The contrast that is used with ultrasound is different from that of CT scans. CT scanners use a dye that can be potentially hazardous. This dye can lead to serious kidney damage in some patients. Ultrasound contrast, on the other hand, is made of small micro bubbles filled with a harmless gas. Because of the way ultrasound works, these micro bubbles reflect the sound waves differently than the surrounding tissue and thus provide contrast between normal tissue and abnormal tissue. The contrast used in this study, Definity, and is the same contrast used by Cardiologists.