# Enhancement of High-Intensity Focused Ultrasound Heating by Short-Pulse Generated Cavitation
## 3 High-Speed Observation of Cavitation Bubbles Induced by Trigger HIFU Exposure
### Lifetime of Cavitation during and after HIFU Exposure  
The dissolution time of air microbubbles without any shells is on the order of tens of milliseconds [55], which is much shorter than that of encapsulated contrast agents with slightly soluble gas. Cavitation bubbles are considered to be free bubbles and mainly consist of vapor and dissolved gases in an ambient medium. Therefore, a high-speed monitoring method is necessary to detect cavitation bubbles before they dissolve quickly. A high-speed ultrasonic imaging using plane-wave transmission at a frame rate of thousands of frames/s [56] has been used to monitor residual cavitation microbubbles after the exposure of a high-intensity pulse [57; 58]. The two studies  
Figure 4: 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. showed that the lifetime of cavitation bubbles was less than a few tens of milliseconds in tissue samples of sheep thigh muscle [57] or chicken breast [58]. In the trigger HIFU exposure, cavitation bubbles induced by the trigger pulse are forced to oscillate by the heating burst. When bubbles oscillate under alternating pressure, the effect of rectified diffusion [59; 60; 61] contributes to suppressing bubble dissolution. An attractive force for oscillating bubbles such as Bjerknes force [62] causes the bubble coalescence [63], which reduces the pressure contribution from the surface tension inversely proportional to the bubble radius then slows the gas dissolution to the surrounding medium. The fission of collapsing bubbles owing to the surface instability induces the fragmentation of the bubbles. The fission process has an additional damping effect on the energy dissipation of a spherically oscillating bubble [64], which can lead to the greater enhancement of the ultrasonic heating by the heating burst. Figure 5 shows the high-speed images of bubble clouds generated by a trigger pulse and cavitation bubbles oscillated by a heating burst in a HIFU focal region [36]. The target was a PAA gel including 15% BSA placed in a water tank. They were taken with a high-speed camera (Phantom v310, Vision Research, Wayne, PA, USA) at a frame rate of 3.2 kfps with an exposure time of 20 us. A 690-nm pulse laser with a pulse width of 20 ns (CAVILUX Smart, Cavitar, Tampere, Finland) was used as a backlight source. The temperature and dissolved oxygen in water were kept at 20 \({}^{\circ}\)C and 23%-28%, respectively. The acoustic source was a 256-element array transducer (Imasonic, Voray sur l'Ognon, France) with both diameter and focal length of 120 mm. The transducer was connected to 128-channel stair case voltage drive amplifiers [65] (Microsonic, Tokyo, Japan) by electrically combining two adjacent elements and driven at 1.2 MHz. The applied trigger HIFU sequence is shown in Figure 1a. The instantaneous total acoustic power of the trigger pulse was 1200 W and that of the heating burst was 21 W. The duration times of the trigger pulse and heating burst were 0.1 and 100 ms, respectively. Figure 6 shows the ultrasonic images of a different type of PAA gel phantom containing a 0.5% graphite layer with a thickness of 0.75 mm to investigate ultrasonic contrast ratio between bubbles and linear scatterers [36]. A pulse inversion (PI) method [66] with two-plane wave transmissions was used to monitor residual bubbles in the graphite gel plane 0.9 ms after the stop of the heating burst exposure with duration times of 0, 1, 10, or 100 ms. The differences in PI images before and after the HIFU exposure were calculated to reduce the effect of graphite particles. Figure 5: High-speed images of generated and oscillating cavitation bubbles in a PAA gel including 15% BSA by the sequence shown in Figure 1a [36]. A 690-nm pulse laser with a pulse width of 20 ns was used as a backlight source.