Brain imaging helps to understand how the brain works, aids in the diagnosis of neurological diseases and guides treatments. Positron emission tomography or PET is an imaging technique that uses trace amounts of radioactive drugs to visualize the function and biochemistry of the brain.

Imaging researchers now have developed new PET tracers to detect changes in the brain caused by Alzheimer's dementia and other neurodegenerative disease. These diseases damage and ultimately kill large numbers of brain cells (neurons) and thus lead to severe disability and death.

Neurodegenerative diseases cause specific patterns of injury and biochemical abnormalities in the brain. Until recently, these changes could only be measured after death by examining brain tissue using a microscope. One of the exciting developments in PET imaging is the availability of new agents that can detect beta-amyloid plaques, one of the key abnormalities in Alzheimer's disease, in the living human brain. Plaques may develop in the brain over a decade before Alzheimer's symptoms develop.

Neurologists and other dementia specialists currently rely primarily on information gathered from the patient and family, physical examination and cognitive tests to diagnose Alzheimer's dementia. In some cases, determining the cause of a patient's cognitive problems can be challenging, and now PET imaging can help doctors and patients be more confident in the diagnosis.

Two clinically used PET imaging tests for patients are being evaluated for dementia. A PET tracer called FDG measures the brain's use of glucose, a simple sugar that serves as the brain's major source of energy. In dementia due to Alzheimer's disease, decreased glucose metabolism in specific brain regions supports a diagnosis of Alzheimer's disease.

The other PET imaging test for patients with cognitive impairment uses a different PET tracer, florbetapir, which binds to beta-amyloid plaques that occur in Alzheimer's disease. This PET tracer was approved for clinical use by the Food and Drug Administration in April 2012. Amyloid PET imaging can show the presence or absence of abnormally increased plaques in the brain. Low plaque levels (a negative amyloid PET study) reduce the likelihood that a patient's cognitive problems are due to Alzheimer's disease. Higher plaque levels are present in Alzheimer's disease, but a positive amyloid PET scan can occur with other neurologic diseases and in older people without cognitive problems.

Both FDG and amyloid PET are only part of the evaluation of patients with dementia or other cognitive disorders. Neither of these tests alone can make specific diagnoses. PET imaging in patients with cognitive impairment should be ordered by physicians experienced in the diagnosis and treatment of patients of these conditions when the results will help in clinical decision making.

Dr. Jonathan McConathy is an assistant professor of radiology at Washington University who is board certified in diagnostic radiology and nuclear medicine. For information about brain PET studies at Washington University, call 314-362-4738.

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Senior Focus: New imaging device helps detect brain changes