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

476 16. Ultrasonicsslow to allow the previous pulse to drop below the noise level before the next pulseis sent. One of the coordinates on the display is the position x B representing thelocation the transducer was at the time of arrival t B of a refection from a reflectorR (R 1 or R 2 or R 3 ) as shown in Figure 16.11, depending on whether the beamin the slice y passes over them. It follows that t B represents the other coordinateof the point (x B , t B ). If the display is assigned a gray scale, the brightness ofthe display corresponds to the amplitude of the reflection. In order to establish an“on-off” scale, a threshold may be assigned to that only echoes exceeding a certainamplitude are displayed. The B-scan procedure is extensively used in medicalultrasonics, especially in obstetrics.C-ScanThe C-scan procedure differs from B-scan in that, while the transducer scansalong the x-axis in consecutive steps y i , the coordinate on the display is (x, y).The display is nonzero only if a reflecting surface R lies within the beam rangealong z centered at (x, y). This configuration would ordinarily result in the echoesfrom the front and back surfaces of the workpiece swamping the display, but theyare disallowed by setting up a range gate to accept and transmit echoes only ifthey arise from regions below the top surface and above the bottom surface of theworkpiece. It is also apparent that the range gate can be narrowed to accept signalsfrom a slice thinner in the z-direction. A threshold may also be prescribed withinthe gate so that only signals above a set amplitude will be displayed. A gray scalecan also be assigned.In both the B-scan and A-scan procedures, a computer can be utilized for datastorage and image enhancement. Color monitors can be useful for pseduocolorgray scales. 2ReferencesAtchley, A. A. and Crum, L.A. 1988. Acoustic cavitation and bubble dynamics. In: Ultrasound:Its Chemical, Physical and Biological Effects. Suslink, K. S. (ed.). New York:VCH Publishers, pp. 1–64.Bardeen, J. B., Cooper, L. N., and Schrieffer, J. R. 1957. Theory of superconductivity.Physics Review 108:1175.Blake, J. R., Boulton-Stone, J. M., and Thomas, N. H. (eds.). 1994. Bubble Dynamics andInterface Phenomena. Dordrecht, the Netherlands: Kluwer.Blitz, Jack. 1967. Fundamentals of Ultrasonics, 2nd ed. New York: Plenum Press. (A bitdated, but this remains a very clear exposition of the ultrasonic field.)Brown, B. and Goodman, J. E. 1965. High Intensity Ultrasonics. London: Iliffe.Crum, L. A. 1994. Sonoluminescence. Physics Today. September: pp. 22–29.2 “Color” displays for ultrasound analyses do not represent optical colors of the ultrasound targets, perse. Rather, they indicate values assigned to specific parameters of ultrasound reflections.