The studies are published in the Archives of Neurology.
The special PET scans use radioactive tracers to highlight amyloid protein plaques in the brain, which are a hallmark of Alzheimer's disease. They may allow doctors to diagnose Alzheimer's earlier -- even before any symptoms appear. But many people with amyloid plaques in their brain don't have Alzheimer's disease.
A progressive brain disease that leads to a decline in memory and other cognitive abilities,Alzheimer's disease is the most common form of dementia. According to the Alzheimer's Association, 5.4 million Americans have Alzheimer's disease in 2011, a figure that includes one in eight Americans older than 65.
One study shows that PET imaging using florbetapir F 18 as a tracer was able to distinguish between 68 people with suspected Alzheimer's disease, 60 people who showed signs of mild cognitive impairment, and 82 healthy older people with no signs of cognitive impatient.
The other study looked at PET scans using fluorine 18-labeled flutemetamol tracer among seven people with normal pressure hydrocephalus, a progressive condition that causes dementia and often mimics Alzheimer's disease. These study participants had undergone brain tissue biopsies during a procedure to treat normal pressure hydrocephalus. Biopsy results correlated with those seen via PET scans.
Checking for Alzheimer's
"What you see on the scans directly reflects the amount of amyloid protein in the brain," says Adam S. Fleischer, MD, a neurologist from Banner Alzheimer's Institute in Phoenix. Fleischer is a researcher on the florbetapir F 18 study.
It is not always that straightforward, he says. Up to one-third of people older than 65 will have amyloid protein in their brains and no cognitive impairment, he says.
"It is not clear how amyloid imaging in the brain will predict whether or not you will develop Alzheimer's disease," Fleischer says. "If you have amyloid in your brain and dementia, it's highly or most likely that your memory problems are from Alzheimer's disease."
Highly likely does not mean 100%, he says.
The tests may ultimately help rule out Alzheimer's disease, instead of rule it in, Fleischer says.
"If you don't have amyloid protein in your brain, even if you have dementia, it's not Alzheimer's disease," he says. "It's a slam dunk."
Many drug companies are working on drugs that target amyloid plaques in the brain, and the only way to determine if these drugs are effective is with this screening technology, he says.
"Before we had to wait for symptoms, but now we can see if there is evidence that these drugs are removing amyloid," he says.
Dementia is a late stage of Alzheimer's disease, he says. "The pathology starts in the brain 10 to 20 years before symptoms," he says. "We need ways to identify early Alzheimer's disease in living patients and that is where amyloid imaging comes into play."
Predicting Alzheimer's Disease
William J. Jagust, MD, a neurologist at the Helen Wills Neuroscience Institute at the University of California at Berkeley, says that "we are all very excited because these tests may help us predict who may get Alzheimer's disease, and allow us to intervene before it starts, but we still don't have a good treatment for Alzheimer's disease."
Jagust wrote an editorial accompanying the new research.
"It could be a really big deal if we have an effective drug," he says. "A high proportion of people who don't have full-blown Alzheimer's disease do have amyloid in brain, so this gives us an opportunity to test drugs."
It's not always black and white, Jagust says. "Some tests are positive and others are negative. But the value of intermediate or borderline scans is unknown."
Marc L. Gordon, MD, a neurologist and Alzheimer's disease researcher at the Feinstein Institute for Medical Research in Manhasset, N.Y., says the new tests "tell you whether you have amyloid binding in your brain, not whether or not you have Alzheimer's disease."
"In the future, if we develop medications that are disease-modifying and effective amyloid-based therapies, these tests may be useful," he says.
"Stay tuned," Gordon says. "People who have no symptoms should not be getting PET imaging because we can't tell a young, healthy person that doesn't have dementia that they will or won't develop it."