What is fMRI?
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 (see Fig 1). These images demonstrate the anatomy of the subject's brain at high resolution, and 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. Different parts of the brain respond to the radio waves differently, and emit slightly different radio signals depending, among other things, on the local water and fat content. The computer that receives all of the signals, therefore, is able to distinguish one brain structure from another, and produce remarkable anatomical images, such as that shown in Figure 1.
Fig 1: Conventional MRI of the human brain
The subject is facing to the right (the nose is in the lower right), and this slice is taken straight down the middle of the brain. Note the remarkable anatomical detail.
Functional MRI, or fMRI, was developed in the early 1990s and 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. Not all iron atoms do this: for the case of blood, only iron not bound to oxygen does this ("deoxyhemoglobin"). 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. The blood is presumably needed to provide extra oxygen and fuel (glucose) for the active brain cells. The result, however, is that 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. In the early 1990s, it was shown that an MRI scanner can be used to 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. The result is usually displayed as a patchy area of color, representing the brain area activated, superimposed upon a conventional, high-resolution, gray-scale image of the subject's brain (Fig 2). The signal is often called a BOLD signal, standing for Blood Oxygen Level Dependent signal.
Fig 2: fMRI image
Note that the yellow-orange area indicates the part of the brain that was significantly activated during the study. This is the ventral striatum, which was activated in this case by unpredicted squirts of juice into the subject's mouth. This area of activation is shown superimposed upon a high-resolution, 'conventional' MRI scan of the subject's brain, shown in gray-scale. To orient yourself, imagine that the subject is facing you, and you are looking at a slice of his brain taken from the mid-to-frontal part of his head.
In summary, 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.