Xin Ying, Zhang Aili, Xu Lisa X, Brian Fowlkes J
School of Biomedical Engineering, Shanghai Jiao Tong University, Shanghai 200030, China; 400 Med-X Research Institute, Shanghai Jiao Tong University, 1954 Huashan Road, Shanghai 200030, China e-mail:
Fellow ASME School of Biomedical Engineering, Shanghai Jiao Tong University, Shanghai 200030, China; 400 Med-X Research Institute, Shanghai Jiao Tong University, 1954 Huashan Road, Shanghai 200030, China e-mail:
J Biomech Eng. 2017 Sep 1;139(9). doi: 10.1115/1.4037150.
Acoustic droplet vaporization has the potential to shorten treatment time of high-intensity focused ultrasound (HIFU) while minimizing the possible effects of microbubbles along the propagation path. Distribution of the bubbles formed from the droplets during the treatment is the major factor shaping the therapeutic region. A numerical model was proposed to simulate the bubble area evolution during this treatment. Using a linear acoustic equation to describe the ultrasound field, a threshold range was defined that determines the amount of bubbles vaporized in the treated area. Acoustic parameters, such as sound speed, acoustic attenuation coefficient, and density, were treated as a function of the bubble size distribution and the gas void fraction, which were related to the vaporized bubbles in the medium. An effective pressure factor was proposed to account for the influence of the existing bubbles on the vaporization of the nearby droplets. The factor was obtained by fitting one experimental result and was then used to calculate bubble clouds in other experimental cases. Comparing the simulation results to these other experiments validated the model. The dynamic change of the pressure and the bubble distribution after exposure to over 20 pulses of HIFU are obtained. It is found that the bubble area grows from a grainlike shape to a "tadpole," with comparable dimensions and shape to those observed in experiments. The process was highly dynamic with the shape of the bubble area changing with successive HIFU pulses and the focal pressure. The model was further used to predict the shape of the bubble region triggered by HIFU when a bubble wall pre-exists. The results showed that the bubble wall helps prevent droplet vaporization on the distal side of the wall and forms a particularly shaped region with bubbles. This simulation model has predictive potential that could be beneficial in applications, such as cancer treatment, by parametrically studying conditions associated with these treatments and designing treatment protocols.
声滴汽化有潜力缩短高强度聚焦超声（HIFU）的治疗时间，同时将微泡沿传播路径可能产生的影响降至最低。治疗过程中液滴形成的气泡分布是塑造治疗区域的主要因素。提出了一个数值模型来模拟该治疗过程中气泡区域的演变。使用线性声学方程描述超声场，定义了一个阈值范围，该范围决定了治疗区域内汽化的气泡数量。声速、声衰减系数和密度等声学参数被视为气泡尺寸分布和气体空隙率的函数，而这些又与介质中汽化的气泡相关。提出了一个有效压力因子来考虑现有气泡对附近液滴汽化的影响。该因子通过拟合一个实验结果获得，然后用于计算其他实验案例中的气泡云。将模拟结果与其他实验进行比较验证了该模型。获得了暴露于20多个HIFU脉冲后压力和气泡分布的动态变化。发现气泡区域从颗粒状生长为“蝌蚪”状，其尺寸和形状与实验中观察到的相当。该过程具有高度动态性，气泡区域的形状随连续的HIFU脉冲和焦点压力而变化。该模型进一步用于预测当存在气泡壁时HIFU触发的气泡区域的形状。结果表明，气泡壁有助于防止壁远端的液滴汽化，并形成一个带有气泡的特殊形状区域。这种模拟模型具有预测潜力，通过参数研究与这些治疗相关的条件并设计治疗方案，可能对诸如癌症治疗等应用有益。
