Doctors may use brain scans to identify strokes, tumors, or other problems that can cause dementia. Also, cortical atrophy—degeneration of the brain's cortex (outer layer)—is common in many forms of dementia and may be visible on a brain scan. The brain's cortex normally appears very wrinkled, with ridges of tissue (called gyri) separated by "valleys" called sulci. In individuals with cortical atrophy, the progressive loss of neurons causes the ridges to become thinner and the sulci to grow wider. As brain cells die, the ventricles (or fluid-filled cavities in the middle of the brain) expand to fill the available space, becoming much larger than normal. Brain scans also can identify changes in the brain's structure and function that suggest Alzheimer's disease.
The most common types of brain scans are computed tomographic (CT) scans and magnetic resonance imaging (MRI). Doctors frequently request a CT or MRI scan of the brain when they are examining a patient with suspected dementia. CT scans, which use X-rays to detect brain structures, can show evidence of brain atrophy, strokes and ischemia, changes to the blood vessels, and other problems such as hydrocephalus and subdural hematomas. MRI scans use magnetic fields and focused radio waves to detect hydrogen atoms in tissues within the body. They can detect the same problems as CT scans but they are better for identifying certain conditions, such as brain atrophy and damage from small strokes or subtle ischemia.
Doctors also may use electroencephalograms (EEGs) in people with suspected seizures, which may occur in some forms of dementia. In an EEG, electrodes are placed on the scalp over several parts of the brain in order to detect and record patterns of electrical activity and check for abnormalities. This electrical activity can indicate cognitive dysfunction in part or all of the brain. Many patients with moderately severe to severe dementia of any sort have abnormal EEGs. An EEG may also be used to detect seizures, which occur in about 10 percent of Alzheimer's disease patients as well as in many other disorders. EEGs also can help diagnose CJD.
Several other types of brain scans allow researchers to watch the brain as it functions. These scans, called functional brain imaging, are not often used as diagnostic tools, but they are important in research and they may ultimately help identify people with dementia earlier than is currently possible. Functional brain scans include functional MRI (fMRI), single photon-emission computed tomography (SPECT), positron emission tomography (PET), and magnetoencephalography (MEG). fMRI uses radio waves and a strong magnetic field to measure the metabolic changes that take place in active parts of the brain. SPECT shows the distribution of blood in the brain, which generally increases with brain activity.
PET scans can detect changes in glucose metabolism, presence of amyloid proteins, oxygen metabolism, and blood flow, all of which can reveal abnormalities of brain function. MEG shows the electromagnetic fields produced by the brain's neuronal activity. Currently, the use of PET to detect the tau protein is only available through research.
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