Human uses of ultrasound: ancient and modern.

For untold millennia certain animals have used ultrasound to probe places where light is unavailable, echo-locating bats being among the most adept. With ultrasonics, bats can quickly and safely 'see' at night in pursuing insects or flying in dark caves. Unable to hear ultrasound, humans have nevertheless made use of it. They did this anciently by taming wolves, with their keen ultrasonic hearing, for aiding in the hunt. Currently, they are doing this by developing technology to detect, generate and process ultrasound for searching in air or other gases, in water or other liquids, and in solids. The story of these technological developments is a large and fascinating mirror of human history involving the advent of such discoveries and inventions as magnetostriction, piezoelectricity, sonar, ultrasonic microscopy, etc.--the list is long. By now we are skilled in probing for underwater objects, the internal structure in materials, organs inside the human body, etc.--again the list is long. A number of different ultrasonic systems can be categorized into one of three key generic approaches: pulse-echo exploration, intensity mapping, and phase-amplitude measurement. In addition, each of these categories can be combined with the others to produce hybrid systems for which an unambiguous categorization is difficult or impossible. Challenging problems remain but solutions are being found. New principles and techniques are being discovered that will improve the use of ultrasound. Employing tomo-holographic techniques to reduce ambiguity in probing three-dimensional objects, near-field techniques to boost resolution and using limited-diffraction beams to provide image construction with ultra high frame rates are cases in point.