On the Impact of Vessel Wall Stiffness on Quantitative Flow Dynamics in a Synthetic Model of the Thoracic Aorta

Abstract

Aortic wall stiffening is a predictive marker for morbidity in hypertensive patients. Arterial pulse wave velocity (PWV) correlates with the level of stiffness and can be derived using non-invasive 4D-flow magnetic resonance imaging (MRI). The objectives of this study were twofold: to develop subject-specific thoracic aorta models embedded into an MRI-compatible flow circuit operating under controlled physiological conditions; and to evaluate how a range of aortic wall stiffness impacts 4D-flow-based quantification of hemodynamics, particularly PWV. Three aorta models were 3D-printed using a novel photopolymer material at two compliant and one nearly rigid stiffnesses and characterized via tensile testing. Luminal pressure and 4D-flow MRI data were acquired for each model and cross-sectional net flow, peak velocities, and PWV were measured. In addition, the confounding effect of temporal resolution on all metrics was evaluated. Stiffer models resulted in increased systolic pressures (112, 116, and 133 mmHg), variations in velocity patterns, and increased peak velocities, peak flow rate, and PWV (5.8 to 7.3 m/s). Lower temporal resolution (20 ms down to 62.5 ms per image frame) impacted estimates of peak velocity and PWV (7.31 down to 4.77 m/s). Using compliant aorta models is essential to produce realistic flow dynamics and conditions that recapitulated in vivo hemodynamics.

Particle Image Velocimetry to Evaluate Pulse Wave Velocity in Aorta Phantom with the lnD–U Method

“…Moreover, under the realistic pulsatile conditions imposed by the mock loop, the phantom diameter variation was around Δ𝐷 = 0.8 mm which was in accordance of a 53-69 years old male Δ𝐷 [32]. In the field of in vitro simulator where rigid glass [33], 3D-printed [15] and silicone [34] models are used, the aorta phantom made up of silicone appeared as a relevant choice to mimic those aorta phantom mechanical responses to pulsatile flow.…”

“…Global diameter and velocity change on the whole volume could provide a more precise PWV evaluation [15]. Finally, changing the boundary conditions to modify wave reflections and inflow conditions could help testing the limits of this lnD-U technique to evaluate elastic properties and test the robustness of this method with the help of the WI calculation to identify reflection-less phases.…”

“…Since this comparison is not achievable from in vivo to ex vivo testing on a living patient, an equivalent method could be implemented with in vitro models and aorta phantoms. This comparison was achieved in Zimmermann et al, 2021 [15] with aorta phantoms, a circulatory mock loop, 4D-flow MRI intra-luminal pressure measurements, transit-time like PWV estimation and tensile tests. This last study provided a volumetric information on the aorta behavior thanks to 4D-MRI and shows large discrepancies in the Young's modulus based tensile tests and PWV estimation.…”

“…This method can be difficult to implement since invasive tools are often needed, the distance measurement is not trivial, and a perfect synchronization of the two devices that measure the pressure pulse is required [13]. 4D-MRI (Magnetic Resonance Imaging) can be used to image the whole aorta but involve much more complex and expensive equipment [14] with a strong impact of temporal resolution on computed PWV [15].…”

“…The main purpose of this article is to provide a methodology to measure PWV and phantom stiffness on an in vitro simulator. Future experiments could be conducted by introducing the bench in an MRI [15] or using US-Doppler on the phantom to mature the present technique and compare the limitations of each methods.…”