Qingyu Ma

Acoustic non-diffracting Airy beam

The acoustic non-diffracting Airy beam as its optical counterpart has unique features of self-bending and self-healing. The complexity of most current designs handicaps its applications. A simple design of an acoustic source capable of generating multi-frequency and broad-band acoustic Airy beam has been theoretically demonstrated by numerical simulations. In the design, a piston transducer is corrugated to induce spatial phase variation for transducing the Airy function. The pistons surface is grooved in a pattern that the width of each groove corresponds to the half wavelength of Airy function. The resulted frequency characteristics and its dependence on the size of the piston source are also discussed. This simple design may promote the wide applications of acoustic Airy beam particularly in the field of medical ultrasound.

Acoustic dipole radiation based conductivity image reconstruction for magnetoacoustic tomography with magnetic induction

Based on the acoustic dipole radiation theory, a tomograhic conductivity image reconstruction algorithm is developed for the magnetoacoustic tomography with magnetic induction (MAT-MI) in a cylindrical measurement configuration. It has been experimentally proved for a tissue-like phantom that not only the configuration but also the inner conductivity distribution can be reconstructed without any borderline stripe. Furthermore, the spatial resolution also can be improved without the limitation of acoustic vibration. The favorable results have provided solid verification for the feasibility of conductivity image reconstruction and suggested the potential applications of MAT-MI in the area of medical electrical impedance imaging.

Phase-coded approach for controllable generation of acoustical vortices

Based on the acoustic radiation of point source, a phase-coded approach is proposed as a general technology to generate controllable acoustical vortices. For N sources, acoustical vortices can be generated with the phase difference of 2πm/N for the source topological charge m. It is proved that more circular pressure distributions of acoustical vortices with higher pressure peak amplitude can be generated for more sources. The number and spiral direction of phase twists are demonstrated to be determined by m and the maximum topological charge of acoustical vortices is |L|=Fix[(N−1)/2], where Fix(x) rounds the element x toward zero. To produce acoustical vortices with a maximum topological charge L, the minimum source number of Nmin=max(2|L|+1,4) should be employed with the phase difference of 2πL/Nmin and the phase difference resolution is also demonstrated to be π. The phase-coded approach has been verified by a 6-source experiment. The measured distributions of pressure and phase as well as the topologi...

Pressure distribution based optimization of phase-coded acoustical vortices

Based on the acoustic radiation of point source, the physical mechanism of phase-coded acoustical vortices is investigated with formulae derivations of acoustic pressure and vibration velocity. Various factors that affect the optimization of acoustical vortices are analyzed. Numerical simulations of the axial, radial, and circular pressure distributions are performed with different source numbers, frequencies, and axial distances. The results prove that the acoustic pressure of acoustical vortices is linearly proportional to the source number, and lower fluctuations of circular pressure distributions can be produced for more sources. With the increase of source frequency, the acoustic pressure of acoustical vortices increases accordingly with decreased vortex radius. Meanwhile, increased vortex radius with reduced acoustic pressure is also achieved for longer axial distance. With the 6-source experimental system, circular and radial pressure distributions at various frequencies and axial distances have been measured, which have good agreements with the results of numerical simulations. The favorable results of acoustic pressure distributions provide theoretical basis for further studies of acoustical vortices.

Deep-level stereoscopic multiple traps of acoustic vortices

Based on the radiation pattern of a planar piston transducer, the mechanisms underlying the generation of axially controllable deep-level stereoscopic multiple traps of acoustic vortices (AV) using sparse directional sources were proposed with explicit formulae. Numerical simulations for the axial and cross-sectional distributions of acoustic pressure and phase were conducted for various ka (product of the wave number and the radius of transducer) values at the frequency of 1u2009MHz. It was demonstrated that, for bigger ka, besides the main-AV (M-AV) generated by the main lobes of the sources, cone-shaped side-AV (S-AV) produced by the side lobes were closer to the source plane at a relatively lower pressure. Corresponding to the radiation angles of pressure nulls between the main lobe and the side lobes of the sources, vortex valleys with nearly pressure zero could be generated on the central axis to form multiple traps, based on Gorkov potential theory. The number and locations of vortex valleys could be ...

Linear phase distribution of acoustical vortices

Linear phase distribution of phase-coded acoustical vortices was theoretically investigated based on the radiation theory of point source, and then confirmed by experimental measurements. With the proposed criterion of positive phase slope, the possibility of constructing linear circular phase distributions is demonstrated to be determined by source parameters. Improved phase linearity can be achieved at larger source number, lower frequency, smaller vortex radius, and/or longer axial distance. Good agreements are observed between numerical simulations and measurement results for circular phase distributions. The favorable results confirm the feasibility of precise phase control for acoustical vortices and suggest potential applications in particle manipulation.

Near-field multiple traps of paraxial acoustic vortices with strengthened gradient force generated by sector transducer array

In order to improve the capability of particle trapping close to the source plane, theoretical and experimental studies on near-field multiple traps of paraxial acoustic vortices (AVs) with a strengthened acoustic gradient force (AGF) generated by a sector transducer array were conducted. By applying the integration of point source radiation, numerical simulations for the acoustic fields generated by the sector transducer array were conducted and compared with those produced by the circular transducer array. It was proved that strengthened AGFs of near-field multiple AVs with higher peak pressures and smaller vortex radii could be produced by the sector transducer array with a small topological charge. The axial distributions of the equivalent potential gradient indicated that the AGFs of paraxial AVs in the near field were much higher than those in the far field, and the distances at the near-field vortex antinodes were also proved to be the ideal trapping positions with relatively higher AGFs. With the ...

Accurate acoustic power measurement for low-intensity focused ultrasound using focal axial vibration velocity

Low-intensity focused ultrasound is a form of therapy that can have reversible acoustothermal effects on biological tissue, depending on the exposure parameters. The acoustic power (AP) should be chosen with caution for the sake of safety. To recover the energy of counteracted radial vibrations at the focal point, an accurate AP measurement method using the focal axial vibration velocity (FAVV) is proposed in explicit formulae and is demonstrated experimentally using a laser vibrometer. The experimental APs for two transducers agree well with theoretical calculations and numerical simulations, showing that AP is proportional to the square of the FAVV, with a fixed power gain determined by the physical parameters of the transducers. The favorable results suggest that the FAVV can be used as a valuable parameter for non-contact AP measurement, providing a new strategy for accurate power control for low-intensity focused ultrasound in biomedical engineering.

Regulation of multiple off-axis acoustic vortices with a centered quasi-plane wave

In treating a circular point-source array using the phase-coded method, a composite acoustic field of multiple off-axis acoustic vortices (AVs) with a centered quasi-plane wave is proposed which is superimposed by an AV beam and a co-axial non-AV beam generated by the even- and odd-numbered source arrays, respectively. The acoustic pressure and the phase of the composite acoustic field are derived using explicit formulae and demonstrated by numerical simulations. Off-axis sub-AVs (SAVs) are shown to be formed at the intersections of the radial pressure distributions of the AV and non-AV beams for the same pressure amplitude and opposite phases. Off-axis SAVs can be generated on a circumference centered with a regular polygon of quasi-plane waves. The radii and azimuthal angles of the off-axis AVs and the radius of the centered quasi-plane wave are determined by the topological charge and the initial phase difference of the acoustic beams. With the established 16-source experimental system, the generation ...

Acoustic radiation torque of an acoustic-vortex spanner exerted on axisymmetric objects

Based on the analysis of the wave vector of an acoustic-vortex (AV) spanner, the radiation torque of object rotation is investigated. It is demonstrated that the rotation of an axisymmetric disk centered on the AV spanner is mainly driven by the acoustic radiation force. The radiation torque exerted on a small-radius object is inversely associated with the topological charge in the center AV, and it is enhanced significantly for a larger AV with a higher topological charge. With the sixteen-source experimental setup, radius dependencies of radiation torque for AV spanners with different topological charges are verified by quantitative laser-displacement measurements using disks with different radii. The favorable results demonstrate that the radiation torque is more applicable than the orbital angular momentum in describing the driving capability of an AV spanner and can be used as an effective tool in clinical applications to manipulate objects with a feature size at the wavelength-scale inside body.