For years, many people suffering a stroke have not been able to get a critical drug due to time limits on its use. Now, a new study suggests that treatment time window can be doubled.

Experts said the findings could open up the treatment option for many more stroke victims.

The drug is called tissue plasminogen activator, or tPA. It can dissolve the blood clot causing a person's stroke, and protect brain tissue from any further damage. However, tPA has to be given within a certain time frame to work.

Long-time guidelines stated that the drug should be infused within three hours of the first stroke symptoms -- or, for certain patients, within 4.5 hours.

That has limited its use. One problem is that people often do not recognize the signs of a stroke and delay going to the hospital, explained Dr. Randolph Marshall, a professor of neurology at Columbia University, in New York City.

He wrote an editorial published with the study.

Another issue is that some people wake up in the midst of a stroke, and it's unclear when the symptoms began.

So, at least as of 2013, only about 6% of patients hospitalized for an ischemic stroke received tPA, according to Marshall.

Ischemic strokes are caused by a clot that blocks blood flow to the brain, and they account for the large majority of all strokes.

In the new trial, researchers looked at whether they could expand the tPA time frame to nine hours in certain ischemic stroke patients. Specifically, that meant cases where brain imaging showed the drug could still be beneficial.

The study took advantage of a newer technology called "perfusion" imaging.

"It allows us to see the area of brain damage, and whether there is still tissue that's salvageable," Marshall explained. It's done using either a CT or MRI scanner.

When patients did have savable brain tissue, the trial found, giving tPA as late as nine hours after the first symptoms still brought benefits. Three months later, over 35% of those patients had no or minimal disability from their stroke.

That compared with 29.5% of patients who were given infusions of a placebo (along with other standard stroke care).

The benefit did come at a price. About 6% of tPA patients had bleeding in the brain -- a known side effect of the drug. But that figure is similar to what's seen when tPA is given earlier, too, Marshall pointed out.

Dr. Geoffrey Donnan, a neurologist at the University of Melbourne, in Australia, led the study.

He said the findings suggest that perfusion imaging should be done whenever a stroke patient either woke up with symptoms, or can be treated within 4.5 to nine hours of their first symptoms.

Perfusion imaging is not universally available, though. Right now, it's mainly used at larger hospitals with comprehensive stroke care, according to Marshall.

The findings are based on 225 stroke patients at hospitals in Australia, Taiwan, China and New Zealand. All underwent perfusion imaging and were shown to have savable brain tissue. Donnan's team randomly assigned them to have an infusion of either tPA or a placebo fluid.

Many of the patients had woken up with stroke symptoms. Others were known to have suffered their first symptoms within 4.5 to nine hours of being randomly assigned to their treatment.

Still, tPA is not the only stroke treatment option. Doctors can also physically remove the blood clot, using a scope that's threaded into the blocked brain artery. In fact, that procedure -- called a thrombectomy -- can be done as late as 24 hours after the first symptoms in some patients.

It's not known whether giving tPA in the later window would be preferable to a thrombectomy, according to Marshall.

"I think that will be the next big trial -- to compare this with thrombectomy," he said.

For now, Marshall and Donnan both stressed that people should learn the signs and symptoms of stroke, and never delay medical care.

The most common symptoms include sudden weakness or numbness in the face or arm (usually on one side of the body), slurred speech, and sudden difficulty walking or seeing, according to the American Stroke Association.

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