The study will enroll 403 participants with possible diagnosis of acute stroke. Individuals participating in the study will be injected with a contrast solution via an intravenous line. A small probe will be used to obtain images of blood vessels in the head. The study researchers will measure vital signs prior to injection of the contrast solution, 5 minutes after the injection, and at the end of the ultrasound. Each participant will have a 5-minute mental function assessment and a brief neurological exam. Participants will undergo at least 3 ultrasounds. The total time of each ultrasound is less than 30 minutes.