Acoustical Imaging of Biological Tissue—Holography and Direct Imaging

Abstract

Ultrasound has been used for some time for the purpose of imaging biological tissue structures. The primary impetus for this has been the diagnostic usefulness of such images. Both transmission and reflection techniques have been employed, and since the earliest visual imaging by acoustical holography the possible medical use has been proposed. Holographic imaging offers several distinct advantages over direct-imaging techniques and has, at the same time, some disadvantages. The primary and overriding advantage of holographic imaging is, of course, the recording of an entire three-dimensional object scene in one step for subsequent image generation. The possibility also exists of constructing virtual images with the subjective character of realistic object structures. Characteristic of the holographic imaging process are such factors as detector resolution, detector sensitivity, amount of data recorded, coherent illumination, etc. All of these factors are of importance in acoustical holography, together with the well-known problems of image aberration and distortion. Techniques and recent results are presented concerning several specific areas, including: techniques for the reduction of image distortion; techniques for the improvement of acoustic capture angle or numerical aperture of acoustical detectors; and a new technique for producing acoustical holograms. This new technique will reduce or eliminate the coherent illumination problems of holography, as well as reduce or eliminate unwanted object signals. This technique can be accomplished by controlling the coherency of the acoustic fields.