differently magnetic when it is carrying oxygen, in comparison to when it is oxygen depleted.

The scanner is essentially a large electromagnet that aligns the proton spin of hydrogen atoms in the body, plus a radio frequency pulse that knocks them out of alignment.

Like shaking a compass, the protons move back into alignment again. The speed of return depends on the density of the body tissue, just like the speed of a compass needle returning to north depends on the density of the liquid in which it rests.

fMRI tunes in to the different magnetic echoes, or more technically, the magnetic resonance, of the protons realigning in oxygenated and deoxygenated blood.

As more active brain areas need more oxygenated blood, it’s possible to infer which tasks or mental activities are most associated with activity in certain brain areas by statistically comparing maps of magnetic resonance differences when people undertake different mental tasks in the scanner.

Although the technique can pinpoint where these changes take place in the brain, down to about the nearest millimetre, blood flow is not the same as actual brain activity, so it is not a precise measure.

Furthermore, changes can only be tracked in time slices of a second or more, clearly missing some of the changes in the fast moving brain, and statistical choices during analysis can affect the outcome sometimes as much as the task itself.

But despite the disadvantages, and with data from other types of study and imaging techniques, fMRI has become an essential scientific tool in the quest to understand the link between the mind and brain.

The piece has interviews with neuroscientists Karl Friston and Russ Poldrack, both involved in fMRI since its early days, who talk about the genuine progress and unfortunate hype that surrounds the technique.

A fantastic look back of the first two decades of fMRI and the other sections of the NeuroPod podcast are equally as interesting.
 

Link to October’s NeuroPod.

2 Comments

  1. Posted October 30, 2011 at 11:26 pm | Permalink

    Just an observation that I found interesting:
    Belliveau’s approach required exogenous contrast agents. Ogawa (also credited with BOLD) published a paper in 1992 [1] detailing fMRI with only BOLD, i.e. without any exogenous contrast.

    [1] S. Ogawa et al., “Intrinsic signal changes accompanying sensory stimulation: functional brain mapping with magnetic resonance imaging.,” Proceedings of the National Academy of Sciences of the United States of America, vol. 89, no. 13, pp. 5951-5955, Jul. 1992.

  2. Lauren
    Posted November 7, 2011 at 4:33 pm | Permalink

    Just a tale: as a grad student in the mid 90’s I helped some folks at the U of Pittsburgh map the visual field inside an fMRI for several years. I was the rat inside the machine. They had trouble finding subjects, but for many reasons I didn’t mind the pain (the ring under my head) or the noise or the 3-4 hour ordeal. After about 3 years the lead researcher asked me what I was doing in there because my signals were so good. I had learned to meditate. They took me to a different lab for EEG study and found that I could get into an alpha state immediately and task there.

    The condition that most led to this was a study design flaw, essentially. After a round of trials a message would appear: if you got them all right, then a green message would say “Great Job!”. If you got at least one wrong a red message would tell you how many you got wrong. I realized that one red message led to more red messages, so I shut my eyes at the end of a trial and ignored the messages. Then I just went further and further into my mind until I no longer could feel the wooden controller with buttons to push in my hand. I couldn’t have located my body at all. Very interesting.

    Also interesting is that at some point, the physicist in charge took me aside and warned me to stop coming. Why? He didn’t know, but thought it best that I stop, after 100’s of hours – I couldn’t even tell you how many really. It’s my favorite excuse for bad behavior or thought! Though I must say that I still wonder what effects it may have had on me…


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