• Question: What is fMRI and how was it discovered?

    Asked by ir123 to Tom on 22 Jun 2010 in Categories: .
    • Photo: Tom Hartley

      Tom Hartley answered on 22 Jun 2010:

      Hi ir123,

      fMRI stands for functional magnetic resonance imaging. It is a way of using an MRI scanner to capture images of the brain in action, from these we can figure out what different parts of the brain do (the “functional” bit).

      fMRI is an amazing technology that builds on several earlier discoveries and technologies. This is a pretty complicated process, and I am going to simplify a bit, but it is still hard to explain – so take a deep breath.

      In the strong magnetic field of the MRI scanner, protons in the body spin at a characteristic speed. Each gives out a tiny radio signal at this frequency, but as each proton is doing is doing its own thing, the signals cancel out and cant be detected. By sending a pulse of radiowaves INTO the body, the spins can be made to “align” with one another, and the radio signal coming OUT from the protons can be detected for a short time. This is the “magnetic resonance” phenomenon.

      The frequency of the radio signal depends on the strength of the magnetic field. Because the strength of the field varies we can separate out signals coming from different places (parts of the brain). By very carefully changing the strength of the field, we can build up an image in 3D (this is the “imaging” part).

      The time it takes for the radio signal coming out of the brain to die away depends on the the protons’ surroundings. For example, it dies away more quickly in “old” blood which has already given up its oxygen, than in “fresh” blood which still has its oxygen. When part of the brain is busy, fresh blood floods in to provide the brain cells with extra oxygen and energy. This change in bloodflow means that the radiosignal doesn’t die away so quickly – so the new blood shows up as a very slightly brighter spot on our images (if we wait the right length of time after putting the radio pulse IN). So by looking at fMRI images we can see which parts of the brain are getting more blood.

      fMRI uses a technique called Echo Planar Imaging to get these images very quickly (a picture of the whole brain takes about 2-3 s). So we have a movie of how the brain activity (actually bloodflow) changes over time.

      Magnetic resonance was first used for chemistry (where it’s known as nuclear magnetic resonance, NMR) and was discovered by Isidor Rabi in the late 1930s. He got the Nobel Prize for this work. Felix Bloch and Edward Purcell refined the use of magnetic resonance for solids and liquids. They got the Nobel Prize, too. Paul Lauterbur had the ideas that lead to imaging based on NMR and Peter Mansfield developed them to the point where imaging and MRI scanners were a practical possibility in the 1970s. They got the Nobel Prize. Mansfield also developed the Echo Planar Imaging technique which is central to fMRI (allowing images to be captured quickly enough to be useful). So fMRI depends on at least 3 Nobel Prizes and 5 prize winners – and this is only the tip of the iceberg. Which is why fMRI is amazing, complex, brilliant, fantastic, and very, very hard to explain properly. I hope I’ve done a good job.