Broadband Three-Dimensional Focusing for an Ultrasound Scalpel at Megahertz Frequencies

Jiajie He, Xue Jiang, Hualiang Zhao, Chuanxin Zhang, Yan Zheng, Chengcheng Liu, and Dean Ta

Phys. Rev. Applied 16, 024006 – Published 4 August 2021

Abstract

Ultrasound focusing, with strong penetrability and superior biocompatibility, has attracted substantial interest in biomedical science. Conventional focusing techniques generally rely on an active transducer array or curved lens, and acoustic metamaterials have recently emerged as an alternative approach. However, the low operating frequency of an ultrasound focusing metasurface below 1 MHz and the narrow bandwidth hinder its broad applications in practice. Here, we experimentally realize broadband ultrasound focusing at megahertz frequencies based on a compact and passive metasurface. Three-dimensional spot focusing, ranging from 0.5 to 1.4 MHz, with high spatial resolution (Full width at half maximum of 0.63λ and full length at half maximum of 2.75λ at 1 MHz) and tunable focal length (18.5–71 mm), is demonstrated. The focusing pattern is further enriched by realizing a broadband ultrasound scalpel, which effectively converges sound energy within a sharp “needle” area. Additionally, accompanied by an enhancement of ultrasound intensity, a prominent temperature rise appears in the focal region, the dependence of which on the ultrasound frequency is investigated and the optimized operating frequency is revealed. Broadband and subwavelength ultrasound spot focusing and a scalpel at megahertz frequencies would promote applications in particle manipulation, nondestructive detection, diagnostic sonography, and ultrasound therapy.

Received 4 May 2021
Revised 1 July 2021
Accepted 20 July 2021

DOI:https://doi.org/10.1103/PhysRevApplied.16.024006

Physics Subject Headings (PhySH)

Acoustic metamaterials Acoustic wave phenomena Medical physics & public health Ultrasonics Underwater acoustics

Devices

Nonlinear DynamicsGeneral Physics

Authors & Affiliations

Jiajie He, Xue Jiang ^*, Hualiang Zhao, Chuanxin Zhang, Yan Zheng, Chengcheng Liu , and Dean Ta^†

Center for Biomedical Engineering, School of Information Science and Technology, Fudan University, Shanghai, 200433, China

^*xuejiang@fudan.edu.cn
^†tda@fudan.edu.cn

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Vol. 16, Iss. 2 — August 2021

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Images

Figure 1
(a) Schematic diagram of broadband underwater three-dimensional focusing. FWHM and FLHM represent spatial focusing resolution in radial and axial directions, respectively. (b) Side view of the ultrasound metasurface model. (c) Relationship between tilted angle $α$ and position of serrated unit x for F = 30, 45, and 60 mm. (d) Lattice constant d as a function of tilted angle $α$ at different reference frequencies, $f_{0}$ , from 0.5 to 2.0 MHz.
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Figure 2
(a) Experimental setup of underwater ultrasound measurement system. (b) Photograph of fabricated sample for broadband three-dimensional spot focusing.
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Figure 3
Underwater three-dimensional spot focusing at 1 MHz. (a) Three perpendicular sections of normalized intensity distributions measured in experiments, where the focal spot is observed at $F = 47.025 mm$ . Comparisons between measured (circles) and simulated (lines) results of normalized intensity (b) along the radial direction (x axis on the focal plane), with a measured FWHM of 0.89λ (0.63λ in simulations), and (c) along the axial direction (z axis on x-z plane) with a measured FLHM of 3.54λ (2.75λ in simulations).
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Figure 4
Broadband and tunable focal-length characteristics of underwater ultrasound focusing. (a) Experimentally measured distributions of normalized ultrasound intensity on x-z plane at 0.95, 1, 1.05, and 1.1 MHz. (b) Measured (open circles) and simulated (solid squares) focal length F from 0.5 to 1.4 MHz. (c) FWHM in radial (pink lines) and FLHM in axial (green lines) directions of focal spots within 0.5–1.4 MHz.
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Figure 5
Underwater ultrasound scalpel. (a) Full view of simulated ultrasound intensity distribution at 1 MHz, and photograph of the fabricated sample. Directions l₁ and l₂ denote direction of the longest intensity profile and direction perpendicular to l₁, respectively. Comparisons between measured (circles) and simulated (lines) results of normalized intensity at 1 MHz (b) along l₁ direction $(z = - 1.72 x + 41.97)$ with a FLHM of 13.41λ in experiments (16.81λ in simulations) and (c) along l₂ direction $(z = 0.58 x + 46.69)$ with a FWHM of 1.46λ in experiments (1.33λ in simulation). (d) Summary of simulated and measured ultrasound intensity distributions [white dashed rectangle in (a)] from 0.9 to 1.15 MHz.
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Figure 6
Profiles of temperature rise for (a),(b) underwater 3D spot focusing (on x-z plane and x-y plane), and (c) ultrasound scalpel in simulations, with ultrasound frequency of 1 MHz and exposure time of 5 s. Temperature difference, ΔT, at focal-spot changes over effective frequency range for (d) 3D focusing and (e) ultrasound scalpel.
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Physical Review Applied