Doctors at Abbott Northwestern Hospital have had success treating brain aneurysms with new, thicker “Pipeline’’ stents that direct blood past the potentially deadly bulges in blood vessels in the brain. And they’re studying a spherical WEB stent that does the same for tricky aneurysms that form at the junctures of these vessels.

But an equally intriguing approach to brain aneurysms is being monitored at the Minneapolis hospital: Doing nothing.

Using a new assessment tool called “Phases” this year, interventional neuroradiologists at Abbott receive evidence-based guidance on whether to attack brain aneurysms or leave them alone.

The precise effect isn’t clear yet; next year researchers will compare patients from 2013 and 2015 to see whether the use of Phases changed outcome and procedure rates. But Abbott doctors are noticing.

“Our scheduling rates have gone down in 2015 and the mean size of the aneurysm treated has been higher,” said Dr. Josser Delgado, who treats aneurysms by threading catheters via the groin to blood vessels in the brain. “More patients are opting to enter our aneurysm surveillance program.”

A bulge on a brain vein is unwelcome; some patients who learn they have such a “time bomb” choose treatment even when they aren’t suffering symptoms. But the risk vs. benefit isn’t always clear.

An operation creates as much as a 2 percent risk of complications that includes stroke or brain bleeds. Odds of an untreated aneurysm rupturing within five years aren’t much worse for most patients.

“Most of them are not going to rupture when you look at the grand scheme of things,” Delgado said. “But if and when they rupture, the consequences can be pretty devastating.”

Phases predicts the five-year odds of a rupture, and is based on research examining rupture rates by patients’ ages and demographics. Combined with the new treatments, Delgado said aneurysm care is growing smarter and more effective.

Most treated aneurysms are filled from the inside with mesh coils, though some doctors still perform open surgeries to clip them off from the outside.

Stents such as Medtronic’s Pipeline system expand options, because some aneurysms with wider “necks” can’t be treated with older methods, Delgado said. “It’s allowed us to treat aneurysms that were not treatable before.”