ULTRASONIC ABSORPTIOX MICROSCOPE 



tion by an "ultrasonic microscope" of less 

 resolving power than that of the light mi- 

 croscope. This follows from the fact that 

 many structural components of biological 

 materials or systems, with different ultra- 

 sonic absorption coefficients, may not be de- 

 tectable at all by the light microscope. There 

 is no a 'priori reason why materials with 

 greatly different ultrasonic absorption co- 

 efficients should have either detectably dif- 

 ferent absorption coefficients or indices of 

 refraction for light. 



The principle of operation of the ultra- 

 sonic microscope is illustrated in Fig. 1. 

 High frequency sound waves are generated 

 in a "coupling" medium by a piezoelectric 

 crystal vibrating in a thickness mode. The 

 coupling liquid, which fills the irradiation 

 chamber, serves to conduct the sound to and 

 from the movable specimen which is inter- 

 posed between the crystal and a small ther- 

 moelectric probe. (An array of probes can 

 be incorporated for more rapid acquisition 

 of data). The piezoelectric crystal is excited 

 electrically at a mechanical resonant fre- 

 quency by voltage pulses with a rectangular 

 temporal envelope (carrier frequency equal 

 to the mechanical resonant frequency) of 

 short duration. The small probe detects the 

 acoustic energy level of the sound which 

 passes through the portion of the specimen 

 in its immediate neighborhood. The varia- 

 tion in this transmitted energy level, as a 

 function of the position of the specimen rela- 

 tive to the probe constitutes an acoustic 

 image of the ultrasonically detected struc- 

 ture. 



Two mechanisms are involved in the de- 

 tection of the ultrasound by the thermo- 

 electric probe (8, 9, 10). First, an increase 

 in the temperature of the wire results from 

 the conversion of acoustic energy into heat 

 by the viscous forces acting between the 

 wire and the imbedding fluid medium. Sec- 

 ond, acoustic energy is converted into heat 

 by the absorption of sound in the body of 

 the coupling medium which surrounds the 



SPECIMEN 



THERMOCOUPLE 

 PROBE 



X-CUT QUARTZ' 

 PLATE 



COUPLING 

 LIQUID 



Fig. 1. Schematic representation of the ultra- 

 sonic microscope showing the transducer plate 

 which is excited to produce pulses of ultrasound 

 in the coupling liquid, the specimen under exam- 

 ination which is movable in the coupling liquid, 

 and the thermocouple probe whose junction is 

 placed immediately adjacent to the specimen. 



probe and specimen. The thermoelectric emf 

 of the probe acts as the input to a DC 

 chopper amphfier (noise level less than 0.01 

 microvolt), the output of the latter driving 

 the vertical deflection plates of a cathode-ray 

 oscilloscope (see Fig. 2). Thus, when the 

 sound source is driven by a suitable radio 

 frequency pulse, the cathode-ray beam is 

 transiently deflected from its equilibrium 

 position and the magnitude of this deflec- 

 tion is a measure of the relative amount of 

 acoustic energy detected by the probe. As 

 the specimen is moved parallel to the radi- 

 ating face of the crystal through the pulsed 

 acoustic field, the changing deflection of the 

 cathode-ray beam is observed and recorded. 

 The data are then plotted and a contour 

 "picture" of the disturbance to the sound 

 field distribution, caused by the presence of 

 the specimen, is obtained. The contour pic- 

 ture constitutes an acoustic image of struc- 

 ture in the specimen. 



The electronic instrmnentation used in the 

 first niodel of the ultrasonic absorption mi- 

 croscope is illustrated in the block diagram 

 of Fig. 2. A commercial signal generator is 

 used to obtain the radio frequency energy of 

 predetermined frequency. The signal gen- 

 erator is controlled by a keying unit which 

 activates the generator to produce a pulsed 

 output. The keying unit is, in turn, con- 

 trolled by a digital tuning device which 



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