Spherical Array Research Articles

3D ultrasound imaging by synthetic transmit aperture beamforming using a spherically curved array transducer

Visualization of dermal circulation is important in the field of skin healthcare. We have developed a three-dimensional (3D) photoacoustic (PA) imaging system using a spherically curved array transducer that can visualize the microscale circulation in the skin layers, but limited anatomical information was available around the microvasculature. To provide such anatomical information, this study was aimed at devising a high-quality and high-speed ultrasound (US) imaging framework, particularly, for the spherical array transducer. We tested three synthetic transmit aperture (STA) methods, all-elements, outer-track, and inner-track, for transmission by evaluating the spatial resolution and uniformity of 3D images of point and copper-wire targets. The results demonstrated that the all-elements and outer-track STA methods could provide uniform and clear 3D images. In addition, the outer-track STA could be performed with fewer transmissions than the all-elements STA, and it will be useful for realizing real-time, high-resolution 3D PA/US imaging.

Reconfigurable switching between reflecting/absorbing modes in VO2 assisted graphene-coated hemispherical dielectric hole arrays

In this paper, a graphene-coated dielectric hole array is used to design a reconfigurable switchable optical reflector/absorber device. The design benefits from the collective excitation of localized surface plasmon resonances of graphene-coated hole array, providing simpler fabrication fellow and more compact structure with respect to graphene-coated spherical nanoparticle array with similar plasmonic behavior. Geometrical parametric study of the reflecting mode shows that the device has lots of degrees of freedom for spectrum tuning and can highly tolerate fabrication imperfections. Moreover, the reflection rate is slightly affected by the dielectric substrate height, which can be tuned to achieve strong absorption by backing it with a metallic mirror. The designed absorber efficiently captures a wide range of obliquely incident transverse electric (TE) and transverse magnetic (TM) waves. Also, the operating frequency of both reflecting and absorbing modes can be tuned after fabrication, thanks to the two-dimensional nature of graphene material. Finally, using vanadium dioxide (VO2) phase change material, the switchable reflector\\absorber mode of the device is also exhibited.

PO-711-02 POST- ABLATION UNIPOLAR ELECTROGRAM CHANGES: A PRECLINICAL EVALUATION COMPARING RADIOFREQUENCY VS PULSED FIELD ABLATION USING A “MAP AND ABLATE” MULTIELECTRODE ARRAY CATHETER

A multielectrode spherical array catheter (Globe, Kardium Inc, Canada) capable of mapping and ablation with either radiofrequency (RF) and pulsed field (PF) energy has been developed. To compare acute unipolar electrogram changes in atrial tissue after RF vs PF ablation. Healthy canines underwent transseptal access under general anesthesia, followed by circumferential pulmonary vein isolation, as well as linear and focal ablation as part of an efficacy and safety evaluation. Temperature controlled RF ablation (63ºC for 3 min) or PF using 1.6-2 kV/application (either gated or ungated modes) were used in separate animal cohorts. Only animals where focal/linear ablations were placed on the left atrial roof were selected so as to compare electrogram features at identical anatomic sites in both cohorts. We compared pre-ablation with post-ablation unipolar electrograms (1-225 Hz) recorded at i) the electrodes selected for ablation (Row 1), ii) from immediately adjacent electrode rows (Row 2, 3.5-6mm away), and iii) from electrodes 2 rows away (Row 3, 7-12 mm away; Figure). A total of 21 ablated sites (Row 1) and 27 adjacent Rows (2 & 3) were analyzed in the RFA group; similarly 77 and 81 sites were analyzed in the PFA group. Lesion transmurality was seen across the focal lesions on the left atrial roof for both the RF and PF applications. With both RF and PF ablation, there was complete loss of the negative deflection of the unipolar electrogram, as well as diminution of the R wave accompanied by a current of injury in Row 1. In the RF cohort, Row 2 demonstrated minimal loss of the negative deflection with mild injury current. However, PF ablation resulted in significantly greater loss of the negative deflection and current of injury was observed in Row 2, compared to RF ablation. Furthermore, in the PF group, even Row 3 demonstrated some degree of loss of negative deflection and injury current, whereas in the RF group this row was completely preserved. Loss of negative deflection of the unipolar electrogram, a feature that suggests transmural ablation in the setting of RF ablation is also seen with PF . However, the extent of unipolar changes extend further (7-12 mm) from the site of ablation with PF compared to RF, where these changes were milder and restricted to 3.5-6 mm.View Large Image Figure ViewerDownload Hi-res image Download (PPT)

PO-624-01 FIRST-IN-HUMAN CLINICAL EXPERIENCE OF A “SINGLE-SHOT” MAP-AND-ABLATE MULTIELECTRODE SPHERICAL ARRAY PULSED FIELD ABLATION CATHETER TO ISOLATE PULMONARY VEINS

Pulsed field ablation (PFA) has safety and procedural workflow advantages over conventional thermal ablation modalities. Most “single-shot” PFA catheter technologies are either not linked to electroanatomical mapping systems, require serial re-positioning or cannot easily accommodate varying pulmonary vein (PV) anatomical sizes / shapes. Recently, a multielectrode spherical array catheter (Globe, Kardium Inc, Canada) capable of single-shot “mapping and ablation”, previously capable of radiofrequency (RF) ablation, has now been enhanced to also deliver PFA.

Invertible neural networks for reconstructing acoustic fields

Sound field reconstruction from finite measurement arrays provides a means to interpolate and extrapolate acoustic quantities that describe the field. By assuming a linear projection on a basis that follows a principled source propagation, one can recover accurate estimates of the aforementioned sound fields. However, the recovery of the basis coefficients relies on explicit models of which measurement noise and data incompleteness can profoundly affect the uncertainty of the solution. This work aims to estimate the distribution of the underlying pressure conditioned on the observations of the measured pressure in a room. A framework for approximate inference is adapted for sound field reconstruction by applying generative flow-based models and invertible neural network architectures. In particular, we use conditional normalising flows for fast conditional posterior estimation and uncertainty quantification. The model's evaluation is carried out using experimental data measured with a spherical array and compared to hierarchical Bayes with Markov Chain Monte-Carlo sampling.

Dual-Mode Volumetric Optoacoustic and Contrast Enhanced Ultrasound Imaging With Spherical Matrix Arrays.

Spherical matrix arrays represent an advantageous tomographic detection geometry for non-invasive deep tissue mapping of vascular networks and oxygenation with volumetric optoacoustic tomography (VOT). Hybridization of VOT with ultrasound (US) imaging remains difficult with this configuration due to the relatively large inter-element pitch of spherical arrays. We suggest a new approach for combining VOT and US contrast-enhanced 3D imaging employing injection of clinically-approved microbubbles. Power Doppler (PD) and US localization imaging were enabled with a sparse US acquisition sequence and model-based inversion based on infimal convolution of total variation (ICTV) regularization. In vitro experiments in tissue-mimicking phantoms and in living mouse brain demonstrate the powerful capabilities of the new dual-mode imaging approach attaining 80 μm spatial resolution and a more than 10 dB signal to noise improvement with respect to a classical delay and sum beamformer. Microbubble localization and tracking allowed for flow velocity mapping up to 40 mm/s.

On the selection of the number of beamformers in beamforming-based binaural reproduction

In recent years, spatial audio reproduction has been widely researched with many studies focusing on headphone-based spatial reproduction. A popular format for spatial audio is higher order Ambisonics (HOA), where a spherical microphone array is typically used to obtain the HOA signals. When a spherical array is not available, beamforming-based binaural reproduction (BFBR) can be used, where signals are captured with arrays of a general configuration. While shown to be useful, no comprehensive studies of BFBR have been presented and so its limitations and other design aspects are not well understood. This paper takes an initial step towards developing a theory for BFBR and develops guidelines for selecting the number of beamformers. In particular, the average directivity factor of the microphone array is proposed as a measure for supporting this selection. The effect of head-related transfer function (HRTF) order truncation that occurs when using too many beamformer directions is presented and studied. In addition, the relation between HOA-based binaural reproduction and BFBR is discussed through analysis based on a spherical array. A simulation study is then presented, based on both a spherical and a planar array, demonstrating the proposed guidelines. A listening test verifies the perceptual attributes of the methods presented in this study. These results can be used for more informed beamformer design for BFBR.

On the surface impedance modeling of metasurfaces composed of graphene-coated spherical nanoparticles

In this paper, we discuss the electromagnetic homogenization of graphene-based nanoparticle arrays using the surface impedance approach. For this purpose, we first investigate the contribution of the different harmonics to the equivalent surface impedance of the array. We conclude that the size of the nanoparticles is small compared to the operating wavelength, and the higher-order modes have a negligible impact on the overall response of the array. Then, the accuracy of the homogenization method to describe the macroscopic response of graphene-coated spherical particle arrays is discussed. In particular, it is demonstrated that the surface-impedance modeling provides reliable results when the level of coupling between the nanoparticles is moderate and the model accuracy can be slightly improved by considering the Floquet expansion-based interaction coefficient. Finally, approximate formulas for the array surface impedance, as well as an equivalent circuit model of the metasurface, are extracted.

Fabrication of a Tool Electrode with Hydrophobic Features and Its Stray-Corrosion Suppression Performance for Micro-electrochemical Machining.

Micro-electrochemical machining (micro-ECM) can machine microstructures with excellent surface integrity on difficult-to-cut alloys. During micro-ECM, stray corrosion results in tapered sidewalls of machined structures. To suppress the stray corrosion, a novel tool electrode with the sidewall insulation of a gas film is proposed by fabricating the metal tool sidewall into a hydrophobic surface. The sidewall surface is designed to be characterized with spherical array cavities (radius of 500 nm) acting as the hydrophobic features, enhancing the gas-shielding effect of the gas film under electrolysis. The fabrication process of the hydrophobic sidewall is described in detail, including the self-assembly procedure of a monolayer template of Φ1μm polystyrene microspheres, the copper-electroforming procedure for filling the microspheres' gaps, and the removal procedure for forming spherical array cavities. The fabricated tool electrode (Φ500μm) obtains the hydrophobic features of a contact angle of up to 138°. As a result, bubbles generated on the tool surface can form an air-electrolyte interface instead of a dispersed bubble cluster. Micro-ECM experiments of microstructures verify that the novel tool electrode can improve machining accuracy by suppressing sidewall stray corrosion.

自己整列微粒子の形状転写によるマイクロレンズアレイの製作とモールド変形を利用した非球面化の検討

Aspheric micro-lens arrays have been attracting great attention to improve light distribution control. This study proposes a simple and efficient method for fabricating close-packed aspheric micro-lens array. First, silica particles of ϕ10 μm were self-assembled over ϕ10 mm area using sedimentation method. Then the convex profile of the self-assembled particles was replicated to a spherical micro-lens array mold. The mold material was silicone elastomer or polydimethylsiloxane (PDMS). Then, the mold profile was replicated into ultraviolet (UV) curing resin. By applying uniaxial stretching to the mold during the secondary replication, aspheric micro-lens array was fabricated. Deformation of the PDMS mold under uniaxial stretching was analyzed by using finite element analysis (FEA) for rough estimation. Experimental results showed that intended lens profile was obtained though some defects were observed due to the nonuniformity of the particle diameters. Optical characteristic of elliptic micro-lens array was made clear.

Multiple-Focus Patterns of Sparse Random Array Using Particle Swarm Optimization for Ultrasound Surgery.

This study aims to investigate the feasibility and potential of sparse random arrays driven by the particle swarm optimization (PSO) algorithm to generate multiple-focus patterns and a large scanning range without grating lobes, which extends the scanning range of focused ultrasound in the treatment of brain tumors, opening the blood-brain barrier, and neuromodulation. Operating at 1.1 MHz, a random spherical array with 200 square elements (sparseness 58%) and a sparse random array with 660 square elements (sparseness 41%) driven by PSO are employed to simulate different focus patterns. With the same radius of curvature and diameter of transducer and element size, the scanning range of the off-axis single focus of a random 200-element array is two times that of an ordinary array using symmetric arrangement. The focal volume of multiple-focus patterns of the random array is 18 times that of the single focus. The single focus of the sparse random array with 660 elements could steer up to ±23 mm in the radial direction, without grating lobes. The maximum distance between two foci in a multiple-focus "S"-shaped deflection is approximately 25 mm. Simulation results illustrate the capability of a focused beam steered in 3-D space. Multiple-focus patterns could significantly increase the focal volume and shorten the treatment time for large target volumes. Simulation results show the feasibility and potential of the method combining PSO with a sparse random array to generate flexible focus patterns that can adapt to different needs in different tissue treatments.

Systematic modeling of electrostatic radiation shields for deep space flight

Astronauts are exposed to a unique radiation environment during space missions. This environment is dominated by high charge and energy (HZE) ions with sources that include solar energetic particles (SEPs), galactic cosmic rays (GCRs), and trapped electrons and protons around planetary bodies with substantial magnetic fields. Traditional shielding strategies aim to reduce astronaut radiation exposure by increasing vehicle mass, but this is ineffective for the extremely penetrating GCRs. Use of electromagnetic fields has been investigated previously, but these studies have been limited to single point designs. The NASA Active Shielding project has instead developed a workflow that includes a fast and accurate particle propagation code (Active Shielding Particle Pusher), which can be used to rapidly evaluate the shielding efficacy of many different permutations of electromagnetic shielding designs. While some validation of the code has been previously done, additional validation is needed to increase user confidence in simulation results involving more complicated electrostatic conductor configurations. In the present study, we evaluated shielding properties of spherical electrode arrays and planar grid structures against 2 MeV or 6 MeV H+ and 22.5 MeV Fe6+ ions using ASPP, SIMION, and COMSOL, and compared these results to laboratory-based measurements. Many numerical experiments were conducted to reproduce experimental ion beam tests that measure the consequential particle reduction rate in a safe zone compared to the initial number of ions. The experimental visualizations were also compared with the present numerical results and found to be in close agreement with each other for all considered voltage combinations applied to electrode structures. This comparison allows us to use numerical models to predict the behavior of complex shielding structures at much larger scales not feasible in a laboratory environment.

Fabrication of a microlens array featuring a high aspect ratio with a swinging diamond tool

Microlens arrays are gaining wider applications due to their unique optical geometry and excellent optical properties. Ultra-precision diamond turning is preferable for fabricating microlens arrays with high form accuracy. However, the interference between the flank face of the diamond tool and the finished surface restricts its application in machining microlens arrays with a high aspect ratio. In this paper, a novel method based on the slow tool servo turning technique is proposed to fabricate these microlens arrays by introducing a rotational motion (A-axis) to swing the diamond cutting tool and adjust the actual tool clearance angle in real time, which can prevent tool interference effectively. The machining system and method principle are introduced first. Then, the toolpath is generated, and the influence of the tool nose radius is compensated for. Finally, machining experiments of cutting spherical lens arrays with high aspect ratios are conducted on a Nanotech 650UPL ultraprecision lathe, and the machined lenses are inspected with an optical microscope, a stylus profilometer and a white light interferometer. The measurement results of the spherical lens arrays machined by the proposed method and the conventional slow tool servo turning method are compared, which demonstrates the superiority of the proposed method in fabricating high-quality lens arrays featuring a high aspect ratio.

Spatiotemporal Evolution of Temperature During Transient Heating of Nanoparticle Arrays.

Nanoparticles (NPs) are promising agents to absorb external energy and generate heat. Clusters of NPs or NP array heating have found an essential role in several biomedical applications, diagnostic techniques, and chemical catalysis. Various studies have shed light on the heat transfer of nanostructures and greatly advanced our understanding of NP array heating. However, there is a lack of analytical tools and dimensionless parameters to describe the transient heating of NP arrays. Here we demonstrate a comprehensive analysis of the transient NP array heating. Firstly, we develop a set of analytical solutions for the NP array heating and provide a useful mathematical description of the spatial-temporal evolution of temperature for 2D, 3D, and spherical NP array heating. Based on this, we introduce the concept of thermal resolution that quantifies the relationship between minimal heating time, NP array size, energy intensity, and target temperature. Lastly, we define a set of dimensionless parameters that characterize the transition from confined heating to delocalized heating. This study advances the understanding of nanomaterials heating and guides the rational design of innovative approaches for NP array heating.

Gravity‐Controlled and Boundary‐Constrained High‐Throughput Fabrication of Polymeric Miniature Lens Arrays

AbstractThis paper reports a facile and high‐throughput method for the controllable fabrication of polymeric miniature lens arrays. Through filling the miniature holes in an additively manufactured template, the curved lenslets are formed in each hole in s steady state, in which the gravity and surface tension of the liquid resin are balanced. Both shapes and curvatures can be controlled by tuning the gravity force and the boundary condition for the resin in each hole. Taking the circular holes for example, the relationship between the formed curvature and the gravity force is described by both analytical and finite element‐based numerical models. Experimentally, the curvature variation and the effect of various boundary conditions on the lenslet surface shapes are also verified, and further examination on the optical features of the spherical miniature lens arrays including the imaging and focus properties demonstrates the effectiveness of the developed fabrication method.

Critical Review of Basic Methods on DoA Estimation of EM Waves Impinging a Spherical Antenna Array

Direction-of-arrival (DoA) estimation of electromagnetic (EM) waves impinging on a spherical antenna array in short time windows is examined in this paper. Reflected EM signals due to non-line-of-sight propagation measured with a spherical antenna array can be coherent and/or highly correlated in a snapshot. This makes spectral-based methods inefficient. Spectral methods, such as maximum likelihood (ML) methods, multiple signal classification (MUSIC), and beamforming methods, are theoretically and systematically investigated in this study. MUSIC is an approach used for frequency estimation and radio direction finding, ML is a technique used for estimating the parameters of an assumed probability distribution for given observed data, and PWD applies a Fourier transform to the capture response and produces them in the frequency domain. Although they have been previously adapted and used to estimate DoA of EM signals impinging on linear and planar antenna array configurations, this paper investigates their suitability and effectiveness for a spherical antenna array. Various computer simulations were conducted, and plots of root-mean-square error (RMSE) against the square root of the Cramér–Rao lower bound (CRLB) were generated and used to evaluate the performance of each method. Numerical experiments and results from measured data show the degree of appropriateness and efficiency of each method. For instance, the techniques exhibit identical performance to that in the wideband scenario when the frequency f = 8 GHz, f = 16 GHz, and f = 32 GHz, but f = 16 GHz performs best. This indicates that the difference between the covariance matrix of the signal is coherent and that the steering vectors of signals impinging from that angle are small. MUSIC and PWD share the same problems in the single-frequency scenario as in the wideband scenario when the delay sample d = 0. Consequently, the DoA estimation obtained with ML techniques is more suitable, less biased, and more robust against noise than beamforming and MUSIC techniques. In addition, deterministic ML (DML) and weighted subspace fitting (WSF) techniques show better DoA estimation performance than the stochastic ML (SML) technique. For a large number of snapshots, WSF is a better choice because it is more computationally efficient than DML. Finally, the results obtained indicate that WSF and ML methods perform better than MUSIC and PWD for the coherent or partially correlated signals studied.

Transcranial Neuromodulation Array With Imaging Aperture for Simultaneous Multifocus Stimulation in Nonhuman Primates.

Even simple behaviors arise from the simultaneous activation of multiple regions in the brain. Thus, the ability to simultaneously stimulate multiple regions within a brain circuit should allow for better modulation of function. However, performing simultaneous multifocus ultrasound neuromodulation introduces challenges to transducer design. Using 3-D Fullwave simulations, we have designed an ultrasound neuromodulation array for nonhuman primates that: 1) can simultaneously focus on multiple targets and 2) include an imaging aperture for additional functional imaging. This design is based on a spherical array, with 128 15-mm elements distributed in a spherical helix pattern. It is shown that clustering the elements tightly around the 65-mm imaging aperture located at the top of the array improves targeting at shallow depths, near the skull surface. Spherical arrays have good focusing capabilities through the skull at the center of the array, but focusing on off-center locations is more challenging due to the natural geometric configuration and the angle of incidence with the skull. In order to mitigate this, the 64 elements closest to the aperture were rotated toward and focusing on a shallow target, and the 64 elements farthest from the aperture were rotated toward and focusing on a deeper target. Data illustrated that this array produced focusing on the somatosensory cortex with a gain of 4.38 and to the thalamus with a gain of 3.82. To improve upon this, the array placement was optimized based on phase aberration simulations, allowing for the elements with the largest impact on the gain at each focal point to be found. This optimization resulted in an array design that can focus on the somatosensory cortex with a gain of 5.19 and the thalamus with a gain of 4.45. Simulations were also performed to evaluate the ability of the array to focus on 28 additional brain regions, showing that off-center target regions can be stimulated, but those closer to the skull will require corrective steps to deliver the same amount of energy to those locations. This simulation and design process can be adapted to an individual monkey or human skull morphologies and specific target locations within individuals by using orientable 3-D printing of the transducer case and by electronic phase aberration correction.

Sparse Distortionless Modal Beamforming for Spherical Microphone Arrays

Phase-mode array processing which utilizes the spherical harmonics decomposition offers a useful framework for spherical microphone arrays. In the modal domain, one of the major applications of spherical arrays is acoustic beamforming. Usually, beamforming is performed by minimizing the power of the beamformer output with a distortionless constraint towards the look direction. Also, most beamformers model the spectral coefficients of target speech using a Gaussian distribution. In this letter, a beamforming method that minimizes the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\ell _{0}$</tex-math></inline-formula> -norm of the beamformer output in the spherical harmonics domain with a distortionless constraint is proposed. The proposed method assumes a super-Gaussian prior for the target speech and sparsifies the beamformer output which is shown to reduce the residual elements of the interfering speech signals from the beamformer output leading to superior spatial separation. The formulation of the proposed beamformer in the spherical harmonics signal model is presented along with an algorithm to solve it. Simulation results show the effectiveness of the proposed beamformer in spatially filtering the desired speaker signal from the interfering signals using a number of objective measures.

Multiarray Eigenbeam-ESPRIT for 3D Sound Source Localization With Multiple Spherical Microphone Arrays

A 3D sound source localization (SSL) technique for multiple spherical arrays is proposed. Spherical arrays have been popularly used for direction of arrival (DoA) estimation, which serves as important prior knowledge for source separation, dereverberation, and binaural rendering for VR applications. In this work, a parametric 3D SSL technique is proposed that can detect multiple sources using multiple spherical array recordings. From the enriched information by using multiple arrays, we show that the parametric estimation of 3D positions is possible without scanning candidate positions in 3D space. To mitigate the problem of the ambiguous association between DoAs estimated from different arrays, a total covariance matrix including both auto- and cross-covariance matrices between arrays is utilized such that DoAs from arrays are automatically paired to uniquely localize 3D source positions. The joint parameter estimation is based on the generalized joint Schur decomposition of multiple matrices combined with a geometric projection robustly guiding the convergence of the proposed iteration algorithm. Simulations conducted in anechoic and reverberant room conditions reveal that the proposed technique can accurately determine multiple source positions in 3D space without ambiguities.

Experimental study on interaction between wettability configuration and dewing efficiency of surfaces with a cavity array

Factors that dominate dew productivity of a hydrophobic surface with a spherical hydrophilic cavity array are investigated.