Detailed Measurement of Wall Strain with 3D Speckle Tracking in the Aortic Root: A Case of Bionic Support for Clinical Decision Making

Biomechanical characterization of tissue types in murine dissecting aneurysms based on histology and 4D ultrasound-derived strain

Abdominal aortic aneurysm disease is the local enlargement of the aorta, typically in the infrarenal section, causing up to 200,000 deaths/year. In vivo information to characterize the individual elastic properties of the aneurysm wall in terms of rupture risk is lacking. We used a method that combines 4D ultrasound and direct deformation estimation to compute in vivo 3D Green-Lagrange strain in murine angiotensin II-induced dissecting aortic aneurysms, a commonly used mouse model. After euthanasia, histological staining of cross-sectional sections along the aorta was performed in areas where in vivo strains had previously been measured. The histological sections were segmented into intact and fragmented elastin, thrombus with and without red blood cells, and outer vessel wall including the adventitia. Meshes were then created from the individual contours based on the histological segmentations. The isolated contours of the outer wall and lumen from both imaging modalities were registered individually using a coherent point drift algorithm. 2D finite element models were generated from the meshes, and the displacements from the registration were used as displacement boundaries of the lumen and wall contours. Based on the resulting deformed contours, the strains recorded were grouped according to segmented tissue regions. Strains were highest in areas containing intact elastin without thrombus attachment. Strains in areas with intact elastin and thrombus attachment, as well as areas with disrupted elastin, were significantly lower. Strains in thrombus regions with red blood cells were significantly higher compared to thrombus regions without. We then compared this analysis to statistical distribution indices and found that the results of each aligned, elucidating the relationship between vessel strain and structural changes. This work demonstrates the possibility of advancing in vivo assessments to a microstructural level ultimately improving patient outcomes.

Evaluation of aortic stiffness by a new simplified 2D speckle tracking analysis

Arterial stiffness is a marker of cardiovascular damage and an independent predictor of major cardiovascular events, usually assessed via Carotid-Femoral Pulse Wave Velocity (cfPWV). The aim of this study was to test the feasibility and usefulness of aortic strain analysis using a simplified transthoracic echocardiography (TTE) speckle-tracking (ST) based method. 60 consecutive patients with normal blood pressure and aortic dimensions underwent TTE and cfPWV evaluation. Strain analysis was performed on the largest section of the ascending aorta (aA) with a dedicated software; three couples of speckles were identified within the anterior and posterior aortic wall and transverse segments connecting each couple traced. Peak ascending Aorta Strain (PaAS) was defined as the averaged peak percentage deformation of these segments during the cardiac cycle; aortic stiffness index β₂ was defined as 100 × Ln(SBP/DBP)/PaAS. 17 patients were excluded for suboptimal TTE images. In the 43 analyzed individuals (male 58%, age 63 years) PaAS was 5.5 [3.7-8.6] % with excellent inter and intra observer reproducibility (ICC 95% for both). PaAS showed significant correlation with age (r = - 0.64), aA diameter (r = - 0.45), cfPWV (r = - 0.41), stroke volume (r = - 0.41) and heart rate (r = - 0.49). Age remained the only independent determinant of PaAS in a multivariate analysis. β₂ (10.8 [5.9-15.0]) showed significant direct correlation with age, Pulse pressure, aA diameter and cfPWV. A simplified strain analysis of the ascending aorta is technically feasible with good reproducibility; PaAS and β₂ index correlate well with established parameters of arterial stiffness and might represent a new index in evaluating vascular biomechanics.