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

494 17. Commercial and Medical Ultrasound Applicationsthe signal currents by about 100,000 times greater than the threshold values beforethey are fed into the associated amplifiers. Thus, the electron multiplier servesas a wide-band amplifier with good noise characteristics. Its output constitutesthe video signal that is processed through a conventional closed-circuit televisionsystem.Two sealed ultrasonic camera tubes were developed by Jacobs: one version incorporatesquartz crystals permanently sealed to the end of the tube and the featuresinterchangeable crystals which permit operation over the frequency range from 1to 15 MHz. Color also has been introduced to increase the sensitivity of the ultrasonicimage converter, particularly in view of the fact that the human eye is moresensitive to changes in color than to differential changes in display brightness. Thecolor display indicates both relative amplitude and phase of the ultrasonic signalundergoing analysis. In North America, the video method used is the NationalTelevision System Committee (NTSC) color broadcasting standard, and the operatingfrequency is 3.58 MHz. 2 A more dominant video broadcasting method is thePAL system that is used in the United Kingdom, Germany, Spain, Portugal, Italy,China, India, most of Africa, and the Middle East. Several distinguishing featuresof the PAL system are: (a) a better overall picture than NTSC because of the increasedamount of scan lines and (b) because color was part of the standard fromthe beginning, color consistency between stations and TVs is considerably better.There is a down side to PAL, however, as there are fewer frames (25) displayedper second, compared with nearly 30 frames per second of the NTSC system.Acoustic LensAcoustic lens are necessary in the use of electron-acoustic image converters.Acoustic lens can (a) increase the sensitivity of an imaging system through energyconcentration and (b) provide coverage of a larger area by concentrating the imageon the receiving piezoelectric element of the image converter in almost the samefashion as an optical lens reduces a larger picture into a smaller area. For optimalperformance, the velocity of sound in the lens material must differ considerablyfrom that of the surrounding media, and the reflection of energy at the boundarybetween the lens and the surrounding should be minimal. The latter condition isfulfilled automatically when the acoustic impedances of the lens materials andthe surrounding medium match each other. Liquid lens of carbon tetrachloride orchloroform have the same acoustic impedance as water, but their toxicity generallyprecludes their use in industry. Plastic lenses also have been developed, butthe sound propagation velocity in these materials exceeds that of water and theypresent an impedance mismatch between the lenses and water, but not to the degreeof rendering them useless. The relatively high absorption of plastics limitstheir use to frequencies less than 15 MHz. Metallic lenses, which can be made ofaluminum and other metals, possess low-absorption characteristics, so they can2 In the year 2006 analog NTSC television broadcasts are scheduled to cease in the United States infavor of the ATSC Digital Television Standard. This should result in higher definition video imaging.