Showing papers in "IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control in 2019"
TL;DR: This work demonstrates that ultrasound B-mode image reconstruction using machine-learned neural networks is feasible and establishes that networks trained solely in silico can be generalized to real-world imaging in vivo to produce images with significantly reduced speckle.
Abstract: With traditional beamforming methods, ultrasound B-mode images contain speckle noise caused by the random interference of subresolution scatterers. In this paper, we present a framework for using neural networks to beamform ultrasound channel signals into speckle-reduced B-mode images. We introduce log-domain normalization-independent loss functions that are appropriate for ultrasound imaging. A fully convolutional neural network was trained with the simulated channel signals that were coregistered spatially to ground-truth maps of echogenicity. Networks were designed to accept 16 beamformed subaperture radio frequency (RF) signals. Training performance was compared as a function of training objective, network depth, and network width. The networks were then evaluated on the simulation, phantom, and in vivo data and compared against the existing speckle reduction techniques. The most effective configuration was found to be the deepest (16 layer) and widest (32 filter) networks, trained to minimize a normalization-independent mixture of the $\ell _{1}$ and multiscale structural similarity (MS-SSIM) losses. The neural network significantly outperformed delay-and-sum (DAS) and receive-only spatial compounding in speckle reduction while preserving resolution and exhibited improved detail preservation over a nonlocal means method. This work demonstrates that ultrasound B-mode image reconstruction using machine-learned neural networks is feasible and establishes that networks trained solely in silico can be generalized to real-world imaging in vivo to produce images with significantly reduced speckle.
134 citations
TL;DR: The application of a saturation model to the reconstructed vessels is shown to be a valuable tool not only to estimate the measurement times necessary to adequately reconstruct the microvasculature but also for the validation of the measurements.
Abstract: Recently, we proved in the first measurements of breast carcinomas the feasibility of super-resolution ultrasound (US) imaging by motion-model ultrasound localization microscopy in a clinical setup. Nevertheless, pronounced in-plane and out-of-plane motions, a nonoptimized microbubble injection scheme, the lower frame rate and the larger slice thickness made the processing more complex than in preclinical investigations. Here, we compare the results of state-of-the-art contrast-enhanced to super-resolution US imaging and systematically analyze the measurements to get indications for the improvement of image acquisition and processing in the future clinical studies. In this regard, the application of a saturation model to the reconstructed vessels is shown to be a valuable tool not only to estimate the measurement times necessary to adequately reconstruct the microvasculature but also for the validation of the measurements. The parameters from this model can also serve to optimize contrast agent concentration and injection protocols. Finally, for the measurements of well-perfused tumors, we observed between 28% and 50% filling for 90-s examination times.
60 citations
TL;DR: It is found that a two-layer structure, in which the AlN layer is removed, achieves a high Q-factor, which means that a reduction of complexity of the layer structure could be obtained without performance loss.
Abstract: In response to the increased mobile data traffic, there is a growing need for more low-loss RF band filters with steep frequency characteristics, and high-quality ( $Q$ )-factor and low-temperature coefficient of frequency (TCF) resonators are required to achieve this. We previously reported that for a surface acoustic wave (SAW) resonator on a three-layer structure, which is composed of a thin LiTaO3 (LT) plate whose orientation is 50° rotated YX propagation, SiO2 layer, and AlN layer on a Si substrate, a Q-factor several times higher than that of an SAW resonator on a standard 42° rotated YX propagation LiTaO3 (42YX-LT) substrate could be obtained. In this study, we investigated this layer structure and found that a two-layer structure, in which the AlN layer is removed, achieves a high $Q$ -factor. Numerical analyses using a finite element method showed that the acoustic wave energy can be confined to the surface of the two-layer substrate, and the TCF and electromechanical coupling coefficient ( $k^{2}$ ) were improved by optimizing the thickness of each layer. We fabricated and evaluated prototype one-port resonators with the two-layer structure and the standard 42YX-LT SAW substrate with resonant frequencies from 0.95 to 3.6 GHz. An improvement of the Q-factor of 3 to 4 times compared with that of the resonator with standard 42YX-LT substrate was observed for the two-layer structure, which means that a reduction of complexity of the layer structure could be obtained without performance loss. The two-layer structure was applied to a 2.4-GHz band Wi-Fi filter to achieve high performances such as low-loss, better steepness, and high attenuation.
57 citations
TL;DR: Fast acoustic wave sparsely activated localization microscopy (fast-AWSALM) was developed to achieve super-resolved frames with subsecond temporal resolution, by using low-boiling-point octafluoropropane nanodroplets and high frame rate plane waves for activation, destruction, as well as imaging.
Abstract: Localization-based ultrasound super-resolution imaging using microbubble contrast agents and phase-change nanodroplets has been developed to visualize microvascular structures beyond the diffraction limit. However, the long data acquisition time makes the clinical translation more challenging. In this study, fast acoustic wave sparsely activated localization microscopy (fast-AWSALM) was developed to achieve super-resolved frames with subsecond temporal resolution, by using low-boiling-point octafluoropropane nanodroplets and high frame rate plane waves for activation, destruction, as well as imaging. Fast-AWSALM was demonstrated on an in vitro microvascular phantom to super-resolve structures that could not be resolved by conventional B-mode imaging. The effects of the temperature and mechanical index on fast-AWSALM were investigated. The experimental results show that subwavelength microstructures as small as $190~\mu \text{m}$ were resolvable in 200 ms with plane-wave transmission at a center frequency of 3.5 MHz and a pulse repetition frequency of 5000 Hz. This is about a 3.5-fold reduction in point spread function full-width-half-maximum compared to that measured in the conventional B-mode, and two orders of magnitude faster than the recently reported AWSALM under a nonflow/very slow flow situations and other localization-based methods. Just as in AWSALM, fast-AWSALM does not require flow, as is required by current microbubble-based ultrasound super-resolution techniques. In conclusion, this study shows the promise of fast-AWSALM, a super-resolution ultrasound technique using nanodroplets, which can generate super-resolution images in milliseconds and does not require flow.
55 citations
TL;DR: This paper shows evidence on the influence of DRA on the estimation of CR and CNR and on the fact that several methods in the state of the art do alter the DR, and shows that DRA may lead to increased CNR values, under some circumstances.
Abstract: Many adaptive beamformers claim to produce images with increased contrast, a feature that could enable a better detection of lesions and anatomical structures. Contrast is often quantified using the contrast ratio (CR) and the contrast-to-noise ratio (CNR). The estimation of CR and CNR can be affected by dynamic range alterations (DRAs), such as those produced by a trivial gray-level transformation. Thus, we can form the hypothesis that contrast improvements from adaptive beamformers can, partly, be due to DRA. In this paper, we confirm this hypothesis. We show evidence on the influence of DRA on the estimation of CR and CNR and on the fact that several methods in the state of the art do alter the DR. To study this phenomenon, we propose a DR test (DRT) to estimate the degree of DRA and we apply it to seven beamforming methods. We show that CR improvements correlate with DRT with $R^{2}\text {-adj}=0.88$ in simulated data and $R^{2}\text {-adj}=0.98$ in experiments. We also show that DRA may lead to increased CNR values, under some circumstances. These results suggest that claims on lesion detectability, based on CR and CNR values, should be revised.
54 citations
TL;DR: The added value of combining different features computed from a single US acquisition with machine learning to characterize carotid artery plaques is shown.
Abstract: Quantitative ultrasound (QUS) imaging methods, including elastography, echogenicity analysis, and speckle statistical modeling, are available from a single ultrasound (US) radio-frequency data acquisition. Since these US imaging methods provide complementary quantitative tissue information, characterization of carotid artery plaques may gain from their combination. Sixty-six patients with symptomatic ( $n = 26$ ) and asymptomatic ( $n = 40$ ) carotid atherosclerotic plaques were included in the study. Of these, 31 underwent magnetic resonance imaging (MRI) to characterize plaque vulnerability and quantify plaque components. US radio-frequency data sequence acquisitions were performed on all patients and were used to compute noninvasive vascular US elastography and other QUS features. Additional QUS features were computed from three types of images: homodyned-K (HK) parametric maps, Nakagami parametric maps, and log-compressed B-mode images. The following six classification tasks were performed: detection of 1) a small area of lipid; 2) a large area of lipid; 3) a large area of calcification; 4) the presence of a ruptured fibrous cap; 5) differentiation of MRI-based classification of nonvulnerable carotid plaques from neovascularized or vulnerable ones; and 6) confirmation of symptomatic versus asymptomatic patients. Feature selection was first applied to reduce the number of QUS parameters to a maximum of three per classification task. A random forest machine learning algorithm was then used to perform classifications. Areas under receiver-operating curves (AUCs) were computed with a bootstrap method. For all tasks, statistically significant higher AUCs were achieved with features based on elastography, HK parametric maps, and B-mode gray levels, when compared to elastography alone or other QUS alone ( $p ). For detection of a large area of lipid, the combination yielding the highest AUC (0.90, 95% CI 0.80–0.92, $p ) was based on elastography, HK, and B-mode gray-level features. To detect a large area of calcification, the highest AUC (0.95, 95% CI 0.94–0.96, $p ) was based on HK and B-mode gray level features. For other tasks, AUCs varied between 0.79 and 0.97. None of the best combinations contained Nakagami features. This study shows the added value of combining different features computed from a single US acquisition with machine learning to characterize carotid artery plaques.
48 citations
TL;DR: The present study aimed at establishing the feasibility of detecting shear waves created after mitral valve closure and aortic valve closure, the variability of the measurements, and at reporting the normal values of propagation velocity.
Abstract: Left ventricular myocardial stiffness could offer superior quantification of cardiac systolic and diastolic function when compared to the current diagnostic tools. Shear wave elastography in combination with acoustic radiation force has been widely proposed to noninvasively assess tissue stiffness. Interestingly, shear waves can also result from intrinsic cardiac mechanical events (e.g., closure of valves) without the need for external excitation. However, it remains unknown whether these natural shear waves always occur, how reproducible they can be detected and what the normal range of shear wave propagation speed is. The present study, therefore, aimed at establishing the feasibility of detecting shear waves created after mitral valve closure (MVC) and aortic valve closure (AVC), the variability of the measurements, and at reporting the normal values of propagation velocity. Hereto, a group of 30 healthy volunteers was scanned with high-frame rate imaging (>1000 Hz) using an experimental ultrasound system transmitting a diverging wave sequence. Tissue Doppler velocity and acceleration were used to create septal color M-modes, on which the shear waves were tracked and their velocities measured. Overall, the methodology was capable of detecting the transient vibrations that spread throughout the intraventricular septum in response to the closure of the cardiac valves in 92% of the recordings. Reference velocities of 3.2±0.6 m/s at MVC and 3.5±0.6 m/s at AVC were obtained. Moreover, in order to show the diagnostic potential of this approach, two patients (one with cardiac amyloidosis and one undergoing a dobutamine stress echocardiography) were scanned with the same protocol and showed markedly higher propagation speeds: the former presented velocities of 6.6 and 5.6 m/s; the latter revealed normal propagation velocities at baseline, and largely increased during the dobutamine infusion (>15 m/s). Both cases showed values consistent with the expected changes in stiffness and cardiac loading conditions.
42 citations
TL;DR: A noise suppression method based on noise debiasing that can be easily applied to the accelerated SVD method to bridge the gap between real-time implementation and high imaging quality is proposed and validated under different ultrasound imaging parameters.
Abstract: Ultrasound microvessel imaging (UMI) based on the combination of singular value decomposition (SVD) clutter filtering and ultrafast plane wave imaging has recently demonstrated significantly improved Doppler sensitivity, especially to small vessels that are invisible to conventional Doppler imaging. Practical implementation of UMI is hindered by the high computational cost associated with SVD and low blood signal-to-noise ratio (SNR) in deep regions of the tissue due to the lack of transmit focusing of plane waves. Concerning the high computational cost, an accelerated SVD clutter filtering method based on randomized SVD (rSVD) and randomized spatial downsampling (rSD) was recently proposed by our group, which showed the feasibility of real-time implementation of UMI. Concerning the low blood flow SNR in deep imaging regions, here we propose a noise suppression method based on noise debiasing that can be easily applied to the accelerated SVD method to bridge the gap between real-time implementation and high imaging quality. The proposed method experimentally measures the noise-induced bias by collecting the noise signal using the identical imaging sequence as regular UMI, but with the ultrasound transmission turned off. The estimated bias can then be subtracted from the original power Doppler (PD) image to obtain effective noise suppression. The feasibility of the proposed method was validated under different ultrasound imaging parameters [including transmitting voltages and time-gain compensation (TGC) settings] with a phantom experiment. The noise-debiased images showed an increase of up to 15.3 and 13.4 dB in SNR as compared to original PD images on the blood flow phantom and an in vivo human kidney data set, respectively. The proposed noise suppression method has negligible computational cost and can be conveniently combined with the previously proposed accelerated SVD clutter filtering technique to achieve high quality, real-time UMI imaging.
41 citations
TL;DR: This work improves the spatial resolution and visual vascular reconstruction quality of sparsity-based superresolution US imaging from low-frame rate acquisitions, by exploiting the inherent flow of MBs and utilize their motion kinematics and demonstrates the effectiveness of the proposed approach on both simulations and in vivo contrast-enhanced human prostate scan.
Abstract: Ultrasound (US) localization microscopy offers new radiation-free diagnostic tools for vascular imaging deep within the tissue. Sequential localization of echoes returned from inert microbubbles (MBs) with low concentration within the bloodstream reveals the vasculature with capillary resolution. Despite its high spatial resolution, low MB concentrations dictate the acquisition of tens of thousands of images, over the course of several seconds to tens of seconds, to produce a single superresolved image. Such long acquisition times and stringent constraints on MB concentration are undesirable in many clinical scenarios. To address these restrictions, sparsity-based approaches have recently been developed. These methods reduce the total acquisition time dramatically, while maintaining good spatial resolution in settings with considerable MB overlap. Here, we further improve the spatial resolution and visual vascular reconstruction quality of sparsity-based superresolution US imaging from low-frame rate acquisitions, by exploiting the inherent flow of MBs and utilize their motion kinematics. We also provide quantitative measurements of MB velocities and show that our approach achieves higher MB recall rate than the state-of-the-art techniques, while increasing contrast agents concentration. Our method relies on simultaneous tracking and sparsity-based detection of individual MBs in a frame-by-frame manner, and as such, may be suitable for real-time implementation. The effectiveness of the proposed approach is demonstrated on both simulations and an in vivo contrast-enhanced human prostate scan, acquired with a clinically approved scanner operating at a 10-Hz frame rate.
39 citations
TL;DR: The demonstrated S0 mode low loss and wideband acoustic delay lines can potentially enable wide-range and high-resolution delay synthesis that is highly sought after for the self-interference cancellation in full-duplex radios.
Abstract: We present the first group of gigahertz S0 mode low loss and wideband acoustic delay lines (ADLs). The ADLs use a single-phase unidirectional transducers (SPUDT) design to launch and propagate the S0 mode in an X-cut lithium niobate thin film with large electromechanical coupling and low damping. In this work, the theoretical performance bounds of S0 mode ADLs are first investigated, significantly surpassing those in state-of-the-art. The design tradeoffs of S0 mode ADLs, when scaled to the gigahertz frequency range, are also discussed. The fabricated miniature ADLs show a fractional bandwidth (FBW) of 4% and a minimum insertion loss (IL) of 3.2 dB, outperforming the incumbent surface acoustic wave (SAW) counterparts, and covering a wide range of delays from 20 to 900 ns for digitally addressable delay synthesis. Multiple ADLs with center frequencies from 0.9 to 2 GHz have been demonstrated, underscoring their great frequency scalability. The propagation properties of S0 waves in lithium niobate at the gigahertz range are experimentally extracted. The demonstrated ADLs can potentially enable wide-range and high-resolution delay synthesis that is highly sought after for the self-interference cancellation in full-duplex radios.
39 citations
TL;DR: The fabrication of contour-mode resonators (CMRs) with Al.17 N as a piezoelectric layer is demonstrated and the first temperature compensation experimental results for Al.0.83 and 0.17 CMRs are reported, indicating a significant improvement in electromechanical coupling coefficients when compared to pure AlN.
Abstract: In this paper, we demonstrate the fabrication of contour-mode resonators (CMRs) with Al0.83Sc0.17N as a piezoelectric layer. Moreover, we assess the electromechanical coupling and the maximum achieved quality factor from 150 to 500 MHz. In comparison to pure aluminum nitride (AlN) CMRs, our results show electromechanical coupling coefficients of more than a $2\times $ factor higher at around 200 MHz. The highest quality factor is measured on a CMR operating at 388 MHz and is in excess of 1600. From the characterization of devices operating at different frequencies, material parameters of the Al0.83Sc0.17N are extracted such as the stiffness constant, the relative permittivity, and the piezoelectric constant. In particular, the reported d31 piezoelectric constant is equal to −3.9 pm/V. This represents a $2.25\times $ improvement when compared to pure AlN. Finally, we report the first temperature compensation experimental results for Al0.83Sc0.17N CMRs. Our results show that about 1.5 $\mu \text{m}$ of sputtered oxide, deposited on top of a released resonator, allows near zero temperature coefficient of frequency variation for CMRs operating up to 500 MHz.
TL;DR: A model is developed to investigate the combined effects of imaging parameters, bubble signal density, and vascular flow on SR image acquisition time and finds that the estimated minimum time needed for SR increases for slower blood velocities and greater resolution improvement.
Abstract: A number of acoustic super-resolution techniques have recently been developed to visualize microvascular structure and flow beyond the diffraction limit. A crucial aspect of all ultrasound (US) super-resolution (SR) methods using single microbubble localization is time-efficient detection of individual bubble signals. Due to the need for bubbles to circulate through the vasculature during acquisition, slow flows associated with the microcirculation limit the minimum acquisition time needed to obtain adequate spatial information. Here, a model is developed to investigate the combined effects of imaging parameters, bubble signal density, and vascular flow on SR image acquisition time. We find that the estimated minimum time needed for SR increases for slower blood velocities and greater resolution improvement. To improve SR from a resolution of $\lambda $ /10 to $\lambda $ /20 while imaging the microvasculature structure modeled here, the estimated minimum acquisition time increases by a factor of 14. The maximum useful imaging frame rate to provide new spatial information in each image is set by the bubble velocity at low blood flows (<150 mm/s for a depth of 5 cm) and by the acoustic wave velocity at higher bubble velocities. Furthermore, the image acquisition procedure, transmit frequency, localization precision, and desired super-resolved image contrast together determine the optimal acquisition time achievable for fixed flow velocity. Exploring the effects of both system parameters and details of the target vasculature can allow a better choice of acquisition settings and provide improved understanding of the completeness of SR information.
TL;DR: In this article, a coherent multi-transducer ultrasound imaging system consisting of synchronized matrix arrays, each with partly shared field of view (FoV), takes turns to transmit plane waves (PWs).
Abstract: This work extends the effective aperture size by coherently compounding the received radio frequency data from multiple transducers. As a result, it is possible to obtain an improved image, with enhanced resolution, an extended field of view (FoV), and high-acquisition frame rates. A framework is developed in which an ultrasound imaging system consisting of $N$ synchronized matrix arrays, each with partly shared FoV, take turns to transmit plane waves (PWs). Only one individual transducer transmits at each time while all $N$ transducers simultaneously receive. The subwavelength localization accuracy required to combine information from multiple transducers is achieved without the use of any external tracking device. The method developed in this study is based on the study of the backscattered echoes received by the same transducer and resulting from a targeted scatterer point in the medium insonated by the multiple ultrasound probes of the system. The current transducer locations along with the speed of sound in the medium are deduced by optimizing the cross correlation between these echoes. The method is demonstrated experimentally in 2-D for two linear arrays using point targets and anechoic lesion phantoms. The first demonstration of a free-hand experiment is also shown. Results demonstrate that the coherent multi-transducer ultrasound imaging method has the potential to improve ultrasound image quality, improving resolution, and target detectability. Compared with coherent PW compounding using a single probe, lateral resolution improved from 1.56 to 0.71 mm in the coherent multi-transducer imaging method without acquisition frame rate sacrifice (acquisition frame rate 5350 Hz).
TL;DR: Air-coupled piezoelectric micromachined ultrasonic transducers using 36% scandium-doped aluminum nitride (ScAlN) thin-film are presented, showing a large displacement and consequently, a high SPL compared to the state-of-the-art PMUTs and the bulk transducer considering the size and the excitation voltage.
Abstract: In this paper, air-coupled piezoelectric micromachined ultrasonic transducers (PMUTs) using 36% scandium-doped aluminum nitride (ScAlN) thin-film are presented. ScAlN is known to exhibit higher piezoelectric properties compared to pure AlN leading to significant performance improvements in various piezoelectric micro-electromechanical systems (MEMS) applications including PMUTs. Here, the concentration of Sc in the actual sputtered 1- $\mu \text{m}$ -thick ScAlN film was 36%, which is slightly below the maximum at the phase boundary. The ScAlN PMUTs were fabricated from an SOI wafer, where the dry etching of ScAlN film was optimized. The frequency response and displacement sensitivity of the PMUTs were characterized in the air using laser Doppler vibrometry confirming $2\times $ higher transverse piezoelectric coefficient than AlN. The acoustic transmission and reception of the PMUTs were evaluated from a high-sensitivity microphone and pulse-echo measurements. The PMUTs were designed to operate below 100 kHz in order to mitigate the absorption loss, which resulted in a high transmit pressure of 105-dB sound pressure level (SPL) at 10 cm and only 30-dB attenuation at the 2-m range. Through implementing 36% ScAlN film, the presented PMUTs exhibited a large displacement and consequently, a high SPL compared to the state-of-the-art PMUTs and the bulk transducer considering the size and the excitation voltage.
TL;DR: The usefulness of the mode decomposition algorithm is demonstrated on a new health monitoring system for composite structures that performs anomaly imaging using the first arriving mode extracted from sensor array signals acquired from the structure.
Abstract: Lamb waves are characterized by their multimodal and dispersive propagation, which often complicates analysis. This paper presents a method for separation of the mode components and reflected components in sensor signals in an active structural health monitoring (SHM) system. The system is trained using linear chirp signals but works for arbitrary excitation signals. The training process employs the cross-Wigner-Ville distribution (xWVD) of the excitation signal and the sensor signal to separate the temporally overlapped modes in the time-frequency domain. The mode decomposition method uses a ridge extraction algorithm to separate each signal component in the time-frequency distribution. Once the individual modes are separated in the time-frequency domain, they are reconstructed in the time domain using the inverse xWVD operation. The propagation impulse response associated with each component can be directly estimated for chirp inputs. The estimated propagation impulse response can be used to separate the modes resulting from arbitrary excitation signals as long as their frequency components fall in the range of the chirp signal. The usefulness of the mode decomposition algorithm is demonstrated on a new health monitoring system for composite structures. This system performs anomaly imaging using the first arriving mode extracted from sensor array signals acquired from the structure. The anomaly maps are computed using a sparse tomographic reconstruction algorithm. The reconstructed map can locate anomalies on the structure and estimate their boundaries. Comparisons with methods that do not employ mode decomposition and/or sparse reconstruction techniques indicate a substantially better performance for the method of this paper.
TL;DR: The results of this study demonstrate the potentials of RCA 2-D arrays against fully addressed 2-d arrays, which are low channel count, low acoustic intensity mechanical index, and high penetration depth, which makes 3-D imaging at high volume rates possible with equipment in the price range of conventional 2- D imaging.
Abstract: This study evaluates the volumetric imaging performance of two prototyped 62 + 62 row–column-addressed (RCA) 2-D array transducer probes using three synthetic aperture imaging (SAI) emission sequences and two different beamformers. The probes are fabricated using capacitive micromachined ultrasonic transducer (CMUT) and piezoelectric transducer (PZT) technology. Both have integrated apodization to reduce ghost echoes and are designed with similar acoustical features, i.e., 3-MHz center frequency, $\lambda $ /2 pitch, and $24.8\times 24.8\,\,\text {mm}^{2}$ active footprint. Raw RF data are obtained using an experimental research ultrasound scanner, SARUS. The SAI sequences are designed for imaging down to 14 cm at a volume rate of 88 Hz. Two beamforming methods, spatial matched filtering and row–column adapted delay-and-sum, are used for beamforming the RF data. The imaging quality is investigated through simulations and phantom measurements. Both probes on average have similar lateral full-width at half-maximum (FWHM) values, but the PZT probe has 20% smaller cystic resolution values and 70% larger contrast-to-noise ratio (CNR) compared to the capacitive micromachined ultrasonic transducer (CMUT) probe. The CMUT probe can penetrate down to 15 cm, and the PZT probe down to 30 cm. The CMUT probe has 17% smaller axial FWHM values. The matched filter focusing shows an improved B-mode image for measurements on a cyst phantom with an improved speckle pattern and better visualization of deeper lying cysts. The results of this study demonstrate the potentials of RCA 2-D arrays against fully addressed 2-D arrays, which are low channel count (e.g., 124 instead of 3844), low acoustic intensity mechanical index (MI ≤ 0.88 and spatial-peak-temporal-average intensity $I_{\text {spta}}\leq 5.5~\text {mW/cm}^{2}$ ), and high penetration depth (down to 30 cm), which makes 3-D imaging at high volume rates possible with equipment in the price range of conventional 2-D imaging.
TL;DR: Results from the closed-loop characterization of an electrically coupled mode-localized sensor topology including measurements of amplitude ratios over a long duration, stability, noise floor, and the bandwidth of operation are presented.
Abstract: This paper presents results from the closed-loop characterization of an electrically coupled mode-localized sensor topology including measurements of amplitude ratios over a long duration, stability, noise floor, and the bandwidth of operation. The sensitivity of the prototype sensor is estimated to be −5250 in the linear operation regime. An input-referred stability of 84 ppb with respect to normalized stiffness perturbations is achieved at 500 s. When compared to frequency shift sensing within the same device, amplitude ratio sensing provides higher resolution for long-term measurements due to the intrinsic common-mode rejection properties of a mode-localized system. A theoretical framework is established to quantify noise floor associated with measurements validated through numerical simulations and experimental data. In addition, the operating bandwidth of the sensor is found to be 3.5 Hz for 3-dB flatness.
TL;DR: A novel computationally efficient quasi-static ultrasound elastography technique is introduced by optimizing an energy function to obtain the time-delay estimation of all samples between the first two and last two frames of ultrasound images simultaneously, and spatially differentiate the TDE to generate axial strain map.
Abstract: In this paper, a novel computationally efficient quasi-static ultrasound elastography technique is introduced by optimizing an energy function. Unlike conventional elastography techniques, three radio frequency (RF) frames are considered to devise a nonlinear cost function consisting of data intensity similarity term, spatial regularization terms and, most importantly, temporal continuity terms. We optimize the aforesaid cost function efficiently to obtain the time-delay estimation (TDE) of all samples between the first two and last two frames of ultrasound images simultaneously, and spatially differentiate the TDE to generate axial strain map. A novelty in our spatial and temporal regularizations is that they adaptively change based on the data, which leads to substantial improvements in TDE. We handle the computational complexity resulting from incorporation of all samples from all three frames by converting our optimization problem to a sparse linear system of equations. Consideration of both spatial and temporal continuity makes the algorithm more robust to signal decorrelation than the previous algorithms. We name the proposed method GUEST: Global Ultrasound Elastography in Spatial and Temporal directions. We validated our technique with simulation, experimental phantom, and in vivo liver data and compared the results with two recently proposed TDE methods. In all the experiments, GUEST substantially outperforms other techniques in terms of signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and strain ratio (SR) of the strain images.
TL;DR: The method can be used to automatically and to quantitatively characterize the distribution of B-lines for diagnosing pulmonary edema and is able to differentiate between the healthy volunteers and the patients.
Abstract: This paper proposes an automatic method for accurate detection and visualization of B-lines in ultrasound lung scans, which provides a quantitative measure for the number of B-lines present. All the scans used in this study were acquired using a BK3000 ultrasound scanner (BK Ultrasound, Herlev, Denmark) driving a 5.5-MHz linear transducer (BK Ultrasound). Four healthy subjects and four patients, after major surgery with pulmonary edema, were scanned at four locations on each lung for B-line examination. Eight sequences of 50 frames were acquired for each subject yielding 64 sequences in total. The proposed algorithm was applied to all 3200 in-vivo lung ultrasound images. The results showed that the average number of B-lines was 0.28±0.06 (Mean±Std) in scans belonging to the patients compared to 0.03 ± 0.06 (Mean ± Std) in the healthy subjects. Also, the Mann–Whitney test showed a significant difference between the two groups with the $p$ -value of 0.015, and indicating that the proposed algorithm was able to differentiate between the healthy volunteers and the patients. In conclusion, the method can be used to automatically and to quantitatively characterize the distribution of B-lines for diagnosing pulmonary edema.
TL;DR: The results of this pilot clinical study suggest the feasibility of implementing symmetrical ARF to obviate mechanical anisotropy in the kidney cortex when anisOTropy is a confounding factor and implementing asymmetric ARF in the cortex of human kidney allografts to exploit mechanical an isotropy when mechanicalAnisotropic biomarker is a potentially relevant biomarker.
Abstract: The kidney is an anisotropic organ, with higher elasticity along versus across nephrons. The degree of mechanical anisotropy in the kidney may be diagnostically relevant if properly exploited; however, if improperly controlled, anisotropy may confound stiffness measurements. The purpose of this study is to demonstrate the clinical feasibility of acoustic radiation force (ARF)-induced peak displacement (PD) measures for both exploiting and obviating mechanical anisotropy in the cortex of human kidney allografts, in vivo . Validation of the imaging methods is provided by preclinical studies in pig kidneys, in which ARF-induced PD values were significantly higher ( $p , Wilcoxon) when the transducer executing asymmetric ARF was oriented across versus along the nephrons. The ratio of these PD values obtained with the transducer oriented across versus along the nephrons strongly linearly correlated ( $R^{2} = 0.95$ ) to the ratio of shear moduli measured by shear wave elasticity imaging. On the contrary, when a symmetric ARF was implemented, no significant difference in PD was observed ( $p > 0.01$ ). Similar results were demonstrated in vivo in the kidney allografts of 14 patients. The symmetric ARF produced PD measures with no significant difference ( $p > 0.01$ ) between along versus across alignments, but the asymmetric ARF yielded PD ratios that remained constant over a six-month observation period post-transplantation, consistent with stable serum creatinine level and urine protein-to-creatinine ratio in the same patient population ( $p> 0.01$ ). The results of this pilot in vivo clinical study suggest the feasibility of 1) implementing symmetrical ARF to obviate mechanical anisotropy in the kidney cortex when anisotropy is a confounding factor and 2) implementing asymmetric ARF to exploit mechanical anisotropy when mechanical anisotropy is a potentially relevant biomarker.
TL;DR: Experiments were performed to test an analytic model for the spatial averaging filter for a nonlinear focused beam and found it to underestimate incident acoustic pressure due to spatial averaging effects across the hydrophone sensitive element.
Abstract: Acoustic pressure can be measured with a hydrophone. Hydrophone measurements can underestimate incident acoustic pressure due to spatial averaging effects across the hydrophone sensitive element. The spatial averaging filter for a nonlinear focused beam is a low-pass filter that decreases monotonically from 1 to 0 as frequency increases from 0 to infinity. Experiments were performed to test an analytic model for the spatial averaging filter. Nonlinear pressure tone bursts were generated by three source transducers with driving frequencies ranging from 2.5 to 6 MHz, diameters ranging from 19 to 64 mm, and focal lengths ranging from 38 to 89 mm. The nonlinear pressure fields were measured using four needle hydrophones with nominal geometrical sensitive element diameters of 200, 400, 600, and $1000~\mu \text{m}$ . The average root-mean-square difference between theoretical and experimental spatial averaging filters was 5.8% ± 2.6%.
TL;DR: This paper starts from time-domain plane-wave imaging in 2-D and compares it to two algorithms in the f-k domain, coming from the Stolt migration in seismic imaging and the Lu theory of limited diffraction beams in medical imaging, and shows that the reconstruction schemes are generalized to 3-D imaging with matrix arrays.
Abstract: Time-domain plane-wave imaging (PWI) has recently emerged in medical imaging and is now taking to nondestructive testing (NDT) due to its ability to provide images of good resolution and contrast with only a few steered plane waves. Insonifying a medium with plane waves is a particularly interesting approach in 3-D imaging with matrix arrays because it allows to tremendously reduce the volume of data to be stored and processed as well as the acquisition time. However, even if the data volume is reduced with plane wave emissions, the image reconstruction in the time domain with a delay-and-sum algorithm is not sufficient to achieve low computation times in 3-D due to the number of voxels. Other reconstruction algorithms take place in the wavenumber–frequency ( f-k ) domain and have been shown to accelerate computation times in seismic imaging and in synthetic aperture radar. In this paper, we start from time-domain PWI in 2-D and compare it to two algorithms in the f-k domain, coming from the Stolt migration in seismic imaging and the Lu theory of limited diffraction beams in medical imaging. We then extend them to immersion testing configurations where a linear array is facing a plane water–steel interface. Finally, the reconstruction algorithms are generalized to 3-D imaging with matrix arrays. A comparison dwelling on image quality and algorithmic complexities is provided, as well as a theoretical analysis of the image amplitudes and the limits of each method. We show that the reconstruction schemes in the f-k domain improve the lateral resolution and offer a theoretical and numerical computation gain of up to 36 in 3-D imaging in a realistic NDT configuration.
TL;DR: The outstanding performance of the 1–3 KNNS-BNZH/epoxy composite transducer competitive to Pb-based transducers suggests that the lead-free material can serve as a promising alternative to PB-based piezoelectric materials for ultrasonic applications.
Abstract: In this paper, lead-free 0.965(K0.45Na0.55) (Nb0.96Sb0.04)O3-0.0375Bi0.5Na0.5Zr0.85Hf0.15O3 (KNNS-BNZH)/epoxy 1–3 composite was designed and fabricated with the dice-and-fill method. The composite material exhibited a high thickness electromechanical coupling coefficient ( $k_{t} = 0.7$ ), high piezoelectric constant ( $d_{33} = 350$ pC N−1), relatively low mechanical quality factor ( $Q_{m} = 5$ ), and relatively low acoustic impedance. An ultrasonic transducer with a center frequency of 5 MHz was produced based on the 1–3 KNNS-BNZH/epoxy composite, showing a broad bandwidth of 80% (−6 dB) and two-way insertion loss of −30 dB. Ultrasonic and photoacoustic images were further demonstrated. The outstanding performance of the 1–3 KNNS-BNZH/epoxy composite transducer competitive to Pb(Zr1– x Ti x )O3 (PZT)-based transducers suggests that the lead-free material can serve as a promising alternative to Pb-based piezoelectric materials for ultrasonic applications.
TL;DR: In this article, the A1 Lamb wave mode propagating in LiTaO3 at a Euler angle close to (0°, 42°, 0°) is proved to be suitable for high-frequency devices.
Abstract: Acoustic wave devices utilizing plate waves in thin lithium tantalate (LiTaO3) have the potential of achieving high resonance frequency with a suitable electromechanical coupling factor and a relatively small temperature coefficient of frequency (TCF). The influence of Euler angle and plate thickness on the characteristics of plate waves has been investigated. High-frequency resonators using first antisymmetric Lamb wave mode (A1) in (0°, 42°, 0°) LiTaO3 thin plates have been fabricated and optimized. The resonance frequency is as high as 5 GHz, with a relative bandwidth of 7.3% and an impedance ratio of 72 dB. Finally, the TCF of (0°, 42°, 0°) LiTaO3 has been evaluated. Therefore, it is proved that A1 Lamb wave mode propagating in LiTaO3 at a Euler angle close to (0°, 42°, 0°) is suitable for high-frequency devices.
TL;DR: It is suggested that log(VoA) enhances the differentiation of LRNC, IPH, COL, and CAL in human carotid plaques, in vivo, which is clinically relevant to improving stroke risk prediction and medical management.
Abstract: While in vivo acoustic radiation force impulse (ARFI)-induced peak displacement (PD) has been demonstrated to have high sensitivity and specificity for differentiating soft from stiff plaque components in patients with carotid plaque, the parameter exhibits poorer performance for distinguishing between plaque features with similar stiffness. To improve discrimination of carotid plaque features relative to PD, we hypothesize that signal correlation and signal-to-noise ratio (SNR) can be combined, outright or via displacement variance. Plaque feature detection by displacement variance, evaluated as the decadic logarithm of the variance of acceleration and termed “log(VoA),” was compared to that achieved by exploiting SNR, cross correlation coefficient, and ARFI-induced PD outcome metrics. Parametric images were rendered for 25 patients undergoing carotid endarterectomy, with spatially matched histology confirming plaque composition and structure. On average, across all plaques, log(VoA) was the only outcome metric with values that statistically differed between regions of lipid-rich necrotic core (LRNC), intraplaque hemorrhage (IPH), collagen (COL), and calcium (CAL). Further, log(VoA) achieved the highest contrast-to-noise ratio (CNR) for discriminating between LRNC and IPH, COL and CAL, and grouped soft (LRNC and IPH) and stiff (COL and CAL) plaque components. More specifically, relative to the previously demonstrated ARFI PD parameter, log(VoA) achieved 73% higher CNR between LRNC and IPH and 59% higher CNR between COL and CAL. These results suggest that log(VoA) enhances the differentiation of LRNC, IPH, COL, and CAL in human carotid plaques, in vivo , which is clinically relevant to improving stroke risk prediction and medical management.
TL;DR: A new microwave-induced thermoacoustic imaging system with non-contact, airborne ultrasound (US) detection and a piecewise synthetic aperture radar (SAR) algorithm modified for US imaging and enhanced with signal processing techniques is used for image reconstruction, resulting in mm-scale lateral and axial image resolution.
Abstract: Portable and easy-to-use imaging systems are in high demand for medical, security screening, nondestructive testing, and sensing applications. We present a new microwave-induced thermoacoustic imaging system with non-contact, airborne ultrasound (US) detection. In this system, a 2.7 GHz microwave excitation causes differential heating at interfaces with dielectric contrast, and the resulting US signal via the thermoacoustic effect travels out of the sample to the detector in air at a standoff. The 65 dB interface loss due to the impedance mismatch at the air-sample boundary is overcome with high-sensitivity capacitive micromachined ultrasonic transducers with minimum detectable pressures (MDPs) as low as ${278~\mu {\mathrm{ Pa}}_{\mathrm{ rms}}}$ and we explore two different designs—one operating at a center frequency of 71 kHz and another at a center frequency of 910 kHz. We further demonstrate that the air–sample interface presents a tradeoff with the advantage of improved resolution, as the change in wave velocity at the interface creates a strong focusing effect alongside the attenuation, resulting in axial resolutions more than $10\times $ smaller than that predicted by the traditional speed/bandwidth limit. A piecewise synthetic aperture radar (SAR) algorithm modified for US imaging and enhanced with signal processing techniques is used for image reconstruction, resulting in mm-scale lateral and axial image resolution. Finally, measurements are conducted to verify simulations and demonstrate successful system performance.
TL;DR: An interesting finding from this initial investigation is that the solid masses in the female breast, which appear as hypoechoic in the B-mode image, have similarly high coherence to that of surrounding tissue in coherence-based images.
Abstract: Ultrasound is frequently used in conjunction with mammography in order to detect breast cancer as early as possible. However, due largely to the heterogeneity of breast tissue, ultrasound images are plagued with clutter that obstructs important diagnostic features. Short-lag spatial coherence (SLSC) imaging has proven to be effective at clutter reduction in noisy ultrasound images. $M$ -Weighted SLSC and Robust-SLSC (R-SLSC) imaging were recently introduced to further improve image quality at higher lag values, while R-SLSC imaging has the added benefit of enabling the adjustment of tissue texture to produce a tissue signal-to-noise ratio (SNR) that is quantitatively similar to B-mode speckle SNR. This paper investigates the initial application of SLSC, $M$ -Weighted SLSC, and R-SLSC imaging to nine targets in the female breast [two simple cysts, one complicated cyst, two fibroadenomas, one hematoma, one complex cystic and solid mass, one invasive ductal carcinoma (IDC), and one ductal carcinoma in situ (DCIS)]. As expected, R-SLSC beamforming improves cyst and hematoma contrast by up to 6.35 and 1.55 dB, respectively, when compared to the original B-mode image, and similar improvements are achieved with SLSC and $M$ -Weighted SLSC imaging. However, an interesting finding from this initial investigation is that the solid masses (i.e., fibroadenoma, complex cystic and solid mass, IDC, and DCIS), which appear as hypoechoic in the B-mode image, have similarly high coherence to that of surrounding tissue in coherence-based images. This work holds promise for using SLSC, $M$ -Weighted SLSC, and/or R-SLSC imaging to distinguish between fluid-filled and solid hypoechoic breast masses.
TL;DR: A design flow for micromechanical RF channel-select filters with tiny fractional bandwidths capable of eliminating strong adjacent channel blockers directly after the antenna, hence reducing the dynamic range requirement of subsequent stages in an RF front-end is introduced.
Abstract: This Part I of two papers introduces a design flow for micromechanical RF channel-select filters with tiny fractional bandwidths capable of eliminating strong adjacent channel blockers directly after the antenna, hence reducing the dynamic range requirement of subsequent stages in an RF front-end. Much like VLSI transistor circuit design, the mechanical circuit design flow described herein is hierarchical with a design stack built upon vibrating micromechanical disk building blocks capable of ${Q}$ ’s exceeding 10 000 that enable low-filter passband loss for tiny fractional bandwidths. Array composites of half-wavelength coupled identical vibrating disks constitute a second level of hierarchy that reduces the filter termination impedance. A next level of hierarchy couples array composites with full-wavelength beams to affect fully balanced differential operation. Finally, identical differential blocks coupled with quarter-wavelength beams generate the desired passband. Part II of this study corroborates the efficacy of this design hierarchy via experimental results that introduce a 39-nm-gap capacitive transducer, voltage-controlled frequency tuning, and differential operation toward demonstration of a 0.1% bandwidth, 223.4-MHz channel-select filter with only 2.7 dB of in-band insertion loss and 50 dB of stopband rejection.
TL;DR: The results illustrated the effect of LIPUS on the dendritic structure, function, and neurotransmitter receptors, which may provide a powerful tool for treating neurodegenerative diseases.
Abstract: Plasticity of synaptic structure and function play an essential role in neuronal development, cognitive functions, and degenerative diseases. Recently, low-intensity pulsed ultrasound (LIPUS) stimulation has been reported as a promising technology for neuromodulation. However, the effect of LIPUS stimulation on the structural and functional synaptic plasticity in rat hippocampus has not yet been addressed. The aim of this study was to investigate whether LIPUS stimulation could affect the dendritic structure, electrophysiological properties, and expression level of glutamate receptors GluN2A, GluN2B, and GluR1 subunits in rat hippocampus. Transcranial LIPUS was delivered to CA1 of the intact hippocampus of rats ( $n = 40$ ) for 10 days (10 min/day) with the following parameters: fundamental frequency of 0.5 MHz, pulse repetition frequency (PRF) of 500 Hz, peak negative pressure of 0.42 MPa, and $I_{\mathrm {spta}}$ of 360 mW/cm2. The effect of LIPUS on dendritic structure, electrophysiological properties, and the expression of neurotransmitter receptors was measured using Golgi staining, electrophysiological recording, and western blotting, respectively. Golgi staining and electrophysiological recordings showed that LIPUS stimulation significantly increased the density of dendritic spines (0.72 ± 0.17 versus 0.94 ± 0.19 spines/ $\mu \text{m}$ , $p ) and the frequency of spontaneous excitatory postsynaptic current (0.37 ± 0.14 versus 1.77 ± 0.37 Hz, $p ) of CA1 hippocampal neurons. Furthermore, the western blotting analysis demonstrated a significant increase in the expression level of GluN2A ( $p ). The results illustrated the effect of LIPUS on the dendritic structure, function, and neurotransmitter receptors, which may provide a powerful tool for treating neurodegenerative diseases.
TL;DR: A dedicated method for uterine-motion quantification by B-mode transvaginal ultrasound is proposed, and promising results motivate toward an extended validation in the context of fertilization procedures.
Abstract: Fertility problems are nowadays being paralleled by important advances in assisted reproductive technologies. Yet the success rate of these technologies remains low. There is evidence that fertilization outcome is affected by uterine motion, but solutions for quantitative analysis of uterine motion are lacking. This work proposes a dedicated method for uterine-motion quantification by B-mode transvaginal ultrasound. Motion analysis is implemented by speckle tracking based on block matching after speckle-size regularization. Sum of absolute differences is the adopted matching metrics. Prior to the analysis, dedicated singular value decomposition (SVD) filtering is implemented to enhance the uterine motion over noise, clutter, and uncorrelated motion induced by neighboring organs and probe movements. SVD and block matching are first optimized by a dedicated ex vivo setup. Robustness to noise and speckle decorrelation is improved by median filtering of the tracking coordinates from surrounding blocks. Speckle tracking is further accelerated by a diamond search. The method feasibility was tested in vivo with a longitudinal study on nine women, aimed at discriminating between four selected phases of the menstrual cycle known to show different uterine behavior. Each woman was scanned in each phase for 4 min; four sites on the uterine fundus were tracked over time to extract strain and distance signals along the longitudinal and transversal directions of the uterus. Several features were extracted from these signals. Among these features, median frequency and contraction frequency showed significant differences between active and quiet phases. These promising results motivate toward an extended validation in the context of fertilization procedures.