Magnetic resonance imaging (MRI) uses radio waves and a strong magnetic field instead of X-rays to produce detailed pictures of internal organs and tissues. First developed in the early 1990s, functional MRI (fMRI) uses the same technology to identify regions of the brain where blood vessels are expanding, chemical changes are taking place, or extra oxygen is being delivered. These are all signs that this part of the brain is, right at that moment, processing information and giving commands to the body. The technique can be used to observe the activation of brain regions in response to all kinds of brief stimulations: sounds, visual images and touch.
This is important because while doctors know the general areas of the brain where speech, sensation, memory and other primary functions occur, the exact locations can vary in individuals. And sometimes injuries or diseases (strokes or brain tumors) will cause functions to shift to other parts of the brain. Being able to determine precisely which areas of the brain are handling critical functions like thought, speech, movement and sensation can help neurosurgeons better plan surgeries and avoid damaging those areas. fMRI can also help assess the extent of damage from stroke, trauma or diseases like Alzheimer's.
Although it uses a conventional MRI scanner, fMRI differs from MRI: it takes advantage of two additional phenomena. First, blood contains iron, which is part of the hemoglobin inside red blood cells that carries oxygen through the body. Iron atoms cause small distortions in the magnetic field around them. Second, whenever any part of the brain becomes active, the blood vessels in that area dilate, causing more blood to rush in and provide extra oxygen and fuel (in the form of a sugar called glucose) to the active brain cells.
In fMRI, patients are asked to perform a variety of specific tasks corresponding to specific brain functions. This causes a small change in the magnetic field, which can be detected by the scanner. For example, if a patient lying in a scanner is shown a flash of light the brain's visual cortex will be activated increasing the flow of blood to that region, and the MRI signal will change.