Functional magnetic resonance imaging (fMRI) is a new neuroimaging technique that measures tiny metabolic changes in the brain. It is becoming an important tool of choice for learning how normal, diseased, or injured brains are working. fMRI has been used to study the functioning brain of people with ADHD.

Physicians use fMRI to examine the anatomy of the brain and to determine which part of the brain is handling functions such as thought, speech, movement, and sensations. fMRI permits investigators to simultaneously produce high-resolution anatomical images as well as information regarding changes in blood flow between an active and resting brain. The advantage of fMRI compared to PET or SPECT is that it is noninvasive and the subject is not exposed to ionizing radiation.

The technical explanation of the fMRI reveals that images of blood flow are based on the varying magnetic properties between oxyhemoglobin, blood cells that have oxygen, and deoxyhemoglobin, the blood cells that have carbon dioxide and no oxygen. When brain activity is increased beyond a certain threshold, the amount of oxygen in the blood support to that brain region surpasses the amount of oxygen being used. The ratio of oxyhemoglobin to deoxyhemoglobin increases and is detected by the scanner. The method has commonly been referred to as the blood oxygen level-dependent or BOLD technique. A variation of fMRI, T2 relaxometry, has been utilized to evaluate steady-state perfusion of various brain regions over time.

The traditional MRI unit is a large cylindrical tube surrounded by a circular magnet. The person lies on a moveable examination table that slides into the center of the magnet. In an fMRI examination, the person is asked to perform a particular task during the imaging process that causes increased metabolic activity in the area of the brain responsible for the task. The activity, which includes expanding blood vessels, chemical changes, and the delivery of extra oxygen can then be recorded on the MRI images. The person may be asked to perform a variety of small tasks, such as tapping the thumb against each of the fingers on the same hand, rubbing a block of sandpaper, or answering simple questions. The individual must be perfectly still while the images are being recorded in the circular-shaped tube surrounded by a circular magnet.

Bibliography:

Gozal, David, and Dennis L. Molfese, eds. 2005. Attention deficit hyperactive disorder: From genes to patients. Totowa, NJ: Humana Press.

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