"A positive scan with someone with completely normal memory, there's no indication to panic," Rowe said. "But if your memory is declining, and you've got a positive scan, there could be a serious problem."

If you live long enough, you'll get beta amyloid deposits in your brain, Rowe said. Nearly one-fifth of patients in the study between the ages of 60 and 70 had positive scans. For those between 70 and 80, nearly half had a positive scan. And nearly five out of six patients over 85 in the study tested positive.


"The ones that showed progression over a couple of years already had some subtle memory impairment that was still within the normal range, but starting to get within the [worse] end of the normal range," he said.

Rowe said this progression mirrors what is known about the prevalence of Alzheimer's disease. At 85, about one-quarter of the population is believed to have Alzheimer's, he said, compared to only one-tenth at age 75.

"Every five years the prevalence doubles," he said. "So basically once you're in your 90s, almost 50 percent of the population has Alzheimer's disease. [Our scans are] following the same trajectory, but 10 to 15 years earlier."

Currently, Rowe is taking part in a massive, decade-long follow-up study dubbed AIBL, the Australian Imaging Biomarkers and Lifestyle study. It has 1,000 participants, who are followed every 18 months, with 215 patients getting a repeat PET or MRI scan at 18 month follow ups.

"If further long-term follow up confirms what we believe, that beta amyloid is a very early predictor of Alzheimer's disease, it provides a means of detecting patients and administering therapies" before the disease strikes, Rowe said.

"A big caveat," he added: "once we have therapies."

Currently, while some treatments can help alleviate some of the symptoms of Alzheimer's, no approved therapy can cure or delay the progression of the disease.

"There have been several dozen [trials] that have failed," Mathis said.

One trial that Mathis worked on, published in April in Lancet Neurology, involved a drug that could decrease amyloid presence in the brain but without improving symptoms. But only 28 subjects were used, and a larger, follow-up "powered" study will be performed to see if it works.

Mathis suspects many of these trials fail because they're begun too late.

"All of those trials have been conducted in Alzheimer's patients in moderate to severe disease. It's like treating cancer after it's metastasized, or treating heart disease after it's advanced so far there's no recovery. The brain cells are dead -- you're not going to bring them back....That's why imaging is so important," he added, "to identify those people who potentially could benefit before they have the disease."

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