Reflected ultrasound in the assessment of mitral valve disease.

SOUND WAVES with frequencies greater than 20,000 cycles per second are above the limits of sound audible to the human ear and, therefore, are designated as ultrasound. Over the past several years ultrasonic irradiation has been widely employed as a form of treatment in physical medicine.' Concurrently, other uses of ultrasound have been developed that utilize the particular transmission characteristics of these mechanical waves. Although ultrasound differs from light in being propagated more easily through a liquid or solid than through a gas, its transmission in a suitable medium is similar to light in many respects. It may be directed in a relatively straight beam with considerably less tendency to scatter than that characteristic of lower frequency audible sound, but demonstrates reflection and refraction similar to light. A beam of impulses traversing an acoustically uniform medium will have a portion of its energy reflected if it strikes the boundary of a medium having different acoustical impedance. The acoustical impedance of any substance is determined by the product of its density and the velocity of sound in the substance, and the proportion of ultrasound that is reflected at any interface is directly related to the magnitude of the difference of the acoustic impedance of the two media. Since the angle of reflection is determined by the angle of incidence of the beam, maximum re-