Brain Activity

The mechanism behind fMRI and its uses actually comes from blood flow. When a specific area of the brain is activated, the neurons in that region use more oxygen. Thus, in order to meet this increase in demand, the body must supply more blood to the area. This means that there will be a significantly higher number of hemoglobin molecules in their “oxygen-bound” state than there would be otherwise. Researchers discovered that the oxygenated state of hemoglobin actually carries with it a different kind of electromagnetic influence than the deoxygenated state does. Thus, they were able to pick up on the change in magnetic resonance when an area has an increased amount of oxygenated blood flow. This whole idea is called BOLD, or blood oxygenation level dependent contrast, and it essentially defines the signal that is received in fMRI (how fMRI works article).

Through the convention of BOLD, fMRI has been able to image many different functions and aspects of the brain. Even so, the innovations with fMRI have not stopped there. There have been several different ways in which researchers have been able to alter fMRI to scan for other changes in the brain. This allows them to validate whatever data they collected through the BOLD means of analysis as well as gain different kinds of evidence beyond what BOLD can offer. Some of these other methods include things such as CBV fMRI, CBF fMRI using ASL, and phase contrast MRI (phase contrast fMRI article). CBV stands for cerebral blood volume, and within the scope of fMRI, this essentially means that it is used to measure changes within the diameter of arterioles. CBF stands for cerebral blood flow, which is analyzing the actual tissues of the brain as they receive blood. This is done using ASL, which is short for arterial spin labeling. Arterial spin labeling is a technique in which water protons in a major upstream vessel are spin-polarized using radio frequencies. This causes some electromagnetic change that can then be picked up and used to examine arterial flow. Finally, phase contrast MRI relies on dephasing magnetized protons in a gradient in order to map the direction and speed of blood flow. Studies conducted with phase contrast MRI have been shown to measure blood flow in the specific arteries through white matter and the lenticulostriate arteries in the basal ganglia of a human brain. There have now been studies conducted that include all of these different kinds of fMRI in order to get an even clearer picture of what is going on in the brain.

But what do fMRIs actually tell us about a person’s brain? fMRI scans can be translated into voxels, the basic unit of measurement for 3D images, that can then be translated into activation maps, which overlay the colored voxels on an image of the brain. These maps are a three dimensional visualization of the activity in the brain, with the red flares meaning activation (indicating a change in neural response), fading to blue and then back to gray when activity in that region stops. This information tells us about the brain’s functional activity, which is particularly useful for telling us which parts of the brain are working during certain tasks. Additionally, fMRIs can also tell us crucial information about the aftereffects of strokes and other brain diseases like cancer, as well as serving as a guide for treatments to diseases such as epilepsy. The procedure is also non-invasive and repeatable, which lowers the risk for patients, and can be reliably used to map out in 3D a person’s functional brain.

However, it may be difficult to determine the necessity of some brain regions while certain tasks are being performed, since correlation does not mean causation. Another thing to consider with fMRIs is that they are much better at demonstrating motor areas than language ones, but a combination of various neural techniques may be able to provide researchers and doctors with more accurate information. Additionally, the BOLD signal may be affected by the presence of brain tumors, which may make the information gleaned from fMRIs inaccurate. Other neurological defects may also produce these effects. Analyzing fMRI alone, it may also be difficult to pinpoint the specific origin of neural activation, and many regions of the brain have a relatively large overlap in cognitive tasks and domains (e.g. fronto-parietal attention network is consistently activated whenever a person focuses on a task). Though there may be limitations to the usage of fMRIs, the information can be combined holistically with other techniques such as EEG to evaluate the relationship between electrical brain activity and hemodynamic responses. fMRIs also have the potential to aid in neurosurgery and creating biomarkers for monitoring certain neurological diseases.