Aneurysm Wall Thickness Research Articles

Quantitative Assessment of Abdominal Aortic Aneurysm Geometry

Recent studies have shown that the maximum transverse diameter of an abdominal aortic aneurysm (AAA) and expansion rate are not entirely reliable indicators of rupture potential. We hypothesize that aneurysm morphology and wall thickness are more predictive of rupture risk and can be the deciding factors in the clinical management of the disease. A non-invasive, image-based evaluation of AAA shape was implemented on a retrospective study of 10 ruptured and 66 unruptured aneurysms. Three-dimensional models were generated from segmented, contrast-enhanced computed tomography images. Geometric indices and regional variations in wall thickness were estimated based on novel segmentation algorithms. A model was created using a J48 decision tree algorithm and its performance was assessed using ten-fold cross validation. Feature selection was performed using the χ2-test. The model correctly classified 65 datasets and had an average prediction accuracy of 86.6% (κ=0.37). The highest ranked features were sac length, sac height, volume, surface area, maximum diameter, bulge height, and intra-luminal thrombus volume. Given that individual AAAs have complex shapes with local changes in surface curvature and wall thickness, the assessment of AAA rupture risk should be based on the accurate quantification of aneurysmal sac shape and size.

Indirect measurement of aneurysm wall thickness using digital stethoscope

Objective: No existing in vivo technique can measure aneurysm wall thickness for evaluation of rupture risk. Intracranial aneurysms produce bruits at a special range of frequency that are highly influenced by the wall thickness. Understanding of the mechanism that generates bruits may allow us to learn aneurysm behavior non-invasively.Methods: A new theory was proposed to account for an interaction between an aneurysm and its parent vessel. Four patients with ophthalmic aneurysms were studied with a digital electronic stethoscope before and after endovascular treatment. Energy spectra of bruits were obtained from digital recording at both eyes. Change of energy spectra was used as an objective indication for aneurysm bruits. Additional four cases were obtained from a previous report.Results: Aneurysm bruits are affected by both aneurysm size and wall thickness. These sounds disappear after coil embolization and parent artery occlusion, but not by stenting. Both large and small aneurysms generate sounds at high frequency. Aneurysms at 6 mm produced very low frequency sound. Wall thickness decreases with aneurysm size, and the decrease is more pronounced at 8 mm.Conclusions: Interaction between an intracranial aneurysm and its parent vessel is important in interpretation of aneurysm bruits. An analysis of in vivo measurements shows a rapid decline in wall thickness for 8 mm aneurysms.

Overestimation of cerebral aneurysm wall thickness by black blood MRI?

Overestimation of cerebral aneurysm wall thickness by black blood MRI?

A model for saccular cerebral aneurysm growth by collagen fibre remodelling

The first structural model for saccular cerebral aneurysm growth is proposed. It is assumed that the development of the aneurysm is accompanied by a loss of the media, and that only collagen fibres provide load-bearing capacity to the aneurysm wall. The aneurysm is modelled as an axisymmetric multi-layered membrane, exposed to an inflation pressure. Each layer is characterized by an orientation angle, which changes between different layers. The collagen fibres and fibroblasts within a specific layer are perfectly aligned. The growth and the morphological changes of the aneurysm are accomplished by the turnover of collagen. Fibroblasts are responsible for collagen production, and the related deformations are assumed to govern the collagen production rate. There are four key parameters in the model: a normalized pressure, the number of layers in the wall, an exponent in the collagen mass production rate law, and the pre-stretch under which the collagen is deposited. The influence of the model parameters on the aneurysmal response is investigated, and a stability analysis is performed. The model is able to predict clinical observations and mechanical test results, for example, in terms of predicted aneurysm size, shape, wall stress and wall thickness.

Fluid-structure interaction in abdominal aortic aneurysms: effects of asymmetry and wall thickness

BackgroundAbdominal aortic aneurysm (AAA) is a prevalent disease which is of significant concern because of the morbidity associated with the continuing expansion of the abdominal aorta and its ultimate rupture. The transient interaction between blood flow and the wall contributes to wall stress which, if it exceeds the failure strength of the dilated arterial wall, will lead to aneurysm rupture. Utilizing a computational approach, the biomechanical environment of virtual AAAs can be evaluated to study the affects of asymmetry and wall thickness on this stress, two parameters that contribute to increased risk of aneurysm rupture.MethodsTen virtual aneurysm models were created with five different asymmetry parameters ranging from β = 0.2 to 1.0 and either a uniform or variable wall thickness to study the flow and wall dynamics by means of fully coupled fluid-structure interaction (FSI) analyses. The AAA wall was designed to have a (i) uniform 1.5 mm thickness or (ii) variable thickness ranging from 0.5 – 1.5 mm extruded normally from the boundary surface of the lumen. These models were meshed with linear hexahedral elements, imported into a commercial finite element code and analyzed under transient flow conditions. The method proposed was then compared with traditional computational solid stress techniques on the basis of peak wall stress predictions and cost of computational effort.ResultsThe results provide quantitative predictions of flow patterns and wall mechanics as well as the effects of aneurysm asymmetry and wall thickness heterogeneity on the estimation of peak wall stress. These parameters affect the magnitude and distribution of Von Mises stresses; varying wall thickness increases the maximum Von Mises stress by 4 times its uniform thickness counterpart. A pre-peak systole retrograde flow was observed in the AAA sac for all models, which is due to the elastic energy stored in the compliant arterial wall and the expansion force of the artery during systole.ConclusionBoth wall thickness and geometry asymmetry affect the stress exhibited by a virtual AAA. Our results suggest that an asymmetric AAA with regional variations in wall thickness would be exposed to higher mechanical stresses and an increased risk of rupture than a more fusiform AAA with uniform wall thickness. Therefore, it is important to accurately reproduce vessel geometry and wall thickness in computational predictions of AAA biomechanics.

The geology of a part of Acadia and the nature of the Acadian orogeny across Central and Eastern Maine

<h3>BACKGROUND AND PURPOSE:</h3> Both hemodynamics and aneurysm wall thickness are important parameters in aneurysm pathophysiology. Our aim was to develop a method for semi-quantitative wall thickness assessment on in vivo 7T MR images of intracranial aneurysms for studying the relation between apparent aneurysm wall thickness and wall shear stress. <h3>MATERIALS AND METHODS:</h3> Wall thickness was analyzed in 11 unruptured aneurysms in 9 patients who underwent 7T MR imaging with a TSE-based vessel wall sequence (0.8-mm isotropic resolution). A custom analysis program determined the in vivo aneurysm wall intensities, which were normalized to the signal of nearby brain tissue and were used as measures of apparent wall thickness. Spatial wall thickness variation was determined as the interquartile range in apparent wall thickness (the middle 50% of the apparent wall thickness range). Wall shear stress was determined by using phase-contrast MR imaging (0.5-mm isotropic resolution). We performed visual and statistical comparisons (Pearson correlation) to study the relation between wall thickness and wall shear stress. <h3>RESULTS:</h3> 3D colored apparent wall thickness maps of the aneurysms showed spatial apparent wall thickness variation, which ranged from 0.07 to 0.53, with a mean variation of 0.22 (a variation of 1.0 roughly means a wall thickness variation of 1 voxel [0.8 mm]). In all aneurysms, apparent wall thickness was inversely related to wall shear stress (mean correlation coefficient, −0.35; <i>P</i> &lt; .05). <h3>CONCLUSIONS:</h3> A method was developed to measure the wall thickness semi-quantitatively, by using 7T MR imaging. An inverse correlation between wall shear stress and apparent wall thickness was determined. In future studies, this noninvasive method can be used to assess spatial wall thickness variation in relation to pathophysiologic processes such as aneurysm growth and rupture.

The influence of aorta-aneurysm geometry upon stress in the aneurysm wall

Finite element analysis (FEA), a computer-based method for solving complex structural problems, was used to determine the wall stress distribution in three cases of model infrarenal abdominal aortic aneurysms representing common problems in determining risk of aneurysm rupture. The point of maximum circumferential wall stress in a spherical model aneurysm was located near the junction of the aneurysm and the nondilated aorta, while maximum longitudinal wall stress was located at the point of maximum diameter of the aneurysm. FEA showed that cylindrically shaped constant thickness model aneurysms had a higher maximum circumferential stress ( σ c = 11.9 × 10 5 dyn/cm 2) and a comparable maximum longitudinal wall stress ( σ L = 6.6 × 10 5 dyn/cm 2) when compared with spherical model aneurysms of the same diameter ( σ c = 8.1 × 10 5 dyn/cm 2 and σ L = 6.2 × 10 5 dyn/cm 2). Analysis of the aorta to aneurysm diameter ratio (A:a gradient) indicated that aortic size is important in determining aneurysm wall stress and that the relationship between aortic size and wall stress is dependent upon aneurysm wall thickness. We conclude that the ability of the aneurysm wall to withstand stress in the longitudinal as well as the circumferential direction is an important factor determining aneurysm rupture. Finally, this investigation showed that FEA is a versatile tool for use in studying the mechanics of vascular structures, making it potentially more useful than size alone in estimating the clinical significance of abdominal aortic aneurysms.