Improved Shear Wave Motion Detection Using Pulse-Inversion Harmonic Imaging With a Phased Array Transducer

Investigation of the effects of myocardial anisotropy for shear wave elastography using impulsive force and harmonic vibration

The myocardium is known to be an anisotropic medium where the muscle fiber orientation changes through the thickness of the wall. Shear wave elastography methods use propagating waves which are measured by ultrasound or magnetic resonance imaging (MRI) techniques to characterize the mechanical properties of various tissues. Ultrasound- or MR-based methods have been used and the excitation frequency ranges for these various methods cover a large range from 24-500 Hz. Some of the ultrasound-based methods have been shown to be able to estimate the fiber direction. We constructed a model with layers of elastic, transversely isotropic materials that were oriented at different angles to simulate the heart wall in systole and diastole. We investigated the effect of frequency on the wave propagation and the estimation of fiber direction and wave speeds in the different layers of the assembled models. We found that waves propagating at low frequencies such as 30 or 50 Hz showed low sensitivity to the fiber direction but also had substantial bias in estimating the wave speeds in the layers. Using waves with higher frequency content (>200 Hz) allowed for more accurate fiber direction and wave speed estimation. These results have particular relevance for MR- and ultrasound-based elastography applications in the heart.

Arterial Stiffness Estimation by Shear Wave Elastography: Validation in Phantoms with Mechanical Testing

Arterial stiffness is an independent risk factor found to correlate with a wide range of cardiovascular diseases. It has been suggested that shear wave elastography (SWE) can be used to quantitatively measure local arterial shear modulus, but an accuracy assessment of the technique for arterial applications has not yet been performed. In this study, the influence of confined geometry on shear modulus estimation, by both group and phase velocity analysis, was assessed, and the accuracy of SWE in comparison with mechanical testing was measured in nine pressurized arterial phantoms. The results indicated that group velocity with an infinite medium assumption estimated shear modulus values incorrectly in comparison with mechanical testing in arterial phantoms (6.7 ± 0.0 kPa from group velocity and 30.5 ± 0.4 kPa from mechanical testing). To the contrary, SWE measurements based on phase velocity analysis (30.6 ± 3.2 kPa) were in good agreement with mechanical testing, with a relative error between the two techniques of 8.8 ± 6.0% in the shear modulus range evaluated (40-100 kPa). SWE by phase velocity analysis was validated to accurately measure stiffness in arterial phantoms.

Cardiac shear-wave elastography using a transesophageal transducer: application to the mapping of thermal lesions in ultrasound transesophageal cardiac ablation

Heart rhythm disorders, such as atrial fibrillation or ventricular tachycardia can be treated by catheter-based thermal ablation. However, clinically available systems based on radio-frequency or cryothermal ablation suffer from limited energy penetration and the lack of lesion's extent monitoring. An ultrasound-guided transesophageal device has recently successfully been used to perform High-Intensity Focused Ultrasound (HIFU) ablation in targeted regions of the heart in vivo. In this study we investigate the feasibility of a dual therapy and imaging approach on the same transesophageal device. We demonstrate in vivo that quantitative cardiac shear-wave elastography (SWE) can be performed with the device and we show on ex vivo samples that transesophageal SWE can map the extent of the HIFU lesions. First, SWE was validated with the transesophageal endoscope in one sheep in vivo. The stiffness of normal atrial and ventricular tissues has been assessed during the cardiac cycle (n = 11) and mapped (n = 7). Second, HIFU ablation has been performed with the therapy-imaging transesophageal device in ex vivo chicken breast samples (n  =  3), then atrial (left, n = 2) and ventricular (left n = 1, right n = 1) porcine heart tissues. SWE provided stiffness maps of the tissues before and after ablation. Areas of the lesions were obtained by tissue color change with gross pathology and compared to SWE. During the cardiac cycle stiffness varied from 0.5   ±   0.1 kPa to 6.0   ±   0.3 kPa in the atrium and from 1.3   ±   0.3 kPa to 13.5   ±   9.1 kPa in the ventricles. The thermal lesions were visible on all SWE maps performed after ablation. Shear modulus of the ablated zones increased to 16.3   ±   5.5 kPa (versus 4.4   ±   1.6 kPa before ablation) in the chicken breast, to 30.3   ±   10.3 kPa (versus 12.2   ±   4.3 kPa) in the atria and to 73.8   ±   13.9 kPa (versus 21.2   ±   3.3 kPa) in the ventricles. On gross pathology, the size of the lesions ranged from 0.1 to 1.5 cm(2) in the imaging plane area. Elasticity-estimated depths and widths of the lesions differed respectively with a median of 0.2 mm (first quartile Q1:  -0.8 mm; third quartile Q3: 2.6 mm) for a mean squared error (MSE) of 5.1 mm(2) and a median of 0.2 mm (Q1:  -2.7 mm; Q3: 2.7 mm) for a MSE of 11.1 mm(2) from gross pathology. We have demonstrated the feasibility of the HIFU thermal ablation monitoring using a dual therapy and imaging transesophageal device. The combination of HIFU, ultrasound imaging and SWE on the same transesophageal system could lead to a new clinical device for a safer and controlled treatment of a wide variety of cardiac arrhythmias.

Coded excitation plane wave imaging for shear wave motion detection

Plane wave imaging has greatly advanced the field of shear wave elastography thanks to its ultrafast imaging frame rate and the large field-of-view (FOV). However, plane wave imaging also has decreased penetration due to lack of transmit focusing, which makes it challenging to use plane waves for shear wave detection in deep tissues and in obese patients. This study investigated the feasibility of implementing coded excitation in plane wave imaging for shear wave detection, with the hypothesis that coded ultrasound signals can provide superior detection penetration and shear wave SNR compared with conventional ultrasound signals. Both phase encoding (Barker code) and frequency encoding (chirp code) methods were studied. A first phantom experiment showed an approximate penetration gain of 2 to 4 cm for the coded pulses. Two subsequent phantom studies showed that all coded pulses outperformed the conventional short imaging pulse by providing superior sensitivity to small motion and robustness to weak ultrasound signals. Finally, an in vivo liver case study on an obese subject (body mass index = 40) demonstrated the feasibility of using the proposed method for in vivo applications, and showed that all coded pulses could provide higher SNR shear wave signals than the conventional short pulse. These findings indicate that by using coded excitation shear wave detection, one can benefit from the ultrafast imaging frame rate and large FOV provided by plane wave imaging while preserving good penetration and shear wave signal quality, which is essential for obtaining robust shear elasticity measurements of tissue.

Ultrafast cardiac ultrasound imaging: technical principles, applications, and clinical benefits

Several recent technical advances in cardiac ultrasound allow data to be acquired at a very high frame rate. Retrospective gating, plane/diverging wave imaging, and multiline transmit imaging all improve the temporal resolution of the conventional ultrasound system. The main drawback of such high frame rate data acquisition is that it typically has reduced image quality. However, for given clinical applications, the acquisition of temporally-resolved data might outweigh the reduction in image quality. It is the aim of this paper to provide an overview of the technical principles behind these new ultrasound imaging modalities, to review the current evidence of their potential clinical added value, and to forecast how they might influence daily clinical practice.

Ultrasound shear wave elasticity imaging quantifies coronary perfusion pressure effect on cardiac compliance

Diastolic heart failure (DHF) is a major source of cardiac related morbidity and mortality in the world today. A major contributor to, or indicator of DHF is a change in cardiac compliance. Currently, there is no accepted clinical method to evaluate the compliance of cardiac tissue in diastolic dysfunction. Shear wave elasticity imaging (SWEI) is a novel ultrasound-based elastography technique that provides a measure of tissue stiffness. Coronary perfusion pressure affects cardiac stiffness during diastole; we sought to characterize the relationship between these two parameters using the SWEI technique. In this work, we demonstrate how changes in coronary perfusion pressure are reflected in a local SWEI measurement of stiffness during diastole. Eight Langendorff perfused isolated rabbit hearts were used in this study. Coronary perfusion pressure was changed in a randomized order (0-90 mmHg range) and SWEI measurements were recorded during diastole with each change. Coronary perfusion pressure and the SWEI measurement of stiffness had a positive linear correlation with the 95% confidence interval (CI) for the slope of 0.009-0.011 m/s/mmHg ( R(2) = 0.88 ). Furthermore, shear modulus was linearly correlated to the coronary perfusion pressure with the 95% CI of this slope of 0.035-0.042 kPa/mmHg ( R(2) = 0.83). In conclusion, diastolic SWEI measurements of stiffness can be used to characterize factors affecting cardiac compliance specifically the mechanical interaction (cross-talk) between perfusion pressure in the coronary vasculature and cardiac muscle. This relationship was found to be linear over the range of pressures tested.

Derivation and analysis of viscoelastic properties in human liver: impact of frequency on fibrosis and steatosis staging

Commercially-available shear wave imaging systems measure group shear wave speed (SWS) and often report stiffness parameters applying purely elastic material models. Soft tissues, however, are viscoelastic, and higher-order material models are necessary to characterize the dispersion associated with broadband shear waves. In this paper, we describe a robust, model-based algorithm and use a linear dispersion model to perform shear wave dispersion analysis in traditionally difficult-to-image subjects. In a cohort of 135 non-alcoholic fatty liver disease patients, we compare the performance of group SWS with dispersion analysis-derived phase velocity c(200 Hz) and dispersion slope dc/df parameters to stage hepatic fibrosis and steatosis. Area under the ROC curve (AUROC) analysis demonstrates correlation between all parameters [group SWS, c(200 Hz), and, to a lesser extent dc/df ] and fibrosis stage, whereas no correlation was observed between steatosis stage and any of the material parameters. Interestingly, optimal AUROC threshold SWS values separating advanced liver fibrosis (≥F3) from mild-to-moderate fibrosis (≤F2) were shown to be frequency-dependent, and to increase from 1.8 to 3.3 m/s over the 0 to 400 Hz shear wave frequency range.

Wave propagation of myocardial stretch: correlation with myocardial stiffness

The mechanism of flow propagation during diastole in the left ventricle (LV) has been well described. Little is known about the associated waves propagating along the heart walls. These waves may have a mechanism similar to pulse wave propagation in arteries. The major goal of the study was to evaluate the effect of myocardial stiffness and preload on this wave transmission. Longitudinal late diastolic deformation and wave speed (Vp) of myocardial stretch in the anterior LV wall were measured using sonomicrometry in 16 pigs. Animals with normal and altered myocardial stiffness (acute myocardial infarction) were studied with and without preload alterations. Elastic modulus estimated from Vp (E VP; Moens-Korteweg equation) was compared to incremental elastic modulus obtained from exponential end-diastolic stress-strain relation (E SS). Myocardial distensibility and α- and β-coefficients of stress-strain relations were calculated. Vp was higher at reperfusion compared to baseline (2.6 ± 1.3 vs. 1.3 ± 0.4 m/s; p = 0.005) and best correlated with E SS (r2 = 0.80, p < 0.0001), β-coefficient (r2 = 0.78, p < 0.0001), distensibility (r2 = 0.47, p = 0.005), and wall thickness/diameter ratio (r2 = 0.42, p = 0.009). Elastic moduli (E VP and E SS) were strongly correlated (r2 = 0.83, p < 0.0001). Increasing preload increased Vp and E VP and decreased distensibility. At multivariate analysis, E SS, wall thickness, and end-diastolic and systolic LV pressures were independent predictors of Vp (r2 model = 0.83, p < 0.0001). In conclusion, the main determinants of wave propagation of longitudinal myocardial stretch were myocardial stiffness and LV geometry and pressure. This local wave speed could potentially be measured noninvasively by echocardiography.

Viscoelastic properties of normal and infarcted myocardium measured by a multifrequency shear wave method: comparison with pressure-segment length method

Our aims were (i) to compare in vivo measurements of myocardial elasticity by shear wave dispersion ultrasound vibrometry (SDUV) with those by the conventional pressure-segment length method, and (ii) to quantify changes in myocardial viscoelasticity during systole and diastole after reperfused acute myocardial infarction. The shear elastic modulus (μ1) and viscous coefficient (μ2) of left ventricular myocardium were measured by SDUV in 10 pigs. Young's elastic modulus was independently measured by the pressure-segment length method. Measurements made with the SDUV and pressure-segment length methods were strongly correlated. At reperfusion, μ1 and μ2 in end-diastole were increased. Less consistent changes were found during systole. In all animals, μ1 increased linearly with left ventricular pressure developed during systole. Preliminary results suggest that μ1 is preload dependent. This is the first study to validate in vivo measurements of myocardial elasticity by a shear wave method. In this animal model, the alterations in myocardial viscoelasticity after a myocardial infarction were most consistently detected during diastole.