How does an ultrasound scan work?

Ultrasound uses sound waves that are emitted at a higher frequency than humans can hear. A hand-held wand called a transducer is pressed against the skin. This transducer sends out the sound waves, which bounce off the structures inside the body. These sound waves are then picked up by the transducer as they bounce back. A computer converts the sound waves into two-dimensional moving pictures.
Sonography for diagnostic imaging purposes, otherwise known as an ultrasound scan, employs frequencies of 1 million to 20 million cycles per second. These are produced by a device known as a transducer, which is placed directly on, and occasionally within, the patient. The device contains a material that vibrates upon receiving a voltage charge. Sound waves are then created and either transmitted, refracted, or reflected back to a receiver. The degree of reflection is based on applied frequency, the sound velocity, and the acoustic impedance of the tissue. In other words, different tissues will have different responses to the sound waves.

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