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By John G. Webster (Editor)

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Extra info for 61.Ultrasonics, Ferroelectrics, and Frequency Control

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Others are based on a means of introducing into the vasculature bubbles consisting entirely of gas that does not readily diffuse into blood. Although many early applications of contrast agents lie in the field of cardiac studies, where contrast agents allow unprecedented detailed imaging of anatomic and physiological structure within the heart (57), contrast agents have made their way into other medical fields. An exciting example is the contrast enhancement of tumors, based on the observation that contrast agents preferably fill either the tumor or the immediately surrounding tissue (as a function of tumor type), thus allowing ultrasound to highlight the tumor itself (58,59).

The early hypothesis that cavitation-produced singlet oxygen (perhaps via sonoluminescence) lies behind this process (126) has given way in recent work to new arguments that cavitation activates porphyrins by producing free radicals other than singlet oxygen, probably directly through sonochemical reactions rather than through light production (127,128). However, there are studies suggesting that the action of sonodynamic therapy is based in part on sonoporation. Essentially, ultrasound delivers chemicals into cells where the chemicals would not ordinarily go.

However, because of asymmetry in this process, the bubble grows minutely with each acoustic cycle and significantly over many acoustic cycles. Bubble Dynamics. A bubble, like a spring, has a primary resonant frequency. For a bubble, this frequency varies inversely with its radius and also strongly depends on gas content and surface tension, among other factors. ] A newly formed bubble within a relatively weak acoustic field often has a resonant frequency that is off from the applied frequency, making its temporal variation in volume initially small, symmetric in shape, and simple within an acoustic cycle and over many acoustic cycles.

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61.Ultrasonics, Ferroelectrics, and Frequency Control by John G. Webster (Editor)

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