THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

17.4 Medical Uses of Ultrasound 497Figure 17.10. Ultrasonic holography making use of liquid levitation.The wave used in the reconstruction does not have to be the original, and thisallows the use of ultrasonic waves and the subsequent reconstruction of the imageusing light from a laser. But the size of the image changes in proportion to theratio of the wavelength of the reconstructing wave to the wavelength of the originalilluminating wave. There are a number of methods of making acoustical holograms.One method as shown in Figure 17.10 uses liquid levitation, where the acousticwaves from below the surface of a liquid forms an ultrasonic image at the surfaceof the liquid that can be rendered visible on a photographic plate. The referencebeam may be obtained either by reflecting a portion of the irradiating beam ontothe surface or by generating a separate wave through a second transducer. Theheight of the bulge of the liquid surface due to the impetus of an acoustic wave iscritical; it should be small compared to the wavelength of the light.17.4 Medical Uses of UltrasoundUltrasound use in the medical arts can be classified as being diagnostic or therapeutic.While the diagnostic procedures involving ultrasound have been in usefor a number of years, ultrasonic therapeutics constitutes a newer, rapidly growingdomain. The ultrasound frequencies used in medical applications range from approximately25 kHz used in dental plaque removal to the megahertz range that isrequired for medical imaging. The merits of ultrasound diagnostics include safety,convenience, and capability of detecting medical conditions to which X-rays andother means of diagnosis are insensitive. Because ultrasound is much safer than