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Research Papers

Numerical Study of Bubble Area Evolution During Acoustic Droplet Vaporization-Enhanced HIFU Treatment

[+] Author and Article Information
Ying Xin

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: novelxiaoxin@126.com

Aili Zhang

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: zhangaili@sjtu.edu.cn

Lisa X. Xu

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: lisaxu@sjtu.edu.cn

J. Brian Fowlkes

Department of Radiology,
University of Michigan Health System,
3226C Medical Sciences Building I,
1301 Catherine Street,
Ann Arbor, MI 48109-5667
e-mail: fowlkes@umich.edu

1Corresponding author.

Manuscript received March 31, 2017; final manuscript received June 9, 2017; published online July 13, 2017. Assoc. Editor: Ram Devireddy.

J Biomech Eng 139(9), 091004 (Jul 13, 2017) (8 pages) Paper No: BIO-17-1137; doi: 10.1115/1.4037150 History: Received March 31, 2017; Revised June 09, 2017

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.

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References

Figures

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Fig. 1

(a) Schematic of the physical model and (b) the geometry of the axisymmetric simulation spatial domain

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Fig. 2

Droplets size distribution used in the simulation

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Fig. 4

(a) The relative difference of the bubble area width and length between experimental result and simulation result using different effective pressure factor. (b) The sectional view of the bubble area after 20 pulses when using 3.8 MPa as the effective pressure factor. (c) The overlay of the experimental result [42] (dark dots depicts the bubbles created in the experiment) and the simulation result (white area depicts the bubble area) which was obtained by revolving (b) around z-axis. The scale bar represents 1 mm. (Reprinted with permission from Lo et al. [42]. Copyright 2006 by Elsevier.)

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Fig. 5

The sectional view of the pressure distribution and corresponding bubble area (depicted in the center of the figure) that in the presence of phase shift droplets. The frequency is 750 kHz and the focal pressure is 9.8 MPa. The scale bar represents 1 mm.

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Fig. 6

(a) The overlay of the experimental result [42] (dark dots depicts the bubbles created in the experiment) and the simulation result (white area depicts the bubble area) when the gel was exposed to a focal pressure of 9.8 MPa and 200 pulses. (b) The overlay of the experimental result and the simulation result when the gel was exposed to a focal pressure of 14.7 MPa and 200 pulses. The scale bar represents 2 mm. (Reprinted with permission from Lo et al. [42]. Copyright 2006 by Elsevier.)

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Fig. 7

(a) The sectional view of bubble area when the gel with a pre-existing 1 mm bubble layer was exposed to a focal pressure of 14.7 MPa and 200 pulses. (b) The overlay of the experimental result [42] and the simulation results. The scale bar represents 2 mm. (c) The sectional view of bubble area when the gel with a pre-existing 1 mm curved bubble layer (curvature was 5 mm) was exposed to a focal pressure of 14.7 MPa and 200 pulses. (Reprinted with permission from Lo et al. [42]. Copyright 2006 by Elsevier.)

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