Functional magnetic resonance imaging (fMRI) is a specialized version of an MRI scan. Instead of focusing on just imaging the tissues of the brain, it also tracks the flow of blood to different areas, based on certain mental tasks being performed by the patient. This responsive blood flow is called hemodynamic activity. The availability of functional magnetic resonance imaging has increased greatly since the early 1990s, and it has come to be one of the more common forms of neuroimaging.
It has long been known that changing areas of neural activity in the brain causes changes in blood flow through these areas. When nerve cells in the brain or anywhere else become active, they consume oxygen from the blood. Unlike muscle cells, they do not have nearby reserves of glucose and oxygen, and blood flow must immediately be modified to accommodate increased neural activity. This leads to localized areas of deoxygenated blood where the nerve activity took place, and these areas are what functional magnetic resonance imaging can see.
The magnetic properties of blood differ depending on whether it is oxygenated or deoxygenated. This magnetic signature is what is read by a functional magnetic resonance imaging scanner. The technology for this was first developed in 1990, and has opened up many potential opportunities for developing our understanding of brain function and organization.
When a patient undergoes a functional magnetic resonance imaging procedure, it may last anywhere from 15 minutes to two hours, depending on its purpose. It is very important for the patient to lie completely still throughout this time, in order for usable images to be obtained. This can be problematic for small children or those with certain psychological conditions, but attempts can be made to accommodate the needs of these patients. Subjects may be presented with various sensory information, or be asked to perform mental tasks such as memorization during the course of the examination, while a doctor reads and interprets the areas of brain activity as they are displayed on a screen.
The biggest advantage offered by functional magnetic resonance imaging is that it is completely noninvasive, and does not require the injection of radioactive contrast dye or similar material. This means that the patient is not exposed to unnecessary radiation. The length of the procedure can also be very brief, if necessary.
The disadvantages of these procedures are related to the nature of what they measure. That is to say that hemodynamic responses due to neural activity are sometimes hard to distinguish from responses due to other events in the body. For this reason, when fMRI data are published, they usually take the form of aggregate images, grouped together from data collected over time from many different subjects.