Experimental foundation for in vivo measurement of the elasticity of the aorta in computed tomography angiography.

OBJECTIVE: This study was performed to determine the feasibility of measuring the elastic properties of the arterial wall in vivo. To prove this concept, elastic parameters were calculated from an aortic model of elastic behavior similar to a human aorta using computed tomography angiography (CTA) images. METHODS: We first constructed an aortic model from polydimethylsiloxane (PDMS). This model was inserted into a pulsatile flow loop. The model was then placed inside a computed tomography scanner. To estimate the elasticity values, we measured the cross-sectional area and the pressure changes in the model during each phase of the simulated cardiac cycle. A discrete wavelet transform (DWT) algorithm was applied to the CTA data to calculate the geometric changes in the pulsatile model over a simulated cardiac cycle for various pulsatile rates and elasticity values of the PDMS material. The elastic modulus of the aortic model wall was derived from these geometric changes. The elastic moduli derived from the CTA data were compared with those obtained by testing strips of the same PDMS material in a tensile testing machine. Our two aortic models had elastic values at both extremes of those found in normal human aortas. RESULTS: The results show a good comparison between the elastic values derived from the CTA data and those obtained in a tensile testing machine. In addition, the elasticity values were found to be independent of the pulsatile rate for mixing ratios of 6:1 and 9:1 (p = .12 and p = .22, respectively). CONCLUSIONS: The elastic modulus of a pulsatile aortic model may be measured by electrocardiographically-gated multi-detector CTA protocol. This preliminary study suggests the possibility of determining non-invasively the elastic properties of a living, functioning aorta using CTA data.

How should we measure and report elasticity in aortic tissue?

BACKGROUND: Different stress-strain definitions are used in the literature to measure the elastic modulus in aortic tissue. There is no agreement as to which stress-strain definition should be implemented. The purpose of this study is to show how different results are given by the various definitions of stress-strain used and to recommend a specific definition when testing aortic tissues. METHODS: Circumferential specimens from three patients with ascending thoracic aortic aneurysm (ATAA) were obtained from the greater curvature and their tensile properties were tested uniaxially. Three stress definitions (second Piola-Kirchhoff stress, engineering stress and true stress) and four strain definitions (Almansi-Hamel strain, Green-St. Venant strain, engineering strain and true strain) were used to determine the elastic modulus. RESULTS: We found that the Almansi-Hamel strain definition exhibited the highest non-linear stress-strain relation and consequently may overestimate the elastic modulus when using different stress definitions (second Piola-Kirchhoff stress, engineering stress and true stress). The Green-St. Venant strain definition yielded the lowest non-linear stress-strain relation using different definitions of stress, which may underestimate the values of elastic modulus. Engineering stress and strain definitions are only valid for small strains and displacements, which make them impractical when analysing soft tissues. We show that the effect of varying the stress definition on the elastic modulus measurements is significant for maximum elastic modulus but not when calculating the hypertensive elastic modulus. CONCLUSIONS: It is important to consider which stress-strain definition is employed when analysing soft tissues. Although the true stress-true strain definition exhibits a non-linear relation, we favour it in tissue mechanics because it gives more accurate measurements of the material's response using the instantaneous values.