In vitro pulsatile flow study in compliant and rigid ascending aorta phantoms by stereo particle image velocimetry

4D-Flow MRI and Vector Ultrasound in the In-Vitro Evaluation of Surgical Aortic Heart Valves - a Pilot Study

INTRODUCTION

The aim of this study was the initial investigation of 4D-Flow MRI and Vector Ultrasound as novel imaging techniques in the in-vitro analysis of hemodynamics in anatomical models. Specifically, by looking at the hemodynamic performance of state-of-the-art surgical heart valves in a 3D-printed aortic arch.

METHODS

The mock circulatory loop simulated physiological, pulsatile flow. Two mechanical and three biological aortic valves prostheses were compared in a 3D-printed aortic arch. 4D magnetic resonance imaging and vector flow Doppler ultrasound served as imaging methods. Hemodynamic parameters such as wall shear stress, flow velocities and pressure gradients were analyzed.

RESULTS

The flow analysis revealed characteristic flow-patterns in the 3D-printed aortic arch. The blood-flow in the arch presented complex patterns, including the formation of helixes and vortices. Higher proximal peak velocities and lower flow volumes were found for biological valves.

CONCLUSION

The mock circulatory loop in combination with modern radiological imaging provides a sufficient basis for the hemodynamic comparison of aortic valves.

Aortic strain, flow pattern and wall shear stress in a patient-specific compliant aorta replica using Shake-the-Box

High-fidelity in vitro flow simulator in combination with high-dimensional flow visualization techniques can offer precise and comprehensive evaluation of aortic hemodynamics. However, it is particularly challenging to create a fully transparent aorta replica that faithfully mimics the aortic curvature and stiffness. In this study, we successfully manufactured a patient-specific compliant aorta phantom with a dilated ascending aorta that can be used in vitro hemodynamic study. We conducted pulsatile flow measurement on the deformable aorta replica using advanced 4D particle tracking velocimetry - Shake-the-Box. The aortic distensibility, circumferential strain, flow pattern, wall shear stress (WSS), and turbulent kinetic energy were assessed. Furthermore, the peak velocity field and WSS distribution were compared to in vivo MRI measurements. We found that the distensibility and circumferential strain of our aortic replica fell within the physiological range of young patients. The aortic diameter changed as much as 5.4 mm (42 %) in a cardiac cycle and the aortic distensibility was 9.9 × 10-3 mmHg-1. In addition, the obtained flow pattern and WSS distribution were found in a good agreement with in vivo MRI measurement. In conclusion, the compliant aorta phantom replicated the aortic wall material well. It also faithfully simulated the aortic flow and near-wall hemodynamics. The relatively large lumen dimension change (5.4 mm) in a cardiac cycle suggests the necessity of considering wall deformation in aortic flow simulations. We propose employing this approach for future studies, such as medical treatment training, validation of in silico fluid-structure interaction models, or as a complement to in vivo measurements.

Literature Survey for In-Vivo Reynolds and Womersley Numbers of Various Arteries and Implications for Compliant In-Vitro Modelling

PURPOSE

In-vitro modelling can be used to investigate haemodynamics of arterial geometry and stent implants. However, in-vitro model fidelity relies on precise matching of in-vivo conditions. In pulsatile flow, velocity distribution and wall shear stress depend on compliance, and the Reynolds and Womersley numbers. However, matching such values may lead to unachievable tolerances in phantom fabrication.

METHODS

Published Reynolds and Womersley numbers for 14 major arteries in the human body were determined via a literature search. Preference was given to in-vivo publications but in-vitro and in-silico values were presented when in-vivo values were not found. Subsequently ascending aorta and carotid artery case studies were presented to highlight the limitations dynamic matching would apply to phantom fabrication.

RESULTS

Seven studies reported the in-vivo Reynolds and Womersley numbers for the aorta and two for the carotid artery. However, only one study each reported in-vivo numbers for the remaining ten arteries. No in-vivo data could be found for the femoral, superior mesenteric and renal arteries. Thus, information derived in-vitro and in-silico were provided instead. The ascending aorta and carotid artery models required scaling to 1.5× and 3× life-scale, respectively, to achieve dimensional tolerance restrictions. Modelling the ascending aorta with the comparatively high viscosity water/glycerine solution will lead to high pump power demands. However, all the working fluids considered could be dynamically matched with low pump demand for the carotid model.

CONCLUSION

This paper compiles available human haemodynamic information, and highlights the paucity of information for some arteries. It also provides a method for optimal in-vitro experimental configuration.

Fluid-structure interactions of peripheral arteries using a coupled in silico and in vitro approach

Vascular compliance is considered both a cause and a consequence of cardiovascular disease and a significant factor in the mid- and long-term patency of vascular grafts. However, the biomechanical effects of localised changes in compliance cannot be satisfactorily studied with the available medical imaging technologies or surgical simulation materials. To address this unmet need, we developed a coupled silico-vitro platform which allows for the validation of numerical fluid-structure interaction results as a numerical model and physical prototype. This numerical one-way and two-way fluid-structure interaction study is based on a three-dimensional computer model of an idealised femoral artery which is validated against patient measurements derived from the literature. The numerical results are then compared with experimental values collected from compliant arterial phantoms via direct pressurisation and ring tensile testing. Phantoms within a compliance range of 1.4-68.0%/100 mmHg were fabricated via additive manufacturing and silicone casting, then mechanically characterised via ring tensile testing and optical analysis under direct pressurisation with moderately statistically significant differences in measured compliance ranging between 10 and 20% for the two methods. One-way fluid-structure interaction coupling underestimated arterial wall compliance by up to 14.7% compared with two-way coupled models. Overall, Solaris™ (Smooth-On) matched the compliance range of the numerical and in vivo patient models most closely out of the tested silicone materials. Our approach is promising for vascular applications where mechanical compliance is especially important, such as the study of diseases which commonly affect arterial wall stiffness, such as atherosclerosis, and the model-based design, surgical training, and optimisation of vascular prostheses.