Ultrasound is the term used to describe very high frequency sound waves, above the audible range.
Medical Ultrasound Scanners utilise sound waves in the region of 3-20 megahertz. They are produced by the electrical excitation of a piezo-electric crystal. Such crystals alter shape when an electric current is applied, during which process an ultrasound wave is produced.
In medical Scanner, the sound waves are produced in a hand-held 'transducer', which is applied to the patients body. The Ultrasound wave passes through the body, where it is variously reflected, scattered and absorbed by the tissues and interfaces it passes.
Some of the reflected and scattered ultrasound returns to the transducer, where it is detected and converted back into electrical impulses. After processing, these impulse can be used to produce an image.
By modifying the method of processing the electrical signals, scanners can demonstrate colour images, blood flow, 3-dimensional studies and more, to aid in diagnosis.