Interpreting the waves

THE technology used for the scans is based on that used in magnetic resonance imaging (MRI), a non-invasive test using a strong magnetic field and radio waves to create detailed images of the body.

But instead of creating images of organs and tissues like MRI, funtional MRI (fMRI) looks at blood flow in the brain to detect areas of activity. These changes in blood flow are captured on a computer.

Participating in an fMRI test involves the volunteer lying down before being slid head first into a large, cylindrical imaging machine.

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While the machine is scanning the brain, the volunteer is asked to perform a task that increases oxygenated blood flow to a particular part of the brain. This might involve tapping the thumb against other fingers, looking at pictures or answering questions on a computer screen. These tests can last from a few minutes to more than an hour.

fMRI is based on the idea that blood carrying oxygen from the lungs behaves differently in a magnetic field than blood that has already released its oxygen to the cells. In other words, oxygen-rich blood and oxygen-poor blood have a different magnetic resonance. Scientists know that more active areas of the brain receive more oxygenated blood.

The fMRI picks up this increased blood flow to pinpoint greater activity. The measurement of blood flow, blood volume and oxygen use is called the blood-oxygen-level-dependent (Bold) signal.

In using fMRI techniques for use in lie detection, activated areas of the brain are observed while the subject is making a statement. Depending on what regions are the most active, the technician might determine whether a subject is telling the truth or not.