Gwh Geert Willem Schurink

Initial stress and nonlinear material behavior in patient-specific AAA wall stress analysis.

Rupture risk estimation of abdominal aortic aneurysms (AAA) is currently based on the maximum diameter of the AAA. A more critical approach is based on AAA wall stress analysis. For that, in most cases, the AAA geometry is obtained from CT-data and treated as a stress free geometry. However, during CT imaging, the AAA is subjected to a time-averaged blood pressure and is therefore not stress free. The aim of this study is to evaluate the effect of neglecting these initial stresses (IS) on the patient-specific AAA wall stress as computed by finite element analysis. Additionally, the contribution of the nonlinear material behavior of the AAA wall is evaluated. Thirty patients with maximum AAA diameters below the current surgery criterion were scanned with contrast-enhanced CT and the AAAs were segmented from the image data. The mean arterial blood pressure (MAP) was measured immediately after the CT-scan and used to compute the IS corresponding with the CT geometry and MAP. Comparisons were made between wall stress obtained with and without IS and with linear and nonlinear material properties. On average, AAA wall stresses as computed with IS were higher than without IS. This was also the case for the stresses computed with the nonlinear material model compared to the linear material model. However, omitting initial stress and material nonlinearity in AAA wall stress computations leads to different effects in the resulting wall stress for each AAA. Therefore, provided that other assumptions made are not predominant, IS cannot be discarded and a nonlinear material model should be used in future patient-specific AAA wall stress analyses.

Patient-Specific AAA Wall Stress Analysis: 99-Percentile Versus Peak Stress

OBJECTIVE Biomechanically, rupture of an Abdominal Aortic Aneurysm (AAA) occurs when the stress acting on the wall due to the blood pressure, exceeds the strength of the wall. Peak wall stress estimations, based on CT reconstruction, may be prone to observer variation. This study focuses on the robustness and reproducibility of AAA wall stress assessment and the relation with geometrical features of the AAA. METHODS The AAAs of twenty patients were reconstructed by three operators. Both the peak and 99-percentile stress were used for intra- and inter-operator variability using the intraclass correlation coefficient (ICC). A regression analysis was performed to relate the stress parameters with the maximum diameter. Outliers were analyzed by their geometrical characteristics. RESULTS The intra-operator ICC was 0.73-0.79 for the peak stress and 0.94 for the 99-percentile stress. The inter-operator ICC was 0.71 for the peak stress and 0.95 for the 99-percentile stress. A significant linear relation with the diameter was found only for the 99-percentile stress. CONCLUSIONS The 99-percentile stress is more reproducible than peak wall stress. A significant relation between wall stress and diameter was found. Other geometrical features had no statistical relation with high stress.

Non-linear viscoelastic behavior of abdominal aortic aneurysm thrombus

The objective of this work was to determine the linear and non-linear viscoelastic behavior of abdominal aortic aneurysm thrombus and to study the changes in mechanical properties throughout the thickness of the thrombus. Samples are gathered from thrombi of seven patients. Linear viscoelastic data from oscillatory shear experiments show that the change of properties throughout the thrombus is different for each thrombus. Furthermore the variations found within one thrombus are of the same order of magnitude as the variation between patients. To study the non-linear regime, stress relaxation experiments are performed. To describe the phenomena observed experimentally, a non-linear multimode model is presented. The parameters for this model are obtained by fitting this model successfully to the experiments. The model cannot only describe the average stress response for all thrombus samples but also the highest and lowest stress responses. To determine the influence on the wall stress of the behavior observed the model proposed needs to implemented in the finite element wall stress analysis.

A longitudinal feasibility study on AAA growth vs. ultrasound elastography

To improve risk stratification of abdominal aortic aneurysms, ultrasound elastography could provide insight in changes in mechanical properties of the aneurysmal wall. In this feasibility study, 2-D US elastography was performed in AAA patients in a longitudinal study. Several patients with non-growing, slow growing and fast growing AAAs were imaged using 2D ultrasound. The RF-data were processed to estimate the mechanical properties, e.g., compliance (C), distensibility (D) and incremental Youngs modulus (Einc). An improved RF-based tracking algorithm was used to estimate the volume changes over time, assuming rotational symmetry of the aneurysm. The estimated parameters and possible changes were compared between the three groups. Incremental moduli ranging from 1- 8 MPa were found. The distensibility ranged from 2 to 10 MPa-1. The relative increase in Einc was -10 to 100% for the patients with no growth, 25% -125% for the medium group and 75% to 700% for the patients with fast growth. Distensibility also decreased by a factor 5. The largest increase in stiffness corresponded to the largest increase in diameter. These preliminary findings imply that large changes in mechanical properties occur in the aneurysmal wall in periods of considerable growth. Automated segmentation and higher frame rates might decrease the intra-subject variability. Finally, the inclusion of more patients is required to strengthen the evidence found.

Wall stress analysis of abdominal aortic aneurysms using 3D ultrasound

Wall stress analysis of abdominal aortic aneurysms is a novel tool that has proven high potential to improve risk stratification of abdominal aortic aneurysms (AAAs). Wall stress analysis is based on computed tomography (CT) and magnetic resonance imaging, however, 3D ultrasound (US) has not been used yet. In this study, the feasibility of 3D US based wall stress analysis is investigated and compared to CT. Three-dimensional US and CT data were acquired in 15 patients (diameter 35 - 90 mm). US data were segmented manually and compared to automatically acquired CT geometries by calculating the similarity index (SI) and Hausdorff distance (HD). Wall stresses were simulated at p = 140 mmHg using a non-linear material model (Raghavan & Vorp). The SI of US vs CT was 0.75 - 0.91, with a median HD of 5 - 15 mm, with the higher values found at the proximal and distal sides of the AAA. Wall stresses were in accordance with literature and a good agreement was found between US and CT-based median stresses and inter-quartile stresses. Stress values of US were typically higher, the result of geometrical irregularities, caused by the manual segmentation of the US data. In future work, an automated segmentation approach is the essential point of improvement.

Automated 2D ultrasound fusion imaging of abdominal aortic aneurysms

Recent studies reveal the benefits of ultrasound strain imaging and elastography of abdominal aortic aneurysms (AAAs). However, feasibility and reproducibility of these techniques is not trivial due to the low imaging depth and low contrast of the US data. Beam-steering overcomes these problems in superficial arteries, but is not applicable for AAAs. Multiangle acquisition could improve both aortic wall segmentation and strain imaging in a similar fashion. In this study, an automated technique for fusion of two-dimensional images, acquired manually at different positions, was developed and applied to ultrasound data of AAAs (n = 5). It was attempted to acquire images at -45, 0 and 45 degrees. Feature points were detected using a scale-space approach and were clustered based on anisotropy of the neighborhood. Next, an ellipsoid was fit through the remaining points. By registering these ellipses, the three different images were compounded. Initial results reveal that the method is able to perform automated registration. The estimated angle between the left and middle images was -25° +/16° and was 37° +/15° between the middle and right position (n = 4). The ellipsoid fit showed more variation in lateral direction. However, additional features should be considered for registration. Results suggest that automated wall thickness assessment might be possible using the extracted feature points.

Intraluminal Thrombus in AAA Wall Stress Analysis

In previous research abdominal aortic aneurysm (AAA) wall stress analysis has proven to be more accurate in rupture risk prediction than the clinically used diameter criterion [1]. Together with Philips Medical Systems (Best, NL), a clinical software tool is created that automatically derives the AAA geometry from patient CT data and performs AAA wall stress analysis (Hemodyn package). Using this software package, the role of intraluminal thrombus (ILT) in AAA wall stress analysis is evaluated in this study.Copyright