Atherosclerosis, the fatty plaque that accumulates inside arteries, can occur throughout the body. That includes the arteries in the neck that deliver blood to the brain (see illustration). Known as carotid artery stenosis, this condition leaves people vulnerable to serious complications. If the plaque breaks open, a clot may form and travel up to the brain, interrupting blood flow and causing a stroke. The best way to assess and reduce that dire but uncommon risk has long been controversial.