Array Transducer Research Articles

Ultrasound Concave 2-D Ring Array for Retinal Stimulation.

An ultrasound concave 2-D ring array transducer was designed for applications in visual stimulation of the retina with a long-term goal to restore vision in individuals with intact neurons but suffering blindness due to retinopathies. The array was synthesized and has a frequency of 20 MHz (0.075-mm wavelengths in water), 18-mm focal length (the curvature of the concave array), 1004 elements (with a pitch of 4.0 wavelengths), and inner and outer diameters of 9 and 14 mm, respectively. Wave patterns produced with the array at the focal distance were simulated. Results show that the wave patterns obtained can achieve a full-width-at-half-maximum (FWHM) resolution of 0.147 mm that is very close to the FWHM diffraction limit (0.136 mm). In addition, a scaled experiment at a lower frequency of 2.5 MHz was performed. The result is very close to those obtained with the simulations.

Signal domain adaptation network for limited-view optoacoustic tomography

Optoacoustic (OA) imaging is based on optical excitation of biological tissues with nanosecond-duration laser pulses and detection of ultrasound (US) waves generated by thermoelastic expansion following light absorption. The image quality and fidelity of OA images critically depend on the extent of tomographic coverage provided by the US detector arrays. However, full tomographic coverage is not always possible due to experimental constraints. One major challenge concerns an efficient integration between OA and pulse-echo US measurements using the same transducer array. A common approach toward the hybridization consists in using standard linear transducer arrays, which readily results in arc-type artifacts and distorted shapes in OA images due to the limited angular coverage. Deep learning methods have been proposed to mitigate limited-view artifacts in OA reconstructions by mapping artifactual to artifact-free (ground truth) images. However, acquisition of ground truth data with full angular coverage is not always possible, particularly when using handheld probes in a clinical setting. Deep learning methods operating in the image domain are then commonly based on networks trained on simulated data. This approach is yet incapable of transferring the learned features between two domains, which results in poor performance on experimental data. Here, we propose a signal domain adaptation network (SDAN) consisting of i) a domain adaptation network to reduce the domain gap between simulated and experimental signals and ii) a sides prediction network to complement the missing signals in limited-view OA datasets acquired from a human forearm by means of a handheld linear transducer array. The proposed method showed improved performance in reducing limited-view artifacts without the need for ground truth signals from full tomographic acquisitions.

Three-dimensional ultrasound matrix imaging

Matrix imaging paves the way towards a next revolution in wave physics. Based on the response matrix recorded between a set of sensors, it enables an optimized compensation of aberration phenomena and multiple scattering events that usually drastically hinder the focusing process in heterogeneous media. Although it gave rise to spectacular results in optical microscopy or seismic imaging, the success of matrix imaging has been so far relatively limited with ultrasonic waves because wave control is generally only performed with a linear array of transducers. In this paper, we extend ultrasound matrix imaging to a 3D geometry. Switching from a 1D to a 2D probe enables a much sharper estimation of the transmission matrix that links each transducer and each medium voxel. Here, we first present an experimental proof of concept on a tissue-mimicking phantom through ex-vivo tissues and then, show the potential of 3D matrix imaging for transcranial applications.

A two-channel ultrasonic flowmeter based on AlN piezoelectric micromachined ultrasonic transducers arrays with improved cross-correlation method

This paper presents a highly accurate two-channel ultrasonic flowmeter based on AlN piezoelectric micromachined ultrasonic transducers (PMUTs) to measure flow rate in small-diameter pipes (9.6 mm). The ultrasonic transducers consist of four 10 by 10 PMUTs arrays with resonant frequency of 1 MHz in air. The ultrasonic transducers are excited by continuous sine voltage, and the transmitted and received signals are subjected to cross-correlation operation to obtain the time delay of the ultrasonic wave in the liquid. A dual-channel design of the flowmeter can reduce measurement errors by taking the average value. To reduce errors in the cross-correlation operation, an iterative algorithm is proposed, which effectively improves the measurement accuracy. The flowmeter is evaluated in flow range of 3.5–10 l min−1, and has a small relative error of 0.7%.

Ultrasound imaging of the testes in children and adolescents

Ultrasound, the imaging method of choice to evaluate abnormalities of the testes and the scrotum, provides accurate anatomic details and allows the assessment of perfusion using color Doppler and power Doppler. Ultrasound represents arapid and reliable procedure which in most cases leads to aconclusive diagnosis. The three most common conditions in the clinical picture of acute scrotum are testicular torsion, torsion of the testicular appendages and inflammatory changes of the testis and the epididymis (epididymo-orchitis). Especially in the case of testicular torsion, rapid diagnosis is essential since time is an important factor to initiate organ-preserving therapy. High-frequency linear array transducer (at least 10 MHz), which allows detection of slow flow rates, is recommended.

Small volume blood-brain barrier opening in macaques with a 1 MHz ultrasound phased array

The use of focused ultrasound to open the blood-brain barrier (BBB) has the potential to deliver drugs to specific regions of the brain. The size of the BBB opening and ability to localize the opening determines the spatial extent and is a limiting factor in many applications of BBB opening where targeting a small brain region is desired. Here we evaluate the performance of a system designed for small opening volumes and highlight the unique challenges associated with pushing the spatial precision of this technique. To achieve small volume openings in cortical regions of the macaque brain, we tested a custom 1 MHz array transducer integrated into a magnetic resonance image-guided focused ultrasound system. Using real-time cavitation monitoring, we demonstrated twelve instances of single sonication, small volume BBB opening with average volumes of 59 ± 37 mm3 and 184 ± 2 mm3 in cortical and subcortical targets, respectively. We found high correlation between subject-specific acoustic simulations and observed openings when incorporating grey matter segmentation (R2 = 0.8577), and the threshold for BBB opening based on simulations was 0.53 MPa. Analysis of MRI-based safety assessment and cavitation signals indicate a safe pressure range for 1 MHz BBB opening and suggest that our system can be used to deliver drugs and gene therapy to small brain regions.

Sonographic evaluation of common carotid artery intima-media thickness in adults with coexisting type two (2) diabetes and hypertension: A case-controlled study

Diabetes mellitus (DM) and hypertension (HTN) are the two leading risk factors for atherosclerosis. Carotid intima-media thickness (CIMT) is considered a reliable surrogate measure for detecting subclinical atherosclerosis including its progression and regression. The study aim at evaluating the intima-media thickness of the common carotid artery in adults with coexisting diabetes mellitus and hypertension (DM+HTN) and compare with healthy controls.. A prospective case control study involving 115 adult subjects with coexisting DM+HTN and 144 healthy adult controls was carried out. Common carotid intima-media thickness (CCIMT) was measured at point approximately 1 cm distal to the right and left carotid bulb using the 7-12 MHz, multi-frequency linear array transducer of high resolution, touch Venue 50 ultrasound machine equipped with electronic callipers. The overall mean CCIMT was 1.13±0.40 mm and 0.82±0.23 mm for subjects with coexisting DM and HTN; and healthy controls, respectively. There was a statistically significant difference in CIMT of subjects with DM+HTN compared to healthy controls (p=0.000 each for both males and females, respectively). There were positive correlations between CCIMT and age in subjects with DM+HTN (r=0.260 and r=0.424 for males and females respectively); with healthy controls (r=0.619 and 0.626 for males and females, respectively). There was a statistically significant difference in CCIMT between subjects with DM+HTN and healthy controls. This implies that the increase in CCIMT is associated with both DM and HTN. In addition, age has a significant positive relationship with CCIMT in both subjects with DM+HTN and healthy controls.

Quantification of damage expansion influence on frequency response function of plate for structural health monitoring with integral differential method

This paper presents a feasibility study on damage size quantification throughout the damage expansion procedure using the integral differential method based on the developed frequency response function (FRF) programme. A designed pattern of piezoelectric Micro Fibre Composite (MFC) transducers was integrated with three composite panels for a real time monitoring testing by swept sine vibration input signals under three varied frequency bandwidths ranged from 10 to 1 kHz, 1k-3 kHz and 3k-5 kHz, respectively. The composite panels under pristine stage without any impact damages were tested and documented as a cross-reference for subsequent quantification estimation. The damages were introduced around transducers array, and its expansion process is equivalently simulated by a repetitive impact test procedure. A modified damage index, difference of response (DoR), was derived through integral differential method to assess the FRF outcome change of damaged composite plate relative to the pristine state. Combined with damage geometrical dimensions measured by thermography imaging technology, a quantification formula is derived reversely through numerical analysis, which showed a segmentation linear relation between the DoR and damage size governed by the power and logarithmic functions. The damages under different severity subjected to an additional composite plate were successfully quantified by the segmentation formula, which validate the feasibility of quantification with DoR.

Dual-frequency piezoelectric micromachined ultrasound transducer based on polarization switching in ferroelectric thin films

Due to its additional frequency response, dual-frequency ultrasound has advantages over conventional ultrasound, which operates at a specific frequency band. Moreover, a tunable frequency from a single transducer enables sonographers to achieve ultrasound images with a large detection area and high resolution. This facilitates the availability of more advanced techniques that simultaneously require low- and high-frequency ultrasounds, such as harmonic imaging and image-guided therapy. In this study, we present a novel method for dual-frequency ultrasound generation from a ferroelectric piezoelectric micromachined ultrasound transducer (PMUT). Uniformly designed transducer arrays can be used for both deep low-resolution imaging and shallow high-resolution imaging. To switch the ultrasound frequency, the only requirement is to tune a DC bias to control the polarization state of the ferroelectric film. Flextensional vibration of the PMUT membrane strongly depends on the polarization state, producing low- and high-frequency ultrasounds from a single excitation frequency. This strategy for dual-frequency ultrasounds meets the requirement for either multielectrode configurations or heterodesigned elements, which are integrated into an array. Consequently, this technique significantly reduces the design complexity of transducer arrays and their associated driving circuits.

High throughput system for preparing samples for genomic, epigenetic, transcriptomic, proteomic and metabolomic analysis

Multi-omics considers the analysis of complex biological “omes” (genome, epigenome, transcriptome, proteome, metabolome, and others). This exponentially growing systems biology approach aims to better understand phenotypes in health and disease and as such identify biomarkers of disease and drug targets. The basic workflow is divided into an upstream sample preparation step, and corresponding downstream assays. To prepare samples for these assays biospecimen are treated to extract analytes such as chromatin, DNA, RNA, protein and metabolites. Often these sample preparation steps are done using cavitation. Current tools such as ultrasonic horns, and even many commercial sample preparation instruments, can be highly inconsistent, leading to unreliable assays. In order to overcome the problems of inconsistency, an instrument was designed and built with a transducer array (free field pressures P + = 30 MPa, P– = 12 MPa) capable of processing samples directly in standard 96-well microplates. An array of transducers is mounted below a microplate, coupled to a lens array that focuses acoustic energy into each well of a microplate. Intense cavitation is generated in each well. This process results in consistent analytes extraction that leads to consistently reproducible results. The authors have a financial conflict of interest. [Funded by NIH R33CA191135, R21GM111439, R01DK103849, R42HG010855, U01CA246503, and R44GM122097.]

Demonstration of medical imaging using flexible polymer ultrasound technology

We present a novel ultrasound transducer technology based on piezo polymers. Its fabrication process (PillarWaveTM), based on creating hexagonal micro-structured PVDF-TrFE pillars. This makes it fully flexible and allows scaling to large apertures (>15 × 15 cm2). Large area flexible transducers enable to select relevant field of view in automated post-processing. This can alleviate the need for a skilled sonographer and enable applications of echography outside clinics. To demonstrate its performance, a 128 element 8 MHz linear array is designed and manufactured. Its total thickness is 0.1 mm. Its acoustic characteristics are determined demonstrating 5.2 kPa/V transmit efficiency and 150 mV/Pa sensitivity. The receive bandwidth at -6dB is larger than 100%. A peak pressure in excess of 1 MPa at 4 cm from the array is measured in water. The transducer array is connected to a Verasonics Vantage research medical ultrasound system. The imaging performance of the array is demonstrated experimentally. Real-time imaging of an in vitro- medical phantom using plane wave compounding and delay-and-sum beamforming is demonstrated at 15 frames per second, with point spread functions at -20 dB of 1.3 and 0.92 mm in the lateral and depth directions, respectively. Finally, imaging of a human carotid in vivo is demonstrated.

Characterizing bulk-driven acoustic streaming in air

The time-independent fluid flows induced by the attenuation of acoustic energy known as acoustic streaming have been experimentally measured in air using particle image velocimetry for two high powered ultrasonic transducers. The literature declares that these bulk-driven “Eckart” streaming flows are observed as turbulent jets in the direction of the acoustic wave propagation. The aim of this work is to increase the understanding of the coupling between the acoustic and fluid domains and to validate these well-established theoretical relationships. Langevin horns and focused arrays of parking sensor transducers, which operate at ∼26 and 40 kHz, respectively, and are both capable of producing sound pressure levels of up to 170dB, were used to assess the relationship between the first order acoustic field and the second order streaming velocity field. Streaming velocities of the order of 0.2 and 0.3 m/s were measured for the Langevin horn and the focused array, respectively. Two-step COMSOL Multiphysics models were created for both transducers, first solving the acoustic fields in the frequency domain, then using the results to drive a streaming volume force in a stationary fluid mechanics study. Both laminar and turbulent k-ε fluid mechanics models were compared to the experimental results.

Diagnostic efficacy of quantitative ultrasonography for anterior disc displacement of the temporomandibular joint.

Ultrasonography has been applied as an alternative method in the assessment of temporomandibular joint (TMJ) pathology including anterior disc displacement (ADD). However, a concrete screening or diagnostic method which is feasible in clinical practice has not yet been established. The study aimed to establish a quantitative ultrasonographic method and determine its diagnostic efficacy for ADD of the TMJ. A total of 75 joints were allocated to either the normal disc position (NDP) group or the ADD group using magnetic resonance imaging (MRI) as the reference standard. Longitudinal scans of the lateral articular compartment were obtained by a 14-MHz L-shaped linear array transducer. The width of the lateral joint space (LJS), the upper lateral joint space (ULJS), and the lower lateral joint space (LLJS), as well as the position of the lateral articular disc edge (ADE), were investigated by stepwise logistic regression analysis to identify significant indicators of ADD and to build a diagnostic model. Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and diagnostic accuracy were computed at the optimal cut-off value. MRI detected 25 joints in the NDP group and 50 joints in the ADD group. Correlation analysis indicated that all 4 variables were associated with ADD. With the best performance of the area under the receiver operating characteristic (ROC) curve (AUC) of 0.939, LJS and ULJS were identified as predictors of ADD and subsequently adopted to build a diagnostic model by stepwise logistic regression. The optimal cut-off value of the 2-variable regression model for diagnosing ADD was 0.800, with a sensitivity of 82%, specificity of 96%, PPV of 97.6%, NPV of 72.7%, and an accuracy of 86.7%. The quantitative ultrasonographic diagnostic method showed promising diagnostic efficacy. It has the potential to be used for ADD screening in future clinical practice.

Flexible Ultrasound Transducer With Embedded Optical Shape Sensing Fiber for Biomedical Imaging Applications.

Flexible ultrasound transducers (FUTs), capable of conforming to irregular surfaces, have become a research hotspot in the field of medical imaging. With these transducers, high-quality ultrasound images can be obtained only if strict design criteria are fulfilled. Moreover, the relative positions of array elements must be determined, which are important for ultrasound beamforming and image reconstruction. These two major characteristics present great challenges to the design and fabrication of FUTs compared to that for traditional rigid probes. In this study, an optical shape-sensing fiber was embedded into a 128-element flexible linear array transducer to acquire the real-time relative positions of array elements to produce high-quality ultrasound images. Minimum concave and convex bend diameters of approximately 20 and 25 mm, respectively, were achieved. The transducer was flexed 2000 times, and yet no obvious damage was observed. Stable electrical and acoustic responses confirmed its mechanical integrity. The developed FUT exhibited an average center frequency of 6.35 MHz, and average -6-dB bandwidth of 69.2%. The array profile and element positions measured by the optic shape-sensing system were instantly transferred to the imaging system. Phantom experiments for both spatial resolution and contrast-to-noise ratio proved that FUTs can maintain satisfactory imaging capability despite bending to sophisticated geometries. Finally, color Doppler images and Doppler spectra of the peripheral arteries of healthy volunteers were obtained in real time.

Real-Time Passive Acoustic Mapping With Enhanced Spatial Resolution in Neuronavigation-Guided Focused Ultrasound for Blood-Brain Barrier Opening.

Passive acoustic mapping (PAM) provides the spatial information of acoustic energy emitted from microbubbles during focused ultrasound (FUS), which can be used for safety and efficacy monitoring of blood-brain barrier (BBB) opening. In our previous work with a neuronavigation-guided FUS system, only part of the cavitation signal could be monitored in real time due to the computational burden although full-burst analysis is required to detect transient and stochastic cavitation activity. In addition, the spatial resolution of PAM can be limited for a small-aperture receiving array transducer. For full-burst real-time PAM with enhanced resolution, we developed a parallel processing scheme for coherence-factor-based PAM (CF-PAM) and implemented it onto the neuronavigation-guided FUS system using a co-axial phased-array imaging transducer. Simulation and in-vitro human skull studies were conducted for the performance evaluation of the proposed method in terms of spatial resolution and processing speed. We also carried out real-time cavitation mapping during BBB opening in non-human primates (NHPs). CF-PAM with the proposed processing scheme provided better resolution than that of traditional time-exposure-acoustics PAM with a higher processing speed than that of eigenspace-based robust Capon beamformer, which facilitated the full-burst PAM with the integration time of 10 ms at a rate of 2 Hz. In vivo feasibility of PAM with the co-axial imaging transducer was also demonstrated in two NHPs, showing the advantages of using real-time B-mode and full-burst PAM for accurate targeting and safe treatment monitoring. This full-burst PAM with enhanced resolution will facilitate the clinical translation of online cavitation monitoring for safe and efficient BBB opening.

Modeling split-beam sonar to evaluate fish target strength accuracy

Split-beam echosounders enable estimation of fish size by directly measuring their target strength. This measurement is achieved using compound transducer array geometries with accurate knowledge of resulting beam patterns. The multiple transducer components and required accurate calibration can present challenges in predicting the exact sonar capabilities. We present a model of the split-beam sonar system that can be used to evaluate and optimize sonar performance of a given transducer before committing to a hardware design. The model has been used to generate beam patterns to match prototype instruments and to simulate acoustic signals based on the scattering of sound from particles in a three-dimensional domain. Different split-beam sonar algorithms have been compared to measure the position and target strength of particles in the generated signal. The model's prediction capabilities were evaluated through comparisons with field trials of a prototype system. The field trials were conducted by lowering a calibration target sphere to a range of 200 m in the acoustic beam. Both model prediction and prototype system performance show accuracy of σ = ±0.2 dB at 25m range. Potential future applications of the model include exploring methods of target separation and improving accuracy when presented with complicated target structures.

Dual-Probe Transcranial Full-Waveform Inversion: A Brain Phantom Feasibility Study.

Despite being a low-cost, portable and safe medical imaging technique, transcranial ultrasound imaging is not used widely in adults because of the severe degradation and distortion of signals caused by the skull. Full-waveform inversion (FWI) has recently been found to have potential as an effective method for transcranial ultrasound tomography to obtain high-quality, subwavelength-resolution acoustic models of the brain using low-frequency ultrasound data. In this study is the first demonstration of this method in recovering a high-resolution 2-D reconstruction of a brain and skull ultrasound imaging phantom using experimentally acquired data. A 2:5 scale brain phantom encased within a 3-D-printed skull-mimicking layer was created to simulate a clinical transcranial imaging target. To obtain tomographic ultrasound data on the brain and skull phantom, a tomographic ultrasound acquisition system was designed and implemented using commercially available low-frequency cardiac probes. FWI reconstructions of the brain and skull phantom were performed using the acquired tomographic data and were compared with corresponding synthetic reconstructions. This comparison was used to evaluate the feasibility of the proposed imaging system when employing different transducer array configurations. We demonstrate the successful FWI reconstruction of the brain phantom within the skull mimic from experimentally acquired tomographic ultrasound data. To mitigate the effects of the skull-mimicking material, a reflection-matching algorithm was applied to model the morphology of the skull layer prior to performing the inversion. The findings of this study provide a promising step toward the clinical use of FWI for transcranial ultrasound imaging in adults.

Continuous and Non-Invasive Monitoring of Blood Pressure Based on Wearable Piezoelectric Micromachined Ultrasonic Transducers Array

Continuous and Non-Invasive Monitoring of Blood Pressure Based on Wearable Piezoelectric Micromachined Ultrasonic Transducers Array

Focusing Reflected Ultrasound Using Boundary Element Model for Mid-Air Tactile Presentation.

Ultrasound focusing with curved reflectors has various advantages in mid-air tactile presentation. First, tactile sensations can be presented from various directions without placing a large number of transducers. It also avoids conflicts in the arrangement of transducer arrays with optical sensors and visual displays. Furthermore, the blurring of the focus can be suppressed. We propose a method for focusing reflected ultrasound by solving the boundary integral equation for the sound field on a reflector divided into elements. This method does not require a prior measurement of the response to each transducer at the tactile presentation point, as in the previous method. It enables real-time focusing on arbitrary locations by formulating the relationship between the transducer input and the reflected sound field. This method also enhances the focus intensity by incorporating the tactile presentation's target object into the boundary element model. Numerical simulations and measurements showed that the proposed method could focus ultrasound reflected from a hemispherical dome. A numerical analysis was also performed to determine the region where focus generation with sufficient intensity was possible.

The Effect of Body Mass Index on Tumor Treating Fields (TTFields) Intensity Distribution in the Lungs

Tumor Treating Fields (TTFields) therapy is a locoregional, noninvasive antimitotic cancer treatment modality. TTFields are electric fields that disrupt processes critical for cancer cell viability and tumor progression. They are generated by a portable medical device and delivered noninvasively to the tumor via two pairs of orthogonally placed transducer arrays. TTFields therapy is currently FDA-approved for recurrent and newly diagnosed glioblastoma and pleural mesothelioma, with ongoing studies in other solid tumor types including non-small cell lung cancer (NSCLC). In the phase 3 LUNAR study (NCT02973789), TTFields therapy demonstrated an improvement in overall survival when added to NSCLC standard of care. TTFields effectiveness depends upon several factors, including field intensity; thus, it is relevant to assess the impact of body mass index (BMI) on the distribution of TTFields in the lungs. This study utilized simulation-based approaches to assess the impact of BMI on electric field intensity distribution in the lungs. Three computerized female phantom models (with BMIs of 22, 26 and 30), were used to simulate electric field intensity. Two different arrays sizes (small [13 disks] and large [20 disks]) were applied to the thoracic region of the models. An electric field intensity distribution map for each model was generated, and electric field intensity values across the entirety of the lungs were analyzed. Electric field intensities were above 1 V/cm (amplitude), which is typically considered a sufficient level of TTFields dose to achieve a therapeutic effect, in all lobes in the lungs for all BMI models when large arrays were used. Therapeutic intensities were achieved in all lobes with the small arrays in all but one model (BMI 30), where intensities fell slightly below 1 V/cm in the right middle lobe. Results from this simulation-based study demonstrate that delivery of TTFields at therapeutic intensities is feasible even in patients with a high BMI, provided the appropriately sized array is used. These findings support ongoing clinical studies of TTFields therapy in lung cancer, including the recently completed LUNAR study in advanced NSCLC, and could facilitate optimization of therapeutic TTFields delivery to individual patients, based on their BMI.