Ultrasound is an imaging method in which sound waves are bounced off internal tissues to outline a part of the body. The sound waves are called "echoes" and they are displayed on a video monitor creating a "sonogram." Ultrasonography cannot penetrate bone, but is very good at imaging soft tissues and detecting the presence of fluid, such as blood or urine. It can show the structure of the body’s various organs, such as the liver, spleen, and uterus. The ability, through ultrasound, to visualize abnormalities can help detect tumors and other kinds of problems. Ultrasound is also extremely valuable in cardiology, where it can be used to monitor the flow of blood in the heart and the contractions of the heart muscle, and to assess structural defects.