High power transcranial beam steering for ultrasonic brain therapy

A pilot study of cardiac guided wave elastography: An ex vivo testing in a rodent model with mechanical testing validation

Many heart diseases can change the elasticity of myocardial tissues, making elastography a potential medical imaging strategy for heart disease diagnosis and cardiovascular risk assessment. Among the existing elastography methods, ultrasound elastography is an appealing choice because of ultrasound's inherent advantages of low cost, high safety, wide availability, and deep penetration. The existing investigations of cardiac ultrasound elastography were implemented based on a bulk model of heart tissue, treating the waves generated in the myocardial tissues as shear waves. In this pilot study, we considered the distinct geometric characteristics of heart tissue, i.e., being a layered structure and its dispersive nature as biological tissue. Based on these considerations, we modeled heart tissues as a layered-dispersive structure and developed a new ultrasound elastography method, ultrasonic guided wave elastography, to characterize the myocardial elasticity. The validity of this layered-dispersive model and the reliability of the developed guided wave elastography were first verified on tissue-mimicking phantoms. Then, the guided wave elastography was applied to an ex vivo imaging of a rat heart tissue specimen in real-time during the biaxial planar mechanical testing. The comparison of the real-time myocardial elasticity obtained from guided wave elastography and mechanical testing demonstrated strong matching, verifying the reliability of the developed cardiac guided wave elastography as a potential method for characterizing myocardial elasticity.

Therapeutic ultrasound transducer technology and monitoring techniques: a review with clinical examples

The exponential growth of therapeutic ultrasound applications demonstrates the power of the technology to leverage the combinations of transducer technology and treatment monitoring techniques to effectively control the preferred bioeffect to elicit the desired clinical effect.Objective: This review provides an overview of the most commonly used bioeffects in therapeutic ultrasound and describes existing transducer technologies and monitoring techniques to ensure treatment safety and efficacy.Methods and materials: Literature reviews were conducted to identify key choices that essential in terms of transducer design, treatment parameters and procedure monitoring for therapeutic ultrasound applications. Effective combinations of these options are illustrated through descriptions of several clinical indications, including uterine fibroids, prostate disease, liver cancer, and brain cancer, that have been successful in leveraging therapeutic ultrasound to provide effective patient treatments.Results: Despite technological constraints, there are multiple ways to achieve a desired bioeffect with therapeutic ultrasound in a target tissue. Visualizations of the interplay of monitoring modality, bioeffect, and applied acoustic parameters are presented that demonstrate the interconnectedness of the field of therapeutic ultrasound. While the clinical indications explored in this review are at different points in the clinical evaluation path, based on the ever expanding research being conducted in preclinical realms, it is clear that additional clinical applications of therapeutic ultrasound that utilize a myriad of bioeffects will continue to grow and improve in the coming years.Conclusions: Therapeutic ultrasound will continue to improve in the next decades as the combination of transducer technology and treatment monitoring techniques will continue to evolve and be translated in clinical settings, leading to more personalized and efficient therapeutic ultrasound mediated therapies.

Crack Detection Method for Wind Turbine Tower Bolts Using Ultrasonic Spiral Phased Array

High-strength bolts are crucial load-bearing components of wind turbine towers. They are highly susceptible to fatigue cracks over long-term service and require timely detection. However, due to the structural complexity and hidden nature of the cracks in wind turbine tower bolts, the small size of the cracks, and their variable propagation directions, detection signals carrying crack information are often drowned out by dense thread signals. Existing non-destructive testing methods are unable to quickly and accurately characterize small cracks at the thread roots. Therefore, we propose an ultrasonic phased array element arrangement method based on the Fermat spiral array. This method can greatly increase the fill rate of the phased array with small element spacing while reducing the effects of grating and sidelobes, thereby achieving high-energy excitation and accurate imaging with the ultrasonic phased array. This has significant theoretical and engineering application value for ensuring the safe and reliable service of key wind turbine components and for promoting the technological development of the wind power industry.

Cardiac 2-D Shear Wave Imaging Using a New Dedicated Clinical Ultrasound System: A Phantom Study

OBJECTIVE

The purpose of this study was to assess cardiac shear wave imaging implemented in a new MACH 30 ultrasound machine (SuperSonic Imaging, Aix-en-Provence, France) and interfaced with a linear probe and a phased array probe, in comparison with a previously validated Aixplorer system connected to a linear probe (SuperSonic Imaging) using Elasticity QA phantoms (Models 039 and 049, CIRS Inc., Norfolk, VA, USA).

METHODS

Quantile-quantile plots were used for distribution agreement. The accuracy of stiffness measurement was assessed by the percentage error and the mean percentage error (MPE), and its homogeneity, by the standard deviation of the MPE. A p value <0.01 was considered to indicate statistical significance.

RESULTS

The accuracy of dedicated cardiac sequences for linear probes was similar for the two systems with an MPE of 8 ± 14% versus 20 ± 21% (p = not significant) with the SuperSonic MACH 30 and Aixplorer, respectively, and was influenced by target stiffness and location of the measurement in the field of view, but without drift over time. The optimal transthoracic cardiac probe workspace was located between 4 and 10 cm, with an MPE of 29.5 ± 25% compared with 93.3 ± 130% outside this area (p < 0.0001). In this area, stiffness below 20 kPa was significantly different from the reference (p < 0.0001). The sectorial probe revealed no MPE difference in any of the measurement areas, with no significant lateral or axial gradient.

CONCLUSION

The new Supersonic MACH 30 system upgraded with a sectorial probe and specific cardiac settings provided homogenous stiffness measurements, especially when operating at depths between 4 and 10 cm. These phantom results may be useful in designing future in vivo studies.

Sex Differences in Aging-related Myocardial Stiffening Quantitatively Measured with MR Elastography

Purpose To investigate the feasibility of using quantitative MR elastography (MRE) to characterize the influence of aging and sex on left ventricular (LV) shear stiffness. Materials and Methods In this prospective study, LV myocardial shear stiffness was measured in 109 healthy volunteers (age range: 18-84 years; mean age, 40 years ± 18 [SD]; 57 women, 52 men) enrolled between November 2018 and September 2019, using a 5-minute MRE acquisition added to a clinical MRI protocol. Linear regression models were used to estimate the association of cardiac MRI and MRE characteristics with age and sex; models were also fit to assess potential age-sex interaction. Results Myocardial shear stiffness significantly increased with age in female (age slope = 0.03 kPa/year ± 0.01, P = .009) but not male (age slope = 0.008 kPa/year ± 0.009, P = .38) volunteers. LV ejection fraction (LVEF) increased significantly with age in female volunteers (0.23% ± 0.08 per year, P = .005). LV end-systolic volume (LVESV) decreased with age in female volunteers (-0.20 mL/m2 ± 0.07, P = .003). MRI parameters, including T1, strain, and LV mass, did not demonstrate this interaction (P > .05). Myocardial shear stiffness was not significantly correlated with LVEF, LV stroke volume, body mass index, or any MRI strain metrics (P > .05) but showed significant correlations with LV end-diastolic volume/body surface area (BSA) (slope = -3 kPa/mL/m2 ± 1, P = .004, r2 = 0.08) and LVESV/BSA (-1.6 kPa/mL/m2 ± 0.5, P = .003, r2 = 0.08). Conclusion This study demonstrates that female, but not male, individuals experience disproportionate LV stiffening with natural aging, and these changes can be noninvasively measured with MRE. Keywords: Cardiac, Elastography, Biological Effects, Experimental Investigations, Sexual Dimorphisms, MR Elastography, Myocardial Shear Stiffness, Quantitative Stiffness Imaging, Aging Heart, Myocardial Biomechanics, Cardiac MRE Supplemental material is available for this article. Published under a CC BY 4.0 license.

Ultrasound Shear Wave Elastography in Cardiology

The advent of high-frame rate imaging in ultrasound allowed the development of shear wave elastography as a noninvasive alternative for myocardial stiffness assessment. It measures mechanical waves propagating along the cardiac wall with speeds that are related to stiffness. The use of cardiac shear wave elastography in clinical studies is increasing, but a proper understanding of the different factors that affect wave propagation is required to correctly interpret results because of the heart's thin-walled geometry and intricate material properties. The aims of this review are to give an overview of the general concepts in cardiac shear wave elastography and to discuss in depth the effects of age, hemodynamic loading, cardiac morphology, fiber architecture, contractility, viscoelasticity, and system-dependent factors on the measurements, with a focus on clinical application. It also describes how these factors should be considered during acquisition, analysis, and reporting to ensure an accurate, robust, and reproducible measurement of the shear wave.

Extracellular matrix stiffness controls cardiac fibroblast proliferation via the nuclear factor-Y (NF-Y) transcription factor

The proliferative expansion of cardiac fibroblasts (CF) contributes towards cardiac fibrosis, which results in myocardial stiffening, cardiac dysfunction, and heart failure. CF sense and respond to increased stiffness of their local extracellular matrix, modulating their phenotype towards increased collagen synthesis and higher proliferation, leading potentially to a vicious circle of positive feedback. Here we describe a novel mechanism that mediates increased CF proliferation in response to a pathologically stiff Exteracellular matrix (ECM). The mechanism we describe is independent of the well-characterised mechano-sensitive transcript factors, YAP-TEAD and MKL1-SRF, which our data indicate are only responsible for part of the genes induced by stiffened ECM. Instead, our data identify Nuclear Factor-Y (NF-Y) as a novel mechanosensitive transcription factor, which mediates enhanced CF proliferation in response to a stiff ECM. We show that levels of NF-YA protein, the major regulatory subunit of NF-Y, and NF-Y transcriptional activity, are increased by a stiff ECM. Indeed, NF-Y activity drives the expression of multiple cell-cycle genes. Furthermore, NF-YA protein levels are dependent on FAK signalling suggesting a mechanistic link to ECM composition. Consistent with its role as a mechano-sensor, inhibition of NF-Y using siRNA or dominant negative mutant blocks CF proliferation on plastic in vitro, which models a stiff ECM, whereas ectopic expression of NF-YA increases the proliferation of cells interacting under conditions that model a physiologically soft ECM. In summary, our data demonstrate that NF-Y is a biomechanically sensitive transcription factor that promotes CF proliferation in a model of pathologically stiffened ECM.

Continuous shear wave measurements for dynamic cardiac stiffness evaluation in pigs

Ultrasound-based shear wave elastography is a promising technique to non-invasively assess the dynamic stiffness variations of the heart. The technique is based on tracking the propagation of acoustically induced shear waves in the myocardium of which the propagation speed is linked to tissue stiffness. This measurement is repeated multiple times across the cardiac cycle to assess the natural variations in wave propagation speed. The interpretation of these measurements remains however complex, as factors such as loading and contractility affect wave propagation. We therefore applied transthoracic shear wave elastography in 13 pigs to investigate the dependencies of wave speed on pressure-volume derived indices of loading, myocardial stiffness, and contractility, while altering loading and inducing myocardial ischemia/reperfusion injury. Our results show that diastolic wave speed correlates to a pressure-volume derived index of operational myocardial stiffness (R = 0.75, p < 0.001), suggesting that both loading and intrinsic properties can affect diastolic wave speed. Additionally, the wave speed ratio, i.e. the ratio of systolic and diastolic speed, correlates to a pressure-volume derived index of contractility, i.e. preload-recruitable stroke work (R = 0.67, p < 0.001). Measuring wave speed ratio might thus provide a non-invasive index of contractility during ischemia/reperfusion injury.

Personalized biomechanical insights in atrial fibrillation: opportunities & challenges

INTRODUCTION

Atrial fibrillation (AF) is an increasingly prevalent and significant worldwide health problem. Manifested as an irregular atrial electrophysiological activation, it is associated with many serious health complications. AF affects the biomechanical function of the heart as contraction follows the electrical activation, subsequently leading to reduced blood flow. The underlying mechanisms behind AF are not fully understood, but it is known that AF is highly correlated with the presence of atrial fibrosis, and with a manifold increase in risk of stroke.

AREAS COVERED

In this review, we focus on biomechanical aspects in atrial fibrillation, current and emerging use of clinical images, and personalized computational models. We also discuss how these can be used to provide patient-specific care.

EXPERT OPINION

Understanding the connection betweenatrial fibrillation and atrial remodeling might lead to valuable understanding of stroke and heart failure pathophysiology. Established and emerging imaging modalities can bring us closer to this understanding, especially with continued advancements in processing accuracy, reproducibility, and clinical relevance of the associated technologies. Computational models of cardiac electromechanics can be used to glean additional insights on the roles of AF and remodeling in heart function.

Ultrasound Visualization and Recording of Transient Myocardial Vibrations

OBJECTIVE

A new visualization and recording method used to assess and quantitate autogenic high-velocity motions in myocardial walls to provide a new description of cardiac function is described.

METHODS

The regional motion display (RMD) is based on high-speed difference ultrasound B-mode images and spatiotemporal processing to record propagating events (PEs). Sixteen normal participants and one patient with cardiac amyloidosis were imaged at rates of 500-1000/s using the Duke Phased Array Scanner, T5. RMDs were generated using difference images and spatially integrating these to display velocity as function of time along a cardiac wall.

RESULTS

In normal participants, RMDs revealed four discrete PEs with average onset timing with respect to the QRS complex of -31.7, +46, +365 and +536 ms. The late diastolic PE propagated apex to base in all participants at an average velocity of 3.4 m/s by the RMD. The RMD of the amyloidosis patient revealed significant changes in the appearance of PEs compared with normal participants. The late diastolic PE propagated at 5.3 m/s from apex to base. All four PEs lagged the average timing of normal participants.

CONCLUSION

The RMD method reliably reveals PEs as discrete events and successfully allows reproducible measurement of PE timing and the velocity of at least one PE. The RMD method is applicable to live, clinical high-speed studies and may offer a new approach to characterization of cardiac function.

Comparing Myocardial Shear Wave Propagation Velocity Estimation Methods Based on Tissue Displacement, Velocity and Acceleration Data

Shear wave elastography (SWE) is a promising technique used to assess cardiac function through the evaluation of cardiac stiffness non-invasively. However, in the literature, SWE varies in terms of tissue motion data (displacement, velocity or acceleration); method used to characterize mechanical wave propagation (time domain [TD] vs. frequency domain [FD]); and the metric reported (wave speed [WS], shear or Young's modulus). This variety of reported methodologies complicates comparison of reported findings and sheds doubt on which methodology better approximates the true myocardial properties. We therefore conducted a simulation study to investigate the accuracy of various SWE data analysis approaches while varying cardiac geometry and stiffness. Lower WS values were obtained by the TD method compared with the FD method. Acceleration-based WS estimates in the TD were systematically larger than those based on velocity (∼10% difference). These observations were confirmed by TD analysis of 32 in vivo SWE mechanical wave measurements. In vivo data quality is typically too low for accurate FD analysis. Therefore, our study suggests using acceleration-based TD analysis for in vivo SWE to minimize underestimation of the true WS and, thus, to maximize the sensitivity of SWE to detect stiffness changes resulting from pathology.

Quantitative Evaluation of Biceps Brachii Muscle by Shear Wave Elastography in Stroke Patients

Purpose

The present study aimed to investigate the differences in muscle size and shear wave speed (SWS) values of biceps brachii muscle (BBM) between stroke survivors and healthy controls.

Methods

This study comprised 61 stroke survivors and 24 healthy subjects, examined at Guangzhou First People's Hospital within one year. Each participant underwent ultrasonic examinations for recording some specific measurement indicators, including muscle thickness, cross-sectional area (CSA), and shear wave speed (SWS) of BBM. The muscular tension of the paretic arm was scored using the modified Ashworth scale (MAS). These above-mentioned indexes were compared between stroke survivors and healthy controls. Also, the correlations among SWS and MAS scores were assessed.

Results

When the lifting arm angle was set for 45°, the CSA and muscle thickness of BBM were obviously decreased in the paretic arms of stroke subjects compared to the non-paretic arms as well as the arms of healthy controls. Moreover, the paretic arms had obviously higher SWS than the non-paretic arms and the healthy arms at 45° or 90°. When the angles of paretic arms were lifted at 90° and 45°, respectively, a positive correlation was established between MAS and SWS.

Conclusion

Ultrasonic examination assessing muscle thickness, CSA, and SWS of the BBM could be used as a means of assessment of the paretic arms of stroke survivors.

Optimized Simultaneous Axial Multifocal Imaging via Frequency Multiplexed Focusing

Simultaneous axial multifocal imaging (SAMI) using a single acoustical transmission was developed to enhance the depth of field. This technique transmits a superposition of axial multifoci waveforms in a single transmission, thus increasing the frame rate. However, since all the waveforms are transmitted at a constant center frequency, there is a tradeoff between attenuation and lateral resolution when choosing a constant frequency for all the axial depths. In this work, we developed an optimized SAMI method by adding frequency dependence to each axial multifocus. By gradually increasing the frequency as a function of the focal depth, this method makes it possible to compensate for the gradually increasing F-number in order to achieve constant lateral resolution across the entire field of view. Alternatively, by gradually decreasing the axial multifoci frequencies as a function of depth, enhanced penetration depth and contrast are obtained. This method, termed frequency multiplexed SAMI (FM-SAMI), is described analytically and validated by resolution and contrast experiments performed on resolution targets, tissue-mimicking phantoms, and ex vivo biological samples. This is the first real-time implementation of a frequency multiplexing approach for axial multifoci imaging that facilitates high-quality imaging at an increased frame rate.

Quantitative stiffness assessment of cardiac grafts using ultrasound in a porcine model: A tissue biomarker for heart transplantation

Background

Heart transplantation is the definitive treatment for many cardiovascular diseases. However, no ideal approach is established to evaluate heart grafts and it mostly relies on qualitative interpretation of surgeon based on the organ aspect including anatomy, color and manual palpation. In this study we propose to assess quantitatively the Shear Wave Velocity (SWV) using ultrasound as a biomarker of cardiac viability on a porcine model.

Methods

The SWV was assessed quantitatively using a clinical ultrasound elastography device (Aixplorer, Supersonics Imagine, France) linked to a robotic motorized arm (UR3, Universal Robots, Denmark) and the elastic anisotropy was obtained using a custom ultrasound research system.

SWV was evaluated as function of time in two porcine heart model during 20h at controlled temperature (4°C). One control group (N = 8) with the heart removed and arrested by cold cardioplegia and immerged in a preservation solution. One ischemic group (N = 6) with the organ harvested after 30 min of in situ warm ischemia, to mimic a donation after cardiac death. Hearts graft were revived at two preservation times, at 4 h (N = 11) and 20 h (N = 10) and the parameters of the cardiac function evaluated.

Findings

On control hearts, SWV remained unchanged during the 4h of preservation. SWV increased significantly between 4 and 20h. For the ischemic group, SWV was found higher after 4h (3.04 +/- 0.69 vs 1.69+/-0.19 m/s, p = 0.007) and 20h (4.77+/-1.22 m/s vs 3.40+/-0.75 m/s, p = 0.034) of preservation with significant differences. A good correlation between SWV and cardiac function index was found (r²=0.88) and manual palpation score (r²=0.81).

Interpretation

Myocardial stiffness increase was quantified as a function of preservation time and harvesting conditions. The correlation between SWV and cardiac function index suggests that SWV could be used as a marker of graft viability. This technique may be transposed to clinical transplantation for assessing the graft viability during transplantation process.

Funding

FRM PME20170637799, Agence Biomédecine AOR Greffe 2017, ANR-18-CE18-0015.

Arterial Stiffness Probed by Dynamic Ultrasound Elastography Characterizes Waveform of Blood Pressure

The clinical and economic burdens of cardiovascular diseases pose a global challenge. Growing evidence suggests an early assessment of arterial stiffness can provide insights into the pathogenesis of cardiovascular diseases. However, it remains difficult to quantitatively characterize local arterial stiffness in vivo. Here we utilize guided axial waves continuously excited and detected by ultrasound to probe local blood pressures and mechanical properties of common carotid arteries simultaneously. In a pilot study of 17 healthy volunteers, we observe a  ∼ 20 % variation in the group velocities of the guided axial waves (5.16 ± 0.55 m/s in systole and 4.31 ± 0.49 m/s in diastole) induced by the variation of the blood pressures. A linear relationship between the square of group velocity and blood pressure is revealed by the experiments and finite element analysis, which enables us to measure the waveform of the blood pressures by the group velocities. Furthermore, we propose a wavelet analysis-based method to extract the dispersion relations of the guided axial waves. We then determined the shear modulus by fitting the dispersion relations in diastole with the leaky Lamb wave model. The average shear modulus of all the volunteers is 166.3 ± 32.8 kPa. No gender differences are found. This study shows the group velocity and dispersion relation of the guided axial waves can be utilized to probe blood pressure and arterial stiffness locally in a noninvasive manner and thus promising for early diagnosis of cardiovascular diseases.

Complex Convolutional Neural Networks for Ultrafast Ultrasound Imaging Reconstruction From In-Phase/Quadrature Signal

Ultrafast ultrasound imaging remains an active area of interest in the ultrasound community due to its ultrahigh frame rates. Recently, a wide variety of studies based on deep learning have sought to improve ultrafast ultrasound imaging. Most of these approaches have been performed on radio frequency (RF) signals. However, in- phase/quadrature (I/Q) digital beamformers are now widely used as low-cost strategies. In this work, we used complex convolutional neural networks for reconstruction of ultrasound images from I/Q signals. We recently described a convolutional neural network architecture called ID-Net, which exploited an inception layer designed for reconstruction of RF diverging-wave ultrasound images. In the present study, we derive the complex equivalent of this network, i.e., complex-valued inception for diverging-wave network (CID-Net) that operates on I/Q data. We provide experimental evidence that CID-Net provides the same image quality as that obtained from RF-trained convolutional neural networks, i.e., using only three I/Q images, CID-Net produces high-quality images that can compete with those obtained by coherently compounding 31 RF images. Moreover, we show that CID-Net outperforms the straightforward architecture that consists of processing real and imaginary parts of the I/Q signal separately, which thereby indicates the importance of consistently processing the I/Q signals using a network that exploits the complex nature of such signals.

Ex Vivo Evaluation of Mechanical Anisotropic Tissues with High-Frequency Ultrasound Shear Wave Elastography

The use of imaging devices to assess directional mechanics of tissues is highly desirable. This is because the directional mechanics depend on fiber orientation, and altered directional mechanics are closely related to the pathological status of tissues. However, measuring directional mechanics in tissues with high-stiffness is challenging due to the difficulty of generating localized displacement in these tissues using acoustic radiation force, a general method for generating displacement in ultrasound-based elastography. In addition, common ultrasound probes do not provide rotational function, which makes the measurement of directional mechanics inaccurate and unreliable. Therefore, we developed a high-frequency ultrasound mechanical wave elastography system that can accommodate a wide range of tissue stiffness and is also equipped with a motorized rotation stage for precise imaging of directional mechanics. A mechanical shaker was applied to the elastography system to measure tissues with high-stiffness. Phantom and ex vivo experiments were performed. In the phantom experiments, the lateral and axial resolution of the system were determined to be 144 μm and 168 μm, respectively. In the ex vivo experiments, we used swine heart and cartilage, both of which are considered stiff. The elastography system allows us to acquire the directional mechanics with high angular resolution in the heart and cartilage. The results demonstrate that the developed elastography system is capable of imaging a wide range of tissues and has high angular resolution. Therefore, this system might be useful for the diagnostics of mechanically anisotropic tissues via ex vivo tests.

Assessing cardiac stiffness using ultrasound shear wave elastography

Shear wave elastography offers a new dimension to echocardiography: it measures myocardial stiffness. Therefore, it could provide additional insights into the pathophysiology of cardiac diseases affecting myocardial stiffness and potentially improve diagnosis or guide patient treatment. The technique detects fast mechanical waves on the heart wall with high frame rate echography, and converts their propagation velocity into a stiffness value. A proper interpretation of shear wave data is required as the shear wave interacts with the intrinsic, yet dynamically changing geometrical and material characteristics of the heart under pressure. This dramatically alters the wave physics of the propagating wave, demanding adapted processing methods compared to other shear wave elastography applications as breast tumor and liver stiffness staging. Furthermore, several advanced analysis methods have been proposed to extract supplementary material features such as viscosity and anisotropy, potentially offering additional diagnostic value. This review explains the general mechanical concepts underlying cardiac shear wave elastography and provides an overview of the preclinical and clinical studies within the field. We also identify the mechanical and technical challenges ahead to make shear wave elastography a valuable tool for clinical practice.

Tapering of the interventricular septum can affect ultrasound shear wave elastography: An in vitro and in silico study

Shear wave elastography (SWE) has the potential to determine cardiac tissue stiffness from non-invasive shear wave speed measurements, important, e.g., for predicting heart failure. Previous studies showed that waves traveling in the interventricular septum (IVS) may display Lamb-like dispersive behaviour, introducing a thickness-frequency dependency in the wave speed. However, the IVS tapers across its length, which complicates wave speed estimation by introducing an additional variable to account for. The goal of this work is to assess the impact of tapering thickness on SWE. The investigation is performed by combining in vitro experiments with acoustic radiation force (ARF) and 2D finite element simulations, to isolate the effect of the tapering curve on ARF-induced and natural waves in the heart. The experiments show a 11% deceleration during propagation from the thick to the thin end of an IVS-mimicking tapered phantom plate. The numerical analysis shows that neglecting the thickness variation in the wavenumber-frequency domain can introduce errors of more than 30% in the estimation of the shear modulus, and that the exact tapering curve, rather than the overall thickness reduction, determines the dispersive behaviour of the wave. These results suggest that septal geometry should be accounted for when deriving cardiac stiffness with SWE.

Smart ultrasound device for non-invasive real-time myocardial stiffness quantification of the human heart

Quantitative assessment of myocardial stiffness is crucial to understand and evaluate cardiac biomechanics and function. Despite the recent progresses of ultrasonic shear wave elastography, quantitative evaluation of myocardial stiffness still remains a challenge because of strong elastic anisotropy. In this paper we introduce a smart ultrasound approach for non-invasive real-time quantification of shear wave velocity (SWV) and elastic fractional anisotropy (FA) in locally transverse isotropic elastic medium such as the myocardium. The approach relies on a simultaneous multidirectional evaluation of the SWV without a prior knowledge of the fiber orientation. We demonstrated that it can quantify accurately SWV in the range of 1.5 to 6 m/s in transverse isotropic medium (FA<0.7) using numerical simulations. Experimental validation was performed on calibrated phantoms and anisotropic ex vivo tissues. A mean absolute error of 0.22 m/s was found when compared to gold standard measurements. Finally, in vivo feasibility of myocardial anisotropic stiffness assessment was evaluated in four healthy volunteers on the antero-septo basal segment and on anterior free wall of the right ventricle (RV) in end-diastole. A mean longitudinal SWV of 1.08 0.20 m/s was measured on the RV wall and 1.74 0.51 m/s on the Septal wall with a good intra-volunteer reproducibility (0.18 m/s). This approach has the potential to become a clinical tool for the quantitative evaluation of myocardial stiffness and diastolic function.

A Prototype Therapy System for Boiling Histotripsy in Abdominal Targets Based on a 256-Element Spiral Array

Boiling histotripsy (BH) uses millisecond-long ultrasound (US) pulses with high-amplitude shocks to mechanically fractionate tissue with potential for real-time lesion monitoring by US imaging. For BH treatments of abdominal organs, a high-power multielement phased array system capable of electronic focus steering and aberration correction for body wall inhomogeneities is needed. In this work, a preclinical BH system was built comprising a custom 256-element 1.5-MHz phased array (Imasonic, Besançon, France) with a central opening for mounting an imaging probe. The array was electronically matched to a Verasonics research US system with a 1.2-kW external power source. Driving electronics and software of the system were modified to provide a pulse average acoustic power of 2.2 kW sustained for 10 ms with a 1-2-Hz repetition rate for delivering BH exposures. System performance was characterized by hydrophone measurements in water combined with nonlinear wave simulations based on the Westervelt equation. Fully developed shocks of 100-MPa amplitude are formed at the focus at 275-W acoustic power. Electronic steering capabilities of the array were evaluated for shock-producing conditions to determine power compensation strategies that equalize BH exposures at multiple focal locations across the planned treatment volume. The system was used to produce continuous volumetric BH lesions in ex vivo bovine liver with 1-mm focus spacing, 10-ms pulselength, five pulses/focus, and 1% duty cycle.

Electronic Point Spread Function Rotation Using a Three-Row Transducer for ARFI-Based Elastic Anisotropy Assessment: In Silico and Experimental Demonstration

Degree of anisotropy (DoA) of mechanical properties has been assessed as the ratio of acoustic radiation force impulse (ARFI)-induced peak displacements (PDs) achieved using spatially asymmetric point spread functions (PSFs) that are rotated 90° to each other. Such PSF rotation has been achieved by manually rotating a linear array transducer, but manual rotation is cumbersome and prone to misalignment errors and higher variability in measurements. The purpose of this work is to evaluate the feasibility of electronic PSF rotation using a three-row transducer, which will reduce variability in DoA assessment. A Siemens 9L4, with 3×192 elements, was simulated in Field II to generate spatially asymmetric ARFI PSFs that were electronically rotated 63° from each other. Then, using the finite element method (FEM), PD due to the ARFI excitation PSFs in 42 elastic, incompressible, transversely isotropic (TI) materials with shear moduli ratios of 1.0-6.0 were modeled. Finally, the ratio of PDs achieved using the two rotated PSFs was evaluated to assess elastic DoA. DoA increased with increasing shear moduli ratios and distinguished materials with 17% or greater difference in shear moduli ratios (Wilcoxon, ). Experimentally, the ratio of PDs achieved using ARFI PSF rotated 63° from each other distinguished the biceps femoris muscle from two pigs, which had median shear moduli ratios of 4.25 and 3.15 as assessed by shear wave elasticity imaging (SWEI). These results suggest that ARFI-based DoA assessment can be achieved without manual transducer rotation using a three-row transducer capable of electronically rotating PSFs by 63°. It is expected that electronic PSF rotation will facilitate data acquisitions and improve the reproducibility of elastic anisotropy assessments.

Use of Speckle Tracking Echocardiography to Detect Induced Regional Strain Changes in the Murine Myocardium by Acoustic Radiation Force

BACKGROUND

It is difficult to simulate the abnormal myocardial strain patterns caused by ischemic coronary artery disease (CAD) which are a precursor to heart failure (HF) within an animal model. Simulation of these strain changes could contribute to better understanding of the early formative stages of HF. This is especially important in investigating the poorly understood pathogenesis of heart failure with preserved ejection fraction (HFpEF). Here, we discuss delivery of high intensity focused ultrasound (HIFU) in a murine model to alter left ventricular (LV) regional longitudinal strain (RLS), and use of speckle tracking echocardiography to detect these changes.

METHODS

HIFU pulses (pressure amplitude 1.7 MPa) were generated by amplifying a sinusoidal waveform from a function generator into a piezoelectric transducer. These pulses were then directed extracorporeally towards the anterior LV surface of C57BI6 mice during three time periods (early, mid, and late diastole). Speckle tracking echocardiography was then used to quantify changes in RLS within six segments of the LV.

RESULTS

We observed an increase in LV RLS with acoustic augmentation during all three time periods. This augmentation was most prominent near the anterior apical region in early diastole and near the posterior basilar region during late diastole.

CONCLUSIONS

Our findings demonstrate the application of HIFU to non-invasively induce changes in RLS within a murine model. Our results also reflect the capability of speckle tracking echocardiography to analyze and quantify these changes. These findings represent the first demonstration of ultrasound-induced augmentation in LV RLS within a small animal model.

Concepts and applications of ultrafast cardiac ultrasound imaging

The concept of ultrafast echocardiographic imaging has been around for decades. However, only recent progress in ultrasound machine hardware and computer technology allowed to apply this concept to echocardiography. High frame rate echocardiography can visualize phenomena that have never been captured before. It enables a wide variety of potential new applications, including shear wave imaging, speckle tracking, ultrafast Doppler imaging, and myocardial perfusion imaging. The principles of these applications and their potential clinical use will be presented in this manuscript.

4D Ultrafast Ultrasound Imaging of Naturally Occurring Shear Waves in the Human Heart

The objectives were to develop a novel three-dimensional technology for imaging naturally occurring shear wave (SW) propagation, demonstrate feasibility on human volunteers and quantify SW velocity in different propagation directions. Imaging of natural SWs generated by valve closures has emerged to obtain a direct measurement of cardiac stiffness. Recently, natural SW velocity was assessed in two dimensions on parasternal long axis view under the assumption of a propagation direction along the septum. However, in this approach the source localization and the complex three-dimensional propagation wave path was neglected making the speed estimation unreliable. High volume rate transthoracic acquisitions of the human left ventricle (1100 volume/s) was performed with a 4D ultrafast echocardiographic scanner. Four-dimensional tissue velocity cineloops enabled visualization of aortic and mitral valve closure waves. Energy and time of flight mapping allowed propagation path visualization and source localization, respectively. Velocities were quantified along different directions. Aortic and mitral valve closure SW velocities were assessed for the three volunteers with low standard deviation. Anisotropic propagation was also found suggesting the necessity of using a three-dimensional imaging approach. Different velocities were estimated for the three directions for the aortic (3.4± 0.1 m/s, 3.5± 0.3 m/s, 5.4± 0.7 m/s) and the mitral (2.8± 0.5 m/s, 2.9± 0.3 m/s, 4.6± 0.7 m/s) valve SWs. 4D ultrafast ultrasound alleviates the limitations of 2D ultrafast ultrasound for cardiac SW imaging based on natural SW propagations and enables a comprehensive measurement of cardiac stiffness. This technique could provide stiffness mapping of the left ventricle.

Reconstruction for Diverging-Wave Imaging Using Deep Convolutional Neural Networks

In recent years, diverging wave (DW) ultrasound imaging has become a very promising methodology for cardiovascular imaging due to its high temporal resolution. However, if they are limited in number, DW transmits provide lower image quality compared with classical focused schemes. A conventional reconstruction approach consists in summing series of ultrasound signals coherently, at the expense of frame rate, data volume, and computation time. To deal with this limitation, we propose a convolutional neural network (CNN) architecture, Inception for DW Network (IDNet), for high-quality reconstruction of DW ultrasound images using a small number of transmissions. In order to cope with the specificities induced by the sectorial geometry associated with DW imaging, we adopted the inception model composed of the concatenation of multiscale convolution kernels. Incorporating inception modules aims at capturing different image features with multiscale receptive fields. A mapping between low-quality images and corresponding high-quality compounded reconstruction was learned by training the network using in vitro and in vivo samples. The performance of the proposed approach was evaluated in terms of contrast ratio (CR), contrast-to-noise ratio (CNR), and lateral resolution (LR), and compared with standard compounding method and conventional CNN methods. The results demonstrated that our method could produce high-quality images using only 3 DWs, yielding an image quality equivalent to that obtained with compounding of 31 DWs and outperforming more conventional CNN architectures in terms of complexity, inference time, and image quality.

A direct comparison of natural and acoustic-radiation-force-induced cardiac mechanical waves

Natural and active shear wave elastography (SWE) are potential ultrasound-based techniques to non-invasively assess myocardial stiffness, which could improve current diagnosis of heart failure. This study aims to bridge the knowledge gap between both techniques and discuss their respective impacts on cardiac stiffness evaluation. We recorded the mechanical waves occurring after aortic and mitral valve closure (AVC, MVC) and those induced by acoustic radiation force throughout the cardiac cycle in four pigs after sternotomy. Natural SWE showed a higher feasibility than active SWE, which is an advantage for clinical application. Median propagation speeds of 2.5-4.0 m/s and 1.6-4.0 m/s were obtained after AVC and MVC, whereas ARF-based median speeds of 0.9-1.2 m/s and 2.1-3.8 m/s were reported for diastole and systole, respectively. The different wave characteristics in both methods, such as the frequency content, complicate the direct comparison of waves. Nevertheless, a good match was found in propagation speeds between natural and active SWE at the moment of valve closure, and the natural waves showed higher propagation speeds than in diastole. Furthermore, the results demonstrated that the natural waves occur in between diastole and systole identified with active SWE, and thus represent a myocardial stiffness in between relaxation and contraction.

Evaluation of Myocardial Stiffness in Hypertensive Patients by Intrinsic Wave Propagation of the Myocardial Stretch

The objective of the study was to evaluate myocardial stiffness in hypertensive patients by measuring the intrinsic velocity propagation (IVP) of the myocardial stretch and to explore the correlation between IVP and cardiac systolic and diastolic functions. Eighty-one hypertensive patients and 53 healthy patients were prospectively enrolled in this study. IVP was measured using high-frame rate tissue Doppler (350-450 frames per second). IVP was significantly higher in hypertensive patients than in the control group (1.53 ± 0.39 m/s vs. 1.40 ± 0.19 m/s, p = 0.031). In the hypertensive group, IVP was significantly higher in patients with electrocardiogram (ECG) strain than in those without ECG strain (1.63 ± 0.46 m/s vs. 1.45 ± 0.32 m/s, p = 0.047). Moreover, IVP exhibited a good correlation with interventricular septal thickness at end-diastole (r = 0.434, p < 0.001), left ventricular posterior wall thickness at end-diastole (r = 0.439, p < 0.001), E/A ratio (r = 0.245, p = 0.004) and global longitudinal systolic strain (r = 0.405, p < 0.001). IVP was significantly higher in hypertensive patients, which indicates elevated myocardial stiffness in this cohort of patients. This novel measurement exhibited great potential for use in clinical practice to assess myocardial stiffness in patients with hypertension non-invasively.

From Anatomy to Functional and Molecular Biomarker Imaging and Therapy: Ultrasound Is Safe, Ultrafast, Portable, and Inexpensive

Ultrasound is the most widely used medical imaging modality worldwide. It is abundant, extremely safe, portable, and inexpensive. In this review, we consider some of the current development trends for ultrasound imaging, which build upon its current strength and the popularity it experiences among medical imaging professional users.Ultrasound has rapidly expanded beyond traditional radiology departments and cardiology practices. Computing power and data processing capabilities of commonly available electronics put ultrasound systems in a lab coat pocket or on a user's mobile phone. Taking advantage of new contributions and discoveries in ultrasound physics, signal processing algorithms, and electronics, the performance of ultrasound systems and transducers have progressed in terms of them becoming smaller, with higher imaging performance, and having lower cost. Ultrasound operates in real time, now at ultrafast speeds; kilohertz frame rates are already achieved by many systems.Ultrasound has progressed beyond anatomical imaging and monitoring blood flow in large vessels. With clinical approval of ultrasound contrast agents (gas-filled microbubbles) that are administered in the bloodstream, tissue perfusion studies are now routine. Through the use of modern ultrasound pulse sequences, individual microbubbles, with subpicogram mass, can be detected and observed in real time, many centimeters deep in the body. Ultrasound imaging has broken the wavelength barrier; by tracking positions of microbubbles within the vasculature, superresolution imaging has been made possible. Ultrasound can now trace the smallest vessels and capillaries, and obtain blood velocity data in those vessels.Molecular ultrasound imaging has now moved closer to clinic; the use of microbubbles with a specific affinity to endothelial biomarkers allows selective accumulation and retention of ultrasound contrast in the areas of ischemic injury, inflammation, or neoangiogenesis. This will aid in noninvasive molecular imaging and may provide additional help with real-time guidance of biopsy, surgery, and ablation procedures.The ultrasound field can be tightly focused inside the body, many centimeters deep, with millimeter precision, and ablate lesions by energy deposition, with thermal or mechanical bioeffects. Some of such treatments are already in clinical use, with more indications progressing through the clinical trial stage. In conjunction with intravascular microbubbles, focused ultrasound can be used for tissue-specific drug delivery; localized triggered release of sequestered drugs from particles in the bloodstream may take time to get to clinic. A combination of intravascular microbubbles with circulating drug and low-power ultrasound allows transient opening of vascular endothelial barriers, including blood-brain barrier; this approach has reached clinical trial stage. Therefore, the drugs that normally would not be getting to the target tissue in the brain will now have an opportunity to produce therapeutic efficacy.Overall, medical ultrasound is developing at a brisk rate, even in an environment where other imaging modalities are also advancing rapidly and may be considered more lucrative. With all the current advances that we discuss, and many more to come, ultrasound may help solve many problems that modern medicine is facing.

Parasternal Versus Apical View in Cardiac Natural Mechanical Wave Speed Measurements

Shear wave speed measurements can potentially be used to noninvasively measure myocardial stiffness to assess the myocardial function. Several studies showed the feasibility of tracking natural mechanical waves induced by aortic valve closure in the interventricular septum, but different echocardiographic views have been used. This article systematically studied the wave propagation speeds measured in a parasternal long-axis and in an apical four-chamber view in ten healthy volunteers. The apical and parasternal views are predominantly sensitive to longitudinal or transversal tissue motion, respectively, and could, therefore, theoretically measure the speed of different wave modes. We found higher propagation speeds in apical than in the parasternal view (median of 5.1 m/s versus 3.8 m/s, , n = 9 ). The results in the different views were not correlated ( r = 0.26 , p = 0.49 ) and an unexpectedly large variability among healthy volunteers was found in apical view compared with the parasternal view (3.5-8.7 versus 3.2-4.3 m/s, respectively). Complementary finite element simulations of Lamb waves in an elastic plate showed that different propagation speeds can be measured for different particle motion components when different wave modes are induced simultaneously. The in vivo results cannot be fully explained with the theory of Lamb wave modes. Nonetheless, the results suggest that the parasternal long-axis view is a more suitable candidate for clinical diagnosis due to the lower variability in wave speeds.

Real-Time Transcranial Histotripsy Treatment Localization and Mapping Using Acoustic Cavitation Emission Feedback

Cavitation events generated during histotripsy therapy generate large acoustic cavitation emission (ACE) signals that can be detected through the skull. This article investigates the feasibility of using these ACE signals, acquired using the elements of a 500-kHz, 256-element hemispherical histotripsy transducer as receivers, to localize and map the cavitation activity in real time through the human skullcap during transcranial histotripsy therapy. The locations of the generated cavitation events predicted using the ACE feedback signals in this study were found to be accurate to within <1.5 mm of the centers of masses detected by optical imaging and found to lie to within the measured volumes of the generated cavitation events in >~80 % of cases. Localization results were observed to be biased in the prefocal direction of the histotripsy array and toward its transverse origin but were only weakly affected by focal steering location. The choice of skullcap and treatment pulse repetition frequency (PRF) were both observed to affect the accuracy of the localization results in the low PRF regime (1-10 Hz), but the localization accuracy was seen to stabilize at higher PRFs (≥10 Hz). Tests of the localization algorithm in vitro, for treatment delivered to a bovine brain sample mounted within the skullcap, revealed good agreement between the ACE feedback-generated treatment map and the morphological characteristics of the treated volume of the brain sample. Localization during experiments was achieved in real time for pulses delivered at rates up to 70 Hz, but benchmark tests indicate that the localization algorithm is scalable, indicating that higher rates are possible with more powerful hardware. The results of this article demonstrate the feasibility of using ACE feedback signals to localize and map transcranially generated cavitation events during histotripsy. Such capability has the potential to greatly simplify transcranial histotripsy treatments, as it may provide a non-MRI-based method for monitoring and localizing transcranial histotripsy treatments in real time.

Direct measurement of cardiac stiffness using echocardiographic shearwave imaging during open-chest surgery: A pilot study in human

PURPOSE

Cardiac stiffness is a marker of diastolic function with a strong prognostic significance in many heart diseases that is not measurable in clinical practice. This study investigates whether elastometry, a surrogate for organ stiffness, is measurable in the heart using ShearWave Imaging.

METHODS

In 33 anesthetized patients scheduled for cardiac surgery, ShearWave imaging was acquired epicardially using a dedicated ultrasound machine on the left ventricle parallel to the left anterior descending coronary artery in a loaded heart following the last cardiac beat. Cardiac elastometry was measured offline using the Young modulus with customized software.

RESULTS

Overall, the ejection fraction was 61 ± 10%. E/A and E/e' ratios were 1.0 ± 0.5 and 10.5 ± 4.1, respectively. Cardiac elastometry averaged 15.3 ± 5.3 kPa with a median of 18 kPa. Patients with high elastometry >18 kPa were older (P = .04), had thicker (P = .02) but smaller LV (P = .004), had larger left atria (P = .05) and a higher BNP level (P = .04). We distinguished three different transmural elastometry patterns: higher epicardial, higher endocardial, or uniformly distributed elastometry.

CONCLUSION

Elastometry measurement was feasible for the human heart. This surrogate for cardiac stiffness dichotomized patients with low and high elastometry, and provided three different phenotypes of transmural elastometry with link to diastolic function.

Analysis of multiple shear wave modes in a nonlinear soft solid: Experiments and finite element simulations with a tilted acoustic radiation force

Tissue nonlinearity is conventionally measured in shear wave elastography by studying the change in wave speed caused by the tissue deformation, generally known as the acoustoelastic effect. However, these measurements have mainly focused on the excitation and detection of one specific shear mode, while it is theoretically known that the analysis of multiple wave modes offers more information about tissue material properties that can potentially be used to refine disease diagnosis. This work demonstrated proof of concept using experiments and finite element simulations in a uniaxially stretched phantom by tilting the acoustic radiation force excitation axis with respect to the material's symmetry axis. Using this unique set-up, we were able to visualize two propagating shear wave modes across the stretch direction for stretches larger than 140%. Complementary simulations were performed using material parameters determined from mechanical testing, which enabled us to convert the observed shear wave behavior into a correct representative constitutive law for the phantom material, i.e. the Isihara model. This demonstrates the potential of measuring shear wave propagation in combination with shear wave modeling in complex materials as a non-invasive alternative for mechanical testing.

Focused Ultrasound Platform for Investigating Therapeutic Neuromodulation Across the Human Hippocampus

Pulsed low-intensity focused ultrasound (PLIFUS) has shown promise in inducing neuromodulation in several animal and human studies. Therefore, it is of clinical interest to develop experimental platforms to test repetitive PLIFUS as a therapeutic modality in humans with neurologic disorders. In the study described here, our aim was to develop a laboratory-built experimental device platform intended to deliver repetitive PLIFUS across the hippocampus in seizure onset zones of patients with drug-resistant temporal lobe epilepsy. The system uses neuronavigation targeting over multiple therapeutic sessions. PLIFUS (548 kHz) was emitted across multiple hippocampal targets in a human subject with temporal lobe epilepsy using a mechanically steered piezoelectric transducer. Stimulation was delivered up to 2.25 W/cm2 spatial peak temporal average intensity (free-field equivalent), with 36%-50% duty cycle, 500-ms sonications and 7-s inter-stimulation intervals lasting 140 s per target and repeated for multiple sessions. A first-in-human PLIFUS course of treatment was successfully delivered using the device platform with no adverse events.

Reproducibility of Natural Shear Wave Elastography Measurements

For the quantification of myocardial function, myocardial stiffness can potentially be measured non-invasively using shear wave elastography. Clinical diagnosis requires high precision. In 10 healthy volunteers, we studied the reproducibility of the measurement of propagation speeds of shear waves induced by aortic and mitral valve closure (AVC, MVC). Inter-scan was slightly higher but in similar ranges as intra-scan variability (AVC: 0.67 m/s (interquartile range [IQR]: 0.40-0.86 m/s) versus 0.38 m/s (IQR: 0.26-0.68 m/s), MVC: 0.61 m/s (IQR: 0.26-0.94 m/s) versus 0.26 m/s (IQR: 0.15-0.46 m/s)). For AVC, the propagation speeds obtained on different day were not statistically different (p = 0.13). We observed different propagation speeds between 2 systems (AVC: 3.23-4.25 m/s [Zonare ZS3] versus 1.82-4.76 m/s [Philips iE33]), p = 0.04). No statistical difference was observed between observers (AVC: p = 0.35). Our results suggest that measurement inaccuracies dominate the variabilities measured among healthy volunteers. Therefore, measurement precision can be improved by averaging over multiple heartbeats.

Imaging Heart Dynamics With Ultrafast Cascaded-Wave Ultrasound

The heart is an organ with highly dynamic complexity, including cyclic fast electrical activation, muscle kinematics, and blood dynamics. Although ultrafast cardiac imaging techniques based on pulsed-wave ultrasound (PUS) have rapidly emerged to permit mapping of heart dynamics, they suffer from limited sonographic signal-to-noise ratio (SNR) and penetration due to insufficient energy delivery and inevitable attenuation through the chest wall. We hereby propose ultrafast cascaded-wave ultrasound (uCUS) imaging to depict heart dynamics in higher SNR and larger penetration than conventional ultrafast PUS. To solve the known tradeoff between the length of transmitted ultrasound signals and spatial resolution while achieving ultrafast frame rates (>1000 Hz), we develop a cascaded synthetic aperture (CaSA) imaging method. In CaSA, an array probe is divided into subapertures; each subaperture transmits a train of diverging waves. These diverging waves are weighted in both the aperture (i.e., spatial) and range (i.e., temporal) directions with a coding matrix containing only +1 and -1 polarity coefficients. A corresponding spatiotemporal decoding matrix is designed to recover backscattered signals. The decoded signals are thereafter beamformed and coherently compounded to obtain one high-SNR beamformed image frame. For CaSA with M subapertures and N cascaded diverging waves, sonographic SNR is increased by 10× log ₁₀ (N ×M) (dB) compared with conventional synthetic aperture (SA) imaging. The proposed uCUS with CaSA was evaluated with conventional SA and Hadamard-encoded SA (H-SA) methods in a calibration phantom for B-mode image quality and an in vivo human heart in a transthoracic setting for the quality assessment of anatomical, myocardial motion, and chamber blood power Doppler images. Our results demonstrated that the proposed uCUS with CaSA (4 subapertures, 32 cascaded waves) improved SNR (+20.46 dB versus SA, +14.83 dB versus H-SA) and contrast ratio (+8.44 dB versus SA, +7.81 dB versus H-SA) with comparable spatial resolutions to and at the same frame rates as benchmarks.

Value of Real-Time Shear Wave Elastography Versus Acoustic Radiation Force Impulse Imaging in the Diagnosis of Female Bladder Neck Obstruction

OBJECTIVE

To investigate the application value of shear wave elastography (SWE) and acoustic radiation force impulse imaging (ARFI) in the diagnosis of female bladder neck obstruction (FBNO), we compared the advantages of these 2 methods to provide a more accurate reference for clinical work.

METHODS

From April 2016 to February 2018, 27 patients who were diagnosed with FBNO by cystoscopy and/or urine dynamics testing were selected for the study, together with 24 healthy adults in a case-control study at Liaoning Province People's Hospital. We collected general information from 27 patients with FBNO, and using transperineal 2-dimensional ultrasound detection, shear wave elastography (SWE) and ARFI were used, respectively, to detect the Young's modulus and shear wave velocity (SWV) of the bladder necks. Cystoscopy results were considered to be the gold standard, and receiver operating characteristic (ROC) curves were drawn for ARFI, SWE, and the combined diagnosis of the two. The efficacy of the diagnosis was determined by comparing the areas under the ROC curves and calculating the sensitivity, specificity, and accuracy.

RESULTS

The areas under the ROC curve for the Q-box mean and max value detected by SWE for FBNO patients were 88.4% and 89.9%, respectively, and the sensitivity, specificity, and accuracy were 81.5%, 79.2%, and 80.4%, respectively. The area under the ROC curve detected by ARFI for FBNO patients was 93.7%, and the sensitivity, specificity, and accuracy were 88.9%, 79.2%, and 84.3%, respectively. The sensitivity, specificity, and accuracy of the combined detection of ARFI and SWE were 92.5%, 87.5%, and 90.2%, respectively. The best diagnostic cutoff point of the SWV or the Q-box mean and max value of SWE in FBNO obtained with the ROC curve was SWV = 2.38 m/s (sensitivity, 71.4%; specificity, 82.5%), Q-box mean = 20.2 kPa, Q-box max = 39.8 kPa (sensitivity, 67.5%; specificity, 76.2%). The average shear wave velocity of ARFI, Q-box mean, and Q-box max value of SWE in the control group were 1.89 ± 0.35 m/s, 15.3 ± 3.6 kPa, and 29.2 ± 8.7 kPa, respectively. In the FBNO group, these values were 2.81 ± 0.63) m/s, 27.2 ± 8.6 kPa, and 51.2 ± 12.3 kPa, respectively. The differences were statistically significant (P < .05).

CONCLUSION

The value of the ARFI in the diagnosis of FBNO is a little greater than that of SWE in terms of sensitivity and accuracy, and the combined detection of ARFI and SWE performs better than ARFI or SWE employed separately.

Naturally Occurring Shear Waves in Healthy Volunteers and Hypertrophic Cardiomyopathy Patients

We apply a high frame rate (over 500 Hz) tissue Doppler method to measure the propagation velocity of naturally occurring shear waves (SW) generated by aortic and mitral valves closure. The aim of this work is to demonstrate clinical relevance. We included 45 healthy volunteers and 43 patients with hypertrophic cardiomyopathy (HCM). The mitral SW (4.68 ± 0.66 m/s) was consistently faster than the aortic (3.51 ± 0.38 m/s) in all volunteers (p < 0.0001). In HCM patients, SW velocity correlated with E/e' ratio (r = 0.346, p = 0.04 for aortic SW and r = 0.667, p = 0.04 for mitral SW). A subgroup of 20 volunteers were matched for age and gender to 20 HCM patients. In HCM, the mean velocity of 5.1 ± 0.7 m/s for the aortic SW (3.61 ± 0.46 m/s in matched volunteers, p < 0.0001) and 6.88 ± 1.12 m/s for the mitral SW(4.65 ± 0.77 m/s in matched volunteers, p < 0.0001). A threshold of 4 m/s for the aortic SW correctly classified pathologic myocardium with a sensitivity of 95% and specificity of 90%. Naturally occurring SW can be used to assess differences between normal and pathologic myocardium.

A Contemporary Look at Biomechanical Models of Myocardium

Understanding and predicting the mechanical behavior of myocardium under healthy and pathophysiological conditions are vital to developing novel cardiac therapies and promoting personalized interventions. Within the past 30 years, various constitutive models have been proposed for the passive mechanical behavior of myocardium. These models cover a broad range of mathematical forms, microstructural observations, and specific test conditions to which they are fitted. We present a critical review of these models, covering both phenomenological and structural approaches, and their relations to the underlying structure and function of myocardium. We further explore the experimental and numerical techniques used to identify the model parameters. Next, we provide a brief overview of continuum-level electromechanical models of myocardium, with a focus on the methods used to integrate the active and passive components of myocardial behavior. We conclude by pointing to future directions in the areas of optimal form as well as new approaches for constitutive modeling of myocardium.

Non-invasive Measurement of Dynamic Myocardial Stiffness Using Acoustic Radiation Force Impulse Imaging

Myocardial stiffness exhibits cyclic variations over the course of the cardiac cycle. These trends are closely tied to the electromechanical and hemodynamic changes in the heart. Characterization of dynamic myocardialstiffness can provide insights into the functional state of the myocardium, as well as allow for differentiation between the underlying physiologic mechanisms that lead to congestive heart failure. Previous work has revealed the potential of acoustic radiation force impulse (ARFI) imaging to capture temporal trends in myocardial stiffness in experimental preparations such as the Langendorff heart, as well as on animals in open-chest and intracardiac settings. This study was aimed at investigating the potential of ARFI to measure dynamic myocardial stiffness in human subjects, in a non-invasive manner through transthoracic imaging windows. ARFI imaging was performed on 12 healthy volunteers to track stiffness changes within the interventricular septum in parasternal long-axis and short-axis views. Myocardial stiffness dynamics over the cardiac cycle was quantified using five indices: stiffness ratio, rates of relaxation and contraction and time constants of relaxation and contraction. The yield of ARFI acquisitions was evaluated based on metrics of signal strength and tracking fidelity such as displacement signal-to-noise ratio, signal-to-clutter level, temporal coherence of speckle and spatial similarity within the region of excitation. These were quantified using the mean ARF-induced displacements over the cardiac cycle, the contrast between the myocardium and the cardiac chambers, the minimum correlation coefficients of radiofrequency signals and the correlation between displacement traces across simultaneously acquired azimuthal beams, respectively. Forty-one percent of ARFI acquisitions were determined to be "successful" using a mean ARF-induced displacement threshold of 1.5 μm. "Successful" acquisitions were found to have higher (i) signal-to-clutter levels, (ii) temporal coherence and (iii) spatial similarity compared with "unsuccessful" acquisitions. Median values of these three metrics, between the two groups, were measured to be 13.42dB versus 5.42dB, 0.988 versus 0.976 and 0.984 versus 0.849, respectively. Signal-to-clutter level, temporal coherence and spatial similarity were also found to correlate with each other. Across the cohort of healthy volunteers, the stiffness ratio measured was 2.74 ± 0.86; the rate of relaxation, 7.82 ± 4.69/s; and the rate of contraction, -7.31±3.79 /s. The time constant of relaxation was 35.90 ± 20.04ms, and that of contraction was 37.24 ± 19.85ms. ARFI-derived indices of myocardial stiffness were found to be similar in both views. These results indicate the feasibility of using ARFI to measure dynamic myocardial stiffness trends in a non-invasive manner and also highlightthe technical challenges of implementing this method in the transthoracic imaging environment.

Natural Shear Wave Imaging in the Human Heart: Normal Values, Feasibility, and Reproducibility

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.

Diaphragm shear modulus reflects transdiaphragmatic pressure during isovolumetric inspiratory efforts and ventilation against inspiratory loading

The reference method for the assessment of diaphragm function relies on the measurement of transdiaphragmatic pressure (Pdi). Local muscle stiffness measured using ultrafast shear wave elastography (SWE) provides reliable estimates of muscle force in locomotor muscles. This study aimed at investigating whether SWE could be used as a surrogate of Pdi to evaluate diaphragm function. Fifteen healthy volunteers underwent a randomized stepwise inspiratory loading protocol of 0–60% of maximal isovolumetric inspiratory pressure during closed-airways maneuvers and 0–50% during ventilation against an external inspiratory threshold load. During all tasks, Pdi was measured and SWE was used to assess shear modulus of the right hemidiaphragm (SMdi) at the zone of apposition. Pearson correlation coefficients ( r) and repeated-measures correlation coefficients ( R) were computed to determine within-individual and overall relationships between Pdi and SMdi, respectively. During closed-airways maneuvers, mean Pdi correlated to mean SMdi in all participants [ r ranged from 0.77 to 0.96, all P < 0.01; R = 0.82, 95% confidence intervals (0.76, 0.86), P < 0.01]. During ventilation against inspiratory threshold loading, Pdi swing correlated to maximal SMdi in all participants [ r ranged from 0.40 to 0.90, all P < 0.01; R = 0.70, 95% confidence intervals (0.66, 0.73), P < 0.001]. Changes in diaphragm stiffness as assessed by SWE reflect changes in transdiaphragmatic pressure. SWE provides a new opportunity for direct and noninvasive assessment of diaphragm function.

NEW & NOTEWORTHY Accurate and specific estimation of diaphragm effort is critical for evaluating and monitoring diaphragm dysfunction. The measurement of transdiaphragmatic pressure requires the use of invasive gastric and esophageal probes. In the present work, we demonstrate that changes in diaphragm stiffness assessed with ultrasound shear wave elastography reflect changes in transdiaphragmatic pressure, therefore offering a new noninvasive method for gauging diaphragm effort.

In Vivo Open- and Closed-chest Measurements of Left-Ventricular Myocardial Viscoelasticity using Lamb wave Dispersion Ultrasound Vibrometry (LDUV): A Feasibility Study

Diastolic dysfunction causes close to half of congestive heart failures and is associated with increased stiffness in left-ventricular myocardium. A clinical tool capable of measuring viscoelasticity of the myocardium could be beneficial in clinical settings. We used Lamb wave Dispersion Ultrasound Vibrometry (LDUV) for assessing the feasibility of making in vivo non-invasive measurements of myocardial elasticity and viscosity in pigs. In vivo open-chest measurements of myocardial elasticity and viscosity obtained using a Fourier space based analysis of Lamb wave dispersion are reported. The approach was used to perform ECG-gated transthoracic in vivo measurements of group velocity, elasticity and viscosity throughout a single heart cycle. Group velocity, elasticity and viscosity in the frequency range 50-500 Hz increased from diastole to systole, consistent with contraction and relaxation of the myocardium. Systolic group velocity, elasticity and viscosity were 5.0 m/s, 19.1 kPa, 6.8 Pa·s, respectively. In diastole, the measured group velocity, elasticity and viscosity were 1.5 m/s, 5.1 kPa and 3.2 Pa·s, respectively.

Soft-Tissue Aberration Correction for Histotripsy

Acoustic aberrations caused by natural heterogeneities of biological soft tissue are a substantial problem for histotripsy, a therapeutic ultrasound technique that uses acoustic cavitation to mechanically fractionate and destroy unwanted target tissue without damaging surrounding tissue. These aberrations, primarily caused by sound speed variations, result in severe defocusing of histotripsy pulses, thereby decreasing treatment efficacy. The gold standard for aberration correction (AC) is to place a hydrophone at the desired focal location to directly measure phase aberrations, which is a method that is infeasible in vivo. We hypothesized that the acoustic cavitation emission (ACE) shockwaves from the initial expansion of inertially cavitating microbubbles generated by histotripsy can be used as a point source for AC. In this study, a 500-kHz, 112-element histotripsy phased array capable of transmitting and receiving ultrasound on all channels was used to acquire ACE shockwaves. These shockwaves were first characterized optically and acoustically. It was found that the shockwave pressure increases significantly as the source changes from a single bubble to a dense cavitation cloud. The first arrival of the shockwave received by the histotripsy array was from the outer-most cavitation bubbles located closest to the histotripsy array. Hydrophone and ACE AC methods were then tested on ex vivo porcine abdominal tissue samples. Without AC, the focal pressure is reduced by 49.7% through the abdominal tissue. The hydrophone AC approach recovered 55.5% of the lost pressure. Using the ACE AC method, over 20% of the lost pressure was recovered, and the array power required to induce cavitation was reduced by approximately 31.5% compared to without AC. These results supported our hypothesis that the ACE shockwaves coupled with a histotripsy array with transmit and receive capability can be used for AC for histotripsy through soft tissue.

Relative identifiability of anisotropic properties from magnetic resonance elastography

Although magnetic resonance elastography (MRE) has been used to estimate isotropic stiffness in the heart, myocardium is known to have anisotropic properties. This study investigated the determinability of global transversely isotropic material parameters using MRE and finite-element modeling (FEM). A FEM-based material parameter identification method, using a displacement-matching objective function, was evaluated in a gel phantom and simulations of a left ventricular (LV) geometry with a histology-derived fiber field. Material parameter estimation was performed in the presence of Gaussian noise. Parameter sweeps were analyzed and characteristics of the Hessian matrix at the optimal solution were used to evaluate the determinability of each constitutive parameter. Four out of five material stiffness parameters (Young's modulii E1 and E3 , shear modulus G13 and damping coefficient s), which describe a transversely isotropic linear elastic material, were well determined from the MRE displacement field using an iterative FEM inversion method. However, the remaining parameter, Poisson's ratio, was less identifiable. In conclusion, Young's modulii, shear modulii and damping can theoretically be well determined from MRE data, but Poisson's ratio is not as well determined and could be set to a reasonable value for biological tissue (close to 0.5).

Field Characterization and Compensation of Vibrational Nonuniformity for a 256-Element Focused Ultrasound Phased Array

Multielement focused ultrasound phased arrays have been used in therapeutic applications to treat large tissue volumes by electronic steering of the focus, to target multiple simultaneous foci, and to correct aberration caused by inhomogeneous tissue pathways. There is an increasing interest in using arrays to generate more complex beam shapes and corresponding acoustic radiation force patterns for manipulation of particles such as kidney stones. Toward this end, experimental and computational tools are needed to enable accurate delivery of desired transducer vibrations and corresponding ultrasound fields. The purpose of this paper was to characterize the vibrations of a 256-element array at 1.5 MHz, implement strategies to compensate for variability, and test the ability to generate specified vortex beams that are relevant to particle manipulation. The characterization of the array output was performed in water using both element-by-element measurements at the focus of the array and holography measurements for which all the elements were excited simultaneously. Both methods were used to quantify each element's output so that the power of each element could be equalized. Vortex beams generated using both compensation strategies were measured and compared to the Rayleigh integral simulations of fields generated by an idealized array based on the manufacturer's specifications. Although both approaches improved beam axisymmetry, compensation based on holography measurements had half the error relative to the simulation results in comparison to the element-by-element method.

Magnetic resonance elastography in nonlinear viscoelastic materials under load

Characterisation of soft tissue mechanical properties is a topic of increasing interest in translational and clinical research. Magnetic resonance elastography (MRE) has been used in this context to assess the mechanical properties of tissues in vivo noninvasively. Typically, these analyses rely on linear viscoelastic wave equations to assess material properties from measured wave dynamics. However, deformations that occur in some tissues (e.g. liver during respiration, heart during the cardiac cycle, or external compression during a breast exam) can yield loading bias, complicating the interpretation of tissue stiffness from MRE measurements. In this paper, it is shown how combined knowledge of a material's rheology and loading state can be used to eliminate loading bias and enable interpretation of intrinsic (unloaded) stiffness properties. Equations are derived utilising perturbation theory and Cauchy's equations of motion to demonstrate the impact of loading state on periodic steady-state wave behaviour in nonlinear viscoelastic materials. These equations demonstrate how loading bias yields apparent material stiffening, softening and anisotropy. MRE sensitivity to deformation is demonstrated in an experimental phantom, showing a loading bias of up to twofold. From an unbiased stiffness of [Formula: see text] Pa in unloaded state, the biased stiffness increases to 9767.5 [Formula: see text]1949.9 Pa under a load of [Formula: see text] 34% uniaxial compression. Integrating knowledge of phantom loading and rheology into a novel MRE reconstruction, it is shown that it is possible to characterise intrinsic material characteristics, eliminating the loading bias from MRE data. The framework introduced and demonstrated in phantoms illustrates a pathway that can be translated and applied to MRE in complex deforming tissues. This would contribute to a better assessment of material properties in soft tissues employing elastography.