Annette Caenen

Supersonic Shear Wave Imaging to Assess Arterial Nonlinear Behavior and Anisotropy: Proof of Principle via Ex Vivo Testing of the Horse Aorta

Supersonic shear wave imaging (SSI) is a noninvasive, ultrasound-based technique to quantify the mechanical properties of bulk tissues by measuring the propagation speed of shear waves (SW) induced in the tissue with an ultrasound transducer. The technique has been successfully validated in liver and breast (tumor) diagnostics and is potentially useful for the assessment of the stiffness of arteries. However, SW propagation in arteries is subjected to different wave phenomena potentially affecting the measurement accuracy. Therefore, we assessed SSI in a less complex ex vivo setup, that is, a thick-walled and rectangular slab of an excised equine aorta. Dynamic uniaxial mechanical testing was performed during the SSI measurements, to dispose of a reference material assessment. An ultrasound probe was fixed in an angle position controller with respect to the tissue to investigate the effect of arterial anisotropy on SSI results. Results indicated that SSI was able to pick up stretch-induced stiffening of the aorta. SW velocities were significantly higher along the specimens circumferential direction than in the axial direction, consistent with the circumferential orientation of collagen fibers. Hence, we established a first step in studying SW propagation in anisotropic tissues to gain more insight into the feasibility of SSI-based measurements in arteries.

A versatile and experimentally validated finite element model to assess the accuracy of shear wave elastography in a bounded viscoelastic medium

The feasibility of shear wave elastography (SWE) in arteries for cardiovascular risk assessment remains to be investigated as the arterys thin wall and intricate material properties induce complex shear wave (SW) propagation phenomena. To better understand the SW physics in bounded media, we proposed an in vitro validated finite element model capable of simulating SW propagation, with full flexibility at the level of the tissues geometry, material properties, and acoustic radiation force. This computer model was presented in a relatively basic set-up, a homogeneous slab of gelatin-agar material (4.35 mm thick), allowing validation of the numerical settings according to actual SWE measurements. The resulting tissue velocity waveforms and SW propagation speed matched well with the measurement: 4.46 m/s (simulation) versus 4.63 ± 0.07 m/s (experiment). Further, we identified the impact of geometrical and material parameters on the SW propagation characteristics. As expected, phantom thickness was a determining factor of dispersion. Adding viscoelasticity to the model augmented the estimated wave speed to 4.58 m/s, an even better match with the experimental determined value. This study demonstrated that finite element modeling can be a powerful tool to gain insight into SWE mechanics and will in future work be advanced to more clinically relevant settings.

Supersonic shear wave imaging to assess arterial anisotropy: Ex-vivo testing of the horse aorta

Supersonic shear wave imaging (SSI) has recently emerged as a reliable technique for soft tissue characterization in bulk tissues (e.g. in the context of breast and liver cancer diagnostics). Another promising application of SSI is arterial stiffness assessment, though challenged by complex shear wave (SW) propagation phenomena in this thin-walled setting such as guided waves, dispersion, reflection and refraction on the arterial walls. Therefore, we investigated the sensitivity of SSI to (i) stretch-induced stiffening and (ii) the arterial fiber organization in a simpler ex-vivo arterial setup based on equine aortic tissue, where the SW propagation is deprived of dispersion and guided-wave effects. For this purpose, we conducted simultaneous dynamic mechanical testing of the tissue along with SSI measurements. The probe was rotated around its axis relative to the tissue to investigate whether SSI is able to determine the dominant collagen fiber direction in the tissue. The cyclic behavior of the SW velocities as a response to the dynamic mechanical testing demonstrated the ability of SSI to detect stretch-induced stiffening, though mainly in the circumferential direction. Furthermore, SW velocities were lower when the probe was positioned away from the circumferential direction of the tissue, which could be explained due to the uni-axial testing, the arterial anisotropy and the progressive recruitment of collagen fibers in the circumferential direction. The elasticity modulus assessed from the SSI measurements and the mechanical testing demonstrated the feasibility of SSI to detect the increase in E-modulus as expected from the measured stress-strain curve (factor 2.1 versus 2.3 increase for SSI and mechanical testing respectively). Future work will include performing histology on the investigated tissue to confirm these findings and clarify the link between SSI measurements and the actual fiber orientation.

Experimental observations of shear waves in cylindrical phantoms and excised equine carotid artery

To facilitate the development and application of ultrasound shear wave elastography (SWE) in arteries, it is necessary to understand the nature of shear waves (SWs) propagating in pressurized anisotropic tubes embedded in softer surrounding tissues and blood. Phantom models are widely used to study SWE in tubular settings mimicking arterial geometry without yet considering anisotropy. To investigate SWs behavior in thin-walled structures, we performed SWE measurements in tubular phantoms (inner diameter of 6–6.5 mm; 3 mm and 1 mm wall thickness) with 2 types of boundary conditions: embedded in water or in a softer medium. Furthermore, experiments were performed on excised equine carotid arteries (diameter 5.6 mm, thickness 1 mm).

The effect of stretching on transmural shear wave anisotropy in cardiac shear wave elastography: An ex vivo and in silico study

The feasibility of Shear Wave Elastography (SWE) for assessing fiber organization in anisotropic tissues based on the 3D spatial anisotropy in shear wave (SW) propagation has been demonstrated. In this work, we investigated the performance of SWE in mapping myocardial anisotropy of the left ventricular (LV) wall while increasing the uniaxial stretch. Additionally, a profound study of the occurring SW physics will be realized through finite element (FE) simulations.

Finite element simulations to support the measurement and analysis of Shear Wave Dispersion

In Shear Wave Elastography, quantitative tissue characterization for dispersive shear wave propagation relies on phase speed analysis, which fits a theoretical relationship to the measured frequency-dependent wave velocity. However, this approach is challenged when applied to soft tissues with complex geometries and/or advanced material models, as there are no theoretical dispersion equations available. Therefore, we investigated the potential of an alternative way, i.e. numerical modal analysis, to dispose of a dispersion relationship for simplified tissue settings. The good agreement between the retrieved dispersion characteristics via modal analysis to the ones obtained from theory and temporal finite element simulations, demonstrates the promising power of this technique.

Experimental study on the effect of the cylindrical vessel geometry on arterial shear wave elastography

Shear wave elastography (SWE) is a promising technique for cardiovascular diagnostics including the assessment of arterial stiffness, as it could pick up pressure-induced stiffening of arteries and be sensitive to arterial anisotropy. Previous studies demonstrated the feasibility of SWE in arteries and tissue-mimicking tubular phantoms but all relied on the longitudinal view. Given the circumferential orientation of collagen fibers, investigating arteries in the cross-sectional plane might be more relevant. We therefore investigated and compared SWE in the cross-sectional and longitudinal views using a tissue-mimicking (poly)vinyl alcohol (PVA) tubular phantom. The phantom was pressurized at three different pressure levels of 0mmHg, 30mmHg and 60mmHg to investigate the influence of the changing geometry and curvature on the shear wave propagation. The interaction and comparison between the longitudinally and circumferentially propagating shear wave fronts was studied as well as the influence on shear wave speed (SWS) assessment via a time-of-flight estimation algorithm. SWS as derived from the mechanical assessment of the PVA tissue via pressure-diameter curves and actual SWE measurements gave similar results: 3.8m/s based on the pressure-diameter data versus SWS values ranging from 4.19±0.34m/s to 4.33±0.32m/s in the longitudinal view, and from 4.09±0.65m/s to 5.29±0.52m/s in the cross-sectional view. However, further numerical modeling is necessary to understand the interaction and nature of the circumferential and longitudinal shear waves.

Myocardial stiffness assessment in pediatric cardiology using shear wave imaging

Shear wave elastography (SWE) is a potential tool to support diagnosis and surgical decision making in children with cardiac disorders. However, SWE in this particular setting is challenged by dispersion and 3D shear wave propagation paths, caused by the relatively thin and curved left ventricle with its anisotropic material properties, ultimately complicating the link between SW propagation and tissue stiffness. To study these complex wave phenomena and the accuracy of currently available signal processing algorithms, we used a combined experimental and numerical approach on a generic model of the left ventricle. These experiments revealed that phase speed analysis provided a better estimate of actual stiffness than group speed analysis. On the other hand, our finite element model contributed to gaining insights in the complex wave propagation pattern and allowed 3D-visualization of the SW.