Advanced brain imaging technology may give doctors an additional 10 hours or more to respond to some strokes, researchers said Wednesday, a development that may soon bring major changes to the way hospitals treat one of the leading causes of disability and death.

The research is upending doctors' long-held belief that they have just six hours to save threatened brain tissue from lack of blood flow when a major vessel to the brain is blocked. The new findings suggest they may have as long as 16 hours in many cases; a study published three weeks ago with a different group of stroke victims put the outer limit at 24 hours for some.

Both studies showed such dramatic results that they were cut short to speed up reporting of the information to physicians.

In response to the studies, new stroke treatment guidelines were released Wednesday.

"The big news is that we were all wrong in how we were thinking about how strokes evolve," said Gregory W. Albers, a professor of neurology at Stanford University Medical Center and lead author of the new paper. While some brain tissue dies quickly after a stroke begins, in most patients, collateral blood vessels usually take over feeding a larger area of the brain that is also starved for blood and oxygen, giving doctors many more hours to save that tissue than they previously believed, Albers said.

So the age-old medical belief that "time is brain" — that millions of neurons die each minute after a stroke — must be reconsidered, he said.

"We are quadrupling the stroke treatment window today," Albers said. "It's going to have a massive impact on how stroke is triaged and assessed."

Walter Koroshetz, director of the National Institute of Neurological Disorders and Stroke, which funded the new study, said in a news release: "These striking results will have an immediate impact and save people from life-long disability or death. I really cannot overstate the size of this effect."

Strokes were the fifth-leading cause of death in the United States in 2016, when they killed 142,142 people. About 800,000 people have strokes every year, most of which are first-time events.

The vast majority of strokes are ischemic — a clot or mass blocks a vessel, cutting off the flow of blood to a portion of the brain. Those strokes kill some brain tissue and threaten more in many people.

Doctors can respond with clot-dissolving medication within the first few hours and within six hours have been reaching into the blood vessel with clot-removing devices such as stents.

But the studies show that they may have more time to save brain tissue where the blood supply is being choked off but the tissue has not yet died.

The DEFUSE 3 study looked at 182 people in 38 medical centers who suffered the kinds of blockages in brain arteries that cause 50 to 60 percent of deaths and the most severe kinds of disabilities. About half received typical care, involving blood pressure medication, blood thinners and other medical interventions. The other half had images taken and the clot removal procedure, known as a "thrombectomy," as well as the medications.

The key is the imaging technology developed at Stanford, Albers said. When a CT scan that uses a dye shows a larger area of damaged tissue surrounding the dead tissue, doctors can respond by removing the clot as long as 16 hours after the patient was last known to be well. This is especially important for people who have strokes in their sleep, which may make it impossible to pinpoint when the blockage occurred, or people who live far from medical centers where blood clots can be removed.

The diagnostic test can be read on a cellphone, with pink and green areas denoting the dead and damaged tissue, Albers said.

Fourteen percent of the people who had thrombectomies died, compared with 26 percent in the medical therapy group. Forty-five percent in the thrombectomy group escaped severe disabilities and were able to resume "functional independence."

Albers's work expands on the results of the DAWN study, sponsored by a manufacturer of clot-removing devices. It found that doctors could intervene as late as 24 hours after stroke victims were last known to be well.

Both studies were published in the New England Journal of Medicine. Albers's study was presented at a stroke conference held by the American Heart Association/American Stroke Association in Los Angeles. The organization also announced new treatment guidelines at the meeting, the first revision in five years. The revision extended the window for thrombectomies to as long as 24 hours in some cases. It also broadened the population eligible for clot-busting medication to people with mild strokes, some of whom may benefit from the therapy.

Peter Panagos, a professor of emergency medicine and neurology at Washington University in St. Louis who was not involved in the studies or the guideline update, said the emphasis in stroke care still will be getting patients to care as fast as possible. But now, he said, hospital systems will need to become better organized so that smaller ones test stroke victims quickly and send them to larger centers where thrombectomies can save more brain tissue.