MRI, on the other hand, responds to the prevalence of particular types of atoms in the body. Most MRIs look for hydrogen, usually found in the body as a component of water. Thus an MRI scan distinguishes among tissues based on their water content. Fatty tissues, which have little water, appear bright, while blood vessels or other fluid-filled areas are dark. MRI is particularly useful for seeing details in the brain.
Gray matter has more fluid than white matter, making it easy to distinguish between the two.
In positron emission tomography, or PET, a patient ingests or is injected with a slightly radioactive substance that emits positrons, which can be monitored as the substance moves through the body. Where CT and MRI are mainly valuable in viewing the body s internal structures, PET is also useful in tracking its metabolism. In one common application, for instance, patients are given glucose with positron emitters attached, and their brains are monitored as they perform various tasks. Since the brain uses glucose as it works, a PET image shows where brain activity is high. Brain researchers have used this technique to discover which parts of the brain a person uses to perform specific tasks, such as read a list of words.