Effect of Sac Asymmetry, Neck and Iliac Angle on the Hemodynamic Behavior of Idealized Abdominal Aortic Aneurysm Geometries.

BACKGROUND: Abdominal aortic aneurysms (AAAs) are currently treated based on the universal maximum diameter criterion, but other geometric variables may play a role in the risk of rupture. The hemodynamic environment inside the AAA sac has been shown to interact with several biologic processes which can affect prognosis. AAA geometric configuration has a significant impact in the hemodynamic conditions that develop, which has only been recently realized, with implications for rupture risk estimations. We aim to perform a parametric study to evaluate the effect of aortic neck angulation, angle between the iliac arteries, and sac asymmetry (SA) on the hemodynamic variables of AAAs. METHODS: This study uses idealized AAA models and it is parametrized in terms of 3 quantities as follows: the neck angle, φ (°), iliac angle, θ (°), and SA (%), each of which accepts 3 different values, specifically φ = (0°, 30°, 60°), θ = (40°, 60°, 80°), and SA = (S, °SS, °OS), where the SA can either be on the same side with respect to neck (SS) or on the opposite side (OS). Time average wall shear stress (TAWSS), oscillatory shear index (OSI), relative residence time (RRT), and the velocity profile are calculated for different geometric configurations, while the percentage of the total surface area under thrombogenic conditions, using thresholds previously reported in the literature, is also recorded. RESULTS: In case of an angulated neck and a higher angle between iliac arteries, favorable hemodynamic conditions are predicted with higher TAWSS and lower OSI and RRT values. The area under thrombogenic conditions reduces by 16-46% as the neck angle increases from 0° to 60°, depending on the hemodynamic variable under consideration. The effect of iliac angulation is present but less pronounced with 2.5-7.5% change between the lower and the higher angle. The effect of SA seems to be significant for OSI, with a nonsymmetrical configuration being hemodynamically favorable, which in the presence of an angulated neck is more pronounced for the OS outline. CONCLUSIONS: Favorable hemodynamic conditions develop inside the sac of idealized AAAs with increasing neck and iliac angles. Regarding the SA parameter, asymmetrical configurations most often appear advantageous. Concerning the velocity profile the triplet (φ, θ, SA) may affect outcomes under certain conditions and thus should be taken into account when parametrizing the geometric characteristics of AAAs.

Prediction of abdominal aortic aneurysm growth by artificial intelligence taking into account clinical, biologic, morphologic, and biomechanical variables.

OBJECTIVES: To develop a prediction model that could risk stratify abdominal aortic aneurysms (AAAs) into high and low growth rate groups, using machine learning algorithms based on variables from different pathophysiological fields. METHODS: A cohort of 40 patients with small AAAs (maximum diameter 32-53 mm) who had at least an initial and a follow-up CT scan (median follow-up 12 months, range 3-36 months) were included. 29 input variables from clinical, biological, morphometric, and biomechanical pathophysiological aspects extracted for predictive modeling. Collected data were used to build two supervised machine learning models. A gradient boosting (XGboost) and a support vector machines (SVM) algorithm were trained with 60% and tested with 40% of the data to predict which AAA would achieve a growth rate higher than the median of our study cohort. Receiver operating characteristics (ROC) curves and areas under the curve (AUC) were used for the evaluation of the developed algorithms. RESULTS: XGboost achieved the highest AUC in predicting high compared to low AAA growth rate with an AUC of 81.2% (95% CI from 61.1 to 100%). SVM achieved the second highest performance with an AUC of 68.8% (95% CI from 46.5 to 91%). Based on the best performing algorithm, variable importance was estimated. Diameter-diameter ratio (maximum diameter/neck diameter), Tortuosity from Renal arteries to aortic bifurcation, and maximum thickness of the intraluminal thrombus were found to be the most important factors for model predictions. Other factors were also found to play a significant but less important role. CONCLUSIONS: A prediction model that can risk stratify AAAs into high and low growth rate groups could be developed by analyzing several factors implicated in the multifactorial pathophysiology of this disease, with the use of machine learning algorithms. Future studies including larger patient cohorts and implementing additional risk markers may aid in the establishment of such methodology during AAA rupture risk estimation.

Should the Proximal Part of a Bifurcated Aortic Graft be Kept as Short as Possible? A Computational Study Elucidates on Aortic Graft Hemodynamics for Various Main Body Lengths.

BACKGROUND: It is accepted that surgically placed bifurcated aortic grafts should be shaped as a short proximal main tube with two long distal limbs. We aim to investigate the hemodynamic effect of different main body lengths in bifurcated aortic grafts using 3D computer models. METHODS: Five different idealized models are generated to represent an aorto-bifemoral graft. Distance from renal to femoral arteries is set at 25cm and distance between the femoral arteries is set at 14cm. Values of the main body length taken into account to build the idealized models are 3cm, 6cm, 9cm, 12cm and 15cm. Blood flow resistance, Time Average Wall Shear Stress (TAWSS), Oscillatory Shear Index (OSI) and Relative Residence Time (RRT) are estimated using the constructed 3D models. RESULTS: The total resistance decreased monotonically by as far as 40% as the main body length increased. Appropriate hemodynamic simulations show a maximum TAWSS decrease and a corresponding maximum OSI and RRT increase with elongated main body configurations, indicating a hemodynamic benefit of the "Short" main body configuration. Nevertheless, the differences in these later variables are small, affecting a limited portion of the geometries. CONCLUSION: A long main body of a bifurcated aortic graft results in significantly reduced total resistance in idealized models designed to represent an aorto-bifemoral surgical graft, while the differences observed in TAWSS, OSI and RRT between models are small.

Spatial Distribution of Abdominal Aortic Aneurysm Surface Expansion and Correlation With Maximum Diameter and Volume Growth.

BACKGROUND: Abdominal aortic aneurysm (AAA) growth rate, measured as maximum diameter (Dmax) change over time, is used as a surrogate marker of rupture risk. However, AAA expansion presents significant spatial variability. We aim to record the spatial distribution of regional wall surface expansion. METHODS: Thirty AAAs were retrospectively studied. Each AAA had one baseline and at least one follow-up computed tomography scan. Three-dimensional AAA models were reconstructed, and change in Dmax and total aneurysm volume was recorded to calculate annual growth rates. Regional surface growth was quantified using the VascForm algorithm, which is based on nonrigid point cloud registration and iterative closest point analysis. Maximum and average surface growths were calculated and correlated with the diameter/volume growth rates. Furthermore, to identify potential correlation between maximum thrombus (intraluminal thrombus) thickness and maximum surface growth, as well as between peak wall stress (PWS) and surface growth, their colocalization was examined. RESULTS: The median average annual surface growth was 6% (0%-28%), and the maximum surface growth 24% (11%-238%). There was strong evidence of a moderate correlation between Dmax and average as well as maximum surface growth. Regarding volumes, there was strong evidence of a very strong association with average surface growth rate and a moderate association with maximum surface growth rate (rho: 0.91, P < 0.001; rho: 0.7, P < 0.001, respectively). In 51.6% of the follow-ups, maximum surface growth occurred away from Dmax site. Sixteen cases presented maximum surface growth away and fifteen at the region of maximum initial intraluminal thrombus thickness. AAAs in the former group had significantly thinner initial intraluminal thrombus thickness (11.3 vs 19.5 mm, P < 0.001) than those in the latter. Apart from a single case, maximum surface growth did not occur at the PWS region. CONCLUSIONS: More than half of the lesions display maximum growth away from Dmax, suggesting that a more accurate method of analyzing AAA growth needs to be established in clinical practice that will take into account local surface growth.

Intraluminal Thrombus Deposition Is Reduced in Ruptured Compared to Diameter-matched Intact Abdominal Aortic Aneurysms.

BACKGROUND: The aim of this study is to compare the pattern of intraluminal thrombus (ILT) deposition in diameter-matched ruptured and nonruptured abdominal aortic aneurysms (AAAs). METHODS: We performed a single-center, retrospective study. Ruptured AAAs were collected during 24 months. Diameter-matched intact lesions were randomly selected in a 2:1 ratio and served as controls. ILT cross-sectional area, relative area, maximum thickness, and asymmetric distribution were recorded at the site of maximum aneurysm size and compared between groups. Moreover, additional comparisons were performed inside the group of ruptured AAAs, between the site of maximum size and the site of rupture. RESULTS: Fifteen ruptured cases were compared with 30 nonruptured cases. ILT relative area (37.5% vs. 73.5%, P = 0.004) and maximum thickness (14.5 vs. 28 mm, P= 0.0017) were significantly reduced among ruptured compared to intact AAAs. The latter group presented mostly an anterior eccentric ILT deposition, while the former presented a more symmetrical pattern. The site of rupture was located at the site of maximum size in only 2 cases. In general, ILT was reduced at the site of rupture compared to the site of maximum aneurysm size in ruptured cases but differences did not reach statistical significance. CONCLUSIONS: In similar sized AAAs, ILT is reduced in ruptured compared to nonruptured cases.

Correlation of Intraluminal Thrombus Deposition, Biomechanics, and Hemodynamics with Surface Growth and Rupture in Abdominal Aortic Aneurysm-Application in a Clinical Paradigm.

BACKGROUND: The natural history of abdominal aortic aneurysm (AAA) can be investigated through longitudinal evaluation of localized aneurysm characteristics exploiting clinical images. The major challenge is to identify corresponding regions between follow-ups. We have recently developed an algorithm (VascForm) based on nonrigid registration that can obtain surface correspondence and quantify surface growth distribution. METHODS: A ruptured AAA with an initial computed tomography scan 2 years ago was studied. Following 3-dimensional reconstruction of outer wall and luminal surfaces, the wall/thrombus thickness was obtained. Wall stress distribution was computed with finite element analysis, and computational fluid dynamics simulation was performed. VascForm was applied and allowed for the ruptured wall site to be traced back to the initial wall surface and be correlated with local initial intraluminal thrombus thickness, wall stress, and hemodynamic parameters. It also allowed for the quantification of wall surface growth based on surface element growth. RESULTS: Rupture occurred at the posterolateral side. Initial wall surface growth was in most regions 40%. However, a large section of the posterior wall presented 110% growth. Initial thrombus deposition was more prevalent anteriorly, and a posterior thrombus-free isle was present. Peak wall stress (initial and follow-up) occurred at AAA neck. Nonrigid registration revealed that rupture originated from the vicinity of the initial thrombus-free isle. Furthermore, rupture occurred at the wall region with the largest growth (110%). No clear correlation between hemodynamics and rupture site could be identified. CONCLUSIONS: High local surface growth correlates with rupture site and could therefore potentially become a marker of rupture risk. The ongoing application of this methodology to a large cohort of AAA patients will focus on identifying characteristic features of AAA regions that present high surface growth in follow-up evaluations, to assist in improved rupture risk estimation.

Effect of intraluminal thrombus asymmetrical deposition on abdominal aortic aneurysm growth rate.

PURPOSE: To determine the relationship between asymmetrical intraluminal thrombus (ILT) deposition in abdominal aortic aneurysm (AAA) and growth rate and to explore its biomechanical perspective. METHODS: Thirty-four patients with AAA underwent at least 2 computed tomography scans during surveillance. The volumes of the AAA (VAAA) and thrombus (VILT) and the maximum thrombus thickness (ILTthick) were computed. Thrombus distribution was evaluated by introducing the asymmetrical thrombus deposition index (ATDI), with positive and negative values (-1

A hemodynamic study of blood flow models on various stent graft configurations during aorto-iliac reconstruction.

PURPOSE: To compare the hemodynamic performance of three (Bottom Up non-ballet, Top-Down non-ballet, Top Down ballet) idealized stent graft configurations used during endovascular repair of abdominal aortic aneurysms, under the influence of various rheological models. METHODS: Ten rheological models are assumed and a commercial finite volume solver is employed for the simulation of blood flow under realistic boundary conditions. An appropriate mesh convergence study is performed and five hemodynamic variables are computed: the time average wall shear stress (TAWSS), oscillatory shear index (OSI), relative residence time (RRT), endothelial cell activation potential (ECAP) and displacement force (DF) for all three configurations. RESULTS: The choice of blood flow model may affect results, but does not constitute a significant determinant on the overall performance of the assumed stent grafts. On the contrary, stent graft geometry has a major effect. Specifically, the Bottom Up non-ballet type is characterized by the least favorable performance presenting the lowest TAWSS and the highest OSI, RRT and ECAP values. On the other hand, the Top Down ballet type presents hemodynamic advantages yielding the highest TAWSS and lowest OSI, RRT and ECAP average values. Furthermore, the ballet type is characterized by the lowest DF, although differences observed are small and their clinical relevance uncertain. CONCLUSIONS: The effect of the assumed rheological model on the overall performance of the grafts is not significant. It is thus relatively safe to claim that it is the type of stent graft that determines its overall performance rather than the adopted blood flow model.

Concomitant vascular and orthopedic trauma: 10 points to consider.

Although vascular injuries complicate only 1-2% of patients with orthopedic trauma, they may be encountered in a much higher rate of around 10 % in injuries around the knee and elbow joints following both fractures and dislocations. In case of vascular involvement, specific diagnostic and therapeutic challenges arise and there is a higher risk for significant morbidity (i.e. limb loss) or mortality. In the absence of randomized data, diagnostic and therapeutic algorithms are not always straightforward and clinical practice may be based on experience and local protocols rather than firm evidence. With this article we intend to review available literature regarding concomitant skeletal and vascular trauma in order to provide concise information and clear guidelines of when to operate with least investigations and when to go for a full spectrum of investigations in the absence of hard clinical signs. Additionally, other aspects concerning the manipulation of these patients are discussed, such as the indications of primary amputation, the potential role of endovascular techniques and the value of the Resuscitative Endovascular Balloon Occlusion of the Aorta. In summary, this scoping review summarizes current practices in the diagnostic and therapeutic management of patients with concomitant orthopedic and vascular injuries, discusses different treatment strategies and gives a practical perspective for implementation on every day practice.

The Effect of Blood Rheology and Inlet Boundary Conditions on Realistic Abdominal Aortic Aneurysms under Pulsatile Flow Conditions.

BACKGROUND: The effects of non-Newtonian rheology and boundary conditions on various pathophysiologies have been studied quite extensively in the literature. The majority of results present qualitative and/or quantitative conclusions that are not thoroughly assessed from a statistical perspective. METHODS: The finite volume method was employed for the numerical simulation of seven patient-specific abdominal aortic aneurysms. For each case, five rheological models and three inlet velocity boundary conditions were considered. Outlier- and heteroscedasticity-robust ANOVA tests assessed the simultaneous effect of rheological specifications and boundary conditions on fourteen variables that capture important characteristics of vascular flows. RESULTS: The selection of inlet velocity profiles appears as a more critical factor relative to rheological specifications, especially regarding differences in the oscillatory characteristics of computed flows. Response variables that relate to the average tangential force on the wall over the entire cycle do not differ significantly across alternative factor levels, as long as one focuses on non-Newtonian specifications. CONCLUSIONS: The two factors, namely blood rheological models and inlet velocity boundary condition, exert additive effects on variables that characterize vascular flows, with negligible interaction effects. Regarding thrombus-prone conditions, the Plug inlet profile offers an advantageous hemodynamic configuration with respect to the other two profiles.

Selection of Bifurcated Grafts' Dimensions during Aorto-Iliac Vascular Reconstruction Based on Their Hemodynamic Performance.

During the vascular surgical reconstruction of aorto-iliac occlusive/aneurysmal disease, bifurcated grafts are used where vascular surgeons intra-operatively select the size and the relative lengths of the parent and daughter portions of the graft. Currently, clinical practice regarding the selection of the most favorable geometric configuration of the graft is an understudied research subject: decisions are solely based on the clinical experience of the operating surgeon. This manuscript aims to evaluate the hemodynamic performance of various diameters, D, of bifurcated aortic grafts and relate those with proximal/distal part length ratios (the angle φ between the limbs is used as a surrogate marker of the main body-to-limb length ratio) in order to provide insights regarding the effects of different geometries on the hemodynamic environment. To this end, a computationally intensive set of simulations is conducted, and the resulting data are analyzed with modern statistical regression tools. A negative curvilinear relationship of TAWSS with both φ and D is recorded. It is shown that the angle between limbs is a more important predictor for the variability of TAWSS, while the graft's diameter is an important determinant for the variability of OSI. Large percentages of the total graft area with TAWSS < 0.4 Pa, which correspond to thrombogenic stimulating environments, are only observed for large values of φ and D > 20 mm. This variable ranges from 10% (for the smallest values of φ and D) to 55% (for the largest φ and D values). Our findings suggest that grafts with the smallest possible angle between the limbs (i.e., smallest parent-to-daughter length ratio) present the most favorable hemodynamic performance, yielding the smallest percentage of total graft area under thrombogenic simulating environments. Similarly, grafts with the smallest acceptable diameter should be preferred for the same reason. Especially, grafts with diameters greater than 20 mm should be avoided, given the abrupt increase in estimated thrombogenic areas.

Comparison of the Haemodynamic Performance of Various Treatment Options for Aorto-Iliac Occlusive Disease.

Introduction: Several surgical and endovascular techniques are used during the treatment of aorto-iliac occlusive disease. Aortobifemoral bypass (AoBFB) is the standard of care, but other options such as axillobifemoral (AxBFB) bypass, aorto-iliac kissing stents (KS), and covered endovascular reconstruction of aortic bifurcation (CERAB) are also available. This study aimed to perform a computational comparison of these four modalities to investigate their haemodynamic performance. Report: Eight patient specific anatomies were analysed, with each of the abovementioned techniques used to treat two anatomies. The CT angiograms were segmented from the renal (or axillary) to common femoral arteries and the 3D geometries were exported. A commercial finite volume solver was implemented for numerical simulations. Outcomes that were assessed were pressure drop (ΔP) between the inlet and the outlet for every configuration and haemodynamic indices of Time Average Wall Shear Stress (TAWSS), Oscillatory Shear Index (OSI), and Relative Residence Time (RRT) as markers of a thrombogenic environment. The results indicate that maximum ΔP was observed at peak systole for all models, with values ranging between 12 mmHg and 21 mmHg for the AoBFB, 64 mmHg and 96 mmHg for the AxBFB, 31 mmHg and 46 mmHg for the KS, and 43 mmHg and 46 mmHg for the CERAB configuration. TAWSS, OSI, and RRT varied among different configurations, mostly presenting values well above thrombogenic thresholds. Regarding RRT, the percentage of total surface area presenting such values is 2.5%, 3.2%, 2%, and 4.3% for the AoBFB, AxBFB, KS, and CERAB configurations, respectively. Discussion: Computational modelling indicates a favourable haemodynamic performance of AoBFB compared with the other configurations. This leads to a smaller pressure drop and sconsequently a higher pressure in the outlet of the conduit, which is the perfusion pressure of the limb. Notably, lower patency rates of the latter modalities cannot be explained based on haemodynamic indices.

An adaptive semi-implicit finite element solver for brain cancer progression modeling.

Glioblastoma is the most aggressive and infiltrative glioma, classified as Grade IV, with the poorest survival rate among patients. Accurate and rigorously tested mechanistic in silico modeling offers great value to understand and quantify the progression of primary brain tumors. This paper presents a continuum-based finite element framework that is built on high performance computing, open-source libraries to simulate glioblastoma progression. We adopt the established proliferation invasion hypoxia necrosis angiogenesis model in our framework to realize scalable simulations of cancer, and has demonstrated to produce accurate and efficient solutions in both two- and three-dimensional brain models. The in silico solver can successfully implement arbitrary order discretization schemes and adaptive remeshing algorithms. A model sensitivity analysis is conducted to test the impact of vascular density, cancer cell invasiveness and aggressiveness, the phenotypic transition potential, including that of necrosis, and the effect of tumor-induced angiogenesis in the evolution of glioblastoma. Additionally, individualized simulations of brain cancer progression are carried out using pertinent magnetic resonance imaging data, where the in silico model is used to investigate the complex dynamics of the disease. We conclude by arguing how the proposed framework can deliver patient-specific simulations of cancer prognosis and how it could bridge clinical imaging with modeling.

Acute Testicular Ischaemia Following Endovascular Aneurysm Repair on the Opposite Side to Intentional Internal Iliac Artery Occlusion.

INTRODUCTION: Testicular ischaemia is a potential complication after endovascular aneurysm repair (EVAR), which has only rarely been reported in the literature. This is the report of a patient who presented with acute testicular ischaemia in the immediate post-EVAR period. REPORT: A 65 year old patient underwent EVAR for an aortic and bilateral iliac aneurysms. During the procedure, the right internal iliac artery was intentionally occluded to facilitate treatment of the common iliac aneurysm; however, the left internal iliac artery was preserved. The procedure was uneventful. On the second post-operative day the patient gradually developed symptoms of acute left testicular ischaemia. Clinical and ultrasonographic findings constituted the bases of diagnosis and the patient received conservative treatment with gradual improvement. To the authors' knowledge, this is the ninth case of testicular ischaemia after endovascular aneurysm repair reported in the literature. CONCLUSION: Testicular ischaemia, although rare, is a possible complication post-EVAR. Acute and chronic testicular damage found in association with an abdominal aortic aneurysm or its treatment has not been well studied in the literature and therefore may be under reported.

The Obsolete Maximum Diameter Criterion, the Evident Role of Biomechanical (Pressure) Indices, the New Role of Hemodynamic (Flow) Indices, and the Multi-Modal Approach to the Rupture Risk Assessment of Abdominal Aortic Aneurysms.

Although the therapeutic management of abdominal aortic aneurysms (AAAs) is currently based on the maximum diameter criterion, this has often proved inaccurate and misleading. Conversely, the biomechanical approach, which takes into account the pressure-induced wall stress exerted at every point throughout the aneurysmal surface, has been proven superior in predicting the rupture risk of AAAs, and its value is being increasingly recognized among physicians. More recently, hemodynamic indices, such as flow-induced wall shear stresses, have been indicated as potentially significant determinants of AAA natural history. Ultimately, a statistical model that takes into account all these factors may be relevant for making a sound prediction of the rupture risk of aneurysms and optimizing the management of these patients.

A robust approach for exploring hemodynamics and thrombus growth associations in abdominal aortic aneurysms.

Longitudinal studies of vascular diseases often need to establish correspondence between follow-up images, as the diseased regions may change shape over time. In addition, spatial data structures should be taken into account in the statistical analyses to avoid inferential errors. This study investigates the association between hemodynamics and thrombus growth in abdominal aortic aneurysms (AAAs) while emphasizing on the abovementioned methodological issues. Six AAA surfaces and their follow-ups were three-dimensionally reconstructed from computed-tomography images. AAA surfaces were mapped onto a rectangular grid which allowed identification of corresponding regions between follow-ups. Local thrombus thickness was measured at initial and follow-up surfaces and computational fluid dynamic simulations provided time-average wall shear stress (TAWSS), oscillatory shear index (OSI), and relative residence time. Six Bayesian regression models, which account for spatially correlated measurements, were employed to explore associations between hemodynamics and thrombus growth. Results suggest that spatial regression models based on TAWSS and OSI offer superior predictive performance for thrombus growth relative to alternative specifications. Ignoring the spatial data structure may lead to improper assessment with regard to predictor significance.