A person whose artery is 70 percent blocked or more has four times the risk of stroke as a person with lesser blockage. Severe blockages can be surgically removed.

Currently, doctors use cerebral angiography as the final step in assessing the blockage to determine whether surgery is needed. This method involves inserting a catheter in the leg and following the arteries up into the head. There, a dye is released into the blood that can be seen on x-rays.

But the catheter itself can rupture the plaque and send pieces into the brain, or trigger an arterial response that may form blood clots, leaving patients at risk of stroke not only during the procedure but for several hours afterward. And, said Hirsch, "the risk is exponential depending on their age and their history of stroke."

Hirsch and other studied 72 patients. On each they performed cerebral angiography, transcranial Doppler and duplex carotid ultrasound and found no statistical difference in the results. This shows, Hirsch said, that "if you are basing the indication for (surgery) solely on the measurement of (blockage), you can do it safely and non-invasively. The combination of the two (ultrasound) methods gives you enough information."

However, Hirsch noted, this new method will not find blockages lower in the carotid artery where it emerges from the main artery leading from the heart.