# Non-invasive estimation of material properties of normal and dissected human ascending aortas in vivo: comparison with the ex vivo tensile experiment

## TL;DR

This study compares non-invasive in vivo and ex vivo methods to estimate the mechanical properties of human aortas, finding significant differences in stiffness measurements.

## Contribution

A novel non-invasive method is proposed to estimate in vivo anisotropic mechanical properties of aortas and compare them with ex vivo data.

## Key findings

- In vivo estimates using ex vivo parameters showed up to −33.44% relative error in Young’s modulus ratios compared to ex vivo measurements.
- A 5% variation in initial parameters caused less than 1.5% change in estimated in vivo properties.
- Estimated in vivo circumferential Young’s modulus was consistently lower than ex vivo results for both normal and dissected aortas.

## Abstract

Patient-specific aortic material properties play a critical role in aortic dissection development. In this study, a non-invasive method was employed to assess the in vivo anisotropic mechanical properties of normal and dissected ascending aortas and compare them with their ex vivo material properties.

Biaxial tensile testing was performed on 10 ascending aortic specimens (five patients with type-A aortic dissection and five donors without aortic diseases), with testing data fitted using anisotropic Mooney–Rivlin models. An iterative algorithm was proposed to determine in vivo aortic material properties by matching systolic and diastolic aortic geometries from echocardiography images with those from computed tomography-based computational models. Three settings of initial guesses of material parameters (M01: subject-specific ex vivo parameters; M02: ex vivo parameters of one subject with median stiffness; M03: a 5% variation applied to M02) were investigated in the iterative algorithm for their influence on in vivo property estimation and effective Young’s moduli along the circumferential (YMc) and axial (YMa) directions.

M01-derived in vivo properties had a maximum relative error of −33.44% in YMc/YMa among 10 subjects compared to ex vivo material properties. The median relative error of YMc was −29.40% for M02. Furthermore, a 5% variation in initial parameters caused less than 1.5% change in the estimated in vivo properties. The anisotropy difference between the initial material guess and real aortic tissue would exert a significant impact on YMa estimation but negligible effects on stress distributions.

Overall, in vivo material properties estimated using the proposed method exhibited lower YMc values than the experimental results for normal and dissected ascending aortas.