Efficient Generation of Cavitation Bubbles in Gel Phantom by Ultrasound Exposure with Negative-Followed by Positive-Peak-Pressure-Emphasized Waves

Abstract

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.

Estimation of sonodynamic treatment region with sonochemiluminescence in gel phantom

“…1,2) Sonodynamic treatment is another type of ultrasonic cancer treatment generating reactive oxygen species (ROS) by combining acoustic cavitation and a sensitizer. [3][4][5][6][7][8] When a microbubble is subjected to ultrasound at its resonant frequency, its oscillation amplitude increases to an extreme. This is followed by its catastrophic collapse, at which the gas inside it may receive virtually adiabatic compression causing a temperature increase to thousands of degrees centigrade.…”

Enhancement of High-Intensity Focused Ultrasound Heating by Short-Pulse Generated Cavitation

“…After negative pressure generates cavitation bubbles enough to provide a pressure release surface, positive pressure is converted to negative pressure exceeding the intrinsic threshold and forms cavitation clouds. Either peak negative or positive pressure can be emphasized by superimposing the second-harmonic ultrasound onto the fundamental component [51][52][53][54]. Figure 1.…”

“…Cavitation bubbles generated in a poly(acryl amide) (PAA) gel at an acoustic intensity of 42 kW/cm 2 were observed with a high-speed camera (HPV-2A, Shimadzu, Kyoto, Japan) at a frame rate of 500 kfps with an exposure time of 250 ns. The results are shown in Figure 4 [54]. A 128-element array transducer with both diameter and focal length of 100 mm (Imasonic, Voray sur l'Ognon, France) was placed in a water tank so that the ultrasound focus was located in the gel phantom in degassed water.…”

“…The gel included 22.5% bovine serum albumin (BSA). High-speed images of generated cavitation bubbles in a poly(acryl amide) (PAA) gel including 22.5% bovine serum albumin (BSA) by each sequence at an ultrasound intensity of 42 kW/cm 2 [54]. The frame rate was 500 kfps with an exposure time of 250 ns.…”

“…Cavitation clouds are formed backward from a focal point, in other words, toward a HIFU transducer, through a shock scattering phenomena [34,46,53,54]. In the trigger HIFU heating, the mismatch between the center position of bubble clouds and focal plane of heating bursts may have caused a less predictable coagulation pattern.…”