Clinical validation of a software for quantitative follow-up of abdominal aortic aneurysm maximal diameter and growth by CT angiography

Purpose To compare the reproducibility and accuracy of abdominal aortic aneurysm (AAA) maximal diameter (D-max) measurements using segmentation software, with manual measurement on double-oblique MPR as a reference standard. Materials and methods The local Ethics Committee approved this study and waived informed consent. Forty patients (33 men, 7 women; mean age, 72 years, range, 49–86 years) had previously undergone two CT angiography (CTA) studies within 16 ± 8 months for follow-up of AAA ≥35 mm without previous treatment. The 80 studies were segmented twice using the software to calculate reproducibility of automatic D-max calculation on 3D models. Three radiologists reviewed the 80 studies and manually measured D-max on double-oblique MPR projections. Intra-observer and inter-observer reproducibility were calculated by intraclass correlation coefficient (ICC). Systematic errors were evaluated by linear regression and Bland–Altman analyses. Differences in D-max growth were analyzed with a paired Student's t-test. Results The ICC for intra-observer reproducibility of D-max measurement was 0.992 (≥0.987) for the software and 0.985 (≥0.974) and 0.969 (≥0.948) for two radiologists. Inter-observer reproducibility was 0.979 (0.954–0.984) for the three radiologists. Mean absolute difference between semi-automated and manual D-max measurements was estimated at 1.1 ± 0.9 mm and never exceeded 5 mm. Conclusion Semi-automated software measurement of AAA D-max is reproducible, accurate, and requires minimal operator intervention

Using averaged models from 4D ultrasound strain imaging allows to signifcantly diferentiate local wall strains in calcifed regions of abdominal aortic aneurysms

Abdominal aortic aneurysms are a degenerative disease of the aorta associated with high mortality. To date, in vivo information to characterize the individual elastic properties of the aneurysm wall in terms of rupture risk is lacking. We have used time-resolved 3D ultrasound strain imaging to calculate spatially resolved in-plane strain distributions characterized by mean and local maximum strains, as well as indices of local variations in strains. Likewise, we here present a method to generate averaged models from multiple segmentations. Strains were then calculated for single segmentations and averaged models. After registration with aneurysm geometries based on CT-A imaging, local strains were divided into two groups with and without calcifications and compared. Geometry comparison from both imaging modalities showed good agreement with a root mean squared error of 1.22 ± 0.15 mm and Hausdorff Distance of 5.45 ± 1.56 mm (mean ± sd, respectively). Using averaged models, circumferential strains in areas with calcifications were 23.2 ± 11.7% (mean ± sd) smaller and significantly distinguishable at the 5% level from areas without calcifications. For single segmentations, this was possible only in 50% of cases. The areas without calcifications showed greater heterogeneity, larger maximum strains, and smaller strain ratios when computed by use of the averaged models. Using these averaged models, reliable conclusions can be made about the local elastic properties of individual aneurysm (and long-term observations of their change), rather than just group comparisons. This is an important prerequisite for clinical application and provides qualitatively new information about the change of an abdominal aortic aneurysm in the course of disease progression compared to the diameter criterion

Computational analysis of flow and stress patterns in thoracic aortic aneurysms : a patient specific study

Imperial Users onl

Analysing the cross-section of the abdominal aortic aneurysm neck and its effects on stent deployment

Stent graft devices for the treatment of abdominal aortic aneurysms (AAAs) are being in-creasingly used worldwide. Yet, during modelling and optimization of these devices, as well as in clinical practice, vascular sections are idealized, possibly compromising the effective-ness of the intervention. In this study, we challenge the commonly used approximation of the circular cross-section of the aorta and identify the implications of this approximation to the mechanical assessment of stent grafts. Using computed tomography angiography (CTA) data from 258 AAA patients, the lumen of the aneurysmal neck was analysed. The cross-section of the aortic neck was found to be an independent variable, uncorrelated to other geometrical aspects of the region, and its shape was non-circular reaching elliptical ratios as low as 0.77. These results were used to design a finite element analysis (FEA) study for the assessment of a ring stent bundle deployed under a variety of aortic cross-sections. Re-sults showed that the most common clinical approximations of the vascular cross-section can be a source of significant error when calculating the maximum stent strains (underes-timated by up to 69%) and radial forces (overestimated by up to 13%). Nevertheless, a less frequently used average approximation was shown to yield satisfactory results (5% and 2% of divergence respectively)

Design of a testing device for an anatomical part of the ascending aorta

Aortic aneurysms are life-threatening pathologies that cause thousands of deaths worldwide. The current main clinical criteria for surgical intervention is aortic diameter, although a large percentage of patients with dissection or rupture has a normal diameter. Computation methods have been adopted to model the biomechanical behaviour of biological tissue in view of adding in the diagnosis of this pathology. Furthermore, experimental testing on aneurismatic aortic tissue has been performed to validate these models. The objective of this study is to integrate com- putational mechanical methods into an innovative experimental test with a specifically designed device where material parameters are obtained by inverse methods assisted by Digital Image Correlation (DIC). Axiomatic Design (AD) is taken into consideration to develop the testing device in a clear, methodical, and efficient way. A case study is analysed, and a patient-specific 3D geometry of an Ascending Thoracic Aortic Aneurysm (ATAA) is obtained by segmenting Computed Tomography Angiography (CTA) images. A methodology is presented by attribut- ing a hyperelastic constitutive model to the geometry and executing Finite Element Analysis (FEA). Future work should rely on real experimental tests where Finite Element Model Up- dating (FEMU) should be adopted to fit the constitutive model more accurately to the actual specimen material.O aneurisma da aorta é uma patologia de risco que provoca milhares de mortes mundialmente. O critério atual para intervenção cirúrgica é o diâmetro da aorta, no entanto, uma grande percentagem de pacientes com dissecção ou rutura da aorta apresenta um diâmetro normal. Métodos computacionais têm sido adotados para modelar o comportamento biomecânico de tecido biológico e auxiliar no diagnóstico desta patologia. Testes experimentais nestes tecidos são executados para validar os modelos. O objetivo deste estudo é um contributo para uma plataforma digital integrando métodos computacionais para o desenvolvimento de um mecan- ismo de ensaio experimental, cuja identificação de parâmetros material deve ser auxiliada pela técnica de correlação digital de imagem 3D. Esta abordagem segue um desenvolvimento de pro- duto orientado por simulação numérica, em que a análise computacional é totalmente integrada como parte do projeto mecânico. Teoria Axiomática de Projeto é tida em consideração para desenvolver o dispositivo de uma forma clara, metódica e eficiente. Um caso de estudo é anal- isado e uma geometria da peça anatómica 3D, específica de um paciente, é obtida através da segmentação de imagens de uma angiotomografia. Uma metodologia é apresentada atribuindo um modelo constitutivo hiperelástico ao material e executando análise de elementos finitos. Como trabalho futuro a identificação dos parametros constitutivos deve ser obtida com recurso a métodos inversos avançados baseados em campos de deformação obtidos por correlação digital de imagem

Passive biomechanics of abdominal aortic aneurysms

En esta tesis se estudia la respuesta elástica de aneurismas aórticos abdominales (AAA), buscando ahondar en su conocimiento y con la finalidad de proveer un mejor criterio de decisión para la realización, o no, de una intervención quirúrgica para la reparación de la lesión. Parámetros biomecánicos como la tensión pico de la pared arterial (singlas en inglés: PWS) o el riesgo de ruptura de la pared arterial (siglas en inglés: PWRR) han mostrado ser una alternativa posible y prometedora a ser utilizada para determinar el riesgo de ruptura. De la misma manera, el entender la biomecánica pasiva de los AAA permite realizar una evaluación más correcta de las tensiones, lo que se puede realizar mediante el uso de modelos de material adecuados para los tejidos junto con modelos geométricos fiables en los que se apliquen condiciones de frontera realistas. Esta tesis presenta un novedoso algoritmo iterativo para determinar la geometría cero-presión de un AAA para pacientes específicos, la cual supera las limitaciones de las metodologías existentes y permite una mejor estimación de las tensiones. La importancia de este algoritmo se debe a que los modelos de AAA de pacientes específicos son generados a partir de imágenes médicas de CT (tomografía axial computarizada) sincronizadas en las cuales la arteria está bajo presión, por lo tanto la identificación de la geometría cero-presión de AAAs permite una estimación más realista de la respuesta mecánica de la pared arterial. La metodología permite considerar el comportamiento hiperelástico anisótropo de la pared arterial, su espesor y la presencia del trombo intraluminal (ILT). Resultados en doce geometrías de de AAAs, paciente específico, indican que el algorítmo es computacionalmente tratable y eficiente, a la vez que preserva el volumen global del modelo. Adicionalmente, una comparación de resultados de PWS calculados usando geometría cero-presión y geometría basada en CT al aplicar la presión sistólica indica que los resultados a partir de geometría CT subestiman (significativamente) la tensión pico de la pared arterial en casos de modelos isótropo y anisótropo de la pared arterial. Adicionalmente, en base a los resultados experimentales publicados para la pared arterial del aneurisma y aorta sana, los resutados de esta tesis no encuentran diferencias significativas entre el uso de un modelo de material isótropo o anisótropo. Con respecto al ILT, el cual es un pseudo-tejido que se desarrolla a partir de sangre coagulada y se encuentra en la mayor parte de los AAAs de tamaño relevante, algunos estudios sugieren que las características mecánicas del ILT pueden estar relacionadas con el riesgo de ruptura del AAA, aunque existe una gran controversia en este respecto. Esta tesis investiga como la constitución y topología del ILT influye en la magnitud y localización de las tensiones pico en la pared arterial. El ILT, isótropo y no homogéneo, puede aparecer como un tejido flexible (una capa) o rígido (fibrótico multicapa). El estudio se extendió a 21 AAAs, pacientes específicos, (diámetro: 4.2-5.4 cm) que fueron reconstruidos a partir de imágenes CT y analizados numéricamente empleando el algoritmo de tirón propuesto para identificar la geometría cero presión. Los resultados indican que la PWS está mayormente correlacionada con el volumen de ILT (¿=0.44, p=0.05) y con el espesor de capa mínimo de ILT (¿=0.73, p=0.001) que con el diámetro máximo de AAA (¿=0.05, p=0.82). En promedio la PWS fue un 20% (desv estándar 12%) más alta para modelos en los que se usaron modelos suaves de ILT en lugar de modelos rígidos de ILT (p<0.001). La localización del PWS está altamente correlacionada con los puntos de menor espesor de ILT, en las secciones de máximo diámetro del AAA, y esto fue independiente de la rigidez del ILT. Adicionalmente, la heterogeneidad del ILT, i.e. la composición espacial de trombo suave o rígido, puede influenciar sustancialmente la tensión de la pared arterial. El presente estudio está limitado a identificar la influencia de factores biomecánicos, el cómo estos resultados se trasladan a la evaluación del riesgo de ruptura de AAA debe ser desarrollado a partir de estudios clínicos.The passive biomechanics of abdominal aortic aneurysms (AAA) is studied, seeking to deepen in its knowledge and with the aim of providing better decision criteria to undergo surgical intervention for AAA repair. Biomechanical parameters as the peak wall stress (PWS) or the peak wall rupture risk (PWRR) have shown to be a feasible and promising alternative that can be used to better ascertain the risk of rupture. In addition, the understanding of the passive biomechanics of AAA allows obtaining a more accurate stress assessment, which can be done by using appropriate material models for the tissues along with accurate geometric models and more realistic boundary conditions for the lesion. This thesis presents a novel iterative algorithm to determine the zeropressure geometry of a patient-specific AAA that overcomes limitations on existing methodologies and allows a better estimation of the stresses. The importance of this algorithm lays in that patient-specific AAA models are generated from gated CT (Computer Tomography) medical images in which the artery is under pressure (diastolic), therefore the identification of the AAA zero pressure geometry would allow for a more realistic estimate of the aneurismal wall mechanics. The methodology allows considering the anisotropic hyperelastic behavior of the aortic wall, its thickness and accounts for the presence of the intraluminal thrombus (ILT). The results on twelve patientspecific AAA geometric models indicate that the procedure is computational tractable and efficient, and preserves the global volume of the model. In addition, a comparison of the peak wall stress computed with the zero pressure and CT-based geometries during systole indicate that computations using CTbased geometric models underestimate (significantly) the peak wall stress for both, isotropic and anisotropic material models of the arterial wall. In addition, based on the reported experimental results for aneurysmal and aortic wall mechanics, no significant differences among isotropic and anisotropic material models have been found. With respect to the ILT, which is a pseudo-tissue that develops from coagulated blood and it is found in most AAAs of clinically relevant size, a number of studies have suggested that ILT mechanical characteristics may be related to AAA risk of rupture, even though there is still great controversy on this regard. This thesis investigates how ILT constitution and topology influence the magnitude and location of PWS. ILT is isotropic and inhomogeneous and may appear as a soft (single-layered) or stiff (multilayered fibrotic) tissue. An extended study was conducted involving twenty-one patient-specific AAAs (diameter: 4.2-5.4 cm) which were reconstructed from CT images and biomechanically analyzed using the proposed methodology. Results indicated that PWS correlated stronger with ILT volume (ρ=0.44, p=0.05) and the minimum thickness of the ILT layer (ρ=0.73, p=0.001) than with maximum AAA diameter (ρ=0.05, p=0.82). In average PWS was 20% (SD 12%) higher for FE models that used a soft instead of stiff ILT models (p<0.001). PWS location strongly correlated with sites of minimum ILT thickness in the section of maximum AAA diameter and was independent from the ILT stiffness. In addition, ILT heterogeneity, i.e. the spatial composition of soft and stiff thrombus tissue, can considerably influence the stress in the AAA wall. The present study is limited to the identification of influential biomechanical factors, and how its findings translate to an AAA rupture risk assessment remains to be explored by clinical studies

Synchrotron-based visualization and segmentation of elastic lamellae in the mouse carotid artery during quasi-static pressure inflation

This dataset contains images that were obtained during quasi-static pressure inflation of mouse carotid arteries. Images were taken with phase propagation imaging at the X02DA TOMCAT beamline of the Swiss Light Source synchrotron at the Paul Scherrer Institute in Villigen, Switzerland. Scans of n=12 left carotid arteries (n-6 Apoe-deficient mice, n=6 wild-type mice, all on a C57Bl6J background) were taken at pressure levels of 0, 10, 20, 30, 40, 50, 70, 90 and 120 mmHg. For analysis we selected 75 images from the center of each stack (starting at the center of the stack, and skipping 2 of every three images in both cranial and caudal axial directions) for each sample and for each pressure level, resulting in a total of 75 x 12 x 9 = 8100 analyzed images from 108 different scans. Segmentation, 3D visualization and geometric analysis is presented in the corresponding manuscript. Files are uploaded in 16bit .tif format and are named: mouseid_pressurelevel_stacknumber, with mouseid consisting of either Apoe (Apoe-deficient) or Bl (wild-type) and the mouse number, pressurelevel varies from P0 to P120 and stacknumber indicates which image from the stack has been uploaded