Andreas Wittek

In vivo determination of elastic properties of the human aorta based on 4D ultrasound data

Computational analysis of the biomechanics of the vascular system aims at a better understanding of its physiology and pathophysiology. To be of clinical use, however, these models and thus their predictions, have to be patient specific regarding geometry, boundary conditions and material. In this paper we present an approach to determine individual material properties of human aortae based on a new type of in vivo full field displacement data acquired by dimensional time resolved three dimensional ultrasound (4D-US) imaging. We developed a nested iterative Finite Element Updating method to solve two coupled inverse problems: The prestrains that are present in the imaged diastolic configuration of the aortic wall are determined. The solution of this problem is integrated in an iterative method to identify the nonlinear hyperelastic anisotropic material response of the aorta to physiologic deformation states. The method was applied to 4D-US data sets of the abdominal aorta of five healthy volunteers and verified by a numerical experiment. This non-invasive in vivo technique can be regarded as a first step to determine patient individual material properties of the human aorta.

Method for Aortic Wall Strain Measurement With Three-Dimensional Ultrasound Speckle Tracking and Fitted Finite Element Analysis

BACKGROUND Aortic wall strains are indicators of biomechanical changes of the aorta due to aging or progressing pathologies such as aortic aneurysm. We investigated the potential of time-resolved three-dimensional ultrasonography coupled with speckle-tracking algorithms and finite element analysis as a novel method for noninvasive in vivo assessment of aortic wall strain. METHODS Three-dimensional volume datasets of 6 subjects without cardiovascular risk factors and 2 abdominal aortic aneurysms were acquired with a commercial real time three-dimensional echocardiography system. Longitudinal and circumferential strains were computed offline with high spatial resolution using a customized commercial speckle-tracking software and finite element analysis. Indices for spatial heterogeneity and systolic dyssynchrony were determined for healthy abdominal aortas and abdominal aneurysms. RESULTS All examined aortic wall segments exhibited considerable heterogenous in-plane strain distributions. Higher spatial resolution of strain imaging resulted in the detection of significantly higher local peak strains (p ≤ 0.01). In comparison with healthy abdominal aortas, aneurysms showed reduced mean strains and increased spatial heterogeneity and more pronounced temporal dyssynchrony as well as delayed systole. CONCLUSIONS Three-dimensional ultrasound speckle tracking enables the analysis of spatially highly resolved strain fields of the aortic wall and offers the potential to detect local aortic wall motion deformations and abnormalities. These data allow the definition of new indices by which the different biomechanical properties of healthy aortas and aortic aneurysms can be characterized.

A finite element updating approach for identification of the anisotropic hyperelastic properties of normal and diseased aortic walls from 4D ultrasound strain imaging.

Computational analysis of the biomechanics of the vascular system aims at a better understanding of its physiology and pathophysiology and eventually at diagnostic clinical use. Because of great inter-individual variations, such computational models have to be patient-specific with regard to geometry, material properties and applied loads and boundary conditions. Full-field measurements of heterogeneous displacement or strain fields can be used to improve the reliability of parameter identification based on a reduced number of observed load cases as is usually given in an in vivo setting. Time resolved 3D ultrasound combined with speckle tracking (4D US) is an imaging technique that provides full field information of heterogeneous aortic wall strain distributions in vivo. In a numerical verification experiment, we have shown the feasibility of identifying nonlinear and orthotropic constitutive behaviour based on the observation of just two load cases, even though the load free geometry is unknown, if heterogeneous strain fields are available. Only clinically available 4D US measurements of wall motion and diastolic and systolic blood pressure are required as input for the inverse FE updating approach. Application of the developed inverse approach to 4D US data sets of three aortic wall segments from volunteers of different age and pathology resulted in the reproducible identification of three distinct and (patho-) physiologically reasonable constitutive behaviours. The use of patient-individual material properties in biomechanical modelling of AAAs is a step towards more personalized rupture risk assessment.

High Resolution Strain Analysis Comparing Aorta and Abdominal Aortic Aneurysm with Real Time Three Dimensional Speckle Tracking Ultrasound

OBJECTIVE/BACKGROUND Ultrasound measurement of aortic diameter for aneurysm screening allows supervision of aneurysm growth. Additional biomechanical analysis of wall motion and aneurysm deformation can supply information about individual elastic properties and the pathological state of the aortic wall. Local aortic wall motion was analyzed through imaged aortic segments according to age and pathology. METHODS Sixty-five patients were examined with a commercial four dimensional ultrasound system (4D-US). Three groups were defined: patients with normal aortic diameter and younger than 60 years of age (n = 21); those with normal aortic diameter and older than 60 years of age (n = 25); and those with infrarenal aortic aneurysm (n = 19). A diastolic reference shape of aortic wall segments was obtained and local and temporally resolved wall strain was determined. Indices characterizing the resulting wall strain distribution were determined. RESULTS The analysis of biomechanical properties displayed increasing heterogeneous and dyssynchronous circumferential strain with increasing patient age. Young patients exhibited higher mean strain amplitude. The distribution of the spatial heterogeneity index and local strain ratio was inversely proportional to age. The maximum local strain amplitude was significantly higher in the young (0.26 ± 0.17) compared with the old (0.16 ± 0.07) or aneurysmal aorta (0.16 ± 0.10). Temporal dyssynchrony significantly differed between young (0.13 ± 0.10) and old (aneurysmal 0.31 ± 0.04, non-aneurysmal 0.29 ± 0.05), regardless of aortic diameter. The spatial heterogeneity index and local strain ratio differentiate non-aneurysmal and aneurysmal aorta, regardless of age. CONCLUSIONS 4D-US strain imaging enables description of individual wall motion (kinematics) of the infrarenal aorta with high spatial and temporal resolution. Functional differences between young, old, and aneurysmal aorta can be described by mean (circumferential) strain amplitude, the spatial heterogeneity index, and the local strain ratio. Further investigation is required to refine this new perspective of patient individualized characterization of the pathological AAA wall and eventually to rupture risk stratification.

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

Three-dimensional (3D) wall motion tracking (WMT) based on ultrasound imaging enables estimation of aortic wall motion and deformation. It provides insights into changes in vascular compliance and vessel wall properties essential for understanding the pathogenesis and progression of aortic diseases. In this report, we employed the novel 3D WMT analysis on the ascending aorta aneurysm (AA) to estimate local aortic wall motion and strain in case of a patient scheduled for replacement of the aortic root. Although progression of the diameter indicates surgical therapy, at present we addressed the question for optimal surgical time point. According to the data, AA in our case has enlarged diameter and subsequent reduced circumferential wall strain, but area tracking data reveals almost normal elastic properties. Virtual remodeling of the aortic root opens a play list for different loading conditions to determine optimal surgical intervention in time.

Characterization of the mechanical behavior and pathophysiological state of abdominal aortic aneurysms based on 4D ultrasound strain imaging

Abdominal aortic aneurysms (AAA) are a degenerative disease of the human aortic wall that may lead to weakening and eventually rupture of the wall with high mortality rates. Since the currently established criterion for surgical or endovascular treatment of the disease is imprecise in the individual case and treatment is not free of complications, the need for additional patient-individual biomarkers for short-term AAA rupture risk as basis for improved clinical decision making. Time resolved 3D ultrasound combined with speckle tracking algorithms is a novel non-invasive medical imaging technique that provides full-field displacement and strain measurements of aortic and aneurysmal wall motion. This is patient-individual information that has not been used so far to assess wall strength and rupture risk. The current study uses simple statistical indices of the heterogeneous spatial distribution of in-plane strain components as biomarkers for the pathological state of the aortic and aneurysmal wall. The pathophysiological rationale behind this approach are the known changes in microstructural composition of the aortic wall with progression of AAA development that results in increased stiffening and heterogeneity of the walls mechanical properties and in decreased wall strength. In a comparative analysis of the aortic wall motion of young volunteers without known cardiovascular diseases, aged arteriosclerotic patients without AAA, and AAA patients, mean values of all in-plane strain components were significantly reduced, and the heterogeneity of circumferential strain was significantly increased in the AAA group compared to both other groups. The capacity of the proposed method to differentiate between wall motion of aged, arteriosclerotic patients and AAA patients is a promising step towards a new method for in vivo assessment of AAA wall strength or stratification of AAA rupture risk as basis for improved clinical decision making on surgical or endovascular treatment of AAA.

STRAIN MEASUREMENT OF ABDOMINAL AORTIC ANEURYSM WITH 3D ULTRASOUND SPECKLE TRACKING

The main risk associated with abdominal aortic aneurysm is rupture caused by mechanical failure of the vascular tissue. To predict individual rupture risk it is necessary to determine biomechanical properties of the aneurysm in vivo, which until the present, has been limited to 2-dimensional analysis of the aorta using conventional sonography, CT or MRI [Vorp, 2005]. Recently, 3-dimensional timeresolved speckle tracking ultrasound has been introduced to measure biomechanical strain parameters of the heart in vivo [Kapetanakis, 2005]. The aim of this investigation was to establish and validate this 3D ultrasound method, applied to the abdominal aortic aneurysm.