Читать книгу The Handy Psychology Answer Book - Lisa J. Cohen - Страница 212

Instead of relying on radioactive decay to image the brain, fMRI measures properties related to magnetic activity. MRI scanners do this by creating a large magnetic field. A typical research MRI scanner has a field strength of 3 Tesla, which is 60,000 times the strength of the Earth’s magnetic field! Most fMRIs measure changes in the magnetic field related to the level of oxygen in the blood, commonly referred to as the blood oxygenation level dependent or BOLD signal. Therefore, like PET scans, fMRIs use a measure of blood flow as an indicator of general activity in specific regions of the brain. The scanners take multiple scans simultaneously, covering the entire brain, with a new scan about every 2 seconds. As mentioned above, fMRI is cheaper than PET, is far more fine-grained across time and space and does not expose subjects to radioactivity. But it also has limitations. The strength of the magnetic field can be very dangerous if any metal is in the body. Therefore fMRI cannot be used in people with metal pins, pacemakers, or other metal inserts in their body. Moreover fMRI is very sensitive to motion and so is difficult to use with children or other restless people.