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Clinical Updates
PET provides dementia diagnosis based on brain activity
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Positron Emission Tomography (PET) offers direct evidence of brain metabolism that makes it a highly accurate tool for diagnosing dementia patients. Studies examining brain tissue at autopsy have confirmed that PET substantially improves diagnostic accuracy. While autopsy examination confirmed the diagnosis of Alzheimer’s disease made with PET in 90 percent of cases, the presence or absence of Alzheimer’s disease was accurately determined without PET only 60 to 70 percent of the time.
Symptoms of dementia include the erosion of recent and remote memory and impairment of language, motor activity, recognition and executive function. An expert evaluation of dementia usually includes a battery of tests covering different cognitive domains — such as language memory, visual memory, processing speed, etc. — and lab tests for medical conditions, such as anemia or infectious disease, that can affect brain function. The evaluation often includes a computed tomography (CT) or magnetic resonance imaging (MRI) study. These imaging studies are important because they can reveal conditions such as tumors, recent stroke or hydrocephalus that may require urgent treatment.
Such conditions account for only a very small percentage of dementia cases. Without direct evidence of metabolic activity, distinguishing dementia conditions is difficult. Among the geriatric population where it is the most prevalent cause of dementia, Alzheimer’s disease is frequently treated as a diagnosis of exclusion when laboratory tests and physical imaging studies fail to identify the source of dementia symptoms. Alzheimer’s disease and less common neurological conditions that produce similar symptoms but call for different treatments can often not be differentiated without the aid of functional imaging.
PET scanning and interpretation
Prior to a brain PET, patients are given 18F-fluorodeoxyglucose (FDG), a form of glucose with a radioactive attachment that makes it visible to the PET scanner. After 30 to 40 minutes in a dimly lit room to allow the glucose to circulate and to encourage a restful brain state, the patient’s head is positioned in the imaging ring and the image is collected in a process that usually takes about 10 minutes.
PET provides information on brain activity by tracking the location of FDG in the brain. Energy for brain activity is supplied almost exclusively by glucose, so FDG is seen on PET in areas where the brain is expending energy. The most energy-expensive brain activity that changes during the timeframe of an imaging study is related to restoring the gradient of sodium, potassium and calcium ions after synaptic firing. By mapping the distribution of FDG, PET reveals the underlying brain activity.
Different neurological conditions are associated with characteristic areas of altered metabolism that are visible in a brain PET scan. Alzheimer’s disease is characterized by areas of decreased metabolism in the back of the brain (specifically in the parietal, temporal and cingulate cortexes but not the occipital cortex), while frontotemporal dementia, which is often misdiagnosed as Alzheimer’s disease, is characterized by altered metabolism in the anterior part of the brain.
It is important to accurately differentiate the disorders that produce symptoms of dementia because they call for different treatments. Misdiagnosing frontotemporal dementia patients and treating them with drugs for Alzheimer’s will not only fail to improve symptoms, but may even make them worse.
The UCLA advantage
Because PET’s inventor, Michael Phelps, Ph.D., is chairman of the Department of Molecular and Medical Pharmacology, UCLA will always be strongly associated with the functional imaging technology. Continuing to build on that strong advantage, UCLA experts have amassed more experience that those at any other center in interpreting brain PET images in dementia patients in whom definitive diagnoses are established.
UCLA offers a PET consultation service to share its expertise with centers across the U.S. and around the world. UCLA produced the first brain PET quantification software approved by the Food and Drug Administration to provide region-by-region analysis of brain metabolism, and is also a leader in research and publishing in the area of brain PET.
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