Acoustic Lens Research Articles

Acoustic Forceps Based on Focused Acoustic Vortices with Different Topological Charges.

For enhanced energy concentration with improved flexibility for object manipulation, a focused acoustic vortex (FAV) is designed using a sector planar piston transducer array and acoustic lens that can produce the effective concentration of the acoustic field to perform the focusing function. Compared to the Gaussian beam, which tends to cause the object to deviate from the axis of acoustic propagation, FAVs can form a central valley region to firmly bind the objects, thus preventing off-target effects. The heat energy in the paraxial region is transferred to the vortex center in the form of heat transfer so that the temperature-sensitive liposomes captured can quickly release drugs, which has a good effect on targeted drug administration. The focused acoustic wave stopped acting on the tissue (gel) for 2 s, the temperature of the vortex center continued to rise, reaching 41.5 °C at the moment of 3.7 s, at which point the liposomes began to release the drug. The FAVs capture the drug and use its thermal effect to achieve accurate and rapid treatment. The simulation results show that the drug release temperature of temperature-sensitive liposomes can be achieved by controlling the action time of the vortices. This study provides a reliable theoretical basis for the clinical application of targeted drugs.

Passive directivity detection of acoustic sources based on acoustic Luneburg lens.

This article reports an acoustic Luneburg lens (ALL) design with graded refractive index for passive directivity detection of acoustic sources. The refractive index profile of the lens is realized based on square pillars with graded variation of their dimensions. Numerical and experimental studies are conducted to investigate the performance of directivity detection. The results demonstrate that the lens designed and developed in this study is capable of precisely detecting the directivity of one acoustic source. Furthermore, the directivities of two acoustic sources can also be detected with a resolution of 15°. In addition, different methods are investigated, including introducing phase difference by tuning input signals or moving ALL, and increasing the aperture size of ALL, to improve the resolution of dual sources directivity detection.

Rapid quantification of 3D ultrasound fields with wavefront sensing and Schlieren tomography

The rapid and precise characterization of three-dimensional (3D) pressure fields inside water is paramount for ultrasound (US) applications in fields as relevant as biomedicine and acoustic trapping. The most conventional way is to scan point-by-point a needle hydrophone across the field of interest, which is an intrinsically invasive and slow process. With typical acquisition times of hours and even days, this method remains impractical in many realistic scenarios. Alternatively, optical techniques can be used to non-invasively and rapidly measure the changes in light intensity or phase induced by pressure differences. However, these techniques remain largely qualitative: extracting precise pressure values can require extensive calibration, and complex processing, or can be limited to low-pressure ranges. Here, we report how combining wavefront sensing and Schlieren tomography enables rapid and direct quantification of 3D pressure fields while obviating any calibration steps. By simultaneously capturing optical phase and intensity information of the US-perturbed fluid using a Wavefront Sensor and Schlieren projections, respectively, 3D pressure fields over several millimeters cubic can be reconstructed after a few seconds. We present a detailed description of the approach and prove its feasibility by characterizing the US field after an acoustic lens, which is in excellent agreement with calibrated hydrophone measurements and simulations. These results are a significant step forward toward the precise and real-time characterization of ultrasound patterns.

Restoring the imaging quality of circular transducer array-based PACT using synthetic aperture focusing technique integrated with 2nd-derivative-based back projection scheme

Circular-array-based photoacoustic computed tomography (CA-PACT) is a promising imaging tool owing to its broad acoustic detection coverage and fidelity. However, CA-PACT suffers from poor image quality outside the focal zone along both elevational and lateral dimensions. To address this challenge, we proposed a novel reconstruction strategy by integrating the synthetic aperture focusing technique (SAFT) with the 2nd derivative-based back projection (2nd D-BP) algorithm to restore the image quality outside the focal zone along both the elevational and lateral axes. The proposed solution is a two-phase reconstruction scheme. In the first phase, with the assistance of an acoustic lens, we designed a circular array-based SAFT algorithm to restore the resolution and SNR along the elevational axis. The acoustic lens pushes the boundary of the upper limit of the SAFT scheme to achieve enhanced elevational resolution. In the second phase, we proposed a 2nd D-BP scheme to improve the lateral resolution and suppress noises in 3D imaging results. The 2nd D-BP strategy enhances the image quality along the lateral dimension by up-converting the high spatial frequencies of the object’s absorption pattern. We validated the effectiveness of the proposed strategy using both phantoms and in vivo human experiments. The experimental results demonstrated superior image quality (7-fold enhancement in elevational resolution, 3-fold enhancement in lateral resolution, and an 11-dB increase in SNR). This strategy provides a new paradigm in the PACT system as it significantly enhances the spatial resolution and imaging contrast in both the elevational and lateral dimensions while maintaining a large focal zone.

Reconfigurable coding acoustic meta-lens based on helical metamaterials

Acoustic lenses play an important role in acoustic sensing and imaging. We propose a reconfigurable coding meta-lens (RCML)based on helical metamaterials in the air. The proposed RCML consists of many reassemblable helical meta-units and tunnel-units that have stable phase-shift difference π; these two kinds of antiphase units are called logical “1{\\Prime} and “0”. By specific array arrangement, the units “1” and “0” can form RCML with different discretization particle sizes. We suggest a set of design and reconfiguration methods for RCML, which can modify the focus via small-scale adjustment. Both simulations and experiments demonstrate that the RCML can realize acoustic convergence over a wide designed frequency band with a compact volume. For low-frequency acoustic waves, the thickness of the RCML is only 1/5.7 the wavelength. Moreover, RCML exhibit accurate focus point control and great amplification effect. Our study has great potential in versatile applications, including the design of acoustic sensors, energy collection devices and imaging systems.

3D underwater acoustic Luneburg lens based on gradient face-centered-cubic phononic crystals

A Luneburg lens is a gradient refractive index lens that can focus plane waves on a point at the perimeter without aberration. Three-dimensional (3D) Luneburg lens for airborne sound has been well investigated in recent years. However, constructing a 3D Luneburg lens for underwater sound is a challenging task due to the difficulties in the designing and fabricating of the desired isotropic underwater acoustic materials. This work presents the practical implementation of a 3D Luneburg lens for underwater sound. Such a 3D Luneburg lens is designed based on 3D gradient face-centered-cubic phononic crystals, which have quasi-isotropic refractive index patterns and can be fabricated with photosensitive resin by 3D printing. The experimental results show that the lens can realize the omnidirectional imaging of underwater sound from 30 to 38 kHz. This 3D underwater acoustic Luneburg lens may prompt the potential applications in underwater acoustic wide-angle retroreflectors, sonars, and biomedical imaging devices.

Multi-neuronal recording in unrestrained animals with all acousto-optic random-access line-scanning two-photon microscopy.

To understand how neural activity encodes and coordinates behavior, it is desirable to record multi-neuronal activity in freely behaving animals. Imaging in unrestrained animals is challenging, especially for those, like larval Drosophila melanogaster, whose brains are deformed by body motion. A previously demonstrated two-photon tracking microscope recorded from individual neurons in freely crawling Drosophila larvae but faced limits in multi-neuronal recording. Here we demonstrate a new tracking microscope using acousto-optic deflectors (AODs) and an acoustic GRIN lens (TAG lens) to achieve axially resonant 2D random access scanning, sampling along arbitrarily located axial lines at a line rate of 70 kHz. With a tracking latency of 0.1 ms, this microscope recorded activities of various neurons in moving larval Drosophila CNS and VNC including premotor neurons, bilateral visual interneurons, and descending command neurons. This technique can be applied to the existing two-photon microscope to allow for fast 3D tracking and scanning.

Reverberant optical coherence elastography using multifocal acoustic radiation force.

In this study, we introduce a multifocal acoustic radiation force source that combines an ultrasound transducer and a 3D-printed acoustic lens for application in reverberant optical coherence elastography (Rev-OCE). An array of plano-concave acoustic lenses, each with an 11.8 mm aperture diameter, were used to spatially distribute the acoustic energy generated by a 1 MHz planar ultrasound transducer, producing multiple focal spots on a target plane. These focal spots generate reverberant shear wave fields detected by the optical coherence tomography (OCT) system. The effectiveness of the multifocal Rev-OCE system in probing mechanical properties with high resolution is demonstrated in layered gelatin phantoms.

High-resolution silicon photonics focused ultrasound transducer with a sub-millimeter aperture.

We present an all-optical focused ultrasound transducer with a sub-millimeter aperture and demonstrate its capability for high-resolution imaging of tissue ex vivo. The transducer is composed of a wideband silicon photonics ultrasound detector and a miniature acoustic lens coated with a thin optically absorbing metallic layer used to produce laser-generated ultrasound. The demonstrated device achieves axial resolution and lateral resolutions of 12 μm and 60 μm, respectively, well below typical values achieved by conventional piezoelectric intravascular ultrasound. The size and resolution of the developed transducer may enable its use for intravascular imaging of thin fibrous cap atheroma.

Multifunctional Acoustic Device Based on a Phononic Crystal with Independently Controlled Asymmetric Rotating Rods

A reconfigurable phononic crystal (PnC) is proposed where elastic properties can be modulated by rotation of asymmetric solid scatterers immersed in water. The scatterers are metallic rods with a cross section of ${120}^{\ensuremath{\circ}}$ circular sector. Orientation of each rod is independently controlled by an external electric motor that allows continuous variation of the local scattering parameters and dispersion of sound in the entire crystal. Due to asymmetry of the scatterers, the crystal band structure possesses highly anisotropic band gaps. Synchronous rotation of all the scatterers by a definite angle changes the regime of reflection to the regime of transmission and vice versa. The same mechanically tunable structure functions as a gradient index medium by incremental, angular reorientation of rods along both row and column, and, subsequently, can serve as a tunable acoustic lens, an acoustic beam splitter, and finally an acoustic beam steerer.

Diffraction-Limited Focusing of Acoustic Waves by a Mesoscopic Flat Janus Lens

Anisotropic focusing by a mesoscopic (Mie size parameter of about 18) acoustic cubic lens based on V-shaped plate structures has been simulated numerically and confirmed experimentally. It has been shown for the first time that this lens with an edge dimension of about three wavelengths ensures the focusing of an acoustic wave in air into a diffraction-limited region. In the inverse geometry of the structure, the lens completely reflects the incident acoustic wave.

Multifocal skull-compensated transcranial focused ultrasound system for neuromodulation applications based on acoustic holography

Transcranial focused ultrasound stimulation is a promising therapeutic modality for human brain disorders because of its noninvasiveness, long penetration depth, and versatile spatial control capability through beamforming and beam steering. However, the skull presents a major hurdle for successful applications of ultrasound stimulation. Specifically, skull-induced focal aberration limits the capability for accurate and versatile targeting of brain subregions. In addition, there lacks a fully functional preclinical neuromodulation system suitable to conduct behavioral studies. Here, we report a miniature ultrasound system for neuromodulation applications that is capable of highly accurate multiregion targeting based on acoustic holography. Our work includes the design and implementation of an acoustic lens for targeting brain regions with compensation for skull aberration through time-reversal recording and a phase conjugation mirror. Moreover, we utilize MEMS and 3D-printing technology to implement a 0.75-g lightweight neuromodulation system and present in vivo characterization of the packaged system in freely moving mice. This preclinical system is capable of accurately targeting the desired individual or multitude of brain regions, which will enable versatile and explorative behavior studies using ultrasound neuromodulation to facilitate widespread clinical adoption.

Acoustic beam splitter based on acoustic metamaterial Luneburg lens

This paper proposes an acoustic beam splitter based on acoustic Luneburg lens. A two-dimensional acoustic Luneburg lens with gradient refractive index is achieved to provide a practical realization for the acoustic beam splitter. The acoustic wave propagation through the beam splitter was explored theoretically, numerically and experimentally. Results show that three output channels can be achieved using a point source on the edge of the beam splitter in a broadband frequency range. Furthermore, the output channel directions can be tuned via changing the design parameter.

Experimental characterization of a shape optimized acoustic lens: Application to compact speakerphone design

This work presents the shape optimization and subsequent experimental validation of an acoustic lens with application to a compact loudspeaker, such as found in commercial speakerphones. The shape optimization framework is based on a combined lumped parameter and boundary element method model using free form deformation geometry parameterization. To test the optimized design, the loudspeaker lens is three-dimensionally printed and experimentally characterized under anechoic conditions on a finite baffle with respect to its off-axis frequency response. The overall tendencies of the frequency responses agree well between measurement and simulations within the optimization frequency range and at low frequencies. The optimization process is applied to a model including acoustic lumped parameter approximations. The shortcomings of the assumptions made in the model are revealed by laser Doppler vibrometer measurements of the loudspeaker driver and modelling of the mechanical vibrations of the lens.

Ultra-broadband acoustic metasurface for speaker directivity control

Acoustic metasurfaces have shown remarkable capabilities for shaping acoustic wavefronts. However, most thin metamaterials can only work well for a narrow range of acoustic frequencies. This limitation has thus far dramatically limited their potential for human-based audio and acoustic applications. Here we present a generic acoustic metasurface design approach to reshape acoustic wavefronts in a broad bandwidth. As a demonstration, we designed two acoustic lenses. The first acoustic lens converts a 4-in. speaker into an omnidirectional source, covering a frequency range from 1 to 10kHz. The second acoustic lens delivers super-directionality for a 1-in. speaker from 1 to 20 kHz. Experimental measurements showed great agreement with the simulations. Our design approach enables the design of ultra-broadband metasurfaces useful for audio applications.

Quantitative biomechanics using reverberant optical coherence elastography

In this presentation, I will introduce and discuss recent developments in our effort for quantitative biomechanics of tissues and organs using a multifocal acoustic radiation force source (that combines an ultrasound transducer and a 3D-printed acoustic lens) in reverberant optical coherence elastography (Rev-OCE). An array of plano-concave acoustic lenses, each with an 11.8 mm aperture diameter, were used to spatially distribute the acoustic energy generated by a 1 MHz planar ultrasound transducer, producing multiple focal spots on a target plane. These focal spots generate reverberant shear wave fields detected by the Optical Coherence Tomography (OCT) system. The effectiveness of the multifocal Rev-OCE system in probing mechanical properties with high resolution is demonstrated in layered gelatin phantoms, tissues ex vivo, and whole mouse embryos in vivo.

Binary acoustic metasurfaces for transcranial focused ultrasound

Transcranial focused ultrasound is a promising technique for noninvasive and spatially targeted diagnosis and treatment of brain diseases. Acoustic lenses were designed to correct the skull-induced beam aberration, but these designs could only generate static focused ultrasound beams inside the brain. Here, we designed and 3D printed binary acoustic metasurfaces (BAMs) for skull aberration correction and dynamic ultrasound beam focusing. BAMs were designed by binarizing the phase distribution at the surface of the metasurfaces. The phase distribution was calculated based on time reversal to correct the skull-induced phase aberration. The binarization enabled the ultrasound beam to be dynamically steered along wave propagation direction by adjusting the operation frequency of the incident ultrasound wave. The designed BAMs were manufactured by 3D printing with two coding bits, a polylactic acid unit for bit “1” and a water unit for bit “0.” BAMs for single- and multi-point focusing through the human skull were designed, 3D printed, and validated numerically and experimentally. In summary, the proposed BAMs with subwavelength scale in thickness are simple to design, easy to fabric, and capable of correcting skull aberration and achieving dynamic beam steering.

Tailoring nonlinear ultrasound beams using acoustic lenses

Acoustic holographic lenses, i.e., acoustic holograms, can tailor ultrasound beams to precisely construct elaborate focal patterns. Due to their valuable capabilities, acoustic holograms are being rapidly investigated for a wide range of applications in physical acoustics and biomedical ultrasound. Modeling and demonstrating acoustic holograms in linear regimes is well documented in the literature. However, Acoustic holograms have the potential to improve the precision and affordability of therapeutic high-intensity focused ultrasound (HIFU). In this work, we will show the effects of nonlinear propagation on ultrasound fields tailored by acoustic holograms. The intensity-dependent effects of harmonic generation, diffraction, and nonlinear gain on the constructed pressure pattern are discussed. In addition, an accelerated method for simulating high-intensity 3D holographic ultrasound is introduced. We will show how the proposed algorithm greatly reduces the computational load for designing and predicting high-intensity holographic ultrasound patterns. This expedites the procedure of designing the acoustic lens, which allows for a more rapid and seamless implementation, especially in medical settings.

Modulation of the blood-retina-barrier permeability by focused ultrasound: Computational and experimental approaches

The blood-retina-barrier (BRB) is a critical barrier that protects the retinal neurons. It selectively regulates the permeability of molecules, responds to pathologies, and modulates disease progression. While the BRB prevents harmful molecules from the blood from affecting the neurons and is thus essential for retinal health, it also poses a hurdle for therapeutic agents and impedes treatment. The goals are a non-invasive and well-defined transient increase of the BRB’s permeability by mechanical microscale-induced forces, to visualize the permeability change in real-time, and to computationally model the process for an improved understanding and control of the dynamics of larger molecules passage through the BRB. Our approach combines in vivo animal experiments with computational modeling in silico. The in vivo results are used to continuously and iteratively optimize the COMSOL model. In order to modulate BRB’s permeability, we developed a prototype acoustic lens assembly based on COMSOL simulation calculations for the delivery of ultrasonic pressure waves to targeted regions in the retina under optical guidance by light-based imaging. Our data indicate that lower energy focused ultrasound at a PRFP close to 0.8 MPa rapidly increases the permeability of the retinal vessels in a fashion that can be depicted in a real-time fluorescein angiography.

Self-stabilizing three-dimensional particle manipulation via a single-transducer acoustic tweezer

This paper investigates the mechanism of self-stabilizing, three-dimensional Mie particle manipulation in water via an acoustic tweezer with a single transducer. A carefully designed acoustic lens is attached to the transducer to form an acoustic vortex, which provides angular momentum on the trapped polymer sphere and leads to a fast-spinning motion. The sphere can find equilibrium positions spontaneously during the manipulation by slightly adjusting its relative position, angular velocity, and spinning axis. The spinning motion greatly enhances the low-pressure recirculation region around the sphere, resulting in a larger pressure induced drag. Simultaneously, the Magnus effect is induced to generate an additional lateral force. The spinning motion of the trapped sphere links the acoustic radiation force and hydrodynamic forces together, so that the sphere can spontaneously achieve new force balance and follow the translational motion of the acoustic tweezer. Non-spherical objects can also be manipulated by this acoustic tweezer.