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Research Papers

Generation of High-Intensity Focused Ultrasound by Carbon Nanotube Opto-Acoustic Lens

[+] Author and Article Information
L. H. Tong

Department of Modern Mechanics,
University of Science and Technology of China,
Hefei, Anhui 230026, China;
USTC-CityU Joint Advanced Research Centre, Suzhou, Jiangsu 215123, China

C. W. Lim

Department of Civil and
Architectural Engineering,
City University of Hong Kong,
Kowloon, Hong Kong SAR, China;
City University of Hong Kong Shenzhen
Research Institute,
Shenzhen 518057, China;
USTC-CityU Joint Advanced Research Centre,
Suzhou, Jiangsu 215123, China
e-mail: bccwlim@cityu.edu.hk

Y. C. Li

Department of Modern Mechanics,
University of Science and Technology of China,
Hefei, Anhui 230026, China

1Corresponding author.

Contributed by the Applied Mechanics Division of ASME for publication in the JOURNAL OF APPLIED MECHANICS. Manuscript received March 19, 2014; final manuscript received May 26, 2014; accepted manuscript posted May 29, 2014; published online June 13, 2014. Assoc. Editor: Weinong Chen.

J. Appl. Mech 81(8), 081014 (Jun 13, 2014) (11 pages) Paper No: JAM-14-1120; doi: 10.1115/1.4027752 History: Received March 19, 2014; Revised May 26, 2014; Accepted May 29, 2014

A new model of high-intensity focused ultrasound generation by radiation from a composite nanothinfilm made of carbon nanotubes (CNTs) and elastomeric polymer is presented in this paper. The composite nanothinfilm is deposited to the surface of a concave lens and the performance of focused ultrasound generated by an incident pulsed laser onto the lens is analyzed. The analysis and results are verified by comparing with published experimental data and very good agreement is recorded. The opto-acoustic pressure on the symmetric axis and the lateral focal plane are investigated analytically and the result indicates that excellent acoustic performance is found to be present in the vicinity of the focus region. The temporal performance of the focused lens is also investigated both at the focal point and the prefocal zone and very good agreement comparing with experiment is obtained. Conclusively, it is demonstrated theoretically that there exists an optimal input frequency for a pulsed laser at which the performance of the focused lens can be tremendously enhanced. In general, this new analytical model provides new guidelines in the design of high-intensity ultrasound lens, hence opening up promising applications to medical ultrasonography treatment.

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References

Figures

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Fig. 1

(a) The schematic of an opto-acoustic lens and (b) the coordinate system

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Fig. 2

Comparison of focal peak pressure versus laser energy. The inserted figure shows the deviation between theory and experiment at high laser energy. The radius of curvature and diameter of lens used in the experiment are 5.5 mm and 6 mm, respectively.

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Fig. 3

Comparison of theory and experiment: (a) temporal domain and (b) spatial domain. In (b), the output axial positive peak amplitude in voltage is transformed into peak pressure in MPa. The radius of curvature and diameter of lens used in the experiment are 5.5 mm and 6 mm, respectively. The CNT-PDMS nanothinfilm thickness is 16 μm. The Gaussian laser energy on the lens surface is 12 mJ/pulse (effective laser fluence = 390 J/m2/pulse).

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Fig. 4

Comparison of (a) theory and (b) experiment [7] on the lateral plane at x = 5.5 mm. Here, Y-position and Z-position are relatively defined from the focal point (x = 5.5 mm) on the lateral plane. The images illustrate the positive pressure peaks. For comparison, the theoretical peak amplitude in Eq. (28) is normalized in (a). The radius of curvature and diameter of lens used in the experiment are 5.5 mm and 6 mm, respectively. The CNT-PDMS nanothinfilm thickness is 16 μm. The Gaussian laser energy on the lens surface is12 mJ/pulse (effective laser fluence = 390 J/m2/pulse).

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Fig. 5

Frequency response at the focal point. The radius of curvature and diameter of lens are 5.5 mm and 6 mm, respectively. The CNT-PDMS nanothinfilm thickness is 16 μm. The amplitude of laser pulse power is 4.77×1010 W/m2 and the optical absorption coefficient is taken as 2 μm-1.

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Fig. 6

Influence of optical absorption coefficient on pressure for different input frequencies at focal point. The radius of curvature and diameter of lens are 5.5 mm and 6 mm, respectively. The CNT-PDMS thinfilm thickness is 16 μm. The amplitude of laser pulse power is 4.77×1010 W/m2.

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Fig. 7

Comparison of approximate and exact amplitudes of D1 in water. The insertion shows the result for frequency larger than 1 MHz. The optical absorption coefficient is taken as 0.08 μm-1.

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Fig. 8

The convergence of a Fourier series expansion for the Gaussian pulse function. The sum of the first 16 terms (n = 0→16) of the Fourier series is considered.

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