Electrical to Mechanical 



Mechanical to Electrica 



Figure 1. — Piezoelectric eftect 



Figure 2. — Split function concept. Pulsed ultra- 

 sound from transducer is transmitted 10% of the 

 time and received 90% at 400 pulses/sec. 



Ultrasonics, the technology of high 

 frequency sound waves, deals with the 

 transmission of sound or pressure 

 waves through a medium. Sound 

 waves, unlike electromagnetic waves, 

 cannot be transmitted through a vac- 

 uum. The generation of sound waves 

 from a transducer depends on a phe- 

 nomenon known as piezoelectric ef- 

 fect. This effect is produced when 

 electrical energy is applied to a crystal 

 which, when distorted by this electri- 

 cal energy, will produce a mechanical 

 pressure wave. In reverse, the piezo- 

 electric effect occurs when mechani- 

 cal energy distorts the crystal, pro- 

 ducing an electrical potential which 

 can be measured. The technique of 

 recording reflected ultrasound results 



from this reversible behavior. (See 

 Figures 1 and 2.) Sound waves travel 

 through various materials with char- 

 acteristic velocities. The product of 

 the density of the material and the 

 velocity of sound through the given 

 material is called "characteristic acous- 

 tic impedance" (Zl. When the two 

 substances adjacent to each other 

 transmit sound at a different velocity, 

 the ultrasonic reflection (/?) at the 

 boundary is determined by the ratio 

 of the two acoustic impedances as 

 described in the formula 



A f 



^=-f- 



1.5 OO 



5,000,000 cyC/ 





If the two substances have the same 

 acoustic impedance, the numerator 

 becomes zero and there is no reflection. 

 On the other hand, if there is a large 

 difference between the acoustic im- 

 pedances, the result approaches unity, 

 and almost all of the energy is reflect- 

 ed. In between these two extremes 

 some of the sound energy is reflected 

 while that remaining passes through 

 the interface. Since most soft tissues 

 have acoustic impedances that are 

 quite similar, there are relatively weak 

 reflections at the boundaries. The air- 

 tissue interface is the strongest biologi- 

 cal reflector. The bone-tissue inter- 

 face, likewise, produces a very strong 

 reflection. As most reflections are rela- 

 tively weak, sensitive equipment is re- 

 quired to detect those boundaries 

 with the less strongly reflected echoes 

 and interfaces such as fat-muscle. 



What is the resolution of the sys- 

 tem? The frequency of sound deter- 

 mines its wavelength. The resolution 

 is likewise dependent on the wave- 

 length in the axial direction. The high- 

 er the frequenc) . the smaller the 

 wavelength; thus, we have a better 

 resolution capabilits as determined 

 by the formula r = X/ where r is the 

 velocity of the sound in the medium. 

 X is the wavelength, and / is the fre- 

 quency. We assume that the minimum 

 distance between two objects for dis- 



= 3x10"^ 

 = 0.3 mm = A 



A X 1.5 = Minimum Distance 

 Between T\a/o Objects 

 for Discrimination. 



= 0.45 mm For 5 mhz 



Figure 3. — Formula lor resolution. 



crimination must be equal to at least 

 l'/2 wavelengths. (See Figure 3.) By 

 going to higher frequencies, however. 

 we lose penetration in tissue due to 

 sound attenuation: therefore, a com- 

 promise must be made and the fre- 

 quency selected which gives adequate 

 axial resolution, yet adequate penetra- 

 tion through the tissue thickness. For 

 the mammalian models studied the 

 frequency varied between 1 and 2 

 megahertz, which was adequate for 

 penetration through the structure 

 studied. 



Findings 



Although the size dilterence be- 

 tween the 28 foot captive gray whale 

 and the captive Atlantic bottlenosed 

 dolphin {.Titrsiops nuncaliis) necrop- 

 sy model is somewhat different, the 

 anatomical structures of the mammals 

 are known to be similar. 



During periods of illness or mal- 

 nutrition, marine mammals of these 

 species are noted to develop a depres- 

 sion in the dorsal contour posterior to 

 the axilla. It is thought that this de- 

 pression is due to catabolism of blub- 

 ber, areolar fat. and/or muscle mass 

 loss. 



Presuming ihat areolar fat dimin- 

 ishes in volume prior to muscle loss. 



Figure 4. — Radiograph of Turs/ops cross sec- 

 tion demonstrating (1) blubber. (2) areolar lat. 

 (3) muscle group to fascial layer, and (4) second 

 deep muscle group to dorsal spinous process. 



16 



