Structural imaging measures permanency. In other words, it measures the permanent, structural characteristics of the brain. Functional imaging measures more of the variability, the moment-to-moment changes of the brain that are associated with various cognitive processes, such as attention.
We know the brain consumes about 20% of the body’s total energy. A lot of this energy consumption is due to the maintenance of the membrane potential, of ion channels/gates. Now, most of the energy the brain receives is due to the vast vasculature of the brain. So, when the brain’s metabolic needs increases, you can see the increase of the blood supply to that specific region of the brain that is being most used in a given task.
PET scans are used in these situations, since these sorts of scans measure blood flow to a region directly. Other techniques use oxygenation to a brain region via fMRI.
This article will focus on PET scans.
PET, or positron emission tomography, uses radioactive tracers that have been injected into a subject’s bloodstream. The greater the blood flow to a brain region, the greater the signal emitted by the tracer in that specific region. Most commonly used tracers are oxygen-15 and fluorine-18. Oxygen-15 is usually given in the form of water, while fluorine-18 is usually given in the form of glucose.
Interestingly, though, other tracers can be used, such as radioactive neurotransmitters, which are used more commonly to study neural pathways and to study drug effects on the brain (which makes sense, considering drugs alter neurochemistry, which is based on neurotransmitters release and reuptake).
Now, with PET scans, it takes 30 seconds for the tracer to enter the brain, and then another 30 seconds for the radioactivity to reach its peak. See a problem with this?
These two 30-second windows are crucial, because in these windows, there are changes in blood flow related to cognitive tasks. Therefore, the temporal resolution (which is the accuracy that you can measure WHEN a cognitive activity is taking place) of a PET scan is about 30 seconds, which makes it unviable to use in the timing of things.
Regardless, this is how a PET scan works:
The tracer is injected into the bloodstream and it converted back from the unstable radioactive form to the stable form. As it does this, it emits a positron that ends up colliding with an electron. This releases two photons that can then be detected by machinery. A spatial image of the brain is thus constructed.