A method may include intravenously directing first bubbles into a patient's venous circulatory system and to a right side of the patient's heart. The first bubbles may have sizes that fall within a first range. The method may further include monitoring the patient's heart, with ultrasound imaging, to detect presence of the first bubbles on a left side of the patient's heart. Upon detecting the first bubbles on the left side of the patient's heart, the method may further include intravenously directing second bubbles into the patient's venous circulatory system and to the right side of the patient's heart. The second bubbles may have sizes that fall within a second range that is different than the first range. Upon detecting the second bubbles on the left side of the patient's heart, the method may further include initiating treatment to minimize risk of stroke in the patient.