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What is fMRI?

P.Read Montague, Jr., PhD

fMRI stands for functional magnetic resonance imaging. In order to understand how it works, it is first necessary to understand conventional magnetic resonance imaging, or MRI.

MRI is a technique for producing astonishingly detailed images of the brain or other bodily structures. These images are used in virtually all modern hospitals to diagnose a wide variety of brain disorders, including brain tumors, multiple sclerosis, and stroke. MRI scanning uses a very strong magnet and radio waves to produce these spectacular images. The subject lies on a table, with his head surrounded by a large magnet. The magnet causes some of the atoms (or, more precisely, particles inside the atoms, called protons) inside the patient's head to align with the magnetic field. A pulse of radio waves is then directed at the patient's head and some of it is absorbed by the protons, knocking them out of alignment. The protons, however, gradually realign themselves, emitting radio waves as they do. These radio waves are captured by a radio receiver and are sent to a computer, which constructs the brain image. The patient cannot sense either the magnet or the radio waves; in fact, the patient only knows the machine is working because of the noise it makes during scanning.

Functional MRI, or fMRI, developed in the early 1990s, is a variation of magnetic resonance imaging. It uses a conventional MRI scanner, but takes advantage of two additional phenomena. The first is that blood contains iron, which is the oxygen-carrying part of hemoglobin inside red blood cells. Iron atoms cause small distortions in the magnetic field around them. The second key phenomenon underlying fMRI is the physiological principle that whenever any part of the brain becomes active, the small blood vessels in that localized region dilate, causing more blood to rush in. A large amount of freshly oxygenated blood pours into any activated brain structure, reducing the amount of oxygen-free (deoxy) hemoglobin. This causes a small change in the magnetic field, and thus the MRI signal, in the active region.

An MRI scanner can detect this small change in the signal, and thus detect which areas of the brain have been activated. So, for example, if a patient lying in a scanner is suddenly shown a flash of light, the visual cortex in his brain will become activated, blood flow there will quickly increase, and the MRI signal will change.

Both conventional and functional MRI use a powerful magnet and radio waves to produce images of the brain. Conventional MRI images show beautifully detailed anatomy, and are an essential part of modern medicine. In fMRI, the same scanner is optimized to detect small changes in blood flow in the brain in response to scientifically designed stimuli. In principle, fMRI can be used to observe the activation of brain structures in response to almost any kind of brief stimulation, ranging from sounds, to visual images, to gentle touching of the skin. Currently, fMRI is being used across the world as a powerful neuroscientific research tool to study how the brain works, although some medical applications are being discovered as well.

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