Ultrasound is a radiological procedure that uses sound waves with a frequency well above the range of human hearing that can penetrate under the skin and, much like sonar, reflect off structures within the body, producing an image that can be used to diagnose illness.  An image taken with ultrasound equipment is a sonogram.

Because the sound waves are harmless, the procedure can be used on any patient, even pregnant women (who can't have X-Rays) or persons with metal pins in their body (who can't have an MRI). However, the penetration of the waves is limited to the first few inches below the skin, and as such cannot be used to scan structures deep inside the body, such as bones. However, they are very useful for looking for problems in the digestive tract and the organs that are close to the skin, such as the liver and kidneys.

Medical ultrasonography at Wikipedia

Ultrasound at the Mayo Clinic

Ultrasound Lectures from Yale Professor

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