Finite Element Analysis of Abdominal Aortic Aneurysms: Predicted Rupture Risk Correlates With Aortic Wall Histology in Individual Patients

Current Imaging Strategies in Patients with Abdominal Aortic Aneurysms

BACKGROUND

 An abdominal aortic aneurysm (AAA) is defined as a localized dilatation of the abdominal aorta of ≥ 3 cm. With a prevalence of 4-8 %, AAA is one of the most common vascular diseases in Western society. Radiological imaging is an elementary component in the diagnosis, monitoring, and treatment planning of AAA patients.

METHOD

 This is a narrative review article on preoperative imaging strategies of AAA, incorporating expert opinions based on the current literature and standard-of-care practices from our own center. Examples are provided to illustrate clinical cases from our institution.

RESULTS AND CONCLUSION

 Radiological imaging plays a pivotal role in the initial diagnosis and monitoring of patients with AAA. Ultrasound is the mainstay imaging modality for AAA screening and surveillance. Contrast-enhanced CT angiography is currently considered the gold standard for preoperative imaging and image-based treatment planning in AAA repair. New non-contrast MR angiography techniques are robustly applicable and allow precise determination of aortic diameters, which is of critical importance, particularly with regard to current diameter-based surgical treatment guidelines. 3D imaging with multiplanar reformation and automatic centerline positioning enables more accurate assessment of the maximum aortic diameter. Modern imaging techniques such as 4D flow MRI have the potential to further improve individualized risk stratification in patients with AAA.

KEY POINTS

  · Ultrasound is the mainstay imaging modality for AAA screening and monitoring. · Contrast-enhanced CT angiography is the gold standard for preoperative imaging in AAA repair. · Non-contrast MR angiography allows for accurate monitoring of aortic diameters in AAA patients. · Measurement of aortic diameters is more accurate with 3D-CT/MRI compared to ultrasound. · Research seeks new quantitative imaging biomarkers for AAA risk stratification, e. g., using 4D flow MRI.

Completing the view - histologic insights from circular AAA specimen including 3D imaging : A methodologic approach towards histologic analysis of circumferential AAA samples

Abdominal aortic aneurysm (AAA) is a pathologic enlargement of the infrarenal aorta with an associated risk of rupture. However, the responsible mechanisms are only partially understood. Based on murine and human samples, a heterogeneous distribution of characteristic pathologic features across the aneurysm circumference is expected. Yet, complete histologic workup of the aneurysm sac is scarcely reported. Here, samples from five AAAs covering the complete circumference partially as aortic rings are investigated by histologic means (HE, EvG, immunohistochemistry) and a new method embedding the complete ring. Additionally, two different methods of serial histologic section alignment are applied to create a 3D view. The typical histopathologic features of AAA, elastic fiber degradation, matrix remodeling with collagen deposition, calcification, inflammatory cell infiltration and thrombus coverage were distributed without recognizable pattern across the aneurysm sac in all five patients. Analysis of digitally scanned entire aortic rings facilitates the visualization of these observations. Immunohistochemistry is feasible in such specimen, however, tricky due to tissue disintegration. 3D image stacks were created using open-source and non-generic software correcting for non-rigid warping between consecutive sections. Secondly, 3D image viewers allowed visualization of in-depth changes of the investigated pathologic hallmarks. In conclusion, this exploratory descriptive study demonstrates a heterogeneous histomorphology around the AAA circumference. Warranting an increased sample size, these results might need to be considered in future mechanistic research, especially in reference to intraluminal thrombus coverage. 3D histology of such circular specimen could be a valuable visualization tool for further analysis.

Rupture risk parameters upon biomechanical analysis independently change from vessel geometry during abdominal aortic aneurysm growth

Objective

The indication for abdominal aortic aneurysm (AAA) repair is based on a diameter threshold. However, mechanical properties, such as peak wall stress (PWS) and peak wall rupture index (PWRI), influence the individual rupture risk. This study aims to correlate biomechanical and geometrical AAA characteristics during aneurysm growth applying a new linear transformation-based comparison of sequential imaging.

Methods

Patients with AAA with two sequential computed tomography angiographies (CTA) were identified from a single-center aortic database. Patient characteristics included age, gender, and comorbidities. Semiautomated segmentation of CTAs was performed using Endosize (Therenva) for geometric variables (diameter, neck configuration, α/β angle, and vessel tortuosity) and for finite element method A4 Clinics Research Edition (Vascops) for additional variables (intraluminal thrombus [ILT]), vessel volume, PWS, PWRI). Maximum point coordinates from at least one CTA 6 to 24 months before their final were predicted for the final preoperative CTA using linear transformation along fix and validation points to estimate spatial motion. Pearson's correlation and the t test were used for comparison.

Results

Thirty-two eligible patients (median age, 70 years) were included. The annual AAA growth rate was 3.7 mm (interquartile range [IQR], 2.25-5.44; P < .001) between CTs. AAA (+17%; P < .001) and ILT (+43%; P < .001) volume, maximum ILT thickness (+35%; P < .001), β angle (+1.96°; P = .017) and iliac tortuosity (+0.009; P = .012) increased significantly. PWS (+12%; P = .0029) and PWRI (+16%; P < .001) differed significantly between both CTAs. Both mechanical parameters correlated most significantly with the AAA volume increase (r = 0.68 [P < .001] and r = 0.6 [P < .001]). Changes in PWS correlated best with the aneurysm neck configuration. The spatial motion of maximum ILT thickness was 14.4 mm (IQR, 7.3-37.2), for PWS 8.4 mm (IQR, 3.8-17.3), and 11.5 mm (IQR, 5.9-31.9) for PWRI. Here, no significant correlation with any of the aforementioned parameters, patient age, or time interval between CTs were observed.

Conclusions

PWS correlates highly significant with vessel volume and aneurysm neck configuration. Spatial motion of maximum ILT thickness, PWS, and PWRI is detectable and predictable and might expose different aneurysm wall segments to maximum stress throughout aneurysm growth. Linear transformation could thus add to patient-specific rupture risk analysis.

Clinical Relevance

Abdominal aortic aneurysm rupture risk assessment is a key feature in future individualized therapy approaches for patients, since more and more data are obtained concluding a heterogeneous disease entity that might not be addressed ideally looking only at diameter enlargement. The approach presented in this pilot study demonstrates the feasibility and importance of measuring peak wall stress and rupture risk indices based on predicted and actual position of maximum stress points including intraluminal thrombus.

Abdominal aortic aneurysm rupture presenting with focal weakness and altered mental status: a case report

Background

Abdominal aortic aneurysms (AAA) can present asymptomatically and may be found through routine screening or seen incidentally on imaging. Rupture due to weaking of the aortic wall is the main complication of an AAA and leads to approximately 200,000 deaths annually worldwide. Clinically, AAA rupture most frequently presents with abdominal and/or back pain, pulsatile abdominal mass, and hypotension. Here, we present an unusual presentation of embolic cerebrovascular accident associated with an AAA rupture.

Case presentation

A 58-year-old African American man transported to the emergency department via ambulance presents with altered mental status and unilateral extremity weakness. The initial presentation was concerning for acute cerebrovascular accident, acute kidney injury, severe sepsis, and urinary tract infection. Several hours after the initial presentation, the patient’s abdomen began to appear distended and he became hypotensive. An abdominal CT was ordered which showed a large AAA rupture with a retroperitoneal bleed. The patient was transferred to a higher-level medical center for surgical repair.

Conclusion

Abdominal aortic aneurysm rupture can rarely present due to an acute cerebrovascular accident with altered mental status and focal neurologic deficits.

Gene Expression Profiling in Abdominal Aortic Aneurysms

Gene expression profiling of abdominal aortic aneurysms (AAA) indicates that chronic inflammatory responses, active matrix metalloproteinases, and degradation of the extracellular matrix components are involved in disease development and progression. This study investigates intra- and interpersonal RNA genome-wide expression profiling differences (Illumina HumanHT-12, BeadCHIP expression) of 24 AAA biopsies from 12 patients using a single gene and pathway (GeneOntology, GO enrichment) analysis. Biopsies were collected during open surgical AAA repair and according to prior finite element analysis (FEA) from regions with the highest and lowest wall stress. Single gene analysis revealed a strong heterogeneity of RNA expression parameters within the same and different AAA biopsies. The pathway analysis of all samples showed significant enrichment of genes from three different signaling pathways (integrin signaling pathway: fold change FC 1.63, p = 0.001; cholecystokinin receptor pathway: FC 1.60, p = 0.011; inflammation mediated by chemokine signaling pathway: FC 1.45, p = 0.028). These results indicate heterogeneous gene expression patterns within the AAA vascular wall. Single biopsy investigations do not permit a comprehensive characterization of activated molecular processes in AAA disease.

Aberrant Mitochondrial Dynamics: An Emerging Pathogenic Driver of Abdominal Aortic Aneurysm

Abdominal aortic aneurysm (AAA) is defined as a progressive segmental dilation of the abdominal aorta and is associated with high mortality. The characterized features of AAA indicate several underlying mechanisms of AAA formation and progression, including reactive oxygen species production, inflammation, and atherosclerosis. Mitochondrial functions are critical for determining cell fate, and mitochondrial dynamics, especially selective mitochondrial autophagy, which is termed as mitophagy, has emerged as an important player in the pathogenesis of several cardiovascular diseases. The PARKIN/PARIS/PGC1α pathway is associated with AAA formation and has been proposed to play a role in mitochondrial dynamics mediated by the PINK/PARKIN pathway in the pathogenesis underlying AAA. This review is aimed at deepening our understanding of AAA formation and progression, which is vital for the development of potential medical therapies for AAA.

Development of an FEA framework for analysis of subject-specific aortic compliance based on 4D flow MRI

This paper presents a subject-specific in-silico framework in which we uncover the relationship between the spatially varying constituents of the aorta and the non-linear compliance of the vessel during the cardiac cycle uncovered through our MRI investigations. A microstructurally motivated constitutive model is developed, and simulations reveal that internal vessel contractility, due to pre-stretched elastin and actively generated smooth muscle cell stress, must be incorporated, along with collagen strain stiffening, in order to accurately predict the non-linear pressure-area relationship observed in-vivo. Modelling of elastin and smooth muscle cell contractility allows for the identification of the reference vessel configuration at zero-lumen pressure, in addition to accurately predicting high- and low-compliance regimes under a physiological range of pressures. This modelling approach is also shown to capture the key features of elastin digestion and SMC activation experiments. The volume fractions of the constituent components of the aortic material model were computed so that the in-silico pressure-area curves accurately predict the corresponding MRI data at each location. Simulations reveal that collagen and smooth muscle volume fractions increase distally, while elastin volume fraction decreases distally, consistent with reported histological data. Furthermore, the strain at which collagen transitions from low to high stiffness is lower in the abdominal aorta, again supporting the histological finding that collagen waviness is lower distally. The analyses presented in this paper provide new insights into the heterogeneous structure-function relationship that underlies aortic biomechanics. Furthermore, this subject-specific MRI/FEA methodology provides a foundation for personalised in-silico clinical analysis and tailored aortic device development. STATEMENT OF SIGNIFICANCE: This study provides a significant advance in in-silico medicine by capturing the structure/function relationship of the subject-specific human aorta presented in our previous MRI analyses. A physiologically based aortic constitutive model is developed, and simulations reveal that internal vessel contractility must be incorporated, along with collagen strain stiffening, to accurately predict the in-vivo non-linear pressure-area relationship. Furthermore, this is the first subject-specific model to predict spatial variation in the volume fractions of aortic wall constituents. Previous studies perform phenomenological hyperelastic curve fits to medical imaging data and ignore the prestress contribution of elastin, collagen, and SMCs and the associated zero-pressure reference state of the vessel. This novel MRI/FEA framework can be used as an in-silico diagnostic tool for the early stage detection of aortic pathologies.

A computational model for understanding the micro-mechanics of collagen fiber network in the tunica adventitia

Abdominal aortic aneurysm is a prevalent cardiovascular disease with high mortality rates. The mechanical response of the arterial wall relies on the organizational and structural behavior of its microstructural components, and thus, a detailed understanding of the microscopic mechanical response of the arterial wall layers at loads ranging up to rupture is necessary to improve diagnostic techniques and possibly treatments. Following the common notion that adventitia is the ultimate barrier at loads close to rupture, in the present study, a finite element model of adventitial collagen network was developed to study the mechanical state at the fiber level under uniaxial loading. Image stacks of the rabbit carotid adventitial tissue at rest and under uniaxial tension obtained using multi-photon microscopy were used in this study, as well as the force-displacement curves obtained from previously published experiments. Morphological parameters like fiber orientation distribution, waviness, and volume fraction were extracted for one sample from the confocal image stacks. An inverse random sampling approach combined with a random walk algorithm was employed to reconstruct the collagen network for numerical simulation. The model was then verified using experimental stress-stretch curves. The model shows the remarkable capacity of collagen fibers to uncrimp and reorient in the loading direction. These results further show that at high stretches, collagen network behaves in a highly non-affine manner, which was quantified for each sample. A comprehensive parameter study to understand the relationship between structural parameters and their influence on mechanical behavior is presented. Through this study, the model was used to conclude important structure-function relationships that control the mechanical response. Our results also show that at loads close to rupture, the probability of failure occurring at the fiber level is up to 2%. Uncertainties in usually employed rupture risk indicators and the stochastic nature of the event of rupture combined with limited knowledge on the microscopic determinants motivate the development of such an analysis. Moreover, this study will advance the study of coupling microscopic mechanisms to rupture of the artery as a whole.

Inflammasome activity in leucocytes decreases with abdominal aortic aneurysm progression

Abdominal aortic aneurysms (AAAs) are characterized by chronic inflammatory cell infiltration. The present extended immunohistochemistry study aimed to characterize inflammation in AAA and aortic control samples. In specific, the composition of the infiltrating immune cells and the expression of five inflammasome components in these immune cells were evaluated, in order to characterize their role in AAA development. A total of 104 biopsies from 48 AAA patients and 40 healthy specimens from organ donors were evaluated for their grade of inflammation. Infiltrating leukocytes were characterized by specific markers (CD3, CD20 and CD68), intramural localization and inflammasome protein expression [NLR family pyrin domain containing 3 (NLRP3), absent in melanoma 2 (AIM2), apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC), Caspase-1 and Caspase-5]. Macrophages, B and T lymphocytes were detected to a similar extent in grade 1, 2 and 3 AAA specimens, whereas in control samples, B and T lymphocytes were rarely observed in grade 1 lesions. Expression frequencies of NLRP3, AIM2 and Caspase-5 were significantly higher in grade 1 lesions of AAA samples compared with grade 1 lesions in control samples. Finally, AIM2, ASC, and Caspase-5 displayed significantly lower expression frequencies in grade 3 compared with grade 2 AAA specimens, and all inflammasome components were less frequently detected in grade 3 than in grade 1 lesions of AAA. This indicates that inflammasome activities decrease with AAA progression in infiltrating leukocytes. No statistically significant association was found for grade 2 and grade 3 lesions and total leukocyte count, C-reactive protein levels, maximal aortic diameter, plasma cholesterol level or biomechanical parameters (derived from finite element analysis) of the respective patients. Overall, the aortic wall of AAA contained lymphocytes and macrophages with different states of activity. The present data suggested that therapeutic inhibition of specific inflammasome components might counteract AAA development and progression.

Abdominal aortic aneurysms

An abdominal aortic aneurysm (AAA) is a localized dilatation of the infrarenal aorta. AAA is a multifactorial disease, and genetic and environmental factors play a part; smoking, male sex and a positive family history are the most important risk factors, and AAA is most common in men >65 years of age. AAA results from changes in the aortic wall structure, including thinning of the media and adventitia due to the loss of vascular smooth muscle cells and degradation of the extracellular matrix. If the mechanical stress of the blood pressure acting on the wall exceeds the wall strength, the AAA ruptures, causing life-threatening intra-abdominal haemorrhage - the mortality for patients with ruptured AAA is 65-85%. Although AAAs of any size can rupture, the risk of rupture increases with diameter. Intact AAAs are typically asymptomatic, and in settings where screening programmes with ultrasonography are not implemented, most cases are diagnosed incidentally. Modern functional imaging techniques (PET, CT and MRI) may help to assess rupture risk. Elective repair of AAA with open surgery or endovascular aortic repair (EVAR) should be considered to prevent AAA rupture, although the morbidity and mortality associated with both techniques remain non-negligible.

Mercaptoethanol Protects the Aorta from Dissection by Inhibiting Oxidative Stress, Inflammation, and Extracellular Matrix Degeneration in a Mouse Model

Background

The aims of this study were to investigate the effects of mercaptoethanol treatment on the expression of mediators of oxidative stress, inflammation, and extracellular matrix (ECM) degeneration in a mouse aortic dissection (AD) model.

Material/Methods

Twenty-four 8-month-old C57BL/6J mice were divided into three groups and studied for two weeks: 1) the aortic dissection (AD) Model group (N=8) underwent intraperitoneal injection of angiotensin II (Ang II) (5 ml/kg) three times every 24 h; 2) the mercaptoethanol Treated group (N=8) were given oral mercaptoethanol (2.5 mM); the Normal group (N=8) underwent intraperitoneal injection of noradrenaline (5 mg/kg) three times every 24 h. Sections of mouse aorta were prepared for histology with hematoxylin and eosin (H&E) staining; immunohistochemistry was performed to detect levels of: nuclear factor (erythroid-derived 2)-like 2 (NFE2L2), nuclear factor κB (NF-κB), p65, superoxide dismutase-1 (SOD1), glutamate cysteine ligase catalytic subunit (GCLC), tumor necrosis factor α (TNF-α), interleukin-1β (IL-1β), and matrix metalloproteinase-9 (MMP9). Quantitative reverse transcription-polymerase chain reaction (RT-PCR) evaluated mRNA expression of SOD1, GCLC, TNF-α, IL-1β, and MMP9.

Results

Mercaptoethanol treatment inhibited Ang II-induced aortic dissection in AD mice, as shown histologically. Mercaptoethanol treatment reduced the expression levels of NFE2L2, NF-κB, p65, TNF-α, IL-1β and increased the expression levels of SOD1, MMP9, and GCLC.

Conclusions

In an AD mouse model, mercaptoethanol treatment inhibited thoracic and abdominal aortic dissection and reduced aortic tissue expression of mediators of oxidative stress and inflammation and increased the activation of ECM signaling pathways.

Numerical identification of the rupture locations in patient-specific abdominal aortic aneurysmsusing hemodynamic parameters

The rupture of an abdominal aortic aneurysm (AAA) is generally an unexpected event. Up to now, there is no agreement on an accurate criteria to predict the rupture risk of AAAs. This paper aims to numerically investigate the hemodynamics of three ruptured and one non-ruptured patient-specific AAA models to correlate local hemodynamic parameters with the rupture sites, and for the first time, this study introduced helicity as a potential index for the rupture potential of AAAs.3D reconstructions from CT scans were done. The simulation revealed that all the rupture sites were in regions of stagnation with near zero wall shear stress (WSS) but large WSS gradient (WSSG), which may explain the observation by the former researchers that the rupture site in the ruptured AAA has the lowest recorded wall thickness compared to other non-ruptured regions. Moreover, all the ruptures occurred at regions of zero helicity which represents a purely axial or circumferential flow. In addition, this study revealed that the double low region for the non-ruptured AAA was present with a thick layer of plaques, it suggests that the AAA rupture and the formation of atherosclerotic plaques may share a lot common physiological features. However, the fact that there are no plaques present in the walls of three RAAAs also indicates that AAA is not always a result of atherosclerosis. The current computational study may complement the maximum diameter, peak wall stress and other clinically relevant factors in AAA ruptures to identify the rupture sites of AAAs.

Gene Expression Profiling in Abdominal Aortic Aneurysms After Finite Element Rupture Risk Assessment

PURPOSE

To investigate the association between local biomechanical rupture risk calculations from finite element analysis (FEA) and whole-genome profiling of the abdominal aortic aneurysm (AAA) wall to determine if AAA wall regions with highest and lowest estimated rupture risk show different gene expression patterns.

METHODS

Six patients (mean age 74 years; all men) scheduled for open surgery to treat asymptomatic AAAs (mean diameter 55.2±3.5 mm) were recruited for the study. Rupture risk profiles were estimated by FEA from preoperative computed tomography angiography data. During surgery, AAA wall samples of ~10 mm² were extracted from the lowest and highest rupture risk locations identified by the FEA. Twelve samples were processed for RNA extraction and subsequent whole genome expression profiling. Expression of single genes and of predefined gene groups were compared between vessel wall areas with highest and lowest predicted rupture risk.

RESULTS

Normalized datasets comprised 15,079 gene transcripts with expression above background. In biopsies with high rupture risk, upregulation of 18 and downregulation of 18 genes was detected when compared to the low-risk counterpart. Global analysis of predefined gene groups revealed expression differences in genes associated with extracellular matrix (ECM) degradation (p<0.001), matrix metalloproteinase activity (p<0.001), and chemokine signaling (p<0.001).

CONCLUSION

Increased expression of genes involved in degrading ECM components was present in AAA wall regions with highest biomechanical stress, supporting the thesis of mechanotransduction. More experimental studies with cooperation of multicenter vascular biobanks are necessary to understand AAA etiologies and identify further parameters of FEA model complementation.

Histology and Biaxial Mechanical Behavior of Abdominal Aortic Aneurysm Tissue Samples

Abdominal aortic aneurysms (AAAs) represent permanent, localized dilations of the abdominal aorta that can be life-threatening if progressing to rupture. Evaluation of risk of rupture depends on understanding the mechanical behavior of patient AAA walls. In this project, a series of patient AAA wall tissue samples have been evaluated through a combined anamnestic, mechanical, and histopathologic approach. Mechanical properties of the samples have been characterized using a novel, strain-controlled, planar biaxial testing protocol emulating the in vivo deformation of the aorta. Histologically, the tissue ultrastructure was highly disrupted. All samples showed pronounced mechanical stiffening with stretch and were notably anisotropic, with greater stiffness in the circumferential than the axial direction. However, there were significant intrapatient variations in wall stiffness and stress. In biaxial tests in which the longitudinal stretch was held constant at 1.1 as the circumferential stretch was extended to 1.1, the maximum average circumferential stress was 330 ± 70 kPa, while the maximum average axial stress was 190 ± 30 kPa. A constitutive model considering the wall as anisotropic with two preferred directions fit the measured data well. No statistically significant differences in tissue mechanical properties were found based on patient gender, age, maximum bulge diameter, height, weight, body mass index, or smoking history. Although a larger patient cohort is merited to confirm these conclusions, the project provides new insight into the relationships between patient natural history, histopathology, and mechanical behavior that may be useful in the development of accurate methods for rupture risk evaluation.

Regional Mapping of Flow and Wall Characteristics of Intracranial Aneurysms

The evolution of intracranial aneurysms (IAs) is thought to be driven by progressive wall degradation in response to abnormal hemodynamics. Previous studies focused on the relationship between global hemodynamics and wall properties. However, hemodynamics, wall structure and mechanical properties of cerebral aneurysms can be non-uniform across the aneurysm wall. Therefore, the aim of this work is to introduce a methodology for mapping local hemodynamics to local wall structure in resected aneurysm specimens. This methodology combines image-based computational fluid dynamics, tissue resection, micro-CT imaging of resected specimens mounted on 3D-printed aneurysm models, alignment to 3D vascular models, multi-photon microscopy of the wall, and regional mapping of hemodynamics and wall properties. This approach employs a new 3D virtual marking tool for surgeons to delineate the location of the resected specimen directly on the 3D model, while in the surgical suite. The case of a middle cerebral artery aneurysm is used to illustrate the application of this methodology to the assessment of the relationship between local wall shear stress and local wall properties including collagen fiber organization and wall geometry. This methodology can similarly be used to study the relationship between local intramural stresses and local wall structure.

Biomechanical rupture risk assessment of abdominal aortic aneurysms based on a novel probabilistic rupture risk index

A rupture risk assessment is critical to the clinical treatment of abdominal aortic aneurysm (AAA) patients. The biomechanical AAA rupture risk assessment quantitatively integrates many known AAA rupture risk factors but the variability of risk predictions due to model input uncertainties remains a challenging limitation. This study derives a probabilistic rupture risk index (PRRI). Specifically, the uncertainties in AAA wall thickness and wall strength were considered, and wall stress was predicted with a state-of-the-art deterministic biomechanical model. The discriminative power of PRRI was tested in a diameter-matched cohort of ruptured (n = 7) and intact (n = 7) AAAs and compared to alternative risk assessment methods. Computed PRRI at 1.5 mean arterial pressure was significantly (p = 0.041) higher in ruptured AAAs (20.21(s.d. 14.15%)) than in intact AAAs (3.71(s.d. 5.77)%). PRRI showed a high sensitivity and specificity (discriminative power of 0.837) to discriminate between ruptured and intact AAA cases. The underlying statistical representation of stochastic data of wall thickness, wall strength and peak wall stress had only negligible effects on PRRI computations. Uncertainties in AAA wall stress predictions, the wide range of reported wall strength and the stochastic nature of failure motivate a probabilistic rupture risk assessment. Advanced AAA biomechanical modelling paired with a probabilistic rupture index definition as known from engineering risk assessment seems to be superior to a purely deterministic approach.

Biomechanical Rupture Risk Assessment: A Consistent and Objective Decision-Making Tool for Abdominal Aortic Aneurysm Patients

Abdominal aortic aneurysm (AAA) rupture is a local event in the aneurysm wall that naturally demands tools to assess the risk for local wall rupture. Consequently, global parameters like the maximum diameter and its expansion over time can only give very rough risk indications; therefore, they frequently fail to predict individual risk for AAA rupture. In contrast, the Biomechanical Rupture Risk Assessment (BRRA) method investigates the wall's risk for local rupture by quantitatively integrating many known AAA rupture risk factors like female sex, large relative expansion, intraluminal thrombus-related wall weakening, and high blood pressure. The BRRA method is almost 20 years old and has progressed considerably in recent years, it can now potentially enrich the diameter indication for AAA repair. The present paper reviews the current state of the BRRA method by summarizing its key underlying concepts (i.e., geometry modeling, biomechanical simulation, and result interpretation). Specifically, the validity of the underlying model assumptions is critically disused in relation to the intended simulation objective (i.e., a clinical AAA rupture risk assessment). Next, reported clinical BRRA validation studies are summarized, and their clinical relevance is reviewed. The BRRA method is a generic, biomechanics-based approach that provides several interfaces to incorporate information from different research disciplines. As an example, the final section of this review suggests integrating growth aspects to (potentially) further improve BRRA sensitivity and specificity. Despite the fact that no prospective validation studies are reported, a significant and still growing body of validation evidence suggests integrating the BRRA method into the clinical decision-making process (i.e., enriching diameter-based decision-making in AAA patient treatment).

Surrogate Markers of Abdominal Aortic Aneurysm Progression

The natural course of many abdominal aortic aneurysms (AAA) is to gradually expand and eventually rupture and monitoring the disease progression is essential to their management. In this publication, we review surrogate markers of AAA progression. AAA diameter remains the most widely used and important marker of AAA growth. Standardized reporting of reproducible methods of measuring AAA diameter is essential. Newer imaging assessments, such as volume measurements, biomechanical analyses, and functional and molecular imaging, as well as circulating biomarkers, have potential to add important information about AAA progression. Currently, however, there is insufficient evidence to recommend their routine use in clinical practice.

Prediction of Rupture Sites in Abdominal Aortic Aneurysms After Finite Element Analysis

PURPOSE

To associate regions of highest local rupture risk from finite element analysis (FEA) to subsequent rupture sites in abdominal aortic aneurysms (AAA).

METHODS

This retrospective multicenter study analyzed computed tomography angiography (CTA) data from 13 asymptomatic AAA patients (mean age 76 years; 8 men) experiencing rupture at a later point in time between 2005 and 2011. All patients had CTA scans before and during the rupture event. FEA was performed to calculate peak wall stress (PWS), peak wall rupture risk (PWRR), rupture risk equivalent diameters (RRED), and the intraluminal thrombus volume (ILTV). PWS and PWRR locations in the prerupture state were compared with subsequent CTA rupture findings. Visible contrast extravasation was considered a definite (n=5) rupture sign, while a periaortic hematoma was an indefinite (n=8) sign. A statistical comparison was performed between the 13-patient asymptomatic AAA group before and during rupture and a 23-patient diameter-matched asymptomatic AAA control group that underwent elective surgery.

RESULTS

The asymptomatic AAAs before rupture showed significantly higher PWRR and RRED values compared to the matched asymptomatic AAA control group (median values 0.74 vs 0.52 and 77 vs 59 mm, respectively; p<0.0001 for both). No statistical differences could be found for PWS and ILTV. Ruptured AAAs showed the highest maximum diameters, PWRR, and RRED values. In 7 of the ruptured AAAs (2 definite and 5 indefinite rupture signs), CTA rupture sites correlated with prerupture PWRR locations.

CONCLUSION

The location of the PWRR in unruptured AAAs predicted future rupture sites in several cases. Asymptomatic AAA patients with high PWRR and RRED values have an increased rupture risk.

A Review of Computational Methods to Predict the Risk of Rupture of Abdominal Aortic Aneurysms

Computational methods have played an important role in health care in recent years, as determining parameters that affect a certain medical condition is not possible in experimental conditions in many cases. Computational fluid dynamics (CFD) methods have been used to accurately determine the nature of blood flow in the cardiovascular and nervous systems and air flow in the respiratory system, thereby giving the surgeon a diagnostic tool to plan treatment accordingly. Machine learning or data mining (MLD) methods are currently used to develop models that learn from retrospective data to make a prediction regarding factors affecting the progression of a disease. These models have also been successful in incorporating factors such as patient history and occupation. MLD models can be used as a predictive tool to determine rupture potential in patients with abdominal aortic aneurysms (AAA) along with CFD-based prediction of parameters like wall shear stress and pressure distributions. A combination of these computer methods can be pivotal in bridging the gap between translational and outcomes research in medicine. This paper reviews the use of computational methods in the diagnosis and treatment of AAA.

The correlation between computed tomography and duplex evaluation of autogenous vein bypass grafts and their relationship to failure

OBJECTIVE

Duplex ultrasound (DUS) imaging for vein bypass graft (VBG) surveillance is confounded by technical and physiologic factors that reduce the sensitivity for detecting impending graft failure. In contrast, three-dimensional computed tomography angiography (CTA) offers high-fidelity anatomic characterization of VBGs, but its utility in detecting at risk grafts is unknown. The current study analyzed the correlation between DUS and CTA for detection of vein graft stenosis and evaluated the relationship of the observed abnormalities to VBG failure.

METHODS

Consecutive lower extremity VBG patients underwent surveillance with concurrent DUS imaging and CTA at 1 week and at 1, 6, and 12 months postoperatively. A standardized algorithm was used for CT reconstruction and extraction of the lumen geometries at 1-mm intervals. At each interval, CT-derived cross-sectional areas were coregistered and correlated to DUS peak systolic velocities (PSVs) within six predesignated anatomic zones and then analyzed for outcome association. Vein graft failure was defined as pathologic change within a given anatomic zone resulting in thrombosis, amputation, or reintervention within the 6-month period after the observed time point.

RESULTS

The study recruited 54 patients, and 10 (18%) experienced failure ≤18 months of implantation. The expected inverse relationship between cross-sectional area and PSV was only weakly correlated (Spearman rank coefficient = -0.19). Moderate elevations in the PSV ratio (PSVr; 2-3.5) were frequently transient, with 14 of 18 grafts (78%) demonstrating ratio reduction on subsequent imaging. A PSVr ≥3.5 was associated with a 67% failure rate. CT stenosis <50% was highly correlated with success (0 failures); however, high-grade (>80%) CT stenosis was more likely to succeed than to fail (25%). Significant discordance between CT and DUS was found in 18 patients. Although 14 of these patients had CT stenosis >70% with a PSVr <3.5, subsequent failure occurred in only two. Conversely, graft failure occurred in three of four patients with CT stenosis <70% but PSVr >3.5. Focused analysis of these patients using computational fluid dynamic modeling demonstrated that vein side branches, local tortuosity, regional diameter variations, and venovenostomies were the drivers of these discrepancies.

CONCLUSIONS

This analysis demonstrated that a PSVr ≥3.5 is strongly correlated with VBG failure, whereas the natural history of moderately elevated PSVr (2-3.5) is largely clinically benign. Although minimum stenosis on the CT scan was highly predictive of success, high-grade CT stenosis was infrequently associated with failure. The interaction of anatomic features with the local flow dynamics was identified as the primary confounder for a direct correlation between CT and DUS imaging.

Finite element analysis in asymptomatic, symptomatic, and ruptured abdominal aortic aneurysms: in search of new rupture risk predictors

OBJECTIVES

To compare biomechanical rupture risk parameters of asymptomatic, symptomatic and ruptured abdominal aortic aneurysms (AAA) using finite element analysis (FEA).

STUDY DESIGN

Retrospective biomechanical single center analysis of asymptomatic, symptomatic, and ruptured AAAs. Comparison of biomechanical parameters from FEA.

MATERIALS AND METHODS

From 2011 to 2013 computed tomography angiography (CTA) data from 30 asymptomatic, 15 symptomatic, and 15 ruptured AAAs were collected consecutively. FEA was performed according to the successive steps of AAA vessel reconstruction, segmentation and finite element computation. Biomechanical parameters Peak Wall Rupture Risk Index (PWRI), Peak Wall Stress (PWS), and Rupture Risk Equivalent Diameter (RRED) were compared among the three subgroups.

RESULTS

PWRI differentiated between asymptomatic and symptomatic AAAs (p < .0004) better than PWS (p < .1453). PWRI-dependent RRED was higher in the symptomatic subgroup compared with the asymptomatic subgroup (p < .0004). Maximum AAA external diameters were comparable between the two groups (p < .1355). Ruptured AAAs showed the highest values for external diameter, total intraluminal thrombus volume, PWS, RRED, and PWRI compared with asymptomatic and symptomatic AAAs. In contrast with symptomatic and ruptured AAAs, none of the asymptomatic patients had a PWRI value >1.0. This threshold value might identify patients at imminent risk of rupture.

CONCLUSIONS

From different FEA derived parameters, PWRI distinguishes most precisely between asymptomatic and symptomatic AAAs. If elevated, this value may represent a negative prognostic factor for asymptomatic AAAs.