Functional MRI scans
Structural scans like the one we used to produce the black and white images, rely on different brain tissues (e.g., grey-matter vs white-matter) having different numbers of hydrogen atoms and therefore appearing lighter or darker. In the case of functional MRI (fMRI) scans, it is a bit more complicated because what is measured for each voxel can constantly change as your brain activity changes. (For more information on how MRI works click here.)
fMRI shows changes in the activity of neurons (nerves) and the connections (synapses) between neurons by mapping out where blood is flowing in the brain. As you might expect, the more active that neurons in a particular area are, the more oxygen-rich blood they require (like a muscle that is lifting weights). To supply this need the body will divert oxygenated blood to where it is most needed, and the MRI scanner can track that. The way it does that is by taking advantage of the same thing that we are so keen to avoid with piercings – the way in which metal disrupts the magnetic field and causes a darker picture (See question 7). In the case of blood, the haemoglobin protein in red blood cells has iron in it (think of people with anaemia taking iron supplements) and this is what disrupts the magnetic field. BUT if the haemoglobin is carrying oxygen then this stops the iron from disrupting the magnetic field. So the regions of the brain that have more oxygen-rich blood will have voxels that show up with a brighter signal than those with oxygen-depleted blood. In scientific papers on fMRI you will often read about BOLD imaging or measurement of the BOLD signal – BOLD stands for Blood-Oxygen Level Dependent. Now you understand why!
