MRI stands for “magnetic resonance imaging”. More clearly, it’s a way to get images of various parts of the body, including the brain, using magnetic fields. To get an MRI scan, a person lies on their back on a scanning table that slides into the MRI scanner, which is basically a really strong, donut-shaped magnet. Radio waves sent from the machine to the targeted area react with the nuclei of hydrogen atoms (called protons) in the body to produce a radio signal that is detected by the machine. The machine records this signal and uses it to produce a picture of the brain.
It’s very safe to get an MRI for most people, but other people may not be able to undergo the procedure. People who dislike being in small spaces may feel claustrophobic in the scanner, and those with certain implanted medical devices (such as aneurysm clips in the brain, heart pacemakers, or cochlear implants) are not allowed into the MRI scanner due to the powerful magnet. A thorough screening is done before anyone enters the MRI to make sure no objects with metal (such as watches, credit cards, or hair clips) are accidentally worn into the scanner. Before going into the MRI scanner, you will be given earplugs to wear. This is because the scanner makes “hammering” noises while preparing and taking pictures. It is very important not to move when you can feel or hear the MRI scanning, as it may blur the pictures being taken. An entire MRI exam can take anywhere from 20 minutes to 1.5 hours, depending on the type of information required.
Unlike an X-ray, which takes pictures of your bones, an MRI takes pictures of non-bony or “soft tissues” of the body. It’s one of the best ways to take a look at the brain, spinal cord, and nerves, and is also used to look at muscles, tendons, and ligaments. An MRI uses no radiation or x-rays, and can be repeated with no known adverse effects.
Similar to an MRI scan is an fMRI, or “functional magnetic resonance imaging”, scan. Instead of just taking pretty pictures of the anatomy of the brain, fMRI tells us which areas of the brain are active at a particular time. It does this by measuring which areas of the brain are using up more oxygen during a task. The areas that get the most oxygen are the most active during the task. This has been a very helpful technique in determining which parts of the brain are in charge of such critical functions as thought, speech, and movement, as well as seeing how activity in these areas are affected by disease, trauma, or drugs. A disadvantage of this technique is that although it is great for determining where activity occurs, it is a relatively slow procedure and thus is poor at telling us precisely when it occurs.
Fortunately, you can get the whole picture if you combine this technique with EEG/ERP.