High-intensity focused ultrasound (HIFU) causes a selective temperature rise in tissue and is used as a noninvasive method for tumor treatment. However, there is a problem in that it typically takes several hours to treat a large tumor. The development of a highly efficient method is required to shorten the treatment time. It is known that cavitation bubbles generated by HIFU enhance HIFU heating. In this study, the enhancement of the heating effect by cavitation was estimated in a numerical simulation solving a bio-heat transfer equation (BHTE) by increasing the absorption coefficients in and out of the volume of cavitation bubbles. The absorption coefficients were obtained by a curve fitting the temperature rise near the focal point between experiment and simulation. The results show that cavitation bubbles caused the increase in ultrasonic absorption not only in but also near the volume of cavitation bubbles.
Cavitation bubbles have much potential for emphasizing therapeutic treatments such as high-intensity focused ultrasound (HIFU) treatment, histotripsy, and sonodynamic therapy. Their highly efficient as well as controlled generation is important to utilize them effectively as well as safely. However, producing negative pressure over the cavitation threshold by focused ultrasound is difficult because of the nonlinear propagation combined with the focal phase shift. We have suggested a dual-frequency ultrasound exposure method, in which N- and P-waves emphasizing either the peak negative or positive pressure, respectively, are synthesized by superimposing the second harmonic onto the fundamental frequency. In this study, high-speed camera observation demonstrated that the exposure with N-waves immediately followed by P-waves could generate cavitation bubbles most efficiently in gel phantom. Furthermore, the measured negative and positive pressure distributions of the N- and P-wave fields, respectively, agreed well with the optically observed distributions of cavitation inception and cavitation cloud growth.
The results of nanoindentation testing strongly rely on load-displacement curves, but an abnormal load-displacement curve with obvious inflection in the unloading portion was commonly observed in previously published papers and the reason is not clear. In this paper, possible reasons involved in a custom-made indentation instrument, such as sensors, control and assembly issues, are analyzed and discussed step by step. Experimental results indicate that non-ideal assembly of the precision driving unit strongly affects the shape of the load-displacement curve and its affecting mechanism is studied by theoretical analysis and finite element simulations. This paper reveals the reason leading to the abnormal load-displacement curve, which is helpful for debugging of indentation instruments and can enhance comparability of indentation results.
High-intensity focused ultrasound (HIFU) causes selective tissue necrosis through heating and is used for a noninvasive treatment of cancer therapy. However, it has a problem of a long treatment time for a large tumor. To improve the throughput of the treatment, the development of a highly efficient method is needed. It is known that cavitation bubbles enhance the heating effect of ultrasound during ultrasonic irradiation because the increase in the energy dissipation is caused by the volumetric oscillation of cavitation bubbles. In this study, cavitation bubbles were generated at multiple spots by changing focal position of high-intensity ultrasound. Immediately after generating the bubbles, the bubbles were exposed to a wide-focused ultrasound which covers all the cavitation sites for the cavitation-enhanced heating in a large region. The behavior of the cavitation bubbles at multiple spots in a tissue-mimicking gel was observed by high-speed photography, and the coagulation performance of the developed sequence was confirmed with an experiment using excised tissue. The results showed high efficacy of the proposed method for to coagulate a large tissue region.
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