Characterizing saccular aortic arch aneurysms from the geometry-flow dynamics relationship

Geometrical Factors Affect Wall Shear Stress in Saccular Aneurysms of the Infrarenal Abdominal Aorta

BACKGROUND

Low wall shear stress (WSS) is predictive of aortic aneurysm growth and rupture. Yet, estimating WSS in a clinical setting is impractical, whereas measuring aneurysm geometry is feasible. This study investigates the association between saccular aneurysm geometry of the infrarenal aorta and WSS.

METHODS

Starting with a nonaneurysmal, patient-specific, computational fluid dynamics model of the aorta, saccular aneurysms of varying geometry were created by incrementally increasing the neck width and sac depth from 1 cm to 4 cm. The aspect ratio (the ratio between sac depth and neck width) varied between 0.25 and 4. The peak WSS, time-averaged WSS (TAWSS), and oscillatory shear index (OSI) were measured within the aneurysm sac.

RESULTS

Decreasing the neck width from 4 cm to 1 cm decreased the peak WSS by 69% and the TAWSS by 83%. Increasing the sac depth from 1 cm to 4 cm decreased the peak WSS by 55% and the OSI by 37%. The aspect ratio was negatively correlated to peak WSS (Rs -0.85; P < 0.001).

CONCLUSIONS

In saccular aneurysms of the infrarenal aorta, a smaller neck width, deeper aneurysm sac, and larger aspect ratio are associated with lower peak WSS.

Saccular and Fusiform Abdominal Aortic Aneurysms Treated With Endovascular Repair Differ in Presentation and Treatment Threshold: Analyses Using a National Clinical Database in Japan

BACKGROUND

Saccular abdominal aortic aneurysms (AAAs) are considered to be at higher risk of rupture than fusiform AAAs, but not much is known about the extent of this risk. Therefore, this study aimed to compare the rupture presentation between fusiform and saccular AAAs.

METHODS AND RESULTS

This is a retrospective cohort study on 27 290 patients who underwent primary endovascular repair for a degenerative AAA between 2016 and 2019, and who were registered in the National Clinical Database in Japan. At operation for nonruptured case, the aneurysm diameter was significantly smaller in saccular AAAs than in fusiform AAAs (median, 44.0 versus 51.0 mm; P<0.001). Similarly, aneurysm diameter at rupture was significantly smaller in saccular AAAs than in fusiform AAAs (median, 55.6 versus 68.0 mm; P<0.001). The likelihood of repair for rupture was significantly higher in saccular AAAs than in fusiform AAAs in the 40- to 54-mm diameter range, in which saccular morphology was found to be an independent risk factor for rupture against fusiform morphology by adjusting for sex and aneurysm diameter (odds ratio, 2.54 [95% CI, 1.75-3.69]). In addition, receiver-operating characteristic curve analysis revealed that the cutoff diameter to predict rupture was smaller in saccular AAAs than in fusiform AAAs (50.5 and 59.5 mm, respectively) based on the Youden index.

CONCLUSIONS

Saccular AAAs presented at smaller diameters than fusiform AAAs in patients with ruptured AAAs treated with endovascular aortic repair, which supports the idea that saccular AAAs should be treated at smaller diameters.

Hemodynamics and wall shear metrics in a pulmonary autograft: Comparing a fluid-structure interaction and computational fluid dynamics approach

OBJECTIVE

In young patients, aortic valve disease is often treated by placement of a pulmonary autograft (PA) which adapts to its new environment through growth and remodeling. To better understand the hemodynamic forces acting on the highly distensible PA in the acute phase after surgery, we developed a fluid-structure interaction (FSI) framework and comprehensively compared hemodynamics and wall shear-stress (WSS) metrics with a computational fluid dynamic (CFD) simulation.

METHODS

The FSI framework couples a prestressed non-linear hyperelastic arterial tissue model with a fluid model using the in-house coupling code CoCoNuT. Geometry, material parameters and boundary conditions are based on in-vivo measurements. Hemodynamics, time-averaged WSS (TAWSS), oscillatory shear index (OSI) and topological shear variation index (TSVI) are evaluated qualitatively and quantitatively for 3 different sheeps.

RESULTS

Despite systolic-to-diastolic volumetric changes of the PA in the order of 20 %, the point-by-point correlation of TAWSS and OSI obtained through CFD and FSI remains high (r > 0.9, p < 0.01) for TAWSS and (r > 0.8, p < 0.01) for OSI). Instantaneous WSS divergence patterns qualitatively preserve similarities, but large deformations of the PA leads to a decrease of the correlation between FSI and CFD resolved TSVI (r < 0.7, p < 0.01). Moderate co-localization between FSI and CFD is observed for low thresholds of TAWSS and high thresholds of OSI and TSVI.

CONCLUSION

FSI might be warranted if we were to use the TSVI as a mechano-biological driver for growth and remodeling of PA due to varying intra-vascular flow structures and near wall hemodynamics because of the large expansion of the PA.

Narrative Review of Risk Assessment of Abdominal Aortic Aneurysm Rupture Based on Biomechanics-Related Morphology

CLINICAL IMPACT

Studies have shown that the biomechanical indicators based on multi-scale models are more effective in accurately assessing the rupture risk of AAA. To meet the need for clinical monitoring and rapid decision making, the typical morphological parameters associated with AAA rupture and their relationships with the mechanical environment have been summarized, which provide a reference for clinical preoperative risk assessment of AAA.

Significant thrombus in saccular aneurysm in a patient with polycythemia: a case report

BACKGROUND

Morphologically, the risk of aortic aneurysm rupture is mainly evaluated based on its type (e.g., fusiform or saccular) and diameter. Based on the finite element analysis, peak wall stress has been identified as a more sensitive and specific predictor of rupture in recent years. Moreover, in finite analysis, the neck of aneurysm is the highest peak wall stress and is associated with the rupture point.

CASE PRESENTATION

A saccular aortic aneurysm (84 mm) was incidentally detected during preoperative examination for chronic empyema in a 74-year-old male patient with a history of polycythemia. Aortic arch graft replacement using an open stent was performed.

CONCLUSIONS

Morphologically, this case was associated with a very high risk of rupture; nevertheless, it did not rupture. In this case, a mural thrombus (likely formed due to polycythemia) covered the neck of aneurysm that is experiencing the highest peak wall stress and is associated with the rupture point. The mural thrombus decreased peak wall stress and could reduce the risk of rupture even for huge saccular aneurysms. Furthermore, the mural thrombus was fully occupied in aneurysms, such as during coil embolization. Thus, polycythemia could decrease the risk of rupture of huge saccular aneurysms.

Clinical presentation, outcomes, and threshold for repair by sex in degenerative saccular vs fusiform aneurysms in the descending thoracic aorta

OBJECTIVE

Saccular-shaped thoracic aortic aneurysms (TAAs) are often treated at smaller diameters compared with fusiform TAAs, despite a lack of strong clinical evidence to support this practice. The aim of this study was to examine differences in presentation, treatment, and outcomes between saccular TAAs and fusiform TAAs in the descending thoracic aorta. We also examined the need for sex-specific treatment thresholds for TAAs.

METHODS

All Vascular Quality Initiative (VQI) patients undergoing thoracic endovascular aneurysm repair (TEVAR) for degenerative TAAs in the descending thoracic aorta from 2012 through 2022 were reviewed. Patients were stratified by urgency: emergent/urgent vs elective repairs (ruptured/symptomatic). Demographics, comorbidities, anatomical/procedural characteristics, and outcomes for fusiform TAAs and saccular TAAs were compared. Cumulative distribution curves were used to plot the proportion of patients who underwent emergent/urgent repair according to sex-stratified aortic diameter.

RESULTS

Among 655 emergent/urgent TEVARs, 37% were performed for saccular TAAs, whereas among 1352 elective TEVARs, 35% had saccular TAA morphology. Compared with fusiform TAAs, saccular TAAs more frequently underwent emergent/urgent (ruptured/symptomatic) TEVAR below the repair threshold in both females (<50 mm: 38% vs 10%; relative risk, 3.39; 95% confidence interval [CI], 2.04-5.70; P < .001), and males (<55 mm: 47% vs 21%; relative risk, 2.26; 95% CI, 1.60-3.18; P < .001). Moreover, among patients with emergent/urgent fusiform TAAs, females presented at smaller diameters compared with males, whereas there was no difference in preoperative aneurysm diameter among patients with saccular TAAs. Regarding outcomes, emergent/urgent treated saccular TAAs had similar postoperative outcomes and 5-year mortality compared with fusiform TAAs. Nevertheless, in the elective cohort, patients with saccular TAAs had similar postoperative mortality compared with those with fusiform TAAs, but a lower rate of postoperative spinal cord ischemia (0.7% vs 3.2%; P = .010). Furthermore, patients with saccular TAAs had a higher rate of 5-year mortality compared with their fusiform counterparts (23% vs 17%; hazard ratio, 1.53; 95% CI, 1.12-2.10; P = .010).

CONCLUSIONS

Patients with saccular TAAs underwent emergent/urgent TEVAR at smaller diameters than those with fusiform TAAs, supporting current clinical practice guideline recommendations that saccular TAAs warrant treatment at smaller diameters. Furthermore, these data support a sex-specific treatment threshold for patients with fusiform TAAs, but not for those with saccular TAAs. Although there were no differences in outcomes following TEVAR between morphologies in the emergent/urgent cohort, patients with saccular TAAs who were treated electively were associated with higher 5-year mortality compared with those with fusiform TAAs.

A rare case of spontaneous thrombosis in saccular cerebral aneurysm in a patient with subarachnoid hemorrhage

Cerebral aneurysm is a known cause of spontaneous subarachnoid hemorrhage (SAH). Furthermore, this condition is often asymptomatic, but the occurrence of a rupture can lead to fatal complications. The incidence of spontaneous thrombosis in saccular aneurysm is rare, with an incidence rate of 1%-2%. The most common sites include the middle cerebral artery (MCA) (41%), posterior communicating artery (PCOMM) (15%), and posterior inferior cerebellar artery (PICA) (11%). A head computed tomography angiography (CTA) with contrast is a common diagnostic tool for detecting SAH in the temporoparietal area, hippocampal gyrus, and right fissure of Sylvie. In some cases, saccular aneurysm can be found in the segment bifurcation of the right middle cerebral artery. A cerebral angiography was carried out, specifically digital subtraction angiography, which revealed the presence of visible blister remnants of aneurysm in the form of spontaneous thrombosis. After 1 year, another angiography evaluation was performed to assess the condition of the patient. Furthermore, the results showed no evidence of recanalization and there were no new neurologic deficits. Although spontaneous thrombosis led to the healing of aneurysm in some cases, secondary recanalization remained a possibility. Therefore, it was essential to monitor any incidence of this complication. Precise knowledge of the mechanism of spontaneous thrombosis could lead to the development of new therapeutic approaches. Spontaneous thrombosis in cases of saccular aneurysm is a rare occurrence, which can provide temporary or permanent benefits to the patient. Therefore, periodic evaluation is required to assess the condition of the patient.

Aortic haemodynamics and wall stress analysis following arch aneurysm repair using a single-branched endograft

Introduction

Thoracic endovascular aortic repair (TEVAR) of the arch is challenging given its complex geometry and the involvement of supra-aortic arteries. Different branched endografts have been designed for use in this region, but their haemodynamic performance and the risk for post-intervention complications are not yet clear. This study aims to examine aortic haemodynamics and biomechanical conditions following TVAR treatment of an aortic arch aneurysm with a two-component single-branched endograft.

Methods

Computational fluid dynamics and finite element analysis were applied to a patient-specific case at different stages: pre-intervention, post-intervention and follow-up. Physiologically accurate boundary conditions were used based on available clinical information.

Results

Computational results obtained from the post-intervention model confirmed technical success of the procedure in restoring normal flow to the arch. Simulations of the follow-up model, where boundary conditions were modified to reflect change in supra-aortic vessel perfusion observed on the follow-up scan, predicted normal flow patterns but high levels of wall stress (up to 1.3M MPa) and increased displacement forces in regions at risk of compromising device stability. This might have contributed to the suspected endoleaks or device migration identified at the final follow up.

Discussion

Our study demonstrated that detailed haemodynamic and biomechanical analysis can help identify possible causes for post-TEVAR complications in a patient-specific setting. Further refinement and validation of the computational workflow will allow personalised assessment to aid in surgical planning and clinical decision making.

ACR Appropriateness Criteria® Thoracoabdominal Aortic Aneurysm or Dissection: Treatment Planning and Follow-Up

As the incidence of thoracoabdominal aortic pathology (aneurysm and dissection) rises and the complexity of endovascular and surgical treatment options increases, imaging follow-up of patients remains crucial. Patients with thoracoabdominal aortic pathology without intervention should be monitored carefully for changes in aortic size or morphology that could portend rupture or other complication. Patients who are post endovascular or open surgical aortic repair should undergo follow-up imaging to evaluate for complications, endoleak, or recurrent pathology. Considering the quality of diagnostic data, CT angiography and MR angiography are the preferred imaging modalities for follow-up of thoracoabdominal aortic pathology for most patients. The extent of thoracoabdominal aortic pathology and its potential complications involve multiple regions of the body requiring imaging of the chest, abdomen, and pelvis in most patients. The ACR Appropriateness Criteria are evidence-based guidelines for specific clinical conditions that are reviewed annually by a multidisciplinary expert panel. The guideline development and revision process support the systematic analysis of the medical literature from peer reviewed journals. Established methodology principles such as Grading of Recommendations Assessment, Development, and Evaluation or GRADE are adapted to evaluate the evidence. The RAND/UCLA Appropriateness Method User Manual provides the methodology to determine the appropriateness of imaging and treatment procedures for specific clinical scenarios. In those instances where peer reviewed literature is lacking or equivocal, experts may be the primary evidentiary source available to formulate a recommendation.

Assessment of Pseudocoarctation of the Aorta with Saccular Aneurysms by Four-Dimensional Flow Magnetic Resonance Imaging and Histological Analysis

In this study, we present the case of a 21-year-old woman with pseudocoarctation of the aorta with saccular aneurysms that were evaluated by four-dimensional flow magnetic resonance imaging and histological analysis. We observed complete occupation of the aneurysm sacs by vortex flow and high peak wall shear stress in the proximal region of the kinked aorta. The aortic replacement was performed for the thoracic aortic aneurysms and the clinical course was uneventful. The aneurysms were histopathologically diagnosed as pseudoaneurysms based on the disappearance of all three layers and their replacement with collagen-rich connective tissues. These findings indicate that abnormal flow dynamics and the resulting abnormal shear stress in the aorta may play central roles in the formation and development of a saccular aneurysm.

Optimization of 4D flow MRI velocity field in the aorta with divergence-free smoothing

Divergence-free smoothing with wall treatment (DFS_wt) method is proposed for processing with four-dimensional (4D) flow magnetic resonance imaging (MRI) data of blood flows to enhance the quality of flow field with physical constraints. The new method satisfies the no-slip wall boundary condition and applies wall function of velocity profile for better estimating the velocity gradient in the near-wall region, and consequently improved wall shear stress (WSS) calculation against the issue of coarse resolution of 4D flow MRI. In the first testing case, blood flow field obtained from 4D flow MRI is well smoothed by DFS_wt method. A great consistency is observed between the post-processed 4D flow MRI data and the computational fluid dynamics (CFD) data in the interested velocity field. WSS has an apparent improvement due to the proposed near-wall treatment with special wall function comparing to the result from original 4D flow MRI data or the DFS-processed data with no wall function. The other five cases also show the same performance that smoothed velocity field and improved WSS estimation are achieved on 4D flow MRI data optimized by DFS_wt. The improvements will benefit the study of hemodynamics regarding the determination of location or the potential possibility of lesions.

Utility of 4D Flow MRI in Thoracic Aortic Diseases: A Literature Review of Clinical Applications and Current Evidence

Despite the recent technical developments, surgery on the thoracic aorta remains challenging and is associated with significant mortality and morbidity. Decisions about when and if to operate are based on a balance between surgical risk and the hazard of aortic rupture. These decisions are sometimes difficult in elective cases of thoracic aortic diseases, including aneurysms and dissections. Abnormal wall stress derived from flow alterations influences disease progression. Therefore, a better understanding of the complex hemodynamic environment inside the aortic lumen will facilitate patient-specific risk assessments of complications, which enable clinicians to provide timely prophylactic interventions. Time-resolved 3D phase-contrast (4D flow) MRI has many advantages for the in vivo assessment of flow dynamics. Recent developments in 4D flow imaging techniques has led to significant advances in our understanding of physiological flow dynamics in healthy subjects and patients with thoracic aortic diseases. In this clinically focused review of thoracic aortic diseases, we demonstrate the clinical advances acquired with 4D flow MRI from published studies. We provide a systematic overview of key evidences and considerations regarding normal thoracic aortas, thoracic aortic aneurysms, aortic dissections, and thoracic aortas with prosthetic graft replacement.

A technique for creating an experimental type Ia endoleak model in the thoracic aorta of swine

PURPOSE

This study aimed to create an animal model of type Ia endoleak that creates persistent problems after thoracic endovascular aortic repair.

MATERIALS AND METHODS

In six swine, thoracic aortic aneurysms were created using the harvested jugular vein. We created a type Ia endoleak using a composite stent-graft comprising the first stent-graft (reverse-tapered: thicker part, 16 mm; thinner part, 10 mm) and the second stent-graft (tapered: thicker part, 18-20 mm; thinner part, 16 mm). This double-component stent-graft was deployed in the abdominal aorta and then moved upward to the proximal entry site of the thoracic aneurysm using the inflated balloon for precise positioning. After the surgical procedure and on postoperative day 8, aortography was performed to detect residual endoleak, and then the swine were euthanized.

RESULTS

A stable aneurysm (mean size of all aneurysms, 16.8 ± 1.72 mm × 11.8 ± 2.32 mm) and type Ia endoleak were successfully observed in all swine. A single stent-graft was sufficient in one of the six swine.

CONCLUSION

A novel technique to create a type Ia endoleak model can be successfully developed in swine.

An infected aneurysm of the vertebral artery following cervical pyogenic spondylitis: a case report and literature review

BACKGROUND

An important complication of pyogenic spondylitis is aneurysms in the adjacent arteries. There are reports of abdominal aortic or iliac aneurysms, but there are few reports describing infected aneurysms of the vertebral artery. Furthermore, there are no reports describing infected aneurysms of the vertebral arteries following cervical pyogenic spondylitis. We report a rare case of an infected aneurysm of the vertebral artery as a complication of cervical pyogenic spondylitis, which was successfully treated by endovascular treatment.

CASE PRESENTATION

Cervical magnetic resonance imaging (MRI) of a 59-year-old man who complained of severe neck pain showed pyogenic spondylitis. Although he was treated extensively by antibiotic therapy, his neck pain did not improve. Follow-up MRI showed the presence of a cyst, which was initially considered an abscess, and therefore, treatment initially included guided tapping and suction under ultrasonography. However, under ultrasonographic examination an aneurysm was detected. The contrast-enhanced computed tomography (CT) scan showed an aneurysm of the vertebral artery. Following endovascular treatment (parent artery occlusion: PAO), the patient's neck pain disappeared completely.

CONCLUSION

Although there are several reports of infected aneurysms of the vertebral arteries, this is the first report describing an infected aneurysm of the vertebral artery as a result of cervical pyogenic spondylitis. Whenever a paraspinal cyst exist at the site of infection, we recommend that clinicians use not only X-ray, conventional CT, and MRI to examine the cyst, but ultrasonography and contrast-enhanced CT as well because of the possibility of an aneurysms in neighboring blood vessels. It is necessary to evaluate the morphology of the aneurysm to determine the treatment required.

4D flow imaging of the thoracic aorta: is there an added clinical value?

Four-dimensional (4D) flow MRI has emerged as a powerful non-invasive technique in cardiovascular imaging, enabling to analyse in vivo complex flow dynamics models by quantifying flow parameters and derived features. Deep knowledge of aortic flow dynamics is fundamental to better understand how abnormal flow patterns may promote or worsen vascular diseases. In the perspective of an increasingly personalized and preventive medicine, growing interest is focused on identifying those quantitative functional features which are early predictive markers of pathological evolution. The thoracic aorta and its spectrum of diseases, as the first area of application and development of 4D flow MRI and supported by an extensive experimental validation, represents the ideal model to introduce this technique into daily clinical practice. The purpose of this review is to describe the impact of 4D flow MRI in the assessment of the thoracic aorta and its most common affecting diseases, providing an overview of the actual clinical applications and describing the potential role of derived advanced hemodynamic measures in tailoring follow-up and treatment.

Sudden Death Caused by Rupture of Spontaneous Ductus Arteriosus Aneurysm in an Adult

A 55-year-old man complained of sudden onset of severe neck pain. This was followed by prompt loss of consciousness and death. Autopsy revealed rupture of a saccular aneurysm, which was considered to have resulted from enlargement of the remaining ductal tissue, and was located on the medial aspect of the uppermost portion of the descending aorta. Dense blood extravasation was noted in the posterior mediastinum and extending to the strap muscles of the neck and larynx. Histological examination of the rupture site revealed disappearance of the medial elastic fibers and thickened intima covered with dense fibrous tissue. Spontaneous ductus arteriosus aneurysm in adults is a rare finding, but widespread use of imaging technologies has revealed that it develops more frequently than previously recognized. Fatal complications may occur even when the aneurysm is relatively small. Therefore, pathologists should be aware of this aneurysm as a potential cause of sudden death.

Wall stress on ascending thoracic aortic aneurysms with bicuspid compared with tricuspid aortic valve

OBJECTIVE

Guidelines for repair of bicuspid aortic valve-associated ascending thoracic aortic aneurysms have been changing, most recently to the same criteria as tricuspid aortic valve-ascending thoracic aortic aneurysms. Rupture/dissection occurs when wall stress exceeds wall strength. Recent studies suggest similar strength of bicuspid aortic valve versus tricuspid aortic valve-ascending thoracic aortic aneurysms; thus, comparative wall stress may better predict dissection in bicuspid aortic valve versus tricuspid aortic valve-ascending thoracic aortic aneurysms. Our aim was to determine whether bicuspid aortic valve-ascending thoracic aortic aneurysms had higher wall stresses than their tricuspid aortic valve counterparts.

METHODS

Patients with bicuspid aortic valve- and tricuspid aortic valve-ascending thoracic aortic aneurysms (bicuspid aortic valve = 17, tricuspid aortic valve = 19) greater than 4.5 cm underwent electrocardiogram-gated computed tomography angiography. Patient-specific 3-dimensional geometry was reconstructed and loaded to systemic pressure after accounting for prestress geometry. Finite element analyses were performed using the LS-DYNA solver (LSTC Inc, Livermore, Calif) with user-defined fiber-embedded material model to determine ascending thoracic aortic aneurysm wall stress.

RESULTS

Bicuspid aortic valve-ascending thoracic aortic aneurysms 99th-percentile longitudinal stresses were 280 kPa versus 242 kPa (P = .028) for tricuspid aortic valve-ascending thoracic aortic aneurysms in systole. These stresses did not correlate to diameter for bicuspid aortic valve-ascending thoracic aortic aneurysms (r = -0.004) but had better correlation to tricuspid aortic valve-ascending thoracic aortic aneurysms diameter (r = 0.677). Longitudinal stresses on sinotubular junction were significantly higher in bicuspid aortic valve-ascending thoracic aortic aneurysms than in tricuspid aortic valve-ascending thoracic aortic aneurysms (405 vs 329 kPa, P = .023). Bicuspid aortic valve-ascending thoracic aortic aneurysm 99th-percentile circumferential stresses were 548 kPa versus 462 kPa (P = .033) for tricuspid aortic valve-ascending thoracic aortic aneurysms, which also did not correlate to bicuspid aortic valve-ascending thoracic aortic aneurysm diameter (r = 0.007).

CONCLUSIONS

Circumferential and longitudinal stresses were greater in bicuspid aortic valve- than tricuspid aortic valve-ascending thoracic aortic aneurysms and were more pronounced in the sinotubular junction. Peak wall stress did not correlate with bicuspid aortic valve-ascending thoracic aortic aneurysm diameter, suggesting diameter alone in this population may be a poor predictor of dissection risk. Our results highlight the need for patient-specific aneurysm wall stress analysis for accurate dissection risk prediction.

Computational Fluid Dynamics and Additive Manufacturing to Diagnose and Treat Cardiovascular Disease

Noninvasive engineering models are now being used for diagnosing and planning the treatment of cardiovascular disease. Techniques in computational modeling and additive manufacturing have matured concurrently, and results from simulations can inform and enable the design and optimization of therapeutic devices and treatment strategies. The emerging synergy between large-scale simulations and 3D printing is having a two-fold benefit: first, 3D printing can be used to validate the complex simulations, and second, the flow models can be used to improve treatment planning for cardiovascular disease. In this review, we summarize and discuss recent methods and findings for leveraging advances in both additive manufacturing and patient-specific computational modeling, with an emphasis on new directions in these fields and remaining open questions.