Computed tomography, or computed axial tomography (CAT), scans show a cross-section of a part of the body, such as the brain. In this technique, a thin x-ray beam is used to produce a series of exposures detected at different angles. The exposures are fed into a computer which overlaps them, yielding a single image analogous to a slice of the organ or body part being scanned. A dye is often injected into the patient so as to improve contrast and obtain images that are clearer than images obtained with regular x rays.
Magnetic resonance imaging (MRI)
Magnetic resonance imaging also produces cross-sectional images of the body, but MRI uses powerful magnetic fields instead of radiation. MRI uses a cylinder housing a magnet that will induce the required magnetic field. The patient lies on a platform inside the scanner. The magnetic field aligns the hydrogen atoms present in the tissue being scanned in a given direction. Following a burst of radio-frequency radiation, the atoms flip back to their original orientation while emitting signals that are fed into a computer for conversion into a two- or three-dimensional image. Dyes can also be injected into patients to produce clearer images.
Positron emission tomography (PET)
Positron emission tomography uses a form of sugar that contains a radioactive atom which emits particles called positrons. The positrons are absorbed to a different extent by cells varying in their metabolic rate. PET scans are especially useful for brain imaging studies and are used to illustrate the differences between brains of people without mental disorders and brains of people with mental disorders. For example, because PET scans can detect brain activity, PET scans of the brains of depressed and non-depressed persons can show researchers where brain activity is decreased in depressed patients. Similar scans have been taken of brains affected by schizophrenia or Alzheimer's disease. Such research can help scientists discover new ways to treat these disorders.
Monique Laberge Ph.D., The Gale Group Inc., Gale, Detroit,