BOSTON (Reuters) - Thirteen percent of healthy adults were found to have some type of undiagnosed -- but likely harmless -- abnormality in the brain, according to a Dutch study published on Wednesday.

The research, led by Meike Vernooij of the Erasmus MC University Medical Center in Rotterdam, is important because brain scans are becoming more common and more detailed, and doctors need to know whether to be concerned if they stumble onto something unexpected.

Vernooij and colleagues looked at MRI scans of 2,000 volunteers over the age of 45. Magnetic resonance imaging or MRI can give a detailed picture of physical brain structures.

Just over 7 percent showed evidence of a brain clot, but the clots were too small to produce symptoms and seemed to be more common with age.

Nearly 2 percent had a brain aneurysm, which is a bulge in a blood vessel that can burst if it becomes too big, causing a stroke. But 32 of the 35 aneurysms were so small, the researchers did not suggest follow-up medical treatment.

The younger volunteers were just as likely to have them as older ones.

The scans also uncovered 32 tumors. All but one were benign.

Thirteen people had more than one abnormality, Aad van der Lugt, another member of the team, said.

As MRI scans become more sensitive, they “will probably increase the number of small brain abnormalities detected” and doctors will need to know which ones can be safely ignored, the researchers wrote in the New England Journal of Medicine.

“Unfortunately, we know little of the natural course of these asymptomatic findings,” Van der Lugt wrote in an e-mail.

“It may well be that the clinical course and relevance of these unexpected asymptomatic findings differ from those of similar symptomatic findings for which persons seek medical treatment,” he added.

Tracking such “incidental” abnormalities “will hopefully provide more information on this that will be useful for both researchers and clinicians,” he said.