Hemodynamic indicators of the formation of tandem intracranial aneurysm based on a vascular restoration algorithm.

Background: Hemodynamic factors are believed to be closely related to IA growth. However, the underlying pathophysiological mechanism that induces the growth sequence in tandem intracranial aneurysms (IAs) remains unclear. Methods and results: This study involved five patients with tandem IAs. Aneurysm models were reconstructed based on image datasets. A novel vascular restoration algorithm was proposed to generate the hypothetical geometry of the healthy parent vessel before each IA formation in the concatenated structure. Detailed hemodynamic patterns and morphological features were revealed under various growth sequences of tandem IAs to investigate the flow-driven mechanism of IA growth. Potential hemodynamic indicators of IA formation were proposed. Results: The patient cases were divided into two groups based on the size difference of tandem IAs. In the group with a similar size of tandem IAs, the position of the vortex core was associated with the site of the secondary aneurysm, while in the group with a significant size difference of the IAs, the position with the maximum curvature of the parent vessel plays a significant role in aneurysm formation. Conclusions: This study preliminarily revealed key hemodynamic and morphological indicators that determine the formation of tandem IAs. The proposed vascular restoration algorithm that provided the pre-aneurysm vasculature might be useful in investigating the flow-driven mechanism of IA growth, thus contributing to the risk evaluation of secondary aneurysm formation.

Association of cerebral infarction with vertebral arterial fenestration using non-Newtonian hemodynamic evaluation.

PURPOSE: Cerebral artery fenestration is a rare vascular anomaly, but its existence has been increasingly documented. The association of cerebral infarction and fenestration is of great clinical interest, and the exact underlying mechanism remains unclear. This study aims to identify risk factors contributing to cerebral infarction by computational hemodynamics analysis. METHODS: Eight patients with image findings of fenestration structure were recruited in this research, in which four suffered fenestration-related cerebral infarction (A series) while the other four (B series) were set as control matched by the fenestration size. Three-dimensional models were reconstructed from the MRA images and computational simulations with non-Newtonian flow model were performed to get interested hemodynamic characteristics. RESULTS: The blood flow pattern was relatively separated along two channels of fenestration in series A compared with series B cases in Group 1-2, however, no significant difference was shown in Group 3-4. Quantitatively, planes were cut in the middle of fenestrations and the ratio of mass flow rate and area was calculated at systolic peak. Results showed that the side of the dominant blood supply was opposite between A and B series, and the dominant side was also opposite between small and large fenestrations. In infarction cases, the basilar top was distributed with larger areas of detrimental hemodynamic indicators and a larger concentrated high viscosity region. CONCLUSION: The flow division condition throughout the fenestration structure has a key impact on further flow redistribution and flow pattern. The blood viscosity has the potential to be a useful tool in identifying the risk factors for cerebral infarction and more emphasis should be placed on the hemodynamic environment at superior cerebellar arteries.

Effects of dispersed fibres in myocardial mechanics, Part I: passive response.

It is widely acknowledged that an imbalanced biomechanical environment can have significant effects on myocardial pathology, leading to adverse remodelling of cardiac function if it persists. Accurate stress prediction essentially depends on the strain energy function which should have competent descriptive and predictive capabilities. Previous studies have focused on myofibre dispersion, but not on fibres along other directions. In this study, we will investigate how fibre dispersion affects myocardial biomechanical behaviours by taking into account both the myofibre dispersion and the sheet fibre dispersion, with a focus on the sheet fibre dispersion. Fibre dispersion is incorporated into a widely-used myocardial strain energy function using the discrete fibre bundle approach. We first study how different dispersion affects the descriptive capability of the strain energy function when fitting to ex vivo experimental data, and then the predictive capability in a human left ventricle during diastole. Our results show that the chosen strain energy function can achieve the best goodness-of-fit to the experimental data by including both fibre dispersion. Furthermore, noticeable differences in stress can be found in the LV model. Our results may suggest that it is necessary to include both dispersion for myofibres and the sheet fibres for the improved descriptive capability to the ex vivo experimental data and potentially more accurate stress prediction in cardiac mechanics.

Role of the Anterior Center-Edge Angle on Acetabular Stress Distribution in Borderline Development Dysplastic of Hip Determined by Finite Element Analysis.

Background: The joint with hip dysplasia is more likely to develop osteoarthritis because of the higher contact pressure, especially in the socket. The lateral center-edge angle (LCEA) is the major indicator for hip dysplasia via radiography. However, the pathological conditions of LCEA angles in the range of 18°-25° are still controversial, which challenges precise diagnosis and treatment decision-making. Objective: The purpose of this study is to investigate the influence of anterior center-edge angle (ACEA) on the mechanical stress distribution of the hip joint, via finite element analysis, to provide insights into the severity of the borderline development dysplasia. Methods: From 2017 to 2019, there were 116 patients with borderline developmental dysplasia of the hip (BDDH) enrolled in this research. Based on the inclusion criteria, nine patients were involved and categorized into three LCEA groups with the maximal ACEA differences. Patient-specific hip joint models were reconstructed from computed tomography scans, and the cartilages, including the labrum, were established via a modified numerical method. The finite element analysis was conducted to compare the stress distributions due to the different ACEA. Results: As ACEA decreased, the maximum stress of the acetabulum increased, and the high stress area developed toward the edge. Quantitative analysis showed that in the cases with lower ACEA, the area ratio of high stress increased, and the contact facies lunata area significantly affected the stress distribution. Conclusion: For patients with BDDH, both the ACEA and the area of facies lunata played essential roles in determining the severity of hip dysplasia and the mechanical mechanism preceding osteoarthritis.

A Mock Circulation Loop for In Vitro Hemodynamic Evaluation of Aorta: Application in Aortic Dissection.

PURPOSE: Aortic dissection (AD) is a catastrophic disease with complex hemodynamic conditions, however, understandings regarding its perfusion characteristics were not sufficient. In this study, a mock circulation loop (MCL) that integrated the Windkessel element and patient-specific silicone aortic phantoms was proposed to reproduce the aortic flow environment in vitro. MATERIALS AND METHODS: Patient-specific normal and dissected aortic phantoms with 12 branching vessels were established and embedded into this MCL. Velocities for aortic branches based on 20 healthy volunteers were regarded as the standardized data for flow division. By altering boundary conditions, the proposed MCL could mimic normal resting and left-sided heart failure (LHF) conditions. Flow rates and pressure status of the aortic branches could be quantified by separate sensors. RESULTS: In normal resting condition, the simulated heart rate and systemic flow rate were 60 bpm and 4.85 L/minute, respectively. For the LHF condition, the systolic and diastolic blood pressures were 75.94±0.77 mmHg and 57.65±0.35 mmHg, respectively. By tuning the vascular compliance and peripheral resistance, the flow distribution ratio (FDR) of each aortic branch was validated by the standardized data in the normal aortic phantom (mean difference 2.4%±1.70%). By comparing between the normal and dissected aortic models under resting condition, our results indicated that the AD model presented higher systolic (117.82±0.60 vs 108.75±2.26 mmHg) and diastolic (72.38±0.58 vs 70.46±2.33 mmHg) pressures, the time-average velocity in the true lumen (TL; 36.95 cm/s) was higher than that in the false lumen (FL; 22.95 cm/s), and the blood transport direction between the TL and FL varied in different re-entries. CONCLUSIONS: The proposed MCL could be applied as a research tool for in vitro hemodynamic analysis of the aorta diseases under various physical conditions.

Hemodynamic effects of size and location of basilar artery fenestrations associated to pathological implications.

Fenestration is a rare congenital abnormality that refers to a segmental duplication of arteries. It is still not clear about the role of fenestrations in the etiology and pathological evolution of vascular diseases. This study aims to investigate the hemodynamic influence brought by various sizes and locations of fenestration in basilar artery models. A series presumptive fenestration models were established based on a normal basilar artery model with various sizes and locations. Identical boundary conditions were utilized in the computational fluid dynamics simulations and different flow patterns in the fenestration and bifurcation regions were comprehensively analyzed. Wall shear stress (WSS)-related parameters such as oscillatory shear index (OSI) and aneurysm formation index (AFI) were computed and compared. The value of WSS on fenestration increased by the fenestration's tortuosity, and nearly-circular fenestration suffered higher WSS than narrow-strips one. Also, high OSI and low AFI value mainly occurred in the bifurcation region, indicating a high level of turbulence and high risk of aneurysm formation. The location of fenestration mainly changed the impact force of blood flow on the bifurcation and the disorder characteristics of blood flow, while the size of fenestration changed the WSS distribution on the proximal inner wall and bifurcation region of fenestration. In summary, the nearly-circular fenestration should be stratified carefully which may results in a high risk inducing unfavorable vascular wall remodeling.

Multi-stage learning for segmentation of aortic dissections using a prior aortic anatomy simplification.

Aortic dissection (AD) is a life-threatening cardiovascular disease with a high mortality rate. The accurate and generalized 3-D reconstruction of AD from CT-angiography can effectively assist clinical procedures and surgery plans, however, is clinically unavaliable due to the lacking of efficient tools. In this study, we presented a novel multi-stage segmentation framework for type B AD to extract true lumen (TL), false lumen (FL) and all branches (BR) as different classes. Two cascaded neural networks were used to segment the aortic trunk and branches and to separate the dual lumen, respectively. An aortic straightening method was designed based on the prior vascular anatomy of AD, simplifying the curved aortic shape before the second network. The straightening-based method achieved the mean Dice scores of 0.96, 0.95 and 0.89 for TL, FL, and BR on a multi-center dataset involving 120 patients, outperforming the end-to-end multi-class methods and the multi-stage methods without straightening on the dual-lumen segmentation, even using different network architectures. Both the global volumetric features of the aorta and the local characteristics of the primary tear could be better identified and quantified based on the straightening. Comparing to previous deep learning methods dealing with AD segmentations, the proposed framework presented advantages in segmentation accuracy.

Flow analysis of aortic dissection: comparison of inflow boundary conditions for computational models based on 4D PCMRI and Doppler ultrasound.

Computational hemodynamics quantifying the flow environment is an important tool in understanding aortic dissection. In this study, various inflow boundaries were applied on a patient-specific model and compared to the individualized velocimetry. The results indicated that the computations generally overestimated the flow volume and underestimated the wall shear stress. By quantifying the accuracy of the simulation results, two inflow settings were suggested. One was individualized, the PCMRI-extracted 4D flow information, and the other was averaged by healthy data, the ultrasound-extracted averaged flow waveform with parabolic velocity profile. This study might contribute to improving the precise computation of aortic dissection hemodynamics.

Computed tomography-based hemodynamic index for aortic dissection.

OBJECTIVE: In this study we aimed to propose a new computed tomography-based hemodynamic indicator to quantify the functional significance of aortic dissection and predict post intervention luminal remodeling. METHODS: Computational hemodynamics and 3D structural analyses were conducted in 51 patients with type B aortic dissection, at initial presentation and at approximately 1 month, 3 months, and 1 year post intervention. A functional index was proposed on the basis of luminal pressure difference. Statistical relationships between the proposed indicator and longitudinal luminal development were analyzed. RESULTS: The computed luminal pressure difference (true lumen pressure minus false lumen pressure) varied overall from positive to negative along the aorta. The first balance position at which the pressure difference equals 0 was proposed as the functional indicator. A more distally located first balance position indicated better functional status. Implantation of stent graft distally shifted this balance position. Patients with the balance position shifted out of the dissected region (43%) presented the highest functional improvement after intervention; whereas those with the balance position shifted to the abdominal region (25%) showed unsatisfactory results. The magnitude of distal shifting of the first balance position at 3 months post intervention was statistically related to the subsequent true lumen expansion and false lumen reduction. CONCLUSIONS: The first balance position of luminal pressure difference quantified the hemodynamic status of the dissected aorta. The magnitude of distal shifting of the balance position after intervention was associated with functional improvement and might be used predict longitudinal aortic remodeling.

Hemodynamic Differences Between Basilar Artery Fenestration and Normal Vertebrobasilar Artery: A Pilot Study.

Background: Basilar artery fenestration has been proposed as a contributor to ischemic stroke, as unique flow patterns induced by fenestration may be related to thrombus formation or insufficiency. This study aimed to evaluate the hemodynamics of basilar artery fenestration (BAF) using computational fluid dynamics (CFD). Methods: Patients with BAF and normal vertebrobasilar system were recruited and separately evaluated using CFD. Specific geometric vascular models were reconstructed based on 3D-rotational angiography (3D-RA). Patients were divided into the BAF group and control group (i.e., patients with the normal vertebrobasilar system). Hemodynamic and geometric variables were calculated and compared between groups using Student's t-test or Wilcoxon rank-sum test. Results: Overall, 24 patients were included, with 12 patients each in the BAF group and the control group. The BAF group had a significantly smaller basilar artery diameter than the control group (3.1 ± 0.51 vs. 3.76 ± 0.4, p = 0.002). Compared to the control group, the BAF group had higher values of maxOSI (median, 0.3 vs. 0.09, p = 0.028), TAWSSG (median, 983.42 vs. 565.39, p = 0.038) in the flow confluence, higher SAR-TAWSSG in bifurcation (median, 70.22 vs. 27.65, p = 0.002) and higher SAR-TAWSSG in basilar artery (median, 48.75 vs. 16.17, p < 0.001) of the vertebrobasilar artery. Conclusions: This pilot study suggested that hemodynamic differences between BAF and normal vertebrobasilar artery across multiple shear flow parameters. The disturbed flow in the BAF may increase the risk of thrombus formation, plaque instability, and subsequent ischemic cerebrovascular events. These should be confirmed by future studies.

Diffusion Tensor Imaging Tractography Detecting Isolated Oculomotor Paralysis Caused by Pituitary Apoplexy.

OBJECTIVES: Pituitary apoplexy (PA)-induced oculomotor palsy, although rare, can be caused by compression on the lateral wall of the cavernous sinus. This study aimed to visualize PA-induced oculomotor nerve damage using diffusion tensor imaging (DTI) tractography. MATERIALS AND METHODS: We enrolled 5 patients with PA-induced isolated oculomotor palsy (patient group) and 10 healthy participants (control group); all underwent DTI tractography preoperatively. Fractional anisotropy (FA) and mean diffusion (MD) values of the cisternal portion of the bilateral oculomotor nerve were measured. DTI tractography was repeated after the recovery of oculomotor palsy. RESULTS: While no statistical difference was observed in FA and MD values of the bilateral oculomotor nerve in the control group (P>0.05), the oculomotor nerve on the affected side was disrupted in the patient group, with a statistical difference in FA and MD values of the bilateral oculomotor nerve (P<0.01). After the recovery of oculomotor palsy, the FA value of the oculomotor nerve on the affected side increased, whereas the MD value decreased (P<0.01). Meanwhile, no significant difference was observed in FA and MD values of the bilateral oculomotor nerve (P>0.05). DTI tractography of the oculomotor nerve on the affected side revealed restoration of integrity. Furthermore, the symptoms of oculomotor palsy improved in all patients 7 days postoperatively. CONCLUSION: DTI tractography could be a helpful adjunct to the standard clinical and paraclinical ophthalmoplegia examinations in patients with PA; thus, this study establishes the feasibility of DTI tractography in this specific clinical setting.

Retrograde branched extension limb assembling stent of pararenal abdominal aortic aneurysm: A longitudinal hemodynamic analysis for stent graft migration.

PURPOSE: Pararenal abdominal aortic aneurysms (PRAAAs) are a life-threatening disease, and hemodynamic analysis may provide greater insight into the effectiveness and long-term outcomes of endovascular aneurysm repair (EVAR). However, the lack of patient-specific boundary conditions on the periphery compromises the accuracy. Windkessel (WK) boundary conditions coupled to hemodynamic follow-up models of a PRAAA patient, aims to provide insights into the link between hemodynamics and poor prognosis. METHOD: One PRAAA patient underwent EVAR and reintervention after one branch of stent-graft (SG) had migrated. Totally five computational follow-up models were studied. Patient-specific flow data acquired via ultrasound were used to define the boundary conditions in the ascending aorta and the following three branches. Coupled zero-dimensional WK models representing the distal vasculature were used to define the outlet boundary conditions under the abdomen. RESULTS: Flow divisions of the main SG branches were 40.7% and 24.7%, respectively. Time-averaged wall shear stress and oscillatory shear index (OSI) increased at the junction connected the SG branch and the stent leading to the right common iliac artery (RCIA) where the stent migrated. The OSI and relative residence time (RRT) value in superior mesenteric artery increased notably after the migration, the RRT continuously increased following the reintervention. CONCLUSION: Unbalanced flow, resulting in locally high-speed flow, high WSS and OSI might significantly affect stent stability. Results suggest that diameters and interconnection design of stents in complex cases should take the flow division into consideration and computational simulations might be considered as a tool for intervention protocol design.

On the Validation of a Multiple-Network Poroelastic Model Using Arterial Spin Labeling MRI Data.

The Multiple-Network Poroelastic Theory (MPET) is a numerical model to characterize the transport of multiple fluid networks in the brain, which overcomes the problem of conducting separate analyses on individual fluid compartments and losing the interactions between tissue and fluids, in addition to the interaction between the different fluids themselves. In this paper, the blood perfusion results from MPET modeling are partially validated using cerebral blood flow (CBF) data obtained from arterial spin labeling (ASL) magnetic resonance imaging (MRI), which uses arterial blood water as an endogenous tracer to measure CBF. Two subjects-one healthy control and one patient with unilateral middle cerebral artery (MCA) stenosis are included in the validation test. The comparison shows several similarities between CBF data from ASL and blood perfusion results from MPET modeling, such as higher blood perfusion in the gray matter than in the white matter, higher perfusion in the periventricular region for both the healthy control and the patient, and asymmetric distribution of blood perfusion for the patient. Although the partial validation is mainly conducted in a qualitative way, it is one important step toward the full validation of the MPET model, which has the potential to be used as a testing bed for hypotheses and new theories in neuroscience research.

Optimization schemes for endovascular repair with parallel technique based on hemodynamic analyses.

Endovascular repair with parallel stent-grafts (SG) is a challenging technique that reconstructs the luminal flow pathways by implanting parallel-placed SGs into the vessel. After treatment, occlusion and shifting of the parallel SGs are sometimes reported, which could be fatal and difficult to be re-operated. These issues are highly related to the local hemodynamic conditions in the stented region. In this study, a patient case treated by the octopus endograft technique (a head-SG with three limb-SGs) and experienced limb-SG occlusion is studied. 3-D models are established based on computed tomography (CT) angiography datasets pretreatment and posttreatment as well as during follow-ups. Hemodynamic quantities such as pressure drop, wall shear stress-related parameters, and flow division in limb-SGs and visceral arteries are quantitatively investigated. Optimizations on the length of the head-SG and diameter of the limb-SGs are analyzed based on various scenarios. The results indicate that when reconstructing the flow pathways via octopus stenting, it is important to ensure the flow distribution as physiologically required with this new morphology. Position (or length) of the head-SG and diameter of the limb-SGs play an important role in controlling flow division, and high time average wall shear stress (TAWSS) around the head-SG acts as a main factor for graft immigration. This study, by proposing optimization suggestions with hemodynamic analyses for a specific case, implicates that pretreatment SG scenarios may assist in wise selection and placement of the device and thus may improve long-term effectiveness of this kind of challenging endovascular repair techniques.

Hemodynamics analyses in treated and untreated carotid arteries of the same patient: A preliminary study based on three patient cases.

Carotid atherosclerotic disease is highly related to cerebrovascular events. Carotid endarterectomy is the common operation method to treat this disease. In this study, hemodynamics analyses are performed on the carotid arteries in three patients, whose right carotid artery had been treated by carotid endarterectomy and the left carotid artery remained untreated. Flow and loading conditions are compared between these treated and untreated carotid arteries and evaluation of the operative results is discussed. Patient-specific models are reconstructed from MDCT data. Intraoperative ultrasound flow measurements are performed on the treated carotid arteries and the obtained data are used as the boundary conditions of the models and the validations of the computational results. Finite volume method is employed to solve the transport equations and the flow and loading conditions of the models are reported. The results indicate that: (i) in two of the three patients, the internal-to-external flow rate ratio in the untreated carotid artery is larger than that in the treated one, and the average overall flow split ratio by summing up the data of both the left and right carotid arteries is about 2.15; (ii) in the carotid bulb, high wall shear stress occurs at the bifurcation near the external carotid artery in all of the cases without hard plaques; (iii) the operated arteries present low time-averaged wall shear stress at the carotid bulb, especially for the treated arteries with patch technique, indicating the possibility of the recurrence of stenosis; (iv) high temporal gradient of wall shear stress (>35 Pa/s) is shown in the narrowing regions along the vessels; and (v) in the carotid arteries without serious stenosis, the maximum velocity magnitude during mid-diastole is 32~37% of that at systolic peak, however, in the carotid artery with 50% stenosis by hard plaques, this value is nearly doubled (64%). The computational work quantifies flow and loading distributions in the treated and untreated carotid arteries of the same patient, contributing to evaluation of the operative results and indicating the recurrent sites of potential atheromatous plaques.

The respiratory ciliary motion produced by dynein activity alone: A computational model of ciliary ultrastructure.

BACKGROUND: Respiratory ciliary motion is enabled by dynein/microtubule activity. Current observation techniques can hardly capture the dynein activation pattern in moving cilia. Here we introduce a computational model to mimic the ciliary ultrastructure and simulate the dynein-driven ciliary motion. METHODS: A three-dimensional model is established to mimic the ``9 + 2'' ciliary ultrastructure. The dynein force is simulated as point loads embedded along the microtubules. The dynein-triggered ciliary motion is solved by using the Finite Element Method along with grid deformation techniques. RESULTS: By comparing the simulated ciliary movement to the observation results, the rationality of different dynein activity hypotheses are evaluated and the dynein activation pattern that can produce the planar beating of lung cilia is proposed. The results also reveal that the dynein force alone can only generate longitudinal microtubule sliding and ciliary bending; to produce the ciliary `curl-up' movement, transverse forces (possibly induced by radial spokes) need to be considered. CONCLUSION: This model provides a platform to investigate various assumptions of dynein activity, facilitating us to evaluate their rationality and propose possible dynein activation patterns.