Ultrasound contrast plane wave imaging

Principle of contrast-enhanced ultrasonography

Sonazoid, an ultrasound contrast agent, has been covered by insurance in Japan since January 2007 for the diagnosis of hepatic mass lesions and is widely used for diagnosing not only primary liver cancer but also liver metastases such as those from breast cancer and colorectal cancer. Contrast-enhanced ultrasound for breast mass lesions has been covered by insurance since August 2012 after phase II and phase III clinical trials showed that the diagnostic performance was significantly superior to that of B-mode and contrast-enhanced magnetic resonance imaging. This paper describes the principles of imaging techniques in contrast-enhanced ultrasonography including the filter, pulse inversion, amplitude modulation, and amplitude-modulated pulse inversion methods. The pulse inversion method, which visualizes the second-harmonic component using the nonlinear scattering characteristics of the contrast agent, is widely used regardless of the contrast agent and target organ because of its high resolution. Sonazoid has a stiffer shell and requires a higher acoustic amplitude than Sonovue to generate nonlinear vibrations. The higher transmitted sound pressure generates more tissue harmonic components. Since pulse inversion allows visualization of the tissue harmonic components, amplitude modulation and amplitude-modulated pulse inversion, which include few tissue harmonic components, are primarily used. Amplitude modulation methods detect nonlinear signals from the contrast agent in the fundamental band. The mechanism of the amplitude modulation is considered to be changes in the echo signal's phase depending on the sound pressure. Since the tissue-derived component is minor in amplitude modulation methods, good contrast sensitivity can be obtained.

Multi-Scale Volumetric Dynamic Optoacoustic and Laser Ultrasound (OPLUS) Imaging Enabled by Semi-Transparent Optical Guidance

Major biological discoveries are made by interrogating living organisms with light. However, the limited penetration of un-scattered photons within biological tissues limits the depth range covered by optical methods. Deep-tissue imaging is achieved by combining light and ultrasound. Optoacoustic imaging exploits the optical generation of ultrasound to render high-resolution images at depths unattainable with optical microscopy. Recently, laser ultrasound has been suggested as a means of generating broadband acoustic waves for high-resolution pulse-echo ultrasound imaging. Herein, an approach is proposed to simultaneously interrogate biological tissues with light and ultrasound based on layer-by-layer coating of silica optical fibers with a controlled degree of transparency. The time separation between optoacoustic and ultrasound signals collected with a custom-made spherical array transducer is exploited for simultaneous 3D optoacoustic and laser ultrasound (OPLUS) imaging with a single laser pulse. OPLUS is shown to enable large-scale anatomical characterization of tissues along with functional multi-spectral imaging of chromophores and assessment of cardiac dynamics at ultrafast rates only limited by the pulse repetition frequency of the laser. The suggested approach provides a flexible and scalable means for developing a new generation of systems synergistically combining the powerful capabilities of optoacoustics and ultrasound imaging in biology and medicine.

rtPA-loaded fucoidan polymer microbubbles for the targeted treatment of stroke

Systemic injection of thrombolytic drugs is the gold standard treatment for non-invasive blood clot resolution. The most serious risks associated with the intravenous injection of tissue plasminogen activator-like proteins are the bleeding complication and the dose related neurotoxicity. Indeed, the drug has to be injected in high concentrations due to its short half-life, the presence of its natural blood inhibitor (PAI-1) and the fast hepatic clearance (0.9 mg/kg in humans, 10 mg/kg in mouse models). Overall, there is a serious need for a dose-reduced targeted treatment to overcome these issues. We present in this article a new acoustic cavitation-based method for polymer MBs synthesis, three times faster than current hydrodynamic-cavitation method. The generated MBs are ultrasound responsive, stable and biocompatible. Their functionalization enabled the efficient and targeted treatment of stroke, without side effects. The stabilizing shell of the MBs is composed of Poly-Isobutyl Cyanoacrylate (PIBCA), copolymerized with fucoidan. Widely studied for its targeting properties, fucoidan exhibit a nanomolar affinity for activated endothelium and activated platelets (P-selectins). Secondly, the thrombolytic agent (rtPA) was loaded onto microbubbles (MBs) with a simple adsorption protocol. Hence, the present study validated the in vivo efficiency of rtPA-loaded Fuco MBs to be over 50 % more efficient than regular free rtPA injection for stroke resolution. In addition, the relative injected rtPA grafted onto targeting MBs was 1/10th of the standard effective dose (1 mg/kg in mouse). As a result, no hemorrhagic event, BBB leakage nor unexpected tissue distribution were observed.

Higher Order Singular Value Decomposition Filter for Contrast Echocardiography

Assessing the coronary circulation with contrast-enhanced echocardiography has high clinical relevance. However, it is not being routinely performed in clinical practice because the current clinical tools generally cannot provide adequate image quality. The contrast agent's visibility in the myocardium is generally poor, impaired by motion and nonlinear propagation artifacts. The established multipulse contrast schemes (MPCSs) and the more experimental singular value decomposition (SVD) filter also fall short to solve these issues. Here, we propose a scheme to process amplitude modulation/amplitude-modulated pulse inversion (AM/AMPI) echoes with higher order SVD (HOSVD) instead of conventionally summing the complementary pulses. The echoes from the complementary pulses form a separate dimension in the HOSVD algorithm. Then, removing the ranks in that dimension with dominant coherent signals coming from tissue scattering would provide the contrast detection. We performed both in vitro and in vivo experiments to assess the performance of our proposed method in comparison with the current standard methods. A flow phantom study shows that HOSVD on AM pulsing exceeds the contrast-to-background ratio (CBR) of conventional AM and an SVD filter by 10 and 14 dB, respectively. In vivo porcine heart results also demonstrate that, compared to AM, HOSVD improves CBR in open-chest acquisition (up to 19 dB) and contrast ratio (CR) in closed-chest acquisition (3 dB).

Superharmonic and Microultrasound Imaging With Plane Wave Beamforming Techniques

Superharmonic contrast imaging (SpHI) suppresses tissue clutter and allows high-contrast visualization of the vasculature. An array-based dual-frequency (DF) probe has been developed for SpHI, integrating a 21-MHz, 256-element microultrasound imaging array with a 2-MHz, 32-element array to take advantage of the broadband nonlinear responses from microbubble (MB) contrast agents. In this work, ultrafast imaging with plane waves was implemented for SpHI to increase the acquisition frame rate. Ultrafast imaging was also implemented for microultrasound B-mode imaging (HFPW B-mode) to enable high-resolution visualization of the tissue structure. Coherent compounding was demonstrated in vitro and in vivo in both imaging modes. Acquisition frame rates of 4.5 kHz and 187 Hz in HFPW B-mode imaging were achieved for imaging up to 21 mm with one and 25 angles, respectively, and 3.5 kHz and 396 Hz in the SpHI mode with one and nine coherently compounded angles, respectively. SpHI images showed suppression of tissue clutter prior to and after the introduction of MBs in vitro and in vivo. The nine-angle coherently compounded 2-D SpHI images of contrast-filled flow channel showed a contrast-to-tissue ratio (CTR) of 26.0 dB, a 2.5-dB improvement relative to images reconstructed from 0° steering. Consistent with in vitro imaging, the nine-angle compounded 2-D SpHI of a Lewis lung cancer tumor showed a 2.6-dB improvement in contrast enhancement, relative to 0° steering, and additionally revealed a region of nonviable tissue. The 3-D display of the volumetric SpHI data acquired from a xenograft mouse tumor using both 0° steering and nine-angle compounding allowed the visualization of the tumor vasculature. A small vessel visible in the compounded SpHI image, measuring around [Formula: see text], is not visualized in the 0° steering SpHI image, demonstrating the superiority of the latter in detecting fine structures within the tumor.

A two-dimensional angular interpolation based on radial basis functions for high frame rate ultrafast imaging

To solve the problem of reduced image quality in plane wave imaging (PWI), coherent plane wave compounding (CPWC) has been introduced, based on a combination of plane wave images from several directions (i.e., with different angles). However, the number of angles needed to reach a reasonable image quality affects the maximum achievable frame rate in CPWC. In this study, we suggest reducing the tradeoff between the image quality and the frame rate in CPWC by employing two-dimensional (2D) interpolation based on radial basis functions. More specifically, we propose constructing a three-dimensional spatio-angular structure to integrate both spatial and angular information into the reconstruction prior to 2D interpolation. The rationale behind our proposal is to reduce the number of transmissions and then apply the 2D interpolation along the angle dimension to reconstruct the missing information corresponding to the angles not selected for CPWC imaging. To evaluate the proposed technique, we applied it to the PWI challenges in the medical ultrasound database. Results show that we can achieve 3× to 4× improvement in frame rate while maintaining acceptable image quality compared to the case of using all the angles.

Ultra-fast ultrasound blood flow velocimetry for carotid artery with deep learning

Accurate measurement of blood flow velocity is important for the prevention and early diagnosis of atherosclerosis. However, due to the uncertainty of parameter settings, the autocorrelation velocimetry methods based on clutter filtering are prone to incorrectly filter out the near-wall blood flow signal, resulting in poor velocimetric accuracy. In addition, the Doppler coherent compounding acts as a low-pass filter, which also leads to low values of blood flow velocity estimated by the above methods. Motivated by this status quo, here we propose a deep learning estimator that combines clutter filtering and blood flow velocimetry based on the adaptive property of one-dimensional convolutional neural network (1DCNN). The estimator is operated by first extracting the blood flow signal from the original Doppler echo signal through an affine transformation of the 1D convolution, and then converting the extracted signal into the desired blood flow velocity using a linear transformation function. The effectiveness of the proposed method is verified by simulation as well as in vivo carotid artery data. Compared with typical velocimetry methods such as high-pass filtering (HPF) and singular value decomposition (SVD), the results show that the normalized root means square error (NRMSE) obtained by 1DCNN is reduced by 54.99 % and 53.50 % for forward blood flow velocimetry, and 70.99 % and 69.50 % for reverse blood flow velocimetry, respectively. Consistently, the in vivo measurements demonstrate that the goodness-of-fit of the proposed estimator is improved by 8.72 % and 4.74 % for five subjects. Moreover, the estimation time consumed by 1DCNN is greatly reduced, which costs only 2.91 % of the time of HPF and 12.83 % of the time of SVD. In conclusion, the proposed estimator is a better alternative to the current blood flow velocimetry, and is capable of providing more accurate diagnosis information for vascular diseases in clinical applications.

Contrast-Enhanced Imaging of Histotripsy Bubble Clouds Using Chirp-Coded Excitation and Volterra Filtering

Histotripsy is a focused ultrasound therapy that ablates tissue via bubble cloud activity. Real-time ultrasound image guidance is used to ensure safe and effective treatment. Plane-wave imaging enables tracking of histotripsy bubble clouds at a high frame rate but lacks adequate contrast. Furthermore, bubble cloud hyperechogenicity is reduced in abdominal targets, making the development of contrast-specific sequences for deep-seated targets an active area of research. Chirp-coded subharmonic imaging was reported previously to enhance histotripsy bubble cloud detection by a modest 4-6 dB compared to the conventional sequence. Incorporating additional steps into the signal processing pipeline could enhance bubble cloud detection and tracking. In this study, we evaluated the feasibility of combining chirp-coded subharmonic imaging with Volterra filtering for enhancing bubble cloud detection in vitro. Chirped imaging pulses were used to track bubble clouds generated in scattering phantoms at a 1-kHz frame rate. Fundamental and subharmonic matched filters were applied to the received radio frequency signals, followed by a tuned Volterra filter to extract bubble-specific signatures. For subharmonic imaging, the application of the quadratic Volterra filter improved the contrast-to-tissue ratio from 5.18 ± 1.29 to 10.90 ± 3.76 dB, relative to the application of the subharmonic matched filter. These findings demonstrate the utility of the Volterra filter for histotripsy image guidance.

Frame rate effects and their compensation on super-resolution microvessel imaging using ultrasound localization microscopy

Ultrasound localization microscopy (ULM) breaks the diffraction limit and allows imaging microvasculature at micrometric resolution while preserving the penetration depth. Frame rate plays an important role for high-quality ULM imaging, but there is still a lack of review and investigation of the frame rate effects on ULM. This work aims to clarify how frame rate influences the performance of ULM, including the effects of microbubble detection, localization and tracking. The performance of ULM was evaluated using an in vivo rat brain dataset (15.6 MHz, 3 tilted plane waves (-5°, 0°, +5°), at a compounded frame rate of 1000 Hz) with different frame rates. Quantification methods, including Fourier ring correlation and saturation parameter, were applied to analyze the spatial resolution and reconstruction efficiency, respectively. In addition, effects on each crucial step in ULM processing were further analyzed. Results showed that when frame rates dropped from 1000 Hz to 250 Hz, the spatial resolution deteriorated from 9.9 μm to 15.0 μm. Applying a velocity constraint was able to improve the ULM performance, but inappropriate constraint may artificially result in high apparent resolution. For the dataset, compared with the results of 1000 Hz frame rate, the velocity was underestimated at 100 Hz with 47.18% difference and the saturation was reduced from 55.00% at 1000 Hz to 43.34% at 100 Hz. Analysis showed that inadequate frame rate generated unreliable microbubble detection, localization and tracking as well as incomplete track reconstruction, resulting in the deterioration in spatial resolution, the underestimation in velocity measurement and the decrease in saturation. Finally, a guidance of determining the frame rate requirement was discussed by considering the required spatial sampling points based on vessel morphology, clutter filtering method, tracking algorithm and acquisition time, which provides indications for future clinical application of ULM method.

Fast Beamforming Method for Plane Wave Compounding Based on Beamspace Adaptive Beamformer and Delay-Multiply-and-Sum

OBJECTIVE

Although the use of coherent plane wave compounding is a promising technique for enabling the attainment of very high frame rate imaging, it achieves relatively low image quality because of data-independent reconstruction. Adaptive beamformers rather than delay-and-sum (DAS) conventional techniques have been proposed to improve the imaging quality. The minimum variance (MV) and delay-multiply-and-sum (DMAS) beamformers have been validated as effective in improving image quality. The MV improves mainly the resolution of the image, while being computationally expensive and having little impact on contrast. The DMAS increases the contrast while over-suppressing the speckle region in the case of 2-D summation for multi-transmission applications.

METHODS

In a new approach, a beamformer based on MV and DMAS is proposed to enhance both spatial resolution and contrast in plane wave imaging. Prior to estimating the weight vector of MV, the backscattered echoes are decorrelated without any spatial smoothing. This enhances the robustness of MV without compromising the improvement in resolution. With a shift from element space to beamspace, MV weights are calculated using the spatial statistics of a set of orthogonal beams, which allows the high-complexity algorithm to be run faster. After that, the MV weights are applied to the DMAS output vector beamformed over different transmissions.

DISCUSSION AND CONCLUSION

The proposed method can result in better contrast resolution, thereby avoiding over-suppression. The complexity of the applied DMAS version is also similar to that of DAS. Imaging results reveal that the proposed method offers improvements over the traditional compounding method in terms of spatial and contrast resolution. It also can achieve a higher image quality compared with some existing adaptive methods applied in the literature.

Ultrasonic imaging using conditional generative adversarial networks

The Full Matrix Capture (FMC) and Total Focusing Method (TFM) combination is often considered the gold standard in ultrasonic nondestructive testing, however it may be impractical due to the amount of time required to gather and process the FMC, particularly for high cadence inspections. This study proposes replacing conventional FMC acquisition and TFM processing with a single zero-degree plane wave (PW) insonification and a conditional Generative Adversarial Network (cGAN) trained to produce TFM-like images. Three models with different cGAN architectures and loss formulations were tested in different scenarios. Their performances were compared with conventional TFM computed from FMC. The proposed cGANs were able to recreate TFM-like images with the same resolution while improving the contrast in more than 94% of the reconstructions in comparison with conventional TFM reconstructions. Indeed, thanks to the use of a bias in the cGANs' training, the contrast was systematically increased through a reduction of the background noise level and the elimination of some artifacts. Finally, the proposed method led to a reduction of the computation time and file size by a factor of 120 and 75, respectively.

High frame-rate contrast enhanced ultrasound (HIFR-CEUS) in the characterization of small hepatic lesions in cirrhotic patients

BACKGROUND

To show the effectiveness of plane wave HighFrame-Rate CEUS (HiFR-CEUS) compared with "conventional" (plane wave) CEUS (C-CEUS) in the characterization of small (< 2 cm) focal liver lesions (FLLs) not easily detected by CT in cirrhotic patients. HiFR-CEUS exploit an ultra-wideband nonlinear process to combine fundamental, second and higher-order harmonic signals generated by ultrasound contrast agents to increase the frame rate. C-CEUS is limited by the transmission principle, and its frame-rate is around 10 FPS. With HiFR-CEUS (Shenzhen Mindray Bio-Medical Electronics Co., China), the frame-rate reached 60 FPS.

MATERIAL AND METHODS

Ultrasound detected small FLLs (< 2 cm) in 63 cirrhotic patients during follow-up (June 2019-February 2020); (7 nodules < 1 cm and were not evaluable by spiral CT). Final diagnosis was obtained with MRI (47) or fine needle aspiration (16 cases) C-CEUS was performed and HiFR-CEUS was repeated after 5 min; 0.8-1.2 ml of contrast media (SonoVue, Bracco, Italy) was used. 57 nodules were better evaluable with HiFR-CEUS; 6 nodules were equally evaluable by both techniques; final diagnosis was: 44 benign lesions (29 hemangiomas, 1 amartoma, 2 hepatic cysts; 2 focal nodular hyperplasias, 3 regenerative macronodules, 3 AV-shunts, 3 hepatic sparing areas and 1 focal steatosis) and 19 malignant one (17 HCCs, 1 cholangioca, 1 metastasis); statistical evaluation for better diagnosis with X2 test (SPSS vers. 26); we used LI-RADS classification for evaluating sensitivity, specificity PPV, NPV and diagnostic accuracy of C- and HFR-CEUS. Corrispective AU-ROC were calculated.

RESULTS

C-CEUS and HiFR-CEUS reached the same diagnosis in 29 nodules (13 nodules > 1 < 1.5 cm; 16 nodules > 1.5 < 2 cm); HiFR-CEUS reached a correct diagnosis in 32 nodules where C-CEUS was not diagnostic (6 nodules < 1 cm; 17 nodules > 1 < 1.5 cm; 9 nodules > 1.5 < 2 cm); C-CEUS was better in 2 nodules (1 < 1 cm and 1 > 1 < 1.5 cm). Some patient's (sex, BMI, age) and nodule's characteristics (liver segment, type of diagnosis, nodule's dimensions (p = 0.65)) were not correlated with better diagnosis (p ns); only better visualization (p 0.004) was correlated; C-CEUS obtained the following LI-RADS: type-1: 18 Nodules, type-2: 21; type-3: 7, type-4: 7; type-5: 8; type-M: 2; HiFR-CEUS: type-1: 38 Nodules, type-2: 2; type-3:4, type-4: 2; type-5: 15; type-M: 2; In comparison with final diagnosis: C-CEUS: TP: 17; TN: 39; FP: 5; FN:2; HIFR-CEUS: TP: 18; TN: 41; FP: 3; FN:1; C-CEUS: sens: 89.5%; Spec: 88.6%, PPV: 77.3%; NPV: 95.1%; Diagn Acc: 88.6% (AU-ROC: 0.994 ± SEAUC: 0.127; CI: 0.969-1.019); HiHFR CEUS: sens: 94.7%; Spec: 93.2%, PPV: 85.7%; NPV: 97.6%; Diagn Acc: 93.2% (AU-ROC: 0.9958 ± SEAUC: 0.106; CI: 0.975-1.017) FLL vascularization in the arterial phase was more visible with HiFR-CEUS than with C-CEUS, capturing the perfusion details in the arterial phase due to a better temporal resolution. With a better temporal resolution, the late phase could be evaluated longer with HiFR-CEUS (4 min C-CEUS vs. 5 min HiFR-CEUS).

CONCLUSION

Both C-CEUS and HIFR-CEUS are good non invasive imaging system for the characterization of small lesions detected during follow up of cirrhotic patients. HiFR-CEUS allowed better FLL characterization in cirrhotic patients with better temporal and spatial resolution capturing the perfusion details that cannot be easily observed with C-CEUS.

A Single-Shot Harmonic Imaging Approach Utilizing Deep Learning for Medical Ultrasound

Tissue Harmonic Imaging (THI) is an invaluable tool in clinical ultrasound owing to its enhanced contrast resolution and reduced reverberation clutter in comparison to fundamental mode imaging. However, harmonic content separation based on high pass filtering suffers from potential contrast degradation or lower axial resolution due to spectral leakage. Whereas nonlinear multi-pulse harmonic imaging schemes, such as amplitude modulation and pulse inversion, suffer from a reduced framerate and comparatively higher motion artifacts due to the necessity of at least two pulse echo acquisitions. To address this problem, we propose a deep-learning-based single-shot harmonic imaging technique capable of generating comparable image quality to pulse amplitude modulation methods, yet at a higher framerate and with fewer motion artifacts. Specifically, an asymmetric convolutional encoder-decoder structure is designed to estimate the combination of the echoes resulting from the half-amplitude transmissions using the echo produced from the full amplitude transmission as input. The echoes were acquired with the checkerboard amplitude modulation technique for training. The model was evaluated across various targets and samples to illustrate generalizability as well as the possibility and impact of transfer learning. Furthermore, for possible interpretability of the network, we investigate if the latent space of the encoder holds information on the nonlinearity parameter of the medium. We demonstrate the ability of the proposed approach to generate harmonic images with a single firing that are comparable to those from a multi-pulse acquisition.

High Frame Rate Ultrasound Imaging by Means of Tensor Completion: Application to Echocardiography

High frame rate ultrasound (US) imaging enables the monitoring of fast-moving organs. In echocardiography, this is especially needed due to the existence of rapidly moving structures, such as the heart valves. In the last two decades, various methods have been proposed to improve the frame rate. Here, we propose a novel method, based on binary coding patterns (BCPs) and tensor completion (TC), to increase the temporal resolution (i.e., frame rate) in the preprocessing stage of conventional focused ultrasound imaging (CFUI). The rationale behind our proposal is to perform, at first, the beamforming of a fraction of the scan lines, randomly selected in each frame based on BCP. Then, we reconstruct the missing scan lines through TC. The latter is an effective technique for recovering missing information from a low-rank tensor, based on a small number of observations using rank minimization. Following our approach, reducing the transmissions events needed to generate an image, the frame rate is increased by the same proportion. We have applied the proposed technique to a pre-beamformed radio frequency (RF) echocardiographic dataset. Our results show that we can improve the frame rate by a factor from 3 to 4, while keeping the structural similarity (SSIM) of the reconstructed tensor and the original one at values higher than 0.98.

Contrast-Enhanced Ultrasound with Optimized Aperture Patterns and Bubble Segmentation Based on Echo Phase

Amplitude modulation (AM) suppresses tissue signals and detects microbubble signals in contrast-enhanced ultrasound (CEUS) and is often implemented with checkerboard apertures. However, possible crosstalk between transmitting and non-transmitting array elements may compromise tissue suppression in AM. Using AM aperture patterns other than the conventional checkerboard approach (one on, one off) may reduce the degree of crosstalk and increase the contrast-to-tissue-ratio (CTR) compared with conventional AM. Furthermore, previous studies have reported that the phase difference between the echoes in AM pulsing sequences may be used to segment tissue and microbubbles and improve tissue signal suppression and the CTR of CEUS images. However, the CTR of the image produced by alternative AM aperture patterns and the effect of segmentation approach on these alternative apertures have not been investigated. We evaluated a number of AM aperture patterns to find an optimal AM aperture pattern that provides the highest CTR. We found that the aperture that uses alternating groups of two elements, AM2, had the highest CTR for the probe evaluated. In addition, a segmentation technique based on echo phase differences (between the full and half-pulses, ΔΦAM, between the complementary half-pulses, ΔΦhalf, and the maximum of the two ΔΦmax) was also considered in the AM aperture optimization process. The segmentation approach increases the CTR by about 25 dB for all apertures. Finally, AM2 segmented with ΔΦmax had a 7-dB higher CTR in a flow phantom and a 6-dB higher contrast in a perfused pig liver than conventional AM segmented with ΔΦAM, and it is the optimal transmit aperture design.

Novel spatio-temporal non-linear beamformers for sparse synthetic aperture ultrasound imaging

The development of two modified non-linear beamformers, Spatio-Temporal Delay Multiply and Sum (ST-DMAS) and Spatio-Temporal Delay Euclidian-Weighted Multiply and Sum (ST-DewMAS) is reported in this paper. A sparse-transmit scheme (with only 8 transmits) on Synthetic Transmit Aperture technique (sparse STA) was chosen to evaluate the beamformers ability to generate the high-resolution Ultrasound image. These methods allow for obtaining superior-quality imaging at enhanced frame rates. The different beamformers of ST-DewMAS, ST-DMAS, Filtered Delay Multiply and Sum (F-DMAS), and Delay and Sum (DAS), were compared in terms of the Axial and Lateral Resolutions, AR and LR, respectively, Contrast-to-Noise Ratio (CNR), Contrast Ratio (CR), and Generalized CNR (GCNR). Experimental results demonstrate that the developed ST-DMAS and ST-DewMAS reconstruction on sparse STA technique resulted in better quality images compared to those obtained using DAS and F-DMAS. Specifically, the metrics of AR, LR CR, CNR, and GCNR showed improvements of more than 25% (for ST-DMAS) and 40 % (for ST-DewMAS) over those from DAS and F-DMAS beamformed images, respectively. Thus, the results demonstrate that the frame rate and image quality of an US system can both be enhanced by ST-DewMAS compared to the beamformers of F-DMAS and DAS.

Adaptive beamforming based on minimum variance (ABF-MV) using deep neural network for ultrafast ultrasound imaging

Ultrafast ultrasound imaging can achieve high frame rate by emitting planewave (PW). However, the image quality is drastically degraded in comparison with traditional scanline focused imaging. Using adaptive beamforming techniques can improve image quality at cost of real-time performance. In this work, an adaptive beamforming based on minimum variance (ABF-MV) with deep neural network (DNN) is proposed to improve the image performance and to speed up the beamforming process of ultrafast ultrasound imaging. In particular, a DNN, with a combination architecture of fully-connected network (FCN) and convolutional autoencoder (CAE), is trained with channel radio-frequency (RF) data as input while minimum variance (MV) beamformed data as ground truth. Conventional delay-and-sum (DAS) beamformer and MV beamformer are utilized for comparison to evaluate the performance of the proposed method with simulations, phantom experiments, and in-vivo experiments. The results show that the proposed method can achieve superior resolution and contrast performance, compared with DAS. Moreover, it is remarkable that both in theoretical analysis and implementation, our proposed method has comparable image quality, lower computational complexity, and faster frame rate, compared with MV. In conclusion, the proposed method has the potential to be deployed in ultrafast ultrasound imaging systems in terms of imaging performance and processing time.

Optimized acoustic streaming generated at oblique incident angles to improve ultrasound thrombolysis effect

BACKGROUND

Combined with thrombolytic drugs and/or microbubbles (MBs), ultrasound (US) has been regarded as a useful tool for thrombolysis treatment by taking its advantages of non-invasive, non-ionization, low cost and accurate targeting of tissues deep in body. Recently, low-intensity pulsed ultrasound (LIPUS), which can cause fewer complications by stable cavitation and acoustic streaming other than more violent effects, has attracted broad attention.

PURPOSE

However, the thrombolysis effect in practice might not achieve expectation because there is not an ideal parallel multilayer structure between the skin and the targeted vessel. Therefore, the current work aims to better elucidate the influence of US incident angle on the generation of acoustic streaming and thrombolysis effect.

METHODS

Systemic numerical and experimental studies, viz., finite element modeling (FEM), particle image velocimetry (PIV) and in vitro thrombolysis measurements, were performed to estimate the acoustical/streaming field pattern, maximum flow velocity and shear stress on the surface of thrombus, as well as the lysis rate generated at different conditions. These methods aim at verifying the hypothesis that streaming-induced vortices can further accelerate the dissolution of the thrombus and optimized thrombolysis effected can be achieved by adjusting US incident angles.

RESULTS

The pool data results showed that the variation trends of the flow velocity and shear stress obtained from FEM simulation and PIV experiments are qualitatively consistent with each other. There exists an optimal incident angle that can maximize the flow velocity and shear stress on the surface of thrombus, so that superior stirring and mixing effect can be generated. Furthermore, as the flow velocity and shear stress on thrombus surface are both highly correlated with the thrombolysis effect (the correlation coefficient R1 = 0.988, R2 = 0.958, respectively), the peak value of lysis rate (increase by at least 5.02%) also occurred at 10°.

CONCLUSIONS

The current results demonstrated that, with appropriately determined incident angle, higher thrombolysis rate could be achieved without increasing the driving pressure. It may shed the light on future US thrombolysis planning strategy that, if combined with other advanced technologies (e.g., machine-learning-based image analysis and image-guided adaptive US emission modulation), more efficient thrombolytic effect could be realized while minimizing undesired side-effects caused by excessively high pressure. This article is protected by copyright. All rights reserved.

Improving Image Quality for Single-Angle Plane Wave Ultrasound Imaging With Convolutional Neural Network Beamformer

Ultrafast ultrasound imaging based on plane wave (PW) compounding has been proposed for use in various clinical and preclinical applications, including shear wave imaging and super resolution blood flow imaging. Because the image quality afforded by PW imaging is highly dependent on the number of PW angles used for compounding, a tradeoff between image quality and frame rate occurs. In the present study, a convolutional neural network (CNN) beamformer based on a combination of the GoogLeNet and U-Net architectures was developed to replace the conventional delay-and-sum (DAS) algorithm to obtain high-quality images at a high frame rate. RF channel data are used as the inputs for the CNN beamformers. The outputs are in-phase and quadrature data. Simulations and phantom experiments revealed that the images predicted by the CNN beamformers had higher resolution and contrast than those predicted by conventional single-angle PW imaging with the DAS approach. In in vivo studies, the contrast-to-noise ratios (CNRs) of carotid artery images predicted by the CNN beamformers using three or five PWs as ground truths were approximately 12 dB in the transverse view, considerably higher than the CNR obtained using the DAS beamformer (3.9 dB). Most tissue speckle information was retained in the in vivo images produced by the CNN beamformers. In conclusion, only a single PW at 0° was fired, but the quality of the output image was proximal to that of an image generated using three or five PW angles. In other words, the quality-frame rate tradeoff of coherence compounding could be mitigated through the use of the proposed CNN for beamforming.

Investigation of the Phase of Nonlinear Echoes From Microbubbles During Amplitude Modulation

Contrast-enhanced ultrasound (CEUS) imaging relies on distinguishing between microbubble and tissue echoes. Amplitude modulation (AM), a nonlinear pulsing scheme, has been developed to take advantage of the amplitude-dependent nonlinearity of microbubble echoes. However, with AM, tissue nonlinear propagation can also degrade the image contrast. Segmentation of CEUS images based on amplitude-dependent phase difference in the echoes, defined in this article as [Formula: see text], has been proposed as an additional method of enhancing contrast-to-tissue ratio as tissue is not expected to create the same degree of [Formula: see text]; however, this has not been robustly investigated. In this work, we evaluate the source of [Formula: see text] through simulations of unshelled versus shelled microbubble oscillation and simulations of nonlinear propagation in tissue. We then validate the simulated [Formula: see text] results with experimental [Formula: see text] measurements during in vitro scattering and imaging in a flow phantom. We show that shelled and unshelled microbubbles resulted in a [Formula: see text] with similar overall magnitude with some differences in trends, and that tissue echoes have a small yet detectable degree of [Formula: see text] due to nonlinear propagation. The results from this work can help inform optimal parameter selection for phase segmentation and implementation on a clinical scanner.

Histotripsy Bubble Cloud Contrast With Chirp-Coded Excitation in Preclinical Models

Histotripsy is a focused ultrasound therapy for tissue ablation via the generation of bubble clouds. These effects can be achieved noninvasively, making sensitive and specific bubble imaging essential for histotripsy guidance. Plane-wave ultrasound imaging can track bubble clouds with an excellent temporal resolution, but there is a significant reduction in echoes when deep-seated organs are targeted. Chirp-coded excitation uses wideband, long-duration imaging pulses to increase signals at depth and promote nonlinear bubble oscillations. In this study, we evaluated histotripsy bubble contrast with chirp-coded excitation in scattering gel phantoms and a subcutaneous mouse tumor model. A range of imaging pulse durations were tested, and compared to a standard plane-wave pulse sequence. Received chirped signals were processed with matched filters to highlight components associated with either fundamental or subharmonic (bubble-specific) frequency bands. The contrast-to-tissue ratio (CTR) was improved in scattering media for subharmonic contrast relative to fundamental contrast (both chirped and standard imaging pulses) with the longest-duration chirped-pulse tested (7.4 [Formula: see text] pulse duration). The CTR was improved for subharmonic contrast relative to fundamental contrast (both chirped and standard imaging pulses) by 4.25 dB ± 1.36 dB in phantoms and 3.84 dB ± 6.42 dB in vivo. No systematic changes were observed in the bubble cloud size or dissolution rate between sequences, indicating image resolution was maintained with the long-duration imaging pulses. Overall, this study demonstrates the feasibility of specific histotripsy bubble cloud visualization with chirp-coded excitation.

3-D Ultrafast Ultrasound Imaging of Microbubbles Trapped Using an Acoustic Vortex

Increasing the local concentration of microbubbles (MBs) within the blood flow plays a crucial role in several medical applications, but there are few imaging modalities available for volumetric tracking of the aggregated MBs in real time. Here we describe a device integrating acoustic vortex tweezers (AVTs) and ultrasound plane-wave imaging (PWI) to achieve the goal of controlling the spatial distribution of MBs in blood vessels and simultaneously monitoring this process using the same probe. Experiments were conducted using a 5-MHz 2-D array ultrasound probe (with three cycles of excitation at an acoustic pressure of 2000 kPa) and 1.2- [Formula: see text]-diameter MBs at a flow rate of 20 mm/s. The AVT waveform was produced by modulating the repetition frequency of the transmitted pulse asymmetrically (4 and 8 kHz at the inflow and outflow ends, respectively). In order to simultaneously capture MBs and carry out imaging with the same probe, the asymmetric AVT pulse signal and the ultrasound-imaging pulse signal were arranged in a staggered series, and the imaging was carried out using plane-wave pulses at nine angles (-7° to 7°) in compounded PWI (volume rate: 200 Hz). Microscopy observations showed that freely suspended MBs could indeed be gathered by the asymmetric AVT in the flow field to form an MBs cluster with a spot size of about [Formula: see text], which could resist the flow to remain at a fixed location for about 22 s. After the asymmetric AVT signal and the ultrasound-imaging pulse signal were turned on for 1 s, the ultrasound 3-D image showed that the signal intensity of the MB clusters increased by 13.1 dB ± 2.9 dB in relation to the background area. These results show that the proposed strategy can be used to accumulate flowing MBs at a desired location and to simultaneously observe this phenomenon. This tool could be used in the future to improve the outcomes of MB-related treatments for various diseases.

Evaluation of Nonlinear Contrast Pulse Sequencing for Use in Super-Resolution Ultrasound Imaging

The use of super-resolution ultrasound (SR-US) imaging greatly improves visualization of microvascular structures, but clinical adoption is limited by long imaging times. This method depends on detecting and localizing isolated microbubbles (MBs), forcing the use of a dilute contrast agent concentration. Contrast-enhanced ultrasound (CEUS) image acquisition times as long as minutes arise as the localization of thousands of MBs are acquired to form a complete SR-US image. In this article, we explore the use of nonlinear CEUS strategies using nonlinear fundamental contrast pulse sequencing (CPS) to increase the contrast-to-tissue ratio (CTR) and compare MB detection effectiveness to linear B-mode CEUS imaging. The CPS compositions of amplitude modulation (AM), pulse inversion (PI), and a combination of the two (AMPI) were studied. A simulation study combined the Rayleigh-Plesset-Marmottant (RPM) model of MB characteristics and a nonlinear tissue model using the k-Wave toolbox for MATLAB (MathWorks Inc., Natick, MA, USA). Validation was conducted using an in vitro flow phantom and in vivo in the rat hind-limb. Imaging was performed with a programmable US scanner (Vantage 256, Verasonics Inc., Kirkland, WA, USA) and customized to transmit a set of basis US pulses from which both B-mode US (frame rate (FR) of 800 Hz) and multiple nonlinear CPS compositions (FR of 200 Hz) could be assessed from identical in vitro and in vivo datasets using a near simultaneous method. The simulations suggest that MB characteristics, such as diameter and motion, help to predict which US imaging strategy will enhance MB detection. The in vitro and in vivo US imaging studies revealed that different subpopulations of polydisperse MB contrast agents were detected by linear imaging and by each different nonlinear CPS composition. The most effective single imaging strategy at a 200-Hz FR was found to be B-mode US imaging. However, a combination of B-mode US imaging with a nonlinear CPS imaging strategy was more effective in detecting MBs in vivo at all depths and was shown to shorten image acquisition time by an average of 33.3%-76.7% when combining one or more CPS sequences.

Coherent Plane Wave Compounding Combined With Tensor Completion Applied for Ultrafast Imaging

To solve the problem of resolution and contrast in plane wave imaging (PWI), coherent plane wave compounding (CPWC) was introduced, in which scanning was performed at different angles, which can achieve the desired image quality by combining the images obtained from PWI at different angles. However, the application of this idea reduces the frame rate in proportion to the number of plane waves (PWs) or angles, so that in this modality, when dealing with some applications such as shear wave imaging (SWI) and strain imaging, there is always a compromise between the frame rate and the image quality. Tensor completion (TC) is a powerful technique to recover missing information of a low-rank tensor from limited observations based on rank minimization. In this article, we present an idea based on TC to make this compromise lighter; in other words, with a smaller number of angles, we can achieve the desired quality of the output image. To evaluate the proposed idea, plane wave imaging challenge in medical ultrasound (PICMUS) datasets was used, which were recorded at 75 different angles. The results of the resolution evaluation showed that using 20% of the coherent PWs and reconstructing other 80% by TC, compared with the situation of using only 20% of the coherent PWs provided a resolution improvement of 14.97% and 17.4% in the simulated and experimental point targets, respectively. Also, the results of the contrast investigation showed that the contrast ratio (CR) improved by 72.6%, 62.9%, and 111.4% in the simulated cyst target data, experimental cyst targets, and in vivo carotid cross section, respectively. The results confirmed that using 20% of the coherent PWs and reconstructing other 80% by TC, the image quality is very close to that obtained by considering all 75 angles, so that the difference in resolution is less than 2% and the difference in contrast to noise ratio (CNR) is less than 5 dB. Therefore, with this idea, it can be said that less compromise is needed; in other words, despite having a higher frame rate, an acceptable quality can be achieved.

Optimization of Microbubble Concentration and Acoustic Pressure for Left Ventricular High-Frame-Rate EchoPIV in Patients

High-frame-rate (HFR) echo-particle image velocimetry (echoPIV) is a promising tool for measuring intracardiac blood flow dynamics. In this study, we investigate the optimal ultrasound contrast agent (UCA: SonoVue) infusion rate and acoustic output to use for HFR echoPIV (PRF = 4900 Hz) in the left ventricle (LV) of patients. Three infusion rates (0.3, 0.6, and 1.2 ml/min) and five acoustic output amplitudes (by varying transmit voltage: 5, 10, 15, 20, and 30 V-corresponding to mechanical indices of 0.01, 0.02, 0.03, 0.04, and 0.06 at 60-mm depth) were tested in 20 patients admitted for symptoms of heart failure. We assess the accuracy of HFR echoPIV against pulsed-wave Doppler acquisitions obtained for mitral inflow and aortic outflow. In terms of image quality, the 1.2-ml/min infusion rate provided the highest contrast-to-background ratio (CBR) (3-dB improvement over 0.3 ml/min). The highest acoustic output tested resulted in the lowest CBR. Increased acoustic output also resulted in increased microbubble disruption. For the echoPIV results, the 1.2-ml/min infusion rate provided the best vector quality and accuracy; mid-range acoustic outputs (corresponding to 15-20-V transmit voltages) provided the best agreement with the pulsed-wave Doppler. Overall, the highest infusion rate (1.2 ml/min) and mid-range acoustic output amplitudes provided the best image quality and echoPIV results.

Ultrafast amplitude modulation for molecular and hemodynamic ultrasound imaging

Ultrasound is playing an emerging role in molecular and cellular imaging thanks to new micro- and nanoscale contrast agents and reporter genes. Acoustic methods for the selective in vivo detection of these imaging agents are needed to maximize their impact in biology and medicine. Existing ultrasound pulse sequences use the nonlinearity in contrast agents' response to acoustic pressure to distinguish them from mostly linear tissue scattering. However, such pulse sequences typically scan the sample using focused transmissions, resulting in a limited frame rate and restricted field of view. Meanwhile, existing wide-field scanning techniques based on plane wave transmissions suffer from limited sensitivity or nonlinear artifacts. To overcome these limitations, we introduce an ultrafast nonlinear imaging modality combining amplitude-modulated pulses, multiplane wave transmissions, and selective coherent compounding. This technique achieves contrast imaging sensitivity comparable to much slower gold-standard amplitude modulation sequences and enables the acquisition of larger and deeper fields of view, while providing a much faster imaging framerate of 3.2 kHz. Additionally, it enables simultaneous nonlinear and linear image formation and allows concurrent monitoring of phenomena accessible only at ultrafast framerates, such as blood volume variations. We demonstrate the performance of this ultrafast amplitude modulation technique by imaging gas vesicles, an emerging class of genetically encodable biomolecular contrast agents, in several in vitro and in vivo contexts. These demonstrations include the rapid discrimination of moving contrast agents and the real-time monitoring of phagolysosomal function in the mouse liver.

Dealiasing High-Frame-Rate Color Doppler Using Dual-Wavelength Processing

Doppler ultrasound is the premier modality to analyze blood flow dynamics in clinical practice. With conventional systems, Doppler can either provide a time-resolved quantification of the flow dynamics in sample volumes (spectral Doppler) or an average Doppler velocity/power [color flow imaging (CFI)] in a wide field of view (FOV) but with a limited frame rate. The recent development of ultrafast parallel systems made it possible to evaluate simultaneously color, power, and spectral Doppler in a wide FOV and at high-frame rates but at the expense of signal-to-noise ratio (SNR). However, like conventional Doppler, ultrafast Doppler is subject to aliasing for large velocities and/or large depths. In a recent study, staggered multi-pulse repetition frequency (PRF) sequences were investigated to dealias color-Doppler images. In this work, we exploit the broadband nature of pulse-echo ultrasound and propose a dual-wavelength approach for CFI dealiasing with a constant PRF. We tested the dual-wavelength bandpass processing, in silico, in laminar flow phantom and validated it in vivo in human carotid arteries ( n = 25 ). The in silico results showed that the Nyquist velocity could be extended up to four times the theoretical limit. In vivo, dealiased CFI were highly consistent with unfolded Spectral Doppler ( r²=0.83 , y=1.1x+0.1 , N=25 ) and provided consistent vector flow images. Our results demonstrate that dual-wavelength processing is an efficient method for high-velocity CFI.

Micro-Bio-Chemo-Mechanical-Systems: Micromotors, Microfluidics, and Nanozymes for Biomedical Applications

Wireless nano-/micromotors powered by chemical reactions and/or external fields generate motive forces, perform tasks, and significantly extend short-range dynamic responses of passive biomedical microcarriers. However, before micromotors can be translated into clinical use, several major problems, including the biocompatibility of materials, the toxicity of chemical fuels, and deep tissue imaging methods, must be solved. Nanomaterials with enzyme-like characteristics (e.g., catalase, oxidase, peroxidase, superoxide dismutase), that is, nanozymes, can significantly expand the scope of micromotors' chemical fuels. A convergence of nanozymes, micromotors, and microfluidics can lead to a paradigm shift in the fabrication of multifunctional micromotors in reasonable quantities, encapsulation of desired subsystems, and engineering of FDA-approved core-shell structures with tuneable biological, physical, chemical, and mechanical properties. Microfluidic methods are used to prepare stable bubbles/microbubbles and capsules integrating ultrasound, optoacoustic, fluorescent, and magnetic resonance imaging modalities. The aim here is to discuss an interdisciplinary approach of three independent emerging topics: micromotors, nanozymes, and microfluidics to creatively: 1) embrace new ideas, 2) think across boundaries, and 3) solve problems whose solutions are beyond the scope of a single discipline toward the development of micro-bio-chemo-mechanical-systems for diverse bioapplications.

Linear Signal Cancellation of Nonlinear Pulsing Schemes in a Verasonics Research Scanner

Contrast-enhanced ultrasound (CEUS) is a real-time imaging technique that allows the visualization of organ and tumor microcirculation by utilizing the nonlinear response of microbubbles. Nonlinear pulsing schemes are used exclusively in CEUS imaging modes in modern scanners. One important aspect of nonlinear pulsing schemes is the near-complete elimination of the linear signals that originate from tissue. Up until now, no study has investigated the performance of Verasonics scanners in eliminating the linear signals during CEUS and, by extension, the optimal pulsing sequences for performing CEUS. The aim of this article was to investigate linear signal cancellation of the Verasonics scanner performing nonlinear pulsing schemes with two different probes (L7-4 linear array and C5-2 convex array). We have considered two pulsing schemes: pulse inversion (PI) and amplitude modulation (AM). We have also compared our results from the Verasonics scanner with a clinical scanner (Philips iU22). We found that the linear signal cancellation of the transmitted pulse by Verasonics scanner was ~40 dB in AM mode and ~30 dB in PI mode when operated at 0.06 MI. The linear signal cancellation performance of Verasonics scanner was comparable with Philips iU22 scanner in focused AM mode and on average 3 dB better than Philips iU22 scanner in focused PI mode.

United Wiener postfilter for plane wave compounding ultrasound imaging

Plane wave compounding (PWC) is a valid method for ultrafast ultrasound imaging. Its imaging quality depends on the beamforming method. The coherence factor (CF) and Wiener postfilter are effective signal processing schemes for aberration correction. However, the CF usually causes over-suppression and brings artifacts. Additionally, the conventional CF and Wiener postfilter cannot fully utilize the spatial coherence in the PWC, which limits the imaging performance and increases the computation. In this paper, we propose a united Wiener postfilter specially for the PWC. The signal and noise power are both estimated through the echo signal matrix, rather than array signal vectors. The method also accords with the theoretical relationship between the CF and Wiener. To evaluate the performance of the proposed method, we conduct simulations, phantom and in vivo experiments and make comparisons with the delay-and-sum (DAS), the CF, the generalized coherence factor (GCF), the conventional Wiener and the scaled Wiener beamformers. Results indicate that our method can offer the better resolution and contrast than the DAS and Wiener. It also solves the over-suppression drawback of the CF. Specifically, the contrast ratio and contrast-to-noise ratio achieve 26.7% and 25.2% improvements in simulations, 28.7% and 32.4% in phantom experiments, respectively. The proposed method also performs well in terms of the speckle signal-to-noise ratio and the generalized contrast-to-noise ratio. Consequently, we believe that the proposed method is effective in enhancing the imaging quality of the PWC.

Analysis of Embedded Optical Interferometry in Transparent Elastic Grating for Optical Detection of Ultrasonic Waves

In this paper, we propose a theoretical framework to explain how the transparent elastic grating structure can be employed to enhance the mechanical and optical properties for ultrasonic detection. Incident ultrasonic waves can compress the flexible material, where the change in thickness of the elastic film can be measured through an optical interferometer. Herein, the polydimethylsiloxane (PDMS) was employed in the design of a thin film grating pattern. The PDMS grating with the grating period shorter than the ultrasound wavelength allowed the ultrasound to be coupled into surface acoustic wave (SAW) mode. The grating gaps provided spaces for the PDMS grating to be compressed when the ultrasound illuminated on it. This grating pattern can provide an embedded thin film based optical interferometer through Fabry-Perot resonant modes. Several optical thin film-based technologies for ultrasonic detection were compared. The proposed elastic grating gave rise to higher sensitivity to ultrasonic detection than a surface plasmon resonance-based sensor, a uniform PDMS thin film, a PDMS sensor with shearing interference, and a conventional Fabry-Perot-based sensor. The PDMS grating achieved the enhancement of sensitivity up to 1.3 × 10-5 Pa-1 and figure of merit of 1.4 × 10-5 Pa-1 which were higher than those of conventional Fabry-Perot structure by 7 times and 4 times, respectively.

Image Quality Enhancement Using a Deep Neural Network for Plane Wave Medical Ultrasound Imaging

Plane wave imaging (PWI), a typical ultrafast medical ultrasound imaging mode, adopts single plane wave emission without focusing to achieve a high frame rate. However, the imaging quality is severely degraded in comparison with the commonly used focused line scan mode. Conventional adaptive beamformers can improve imaging quality at the cost of additional computation. In this article, we propose to use a deep neural network (DNN) to enhance the performance of PWI while maintaining a high frame rate. In particular, the PWI response from a single point target is used as the network input, while the focused scan response from the same point serves as the desired output, which is the main contribution of this method. To evaluate the performance of the proposed method, simulations, phantom experiments and in vivo studies are conducted. The delay-and-sum (DAS), the coherence factor (CF), a previously proposed deep learning-based method and the DAS with focused scan are used for comparison. Numerical metrics, including the contrast ratio (CR), the contrast-to-noise ratio (CNR), and the speckle signal-to-noise ratio (sSNR), are used to quantify the performance. The results indicate that the proposed method can achieve superior resolution and contrast performance. Specifically, the proposed method performs better than the DAS in all metrics. Although the CF provides a higher CR, its CNR and sSNR are much lower than those of the proposed method. The overall performance is also better than that of the previous deep learning method and at the same level with focused scan performance. Additionally, in comparison with the DAS, the proposed method requires little additional computation, which ensures high temporal resolution. These results validate that the proposed method can achieve a high imaging quality while maintaining the high frame rate associated with PWI.

The Advent of Biomolecular Ultrasound Imaging

Ultrasound imaging is one of the most widely used modalities in clinical practice, revealing human prenatal development but also arterial function in the adult brain. Ultrasound waves travel deep within soft biological tissues and provide information about the motion and mechanical properties of internal organs. A drawback of ultrasound imaging is its limited ability to detect molecular targets due to a lack of cell-type specific acoustic contrast. To date, this limitation has been addressed by targeting synthetic ultrasound contrast agents to molecular targets. This molecular ultrasound imaging approach has proved to be successful but is restricted to the vascular space. Here, we introduce the nascent field of biomolecular ultrasound imaging, a molecular imaging approach that relies on genetically encoded acoustic biomolecules to interface ultrasound waves with cellular processes. We review ultrasound imaging applications bridging wave physics and chemical engineering with potential for deep brain imaging.

Ultrasound Super-Resolution Flow Measurement of Suspensions in Narrow Channels

Zinc-air flow batteries provide a scalable and cost-efficient energy storage solution. However, the achieved power density depends on the local flow conditions of the zinc particle suspension in the electrochemical cell. Numerical modeling is challenging due to the complex multiphase fluid and the interaction of flow and electrochemistry. Hence, performing experiments is crucial to investigate the influence of the flow conditions on the electrical performance, which requires flow instrumentation for the opaque suspension. To resolve the flow field across the 2.6-mm-wide flow channel of the investigated zinc-air flow battery (ZAB), a spatial resolution below 100 [Formula: see text] has to be typically achieved. Using ultrasound techniques, the achieved spatial resolution is limited by the trade-off between ultrasound frequency and imaging depth. This trade-off is even more critical for suspensions due to the scattering of the ultrasound, which increases strongly with frequency. We propose super-resolution particle tracking velocimetry (SRPTV) to overcome this limitation by achieving the required spatial resolution at a low ultrasound frequency. SRPTV is based on the super-resolution technique ultrasound localization microscopy, which is adapted to strongly scattering suspensions by using a dual-frequency-phased array and applying a coherence weighting beamformer to suppress speckles, which result from the scattering at the zinc particles of the suspension. The spatial resolution and the velocity uncertainty are characterized through calibration measurement and numerical simulation. A spatial resolution of 66 [Formula: see text] at an excitation wavelength of 330 [Formula: see text] was achieved, which is sufficient for performing flow investigation in an operational ZAB. The measured flow profile reveals shear-thinning properties and wall slip and therefore differs significantly from a parabolic flow profile of a Newtonian fluid. The presented technique offers potential for performing flow investigations of suspensions in small geometries with a spatial resolution beyond the diffraction limit.

Therapeutic Ultrasound Parameter Optimization for Drug Delivery Applied to a Murine Model of Hepatocellular Carcinoma

Ultrasound and microbubble (USMB)-mediated drug delivery is a valuable tool for increasing the efficiency of the delivery of therapeutic agents to cancer while maintaining low systemic toxicity. Typically, selection of USMB drug delivery parameters used in current research settings are either based on previous studies described in the literature or optimized using tissue-mimicking phantoms. However, phantoms rarely mimic in vivo tumor environments, and the selection of parameters should be based on the application or experiment. In the following study, we optimized the therapeutic parameters of the ultrasound drug delivery system to achieve the most efficient in vivo drug delivery using fluorescent semiconducting polymer nanoparticles as a model nanocarrier. We illustrate that voltage, pulse repetition frequency and treatment time (i.e., number of ultrasound pulses per therapy area) delivered to the tumor can successfully be optimized in vivo to ensure effective delivery of the semiconducting polymer nanoparticles to models of hepatocellular carcinoma. The optimal in vivo parameters for USMB drug delivery in this study were 70 V (peak negative pressure = 3.4 MPa, mechanical index = 1.22), 1-Hz pulse repetition frequency and 100-s therapy time. USMB-mediated drug delivery using in vivo optimized ultrasound parameters caused an up to 2.2-fold (p < 0.01) increase in drug delivery to solid tumors compared with that using phantom-optimized ultrasound parameters.

In vitro performance of echoPIV for assessment of laminar flow profiles in a carotid artery stent

Purpose: Detailed blood flow studies may contribute to improvements in carotid artery stenting. High-frame-rate contrast-enhanced ultrasound followed by particle image velocimetry (PIV), also called echoPIV, is a technique to study blood flow patterns in detail. The performance of echoPIV in presence of a stent has not yet been studied extensively. We compared the performance of echoPIV in stented and nonstented regions in an in vitro flow setup.

Approach: A carotid artery stent was deployed in a vessel-mimicking phantom. High-frame-rate contrast-enhanced ultrasound images were acquired with various settings. Signal intensities of the contrast agent, velocity values, and flow profiles were calculated.

Results: The results showed decreased signal intensities and correlation coefficients inside the stent, however, PIV analysis in the stent still resulted in plausible flow vectors.

Conclusions: Velocity values and laminar flow profiles can be measured in vitro in stented arteries using echoPIV.

Imaging With Therapeutic Acoustic Wavelets-Short Pulses Enable Acoustic Localization When Time of Arrival is Combined With Delay and Sum

Passive acoustic mapping (PAM) is an algorithm that reconstructs the location of acoustic sources using an array of receivers. This technique can monitor therapeutic ultrasound procedures to confirm the spatial distribution and amount of microbubble activity induced. Current PAM algorithms have an excellent lateral resolution but have a poor axial resolution, making it difficult to distinguish acoustic sources within the ultrasound beams. With recent studies demonstrating that short-length and low-pressure pulses-acoustic wavelets-have the therapeutic function, we hypothesized that the axial resolution could be improved with a quasi-pulse-echo approach and that the resolution improvement would depend on the wavelet's pulse length. This article describes an algorithm that resolves acoustic sources axially using time of flight and laterally using delay-and-sum beamforming, which we named axial temporal position PAM (ATP-PAM). The algorithm accommodates a rapid short pulse (RaSP) sequence that can safely deliver drugs across the blood-brain barrier. We developed our algorithm with simulations (k-wave) and in vitro experiments for one-, two-, and five-cycle pulses, comparing our resolution against that of two current PAM algorithms. We then tested ATP-PAM in vivo and evaluated whether the reconstructed acoustic sources mapped to drug delivery within the brain. In simulations and in vitro, ATP-PAM had an improved resolution for all pulse lengths tested. In vivo, experiments in mice indicated that ATP-PAM could be used to target and monitor drug delivery into the brain. With acoustic wavelets and time of flight, ATP-PAM can locate acoustic sources with a vastly improved spatial resolution.

Monodisperse versus Polydisperse Ultrasound Contrast Agents: In Vivo Sensitivity and safety in Rat and Pig

Recent advances in the field of monodisperse microbubble synthesis by flow focusing allow for the production of foam-free, highly concentrated and monodisperse lipid-coated microbubble suspensions. It has been found that in vitro, such monodisperse ultrasound contrast agents (UCAs) improve the sensitivity of contrast-enhanced ultrasound imaging. Here, we present the first in vivo study in the left ventricle of rat and pig with this new monodisperse bubble agent. We systematically characterize the acoustic sensitivity and safety of the agent at an imaging frequency of 2.5 MHz as compared with three commercial polydisperse UCAs (SonoVue/Lumason, Definity/Luminity and Optison) and one research-grade polydisperse agent with the same shell composition as the monodisperse bubbles. The monodisperse microbubbles, which had a diameter of 4.2 μm, crossed the pulmonary vasculature, and their echo signal could be measured at least as long as that of the polydisperse UCAs, indicating that microfluidically formed monodisperse microbubbles are stable in vivo. Furthermore, it was found that the sensitivity of the monodisperse agent, expressed as the mean echo power per injected bubble, was at least 10 times higher than that of the polydisperse UCAs. Finally, the safety profile of the monodisperse microbubble suspension was evaluated by injecting 400 and 2000 times the imaging dose, and neither physiologic nor pathologic changes were found, which is a first indication that monodisperse lipid-coated microbubbles formed by flow focusing are safe for in vivo use. The more uniform acoustic response and corresponding increased imaging sensitivity of the monodisperse agent may boost emerging applications of microbubbles and ultrasound such as molecular imaging and therapy.

Nondestructive Detection of Targeted Microbubbles Using Dual-Mode Data and Deep Learning for Real-Time Ultrasound Molecular Imaging

Ultrasound molecular imaging (UMI) is enabled by targeted microbubbles (MBs), which are highly reflective ultrasound contrast agents that bind to specific biomarkers. Distinguishing between adherent MBs and background signals can be challenging in vivo. The preferred preclinical technique is differential targeted enhancement (DTE), wherein a strong acoustic pulse is used to destroy MBs to verify their locations. However, DTE intrinsically cannot be used for real-time imaging and may cause undesirable bioeffects. In this work, we propose a simple 4-layer convolutional neural network to nondestructively detect adherent MB signatures. We investigated several types of input data to the network: "anatomy-mode" (fundamental frequency), "contrast-mode" (pulse-inversion harmonic frequency), or both, i.e., "dual-mode", using IQ channel signals, the channel sum, or the channel sum magnitude. Training and evaluation were performed on in vivo mouse tumor data and microvessel phantoms. The dual-mode channel signals yielded optimal performance, achieving a soft Dice coefficient of 0.45 and AUC of 0.91 in two test images. In a volumetric acquisition, the network best detected a breast cancer tumor, resulting in a generalized contrast-to-noise ratio (GCNR) of 0.93 and Kolmogorov-Smirnov statistic (KSS) of 0.86, outperforming both regular contrast mode imaging (GCNR = 0.76, KSS = 0.53) and DTE imaging (GCNR = 0.81, KSS = 0.62). Further development of the methodology is necessary to distinguish free from adherent MBs. These results demonstrate that neural networks can be trained to detect targeted MBs with DTE-like quality using nondestructive dual-mode data, and can be used to facilitate the safe and real-time translation of UMI to clinical applications.

Phase Modulation Beamforming for Ultrafast Plane-Wave Imaging

In order to improve the spatial resolution for high-frame-rate imaging, a new image formation approach is proposed based on introducing very weak aberration into received data, then combining the multiple results by taking the pixel-wise standard deviation of multiple aberrated images and subtracting the result from the delay-and-sum image. This approach is demonstrated in simulations, tissue-mimicking phantom experiments, and in vivo imaging. Simulations indicate the lateral full-width half-maximum (FWHM) of targets decreases by 38.24% ± 6.38%. In imaging wire targets in a tissue-mimicking phantom at 7.8 MHz, wire target FWHM decreases by 35.91% ± 5.39%. However, contrast was observed to decrease by 1.23 dB and contrast-to-noise ratio (CNR) by 18.5% in phantom studies due to the subtraction of similar images, which increases the number of dark pixels in the image. Finally, the proposed technique is tested in vivo, with images showing improvements similar to those in tissue-mimicking phantoms, including increased separation between closely spaced targets.

Transcranial activation and imaging of low boiling point phase-change contrast agents through the temporal bone using an ultrafast interframe activation ultrasound sequence

PURPOSE

As a cavitation enhancer, low boiling point phase-change contrast agents (PCCA) offer potential for ultrasound-mediated drug delivery in applications including intracerebral hemorrhage or brain tumors. In addition to introducing cavitation, ultrasound imaging also has the ability to provide guidance and monitoring of the therapeutic process by localizing delivery events. However, the highly attenuating skull poses a challenge for achieving an image with useful contrast. In this study, the feasibility of transcranial activation and imaging of low boiling point PCCAs through the human temporal bone was investigated by using a low frequency ultrafast interframe activation ultrasound (UIAU) imaging sequence with singular value decomposition-based denoising.

METHODS

Lipid-shelled PCCAs filled with decafluorobutane were activated and imaged at 37°C in tissue-mimicking phantoms both without and with an ex vivo human skull using the new UIAU sequence and a low frequency diagnostic transducer array at frequencies from 1.5 to 3.5 MHz. A singular value decomposition-based denoising filter was developed and used to further enhance transcranial image contrast. The contrast-to-tissue ratio (CTR) and contrast enhancement (CE) of UIAU was quantitatively evaluated and compared with the amplitude modulation pulse inversion (AMPI) and vaporization detection imaging (VDI) approaches.

RESULTS

Image results demonstrate enhanced contrast in the phantom channel with suppressed background when the low boiling point PCCA was activated both without and with an ex vivo human skull using the UIAU sequence. Quantitative results show that without the skull, low frequency UIAU imaging provided significantly higher image contrast (CTR ≥ 18.56 dB and CE ≥ 18.66 dB) than that of AMPI and VDI (P < 0.05). Transcranial imaging results indicated that the CE of UIAU (≥18.80 dB) was significantly higher than AMPI for free-field activation pressures of 5 and 6 MPa. The CE of UIAU is also significantly higher than that of VDI when PCCAs were activated at 2.5 MHz and 3 MHz (P < 0.05). The CTR (23.30 [20.07-25.56] dB) of denoised UIAU increased by 12.58 dB relative to the non-denoised case and was significantly higher than that of AMPI at an activation pressure of 4 MPa (P < 0.05).

CONCLUSIONS

Results indicate that low boiling point PCCAs can be activated and imaged at low frequencies including imaging through the temporal bone using the UIAU sequence. The UIAU imaging approach provides higher contrast than AMPI and VDI, especially at lower activation pressures with additional removal of electronic noise.

Simultaneous Evalulation of Contrast Pulse Sequences for Super-Resolution Ultrasound Imaging - Preliminary In Vitro and In Vivo Results

Super-resolution ultrasound imaging (SR-US) has enabled a tenfold improvement in resolution of the microvasculature with clinical application in many disease processes such as cancer, diabetes and cardiovascular disease. Plane wave ultrasound (US) platforms in turn are capable of the very high frame rates needed to track microbubble (MB) contrast agents used in SR-US. Both B-mode US imaging and contrast enhanced US imaging (CEUS) have been effectively used in SR-US, with B-mode US having higher signal-to-noise ratio (SNR) and CEUS providing higher contrast-to-tissue ratio (CTR). Lengthy imaging time needed for SR-US to allow perfusion and MB detection is an impediment to clinical adoption. Both SNR and CTR improvements can enhance SR-US imaging by enhancing the detection of MBs thus reducing imaging time. This study simultaneously evaluated nonlinear contrast pulse sequences (CPS) employing different amplitude modulation (AM) and pulse inversion (PI) nonlinear CEUS imaging techniques as well as combinations of the two, (AMPI) with B-mode US imaging. The objective was to improve the detection rate of MB during SR-US. Imaging was performed in vitro and in vivo in the rat hind limb using a Vantage 256 research scanner (Verasonics Inc.). Comparisons of four CPS compositions with B-mode US imaging was made based on the number of MB detected and localized in SR-US images. The use of a PI nonlinear CEUS imaging strategy improved SR-US imaging by increasing the number of MB detected in a sequence of frames by an average of 28.3% and up to 52.6% over a B-mode US imaging strategy, which would decrease imaging time accordingly.

Microbubble Agents: New Directions

Microbubble ultrasound contrast agents have now been in use for several decades and their safety and efficacy in a wide range of diagnostic applications have been well established. Recent progress in imaging technology is facilitating exciting developments in techniques such as molecular, 3-D and super resolution imaging and new agents are now being developed to meet their specific requirements. In parallel, there have been significant advances in the therapeutic applications of microbubbles, with recent clinical trials demonstrating drug delivery across the blood-brain barrier and into solid tumours. New agents are similarly being tailored toward these applications, including nanoscale microbubble precursors offering superior circulation times and tissue penetration. The development of novel agents does, however, present several challenges, particularly regarding the regulatory framework. This article reviews the developments in agents for diagnostic, therapeutic and "theranostic" applications; novel manufacturing techniques; and the opportunities and challenges for their commercial and clinical translation.

Ultrafast three-dimensional microbubble imaging in vivo predicts tissue damage volume distributions during nonthermal brain ablation

Transcranial magnetic resonance imaging (MRI)-guided focused ultrasound (FUS) thermal ablation is under clinical investigation for non-invasive neurosurgery, though its use is restricted to central brain targets due primarily to skull heating effects. The combination of FUS and contrast agent microbubbles greatly reduces the ultrasound exposure levels needed to ablate brain tissue and may help facilitate the use of transcranial FUS ablation throughout the brain. However, sources of variability exist during microbubble-mediated FUS procedures that necessitate the continued development of systems and methods for online treatment monitoring and control, to ensure that excessive and/or off-target bioeffects are not induced from the exposures. Methods: Megahertz-rate three-dimensional (3D) microbubble imaging in vivo was performed during nonthermal ablation in rabbit brain using a clinical-scale prototype transmit/receive hemispherical phased array system. Results:In-vivo volumetric acoustic imaging over microsecond timescales uncovered spatiotemporal microbubble dynamics hidden by conventional whole-burst temporal averaging. Sonication-aggregate ultrafast 3D source field intensity data were predictive of microbubble-mediated tissue damage volume distributions measured post-treatment using MRI and confirmed via histopathology. Temporal under-sampling of acoustic emissions, which is common practice in the field, was found to impede performance and highlighted the importance of capturing adequate data for treatment monitoring and control purposes. Conclusion: The predictive capability of ultrafast 3D microbubble imaging, reported here for the first time, will enable future microbubble-mediated FUS treatments with unparalleled precision and accuracy, and will accelerate the clinical translation of nonthermal tissue ablation procedures both in the brain and throughout the body.

Ultrafast Endoscopic Ultrasonography With Circular Array

Rapid development of ultrafast ultrasound imaging has led to novel medical ultrasound applications, including shear wave elastography and super-resolution vascular imaging. However, these have yet to incorporate endoscopic ultrasonography (EUS) with a circular array, which provides a wider view in the alimentary canal than traditional linear and convex arrays. A coherent diverging wave compounding (CDWC) imaging method was proposed for ultrafast EUS imaging and implemented on a custom circular array. In CDWC, virtual acoustic point sources are allocated and virtually insonified diverging waves from each source are achieved by adjusting all circular array elements' emission time delays. Diverging waves emitted from different virtual sources are coherently compounded, generating synthetic transmit focusing at every location in the image plane. As the field of view of the circular array is centrally symmetric, all virtual sources are equidistantly distributed on a concentric circle of radius r . To achieve the highest frame rate possible with image quality comparable to that obtained with the traditional multi-focus imaging method, the effects of various radii r and virtual source quantities on the compounded image quality were theoretically analyzed and experimentally verified. Simulation, phantom, and ex-vivo experiments were conducted with an 8 MHz, 124-element circular array, with a 5.35 mm radius. When 16 virtual sources were used with r=1.605 mm, image quality comparable to that obtained with the multi-focus approach was achieved at a frame rate of 1000 frames/s. This demonstrates the feasibility of the proposed ultrafast EUS imaging method and promotes further development of multi-functional EUS devices.

Imaging the Activation of Low-Boiling-Point Phase-Change Contrast Agents in the Presence of Tissue Motion Using Ultrafast Inter-frame Activation Ultrasound Imaging

Nanoscale phase-change contrast agents (PCCAs) have been found to have great potential in non-invasive extravascular imaging and therapeutic delivery. However, the contrast-to-tissue ratio (CTR) of PCCA images is usually limited because of either physiological motion or incomplete cancelation of tissue signal. Therefore, to improve the CTR of PCCA images in the presence of physiological motion, a new imaging technique, ultrafast inter-frame activation ultrasound (UIAU) imaging, is proposed and validated. Results of studies with controlled motion in tissue-mimicking phantoms indicate UIAU could provide significantly higher CTRs (maximum: 17.3 ± 0.9 dB) relative to conventional pulse inversion imaging (maximum CTR: 3.4 ± 1.4 dB). UIAU has CTRs up to 16.1 ± 1.0 dB relative to 3.9 ± 2.3 dB for differential imaging in the presence of physiological motion at 20 mm/s. In vivo imaging of PCCAs in the rat liver also reveals the ability of UIAU to enhance PCCA image contrast in the presence of physiological motion.

High-Frequency Multipulse, Plane-Wave Acoustic Contrast Imaging

Multipulse (MP) ultrasound contrast agent (UCA) imaging is a method to increase the contrast-to-background (CBR) ratio in regions of blood flow. Plane-wave imaging allows high frame rates, and with high-frequency ultrasound, fine-spatial and temporal resolution. MP and plane-wave imaging have not been applied to high-frequency ultrasound. Here, an 18-MHz linear array was employed to implement the MP methods of pulse inversion (PI) and amplitude modulation (AM) using high-speed, multiangle, compound plane-wave imaging. A flow of the UCA DEFINITY^© at a dilution ratio of 2000:1 circulating through a 2-mm-diameter flow channel in a tissue-mimicking phantom was used to characterize CBR and compared with cases of standard, multiangle compound plane-wave imaging. The relative improvement of PI and AM versus standard plane-wave imaging ranged from 5 to 10 dB. The CBR was observed to be stable over a 60-min time duration for a 2000:1 dilution ratio and a 2000:1 dilution ratio provided an optimal CBR.

Super-resolution Ultrasound Imaging

The majority of exchanges of oxygen and nutrients are performed around vessels smaller than 100 μm, allowing cells to thrive everywhere in the body. Pathologies such as cancer, diabetes and arteriosclerosis can profoundly alter the microvasculature. Unfortunately, medical imaging modalities only provide indirect observation at this scale. Inspired by optical microscopy, ultrasound localization microscopy has bypassed the classic compromise between penetration and resolution in ultrasonic imaging. By localization of individual injected microbubbles and tracking of their displacement with a subwavelength resolution, vascular and velocity maps can be produced at the scale of the micrometer. Super-resolution ultrasound has also been performed through signal fluctuations with the same type of contrast agents, or through switching on and off nano-sized phase-change contrast agents. These techniques are now being applied pre-clinically and clinically for imaging of the microvasculature of the brain, kidney, skin, tumors and lymph nodes.

A United Sign Coherence Factor Beamformer for Coherent Plane-Wave Compounding with Improved Contrast

In this study, we present a united sign coherence factor beamformer for coherent plane-wave compounding (CPWC). CPWC is capable of reaching an image quality comparable to the conventional B-mode with a much higher frame rate. Conventional coherence factor (CF) based beamformers for CPWC are based on one-dimensional (1D) frameworks, either in the spatial coherence dimension or angular coherence dimension. Both 1D frameworks do not take into account the coherence information of the dimensions of each other. In order to take full advantage of the radio-frequency (RF) data, this paper proposes a united framework containing both spatial and angular information for CPWC. A united sign coherence factor beamformer (uSCF), which combines the conventional sign coherence factor (SCF) and the united framework, is introduced in the paper as well. The proposed beamformer is compared with the conventional 1D SCF beamformers (spatial and angular dimension beamformers) using simulation, phantom and in vivo studies. In the in vivo images, the proposed method improves the contrast ratio (CR) and generalized contrast-to-noise ratio (gCNR) by 197% and 20% over CPWC. Compared with other 1D methods, uSCF also shows an improved contrast and lateral resolution on all datasets.