Background: High intensity focused ultrasound (HIFU) is an emerging non-invasive treatment modality for localized treatment of cancers. While current clinical strategies employ HIFU exclusively for thermal ablation of the target sites, biological responses associated with both thermal and mechanical damage from focused ultrasound have not been thoroughly investigated. In particular, endogenous danger signals from HIFU-damaged tumor cells may trigger the activation of dendritic cells. This response may play a critical role in a HIFU-elicited anti-tumor immune response which can be harnessed for more effective treatment.Methods: Mice bearing MC-38 colon adenocarcinoma tumors were treated with thermal and mechanical HIFU exposure settings in order to independently observe HIFU-induced effects on the host's immunological response.In vivodendritic cell activity was assessed along with the host's response to challenge tumor growth.Results: Thermal and mechanical HIFU were found to increase CD11c+ cells 3.1-fold and 4-fold, respectively, as compared to 1.5-fold observed for DC injection alone. In addition, thermal and mechanical HIFU increased CFSE+ DC accumulation in draining lymph nodes 5-fold and 10-fold, respectively. Moreover, focused ultrasound treatments not only caused a reduction in the growth of primary tumors, with tumor volume decreasing by 85% for thermal HIFU and 43% for mechanical HIFU, but they also provided protection against subcutaneous tumor re-challenge. Further immunological assays confirmed an enhanced CTL activity and increased tumor-specific IFN-γ-secreting cells in the mice treated by focused ultrasound, with cytotoxicity induced by mechanical HIFU reaching as high as 27% at a 10:1 effector:target ratio.Conclusion: These studies present initial encouraging results confirming that focused ultrasound treatment can elicit a systemic anti-tumor immune response, and they suggest that this immunity is closely related to dendritic cell activation. Because DC activation was more pronounced when tumor cells were mechanically lysed by focused ultrasound treatment, mechanical HIFU in particular may be employed as a potential strategy in combination with subsequent thermal ablations for increasing the efficacy of HIFU cancer treatment by enhancing the host's anti-tumor immunity.

The interaction of a lithotripter shock wave (LSW) with laser-generated single vapor bubbles in water is investigated using high-speed photography and pressure measurement via a fiber-optic probe hydrophone. The interaction leads to nonspherical collapse of the bubble with secondary shock wave emission and microjet formation along the LSW propagation direction. The maximum pressure amplification is produced during the collapse phase of the bubble oscillation when the compressive pulse duration of the LSW matches with the forced collapse time of the bubble.

The interaction of laser-generated tandem microbubble (maximum diameter of about $50\mu \mathrm{m}$) with single (rat mammary carcinoma) cells is investigated in a $25\mathrm{\text{−}}\mu \mathrm{m}$ liquid layer. Antiphase and coupled oscillation of the tandem microbubble leads to the formation of alternating, directional microjets (with max microstreaming velocity of $10\mathrm{m}/\mathrm{s}$) and vortices (max vorticity of $350000{\mathrm{s}}^{-1}$) in opposite directions. Localized and directional membrane poration (200 nm to $2\mu \mathrm{m}$ in pore size) can be produced by the tandem microbubble in an orientation and proximity-dependent manner, which is absent from a single oscillating microbubble of comparable size and at the same stand-off distance.

The acoustic fields of a high intensity focused ultrasound (HIFU) transducer operating either at its fundamental $\left(1.1\mathrm{MHz}\right)$ or third harmonic $\left(3.3\mathrm{MHz}\right)$ frequency were measured by a fiber optic probe hydrophone (FOPH). At $1.1\mathrm{MHz}$ when the electric power applied to the transducer was increased from $1.6\text{to}125\mathrm{W}$ , the peak positive∕negative pressures at the focus were measured to be $p^{+}=1.7–23.3\mathrm{MPa}$ and $p^{-}=-1.2–-10.0\mathrm{MPa}$ . The corresponding spatial-peak pulse-average $\left(I_{\mathrm{SPPA}}\right)$ and spatial-average pulse-average $\left(I_{\mathrm{SAPA}}\right)$ intensities were $I_{\mathrm{SPPA}}=77–6000\mathrm{W}∕{\mathrm{cm}}^2$ and $I_{\mathrm{SAPA}}=35–4365\mathrm{W}∕{\mathrm{cm}}^2$ . Nonlinear propagation with harmonics generation was dominant at high intensities, leading to a reduced $-6\mathrm{dB}$ beam size $(\mathrm{L}\times \mathrm{W})$ of the compressional wave $(11.5\times 1.8–8.8\times 1.04\mathrm{mm})$ but an increased beam size of the rarefactional wave $(12.5\times 1.6–13.2\times 2.0\mathrm{mm})$ . Enhancement ratio of absorbed power density in water increased from 1.0 to 3.0. In comparison, the HIFU transducer working at $3.3\mathrm{MHz}$ produced higher peak pressures $(p^{+}=3.0–35.1\mathrm{MPa}$ and $p^{-}=-2.5–-13.8\mathrm{MPa})$ with smaller beam size $(0.5\times 4\mathrm{mm})$ . Overall, FOPH was found to be a convenient and reliable tool for HIFU exposimetry measurement.