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

492 17. Commercial and Medical Ultrasound Applications17.3 Ultrasound ImagingThe application of ultrasound to imaging processes is extremely important in industryand in medicine. Because imaging entails low-intensity ultrasound energy,it provides a valuable nondestructive testing technique. Ultrasonic imaging, whichmay be defined as any technique of providing a visible display of the intensity andphase distributions in an acoustic field, falls into a number of categories: (1) theelectronic-acoustic imaging; (2) B-scanning, most commonly used in medical diagnosis;(3) C-scanning, widely used in nondestructive testing and inspection offlat and cylindrical surfaces; (4) liquid-surface-levitation presentations; (5) liquidcrystal display (LCD), photographic or similar display; (6) light-refractionmethods; and (7) acoustical holography.Electron-acoustic Image ConvertersThe concept of an electron-image converter originated by the Russian scientist S.Ya. Sokolov (1937) who envisioned a device, similar to a video camera tube, inwhich the photosensitive element is replaced by a pressure-sensitive piezoelectricplate. Secondary electrons are given off when the plate is struck by a scanningbeam of electrons. An ultrasound field aimed at the plate influences the electricalpotentials on the faces of the plate. These potentials are proportional to the impressedacoustic pressure, with the result that the electrical potentials modulatethe secondary emission of electrons. The secondary emission that results from theimpingement of an electron beam is a function of the velocity of the primary electronsand also of the plate material. The ratio of the number of secondary electronsleaving the plate to the number of electrons impinging on the plate is called thesecondary emission ratio. When the primary electron voltage is increased fromzero, the secondary emission rate also increases from zero, passes through a maximum,and then decreases. With some piezoelectric materials, the maximum ratioexceeds unity and there are two velocities, one above and one below the point ofmaximum secondary emission, at which the ratio becomes unity. In other materials,the maximum secondary emission ratio never exceeds unity.Smythe et al. (1953) improved on the Sokolov concept by developing anultrasonic-imaging camera that could be operated in either an amplitude-sensitivemode or a phase-sensitive mode. In the amplitude-sensitive mode, a relatively lowvoltage(160–200 V) scanning beam is used. The secondary emission ratio is lessthan zero, which allows the potential of the surface of the (quartz) piezoelectricplate to nearly equal that of the electron gun cathode, in absence of ultrasound.After a scan without ultrasound, the ultrasound is activated and the plate scannedagain. The electrons are distributed over the surface of the plate in direct ratio to thepiezoelectric voltage present at each point of the surface. A corresponding imagecharge forms on an anode located externally to the tube, and the video image isobtained directly from the anode charges. The ultrasound is then turned off and thenegative charge on the piezoelectric surface is removed by inundating the surface