How fMRI Works

fMRI Imaging: How Is an fMRI Done?

An fMRI scan is usually performed on an outpatient basis. This means you will come into the hospital for the scan and leave afterward. During the test you may wear a hospital gown or your own clothes, but you can't bring anything metal (zippers, clips, pins, glasses) into the room, because it could interfere with the MRI machine.

During the test, you lie on a table. Your head may be placed in a brace to hold it still. Then you are slid headfirst into the large, cylindrical MRI machine. You may be given earplugs to mask the sound -- MRI machines tend to be very noisy.

While the machine is scanning your brain, you will be asked to perform a task that increases oxygenated blood flow to a particular part of your brain. For example, you may tap your thumb against your fingers, look at pictures or answer questions on a computer screen. The test can last anywhere from a few minutes to an hour or more. After the scan is done, a specialist called a radiologist will interpret the results.

Although an fMRI test doesn't use radiation, its strong magnetic field and radio waves may not be recommended for certain groups of people including:

Pregnant women
People with an internal defibrillator or pacemaker
Those with artificial heart valves or limbs
People with cochlear implants
People with an infusion catheter
Those with clips used on brain aneurysms
Women with an intrauterine device (IUD)
People with metal pins, screws, plates or surgical staples.

Can an fMRI tell if you're lying?

Although the polygraph test works relatively well, scientists are in search of more accurate lie detection methods, one of which is fMRI. In a study presented at the Society for Neuroscience, researchers gave subjects a playing card and then asked them to lie about the card while undergoing an fMRI scan. When the subjects lied, certain decision-making areas of their brain "lit up" [source: Scientific American]. This appears to be compelling evidence that the fMRI can pick up false statements. Yet many researchers say it isn't proof that fMRI is an accurate lie detector, because areas of the brain that correspond to lying are also involved with a number of other thought processes. More research needs to be done on the technology to ensure that it won't falsely accuse the innocent based on misinterpreted brain signals. Read Can an MRI machine also act as a lie detector? to learn more.