Ultrasound uses reflected sound echoes to study and characterize internal structures and tissues. Many mothers are familiar with ultrasound as a diagnostic tool for examining the fetus during pregnancy. With ultrasound a growing fetus can be imaged, an achievement previously impossible without exposing the fetus to ionizing radiation. During an obstetrical ultrasound, a series of measurements will be made to determine the age and overall health of the fetus. Ultrasound is also important to the non-pregnant woman as it may visualize abnormalities of the ovaries, fallopian tubes or uterus not detectable by clinical examination. Another major application of ultrasound imaging is in the field of heart disease. Ultrasound is an excellent tool for examining the heart because it is “non-invasive” (not traumatic to the body) and can reveal more about heart function than conventional x-ray studies. Cardiac ultrasound eliminates the need for many diagnostic procedures that require thin tubes, or catheters, to be inserted into the heart. Ultrasound is indispensable in detecting fluid collections around the heart and is the best technique for the diagnosis and follow-up of heart valve abnormalities. Ultrasound can reveal information about the shape, texture and composition of tumors and cysts that cannot be seen on conventional x-ray studies. Suspicious lumps found during a mammographic study can be examined with ultrasound to determine whether they are benign cysts or solid masses. Some gallstones and kidney stones may be seen only with ultrasound; ultrasound is the imaging technique of choice for diagnosis of gallstones. An infant’s brain and spinal cord can be easily imaged by ultrasound. As the uses for ultrasound increase, it is providing new ways to image the musculoskeletal system and the prostate gland. During a typical ultrasound examination, the patient lies on a table and a transducer (a device that produces the sound waves and receives their echoes) is placed in contact with the skin. This does not cause the patient any discomfort. The images produced are displayed on a video screen and are frequently recorded as they are being made. A radiologist interprets the images. The frontiers of ultrasound continue to expand. Doppler ultrasound, which visualizes blood flowing through vessels, is being used to study kidney, liver and heart transplants and blood flow to the brain. Intraoperative ultrasound frequently can be the key to the success of many procedures.