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How Does an fMRI Scanner Work?

Article Details
  • Written By: S. Berger
  • Edited By: Shereen Skola
  • Last Modified Date: 06 November 2018
  • Copyright Protected:
    2003-2018
    Conjecture Corporation
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Researchers, doctors, and other professionals can often benefit from being able to view brain activity clearly, with relatively high resolution. Functional magnetic resonance imaging (fMRI) is one method that can show this type of activity, and an fMRI scanner is usually employed for this purpose. This device can have many uses, from diagnosing disease to working as a lie detector. Individuals usually lay on a bed inside of a tube-shaped scanner, which also has the ability to produce magnetic and radio transmissions, when undergoing this type of brain scan.

Put simply, the fMRI scanner measures changes in blood flow to the brain. Active brain cells, or neurons, require more sugars, like glucose, to function than when they are at rest, and the body provides oxygen-rich blood, which also contains sugars, to neurons that are engaged in sending electrical and chemical messages. After providing these nutrients to the neurons, blood can then return to the lungs and heart to repeat the cycle. Tracking these shifts in blood flow allow the scanner to show which brain areas are active during particular tasks.

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Oxygenated blood, being pumped from the lungs and heart toward the brain, responds differently to magnetic fields than blood low in oxygen that is returning to the heart after supplying neurons with nutrients. Specifically, protons in each type of blood align themselves in specific ways when they make contact with these fields. The fMRI scanner takes generates an external magnetic field that causes these distinct responses among protons. Other substances in the body have protons that react to this scanning, but they are screened out later in the imaging process, unlike in other forms of MRI.

Next, the fMRI scanner releases a burst of radio waves toward their target, which hit the proteins. For a brief moment, these waves shake the protons out of their aligned positions as they are absorbed, but the protons return to their alignment afterward. When they do so, they release a radio frequency burst of their own. Scanners contain a cylinder that is lined with coiled sensors which can pick up these radio signals from any direction around the person being scanned. This three-dimensional coverage allows for a complete "snapshot" of brain activity to be assembled by compiling the information from a single burst of radio waves.

Using mathematical transformations, computer software in these devices determine whether blood is highly oxygenated, and where in the brain it is located. Programs in the fMRI scanner also create a complete image of the brain and use various colors to show different levels of oxygen usage and, therefore, brain activity. Multiple bursts of radio waves can provide a view of changing neuronal activity over time, while an individual performs a task. Generally, these scanners are used to investigate activities that take several seconds of neuronal processing, but scans are often conducted over the course of ten minutes or more. The scanners are sensitive, and can register changes that are due to head movements, boredom, or other events that must be accounted for in these readings.

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