Patients with certain anatomic or medical conditions have a greater risk of serious complications following carotid endarterectomy (a surgical procedure that removes blockages in the carotid artery). Recently, these patients have been treated effectively with the implantation of a carotid stent (a metallic mesh tube). When implanting the carotid stent, physicians use devices called embolic protection devices to prevent particles from the blockage from flowing up to the brain. The TriActiv® ProGuard™ System is an embolic protection device that uses a small balloon to temporarily stop blood flow within the diseased carotid artery while the stent is implanted. Then, the TriActiv® ProGuard™ System removes any particles that may have been released from the blockage during stent implantation before blood flow is restored. This study will evaluate the safety and efficacy of the TriActiv® ProGuard™ System during carotid stent placement in patients who are considered to be at a high risk for complications from surgical endarterectomy.