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Alzheimer's Disease Alzheimer's disease is the most common form of dementia, yet its cause is unknown. A disease of aging, Alzheimer's disease is becoming increasingly prevalent as the average lifespan in the United States continues to climb into the 80s. Some have suggested that the number of individuals affected by Alzheimer's disease could quadruple over the next 50 years. This dismal scenario suggests that the enormous financial and social burden of Alzheimer's disease will continue to grow. However, if it were possible to improve our ability to recognize early forms of the disease and to start treatment immediately, we could substantially brighten our future. Indeed, a modest goal of delaying disease onset by just two years would result in 2 million fewer cases of Alzheimer's disease by the year 2050. The past two decades have witnessed rapid advances in the use of imaging techniques for evaluating disease processes in the living human brain. Particularly in the case of Alzheimer's disease, a definitive diagnosis could not be obtained until after the patient had died. However, powerful new tools for imaging the brain's structure and function can now contribute important information to establishing the diagnosis and prognosis in individual patients. Imaging is also likely to play an increasing role in evaluating the effectiveness of new medications designed to slow the progression of Alzheimer's disease. Two especially valuable imaging techniques are magnetic resonance imaging (MRI) and positron emission tomography (PET). MRI works by using a powerful magnet to create highly detailed pictures of brain anatomy, and it can also be applied to evaluate how the brain responds to thinking or movement tasks. The brain slowly shrinks with aging, but this process is accelerated in patients with Alzheimer's disease. The hippocampus, an area of the brain that controls memory, is particularly affected. Shrinkage of the hippocampus may be detected with MRI early in the course of Alzheimer's dementia. However, techniques aimed at evaluating brain function (or how the brain responds to stimuli) tend to be even more sensitive for detecting the very earliest stage of the disease. An advantage of MRI is that it does not use ionizing radiation. However, people who are uncomfortable in small spaces (those with claustrophobia) may sometimes find it difficult to undergo MRI. Also, those with a history of having certain kinds of metal in their body, such as a pacemaker or an internal ear implant, may be unable to undergo MRI. PET imaging provides physicians with the means to examine how the brain uses sugar (glucose) for energy. Because this form of energy drives all brain processes, PET provides a complete picture of brain function. PET works with the injection of a tiny amount of a radioactive drug through a small plastic tube in the patient's hand or arm. (The amount of radiation received is comparable or less than that of many medical imaging tests such as a computed tomography [CT] or a stress test.) The PET scanner then takes a series of pictures of the brain, following the drug as it courses through the patient.
The imaging of brain energy metabolism with PET is an effective test for distinguishing Alzheimer's disease from other forms of dementia. Decreased metabolism in the temporal and parietal regions of the brain is typical and highly suggestive of Alzheimer's disease in a patient with memory changes. This pattern has also been observed in individuals with family members in whom Alzheimer's disease was diagnosed at a young age. PET scanners have become much more widely available over the past few years, as PET is now also used for diagnosing cancers elsewhere in the body. Thus, PET imaging may be used more frequently in the future in the diagnosis of dementia. In fact, the Health Care Financing Administration (HCFA) is currently reviewing whether to approve Medicare reimbursement for clinical PET imaging in cases of suspected Alzheimer's disease. With special drugs, other brain components (e.g., brain receptors) can also be imaged with PET techniques to measure brain areas thought to be involved in the development of dementia. The hallmark of Alzheimer's disease is the abnormal deposition of a protein, known as amyloid, in the brain. Drugs under development include one that can be used with PET to visualize brain amyloid. Amyloid imaging may be highly useful in the future for diagnosing Alzheimer's disease even before a person becomes aware of symptoms and for judging the effectiveness of new therapies designed to break down amyloid.
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