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Extra resources for 61.Ultrasonics, Ferroelectrics, and Frequency Control
As bubbles grow and shrink under the action of an applied acoustic field, they generate effects outside themselves directly via mechanical forces, and indirectly by altering their contents. To appreciate the results of the internal processes of bubbles, we first discuss heat generation BIOLOGICAL EFFECTS OF ULTRASOUND within the interior of bubbles and a possible example of biological effects associated with this process. This theoretical source of localized rather than bulk heating remains unobserved directly in vivo although in principle it is important in vivo.
Longer pulses of sound with high amplitude also alter the contraction of heart muscle in frogs (83). As yet no mechanism to create these bioeffects has been identified, although cavitation is likely, given the acoustic regime brought to bear in their studies. In principle, these results could be of concern to those who wish to use ultrasound only for diagnostic purposes. This is particularly true given the trend to increase the pulse amplitude of diagnostic ultrasound machines, which would enhance the opportunity for cavitation.
In ‘‘B’’ mode imaging, a series of ‘‘A’’ mode scans from the diagnostic source are collected together to form two-dimensional maps of the backscatter values as functions of distance and angle relative to the acoustic source/ receiver. In ‘‘M’’ mode imaging, an A mode scan is followed in time, producing a time-distance trace that finds particular use in imaging the heart and its periodic motion. ) Doppler imaging takes advantage of the fact that blood moves relative to the direction of acoustic wave propagation to create images based on the strength and direction of the Doppler shift in the backscattered signal.