Blood flow dynamic improvement with aneurysm repair detected by a patient-specific model of multiple aortic aneurysms

Postendovascular Aneurysmal Repair Increase in Local Energy Loss for Fusiform Abdominal Aortic Aneurysm: Assessments With 4D flow MRI

BACKGROUND

Although endovascular aneurysmal repair (EVAR) is a preferred treatment for abdominal aortic aneurysm (AAA) owing to its low invasiveness, its impact on the local hemodynamics has not been fully assessed.

PURPOSE

To elucidate how EVAR affects the local hemodynamics in terms of energy loss (EL).

STUDY TYPE

Prospective single-arm study.

FIELD STRENGTH/SEQUENCE

A 3.0 T/4D flow MRI using a phase-contrast three-dimensional cine-gradient-echo sequence.

POPULATION

A total of 13 consecutive patients (median [interquartile range] age: 77.0 [73.0, 78.8] years, 11 male) scheduled for EVAR as an initial treatment for fusiform AAA.

ASSESSMENT

4D flow MRI covering the abdominal aorta and bilateral common iliac arteries and the corresponding stent-graft (SG) lumen was performed before and after EVAR. Plasma brain natriuretic peptide (BNP) was measured within 1 week before and 1 month after EVAR. The hemodynamic data, including mean velocity and the local EL, were compared pre-/post-EVAR. EL was correlated with AAA neck angle and with BNP. Patients were subdivided into deformed (N = 5) and undeformed SG subgroups (N = 8) and pre-/post-EVAR BNP compared in each.

STATISTICS

Parametric or nonparametric methods. Spearman's rank correlation coefficients (r). The interobserver/intraobserver variabilities with Bland-Altman plots. A P value < 0.05 is considered significant.

RESULTS

The mean velocity (cm/sec) at the AAA was five times greater after EVAR: 4.79 ± 0.32 vs. 0.91 ± 0.02. The total EL (mW) increased by 1.7 times after EVAR: 0.487 (0.420, 0.706) vs. 0.292 (0.192, 0.420). The total EL was proportional to the AAA neck angle pre-EVAR (r = 0.691) and post-EVAR (r = 0.718). BNP (pg/mL) was proportional to the total EL post-EVAR (r = 0.773). In the deformed SG group, EL (0.349 [0.261, 0.416]) increased 2.4-fold to 0.848 (0.597, 1.13), and the BNP 90.3 (53.6, 105) to 100 (67.2, 123) post-EVAR.

CONCLUSION

The local EL showed a 1.7-fold increase after EVAR. The larger increase in the EL in the deformed SG group might be a potential concern for frail patients.

EVIDENCE LEVEL

1 TECHNICAL EFFICACY: Stage 2.

Non-invasive hemodynamic diagnosis based on non-linear pulse wave theory applied to four limbs

Introduction: Hemodynamic diagnosis indexes (HDIs) can comprehensively evaluate the health status of the cardiovascular system (CVS), particularly for people older than 50 years and prone to cardiovascular disease (CVDs). However, the accuracy of non-invasive detection remains unsatisfactory. We propose a non-invasive HDIs model based on the non-linear pulse wave theory (NonPWT) applied to four limbs. Methods: This algorithm establishes mathematical models, including pulse wave velocity and pressure information of the brachial and ankle arteries, pressure gradient, and blood flow. Blood flow is key to calculating HDIs. Herein, we derive blood flow equation for different times of the cardiac cycle considering the four different distributions of blood pressure and pulse wave of four limbs, then obtain the average blood flow in a cardiac cycle, and finally calculate the HDIs. Results: The results of the blood flow calculations reveal that the average blood flow in the upper extremity arteries is 10.78 ml/s (clinically: 2.5-12.67 ml/s), and the blood flow in the lower extremity arteries is higher than that in the upper extremity. To verify model accuracy, the consistency between the clinical and calculated values is verified with no statistically significant differences (p < 0.05). Model IV or higher-order fitting is the closest. To verify the model generalizability, considering the risk factors of cardiovascular diseases, the HDIs are recalculated using model IV, and thus, consistency is verified (p < 0.05 and Bland-Altman plot). Conclusion: We conclude our proposed algorithmic model based on NonPWT can facilitate the non-invasive hemodynamic diagnosis with simpler operational procedures and reduced medical costs.

Retrograde flow in aortic isthmus in normal and fetal heart disease by principal component analysis and computational fluid dynamics

OBJECTIVES

Reverse flow Retrograde flow (RF) of blood in the aortic isthmus can be observed in different types of fetal heart disease (FHD), including abnormalities in heart structure and function. This study sought to investigate the relationship between RF and blood flow parameters, and develop a computational fluid dynamics (CFD) model to understand the mechanisms underlying this observation.

MATERIAL AND METHODS

A total of 281 fetuses (gestational age [GA] 26.6±.3 weeks) with FHD and 2803 normal fetuses (GA: 26.1±.1 weeks) by fetal echocardiography collected from May 2016 to December 2018. Principal component analysis (PCA) was performed to find the relationship and the CFD model reconstructed from 3D/4D spatio-temporal image correlation (STIC) images to simulate hemodynamics.

RESULTS

There was a significant difference in the percentages of RF between the study (80/201 (39%)) and control (29/2803 (1%)) groups (p < 0.05). The RF occur when the aorta flow rate (left heart) is reduced to 60% by CFD stimulation. Pearson correlation analysis showed significant correlations between flow rate and wall shear stress(WSS) (r = .883, p = 0.047) variables at the AI.

CONCLUSION

Volumetric flow rate of AO or left heart was the main component of the cause of RF. The hemodynamics of the cardiovascular system have highly complex behavior hinge on the turbulent nature of circulating blood flow.

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.

CFD validation using in-vitro MRI velocity data - Methods for data matching and CFD error quantification

Predicting blood flow velocities in patient-specific geometries with Computational Fluid Dynamics (CFD) can provide additional data for diagnosis and treatment planning but the solution can be inaccurate. Therefore, it is crucial to understand the simulation errors and calibrate the numerical model. In-vitro velocity-encoded MRI is a versatile tool to validate CFD. The comparison between CFD and in-vitro MRI velocity data, and the analysis of the simulation error are the objectives of this study. A three-step routine is presented to validate medical CFD data. First, a properly scaled model of the patient-specific geometry is fabricated to achieve high relative resolution in the MRI experiment. Second, the measured flow geometry is matched with the CFD data using one of two algorithms, Coherent Point Drift and Iterative Closest Point. The aligned data sets are then interpolated onto a common grid to enable a point-to-point comparison. Third, the global and local deviations between CFD and MRI velocity data are calculated using different algorithms to reliably estimate the simulation error. The routine is successfully tested with a patient-specific model of a cerebral aneurysm. In conclusion, the methods presented here provide a framework for CFD validation using in-vitro MRI velocity data.

Biomechanical Investigation of Disturbed Hemodynamics-Induced Tissue Degeneration in Abdominal Aortic Aneurysms Using Computational and Experimental Techniques

Abdominal aortic aneurysm (AAA) is the dilatation of the aorta beyond 50% of the normal vessel diameter. It is reported that 4-8% of men and 0.5-1% of women above 50 years of age bear an AAA and it accounts for ~15,000 deaths per year in the United States alone. If left untreated, AAA might gradually expand until rupture; the most catastrophic complication of the aneurysmal disease that is accompanied by a striking overall mortality of 80%. The precise mechanisms leading to AAA rupture remains unclear. Therefore, characterization of disturbed hemodynamics within AAAs will help to understand the mechanobiological development of the condition which will contribute to novel therapies for the condition. Due to geometrical complexities, it is challenging to directly quantify disturbed flows for AAAs clinically. Two other approaches for this investigation are computational modeling and experimental flow measurement. In computational modeling, the problem is first defined mathematically, and the solution is approximated with numerical techniques to get characteristics of flow. In experimental flow measurement, once the setup providing physiological flow pattern in a phantom geometry is constructed, velocity measurement system such as particle image velocimetry (PIV) enables characterization of the flow. We witness increasing number of applications of these complimentary approaches for AAA investigations in recent years. In this paper, we outline the details of computational modeling procedures and experimental settings and summarize important findings from recent studies, which will help researchers for AAA investigations and rupture mechanics.

Impact of the location of the fenestration on Fontan circulation haemodynamics: a three-dimensional, computational model study

OBJECTIVES

There is no consensus or theoretical explanation regarding the optimal location for the fenestration during the Fontan operation. We investigated the impact of the location of the fenestration on Fontan haemodynamics using a three-dimensional Fontan model in various physiological conditions.

METHODS

A three-dimensional Fontan model was constructed on the basis of CT images, and a 4-mm-diameter fenestration was located between the extracardiac Fontan conduit and the right atrium at three positions: superior, middle, and inferior part of the conduit. Haemodynamics in the Fontan route were analysed using a three-dimensional computational fluid dynamic model in realistic physiological conditions, which were predicted using a lumped parameter model of the cardiovascular system. The respiratory effect of the caval flow was taken into account. The flow rate through the fenestration, the effect of lowering the central venous pressure, and wall shear stress in the Fontan circuit were evaluated under central venous pressures of 10, 15, and 20 mmHg. The pulse power index and pulsatile energy loss index were calculated as energy loss indices.

RESULTS

Under all central venous pressures, the middle-part fenestration demonstrated the most significant effect on enhancing the flow rate through the fenestration while lowering the central venous pressure. The middle-part fenestration produced the highest time-averaged wall shear stress, pressure pulse index, and pulsatile energy loss index.

CONCLUSIONS

Despite slightly elevated energy loss, the middle-part fenestration most significantly increased cardiac output and lowered central venous pressure under respiration in the Fontan circulation.

Validation of numerical simulation methods in aortic arch using 4D Flow MRI

Computational fluid dynamics (CFD) are the gold standard in studying blood flow dynamics. However, CFD results are dependent on the boundary conditions and the computation model. The purpose of this study was to validate CFD methods using comparison with actual measurements of the blood flow vector obtained with four-dimensional (4D) flow magnetic resonance imaging (MRI). 4D Flow MRI was performed on a healthy adult and a child with double-aortic arch. The aortic lumen was segmented to visualize the blood flow. The CFD analyses were performed for the same geometries based on three turbulent models: laminar, large eddy simulation (LES), and the renormalization group k–ε model (RNG k–ε). The flow-velocity vector components, namely the wall shear stress (WSS) and flow energy loss (EL), of the MRI and CFD results were compared. The flow rate of the MRI results was underestimated in small vessels, including the neck vessels. Spiral flow in the ascending aorta caused by the left ventricular twist was observed by MRI. Secondary flow distal to the aortic arch was well realized in both CFD and MRI. The average correlation coefficients of the velocity vector components of MRI and CFD for the child were the highest for the RNG k–ε model (0.530 in ascending aorta, 0.768 in the aortic arch, 0.584 in the descending aorta). The WSS and EL values of MRI were less than half of those of CFD, but the WSS distribution patterns were quite similar. The WSS and EL estimates were higher in RNG k–ε and LES than in the laminar model because of eddy viscosity. The CFD computation realized accurate flow distal to the aortic arch, and the WSS distribution was well simulated compared to actual measurement using 4D Flow MRI. However, the helical flow was not simulated in the ascending aorta. The accuracy was enhanced by using the turbulence model, and the RNG k–ε model showed the highest correlation with 4D Flow MRI.

Effects of arterial blood flow on walls of the abdominal aorta: distributions of wall shear stress and oscillatory shear index determined by phase-contrast magnetic resonance imaging

Although abdominal aortic aneurysms (AAAs) occur mostly inferior to the renal artery, the mechanism of the development of AAA in relation to its specific location is not yet clearly understood. The objective of this study was to evaluate the hypothesis that even healthy volunteers may manifest specific flow characteristics of blood flow and alter wall shear or oscillatory shear stress in the areas where AAAs commonly develop. Eight healthy male volunteers were enrolled in this prospective study, aged from 24 to 27. Phase-contrast magnetic resonance imaging (MRI) was performed with electrocardiographic triggering. Flow-sensitive four-dimensional MR imaging of the abdominal aorta, with three-directional velocity encoding, including simple morphological image acquisition, was performed. Information on specific locations on the aortic wall was applied to the flow encodes to calculate wall shear stress (WSS) and oscillatory shear index (OSI). While time-framed WSS showed the highest peak of 1.14 ± 0.25 Pa in the juxtaposition of the renal artery, the WSS plateaued to 0.61 Pa at the anterior wall of the abdominal aorta. The OSI peaked distal to the renal arteries at the posterior wall of the abdominal aorta of 0.249 ± 0.148, and was constantly elevated in the whole abdominal aorta at more than 0.14. All subjects were found to have elevated OSI in regions where AAAs commonly occur. These findings indicate that areas of constant peaked oscillatory shear stress in the infra-renal aorta may be one of the factors that lead to morphological changes over time, even in healthy individuals.

Repression of wall shear stress inside cerebral aneurysm at bifurcation of anterior cerebral artery by stents

The effect of a simple bare metal stent on repression of wall shear stress inside a model cerebral aneurysm was experimentally investigated by two-dimensional particle image velocimetry in vitro. The flow model simulated a cerebral aneurysm induced at the apex of bifurcation between the anterior cerebral artery and the anterior communicating artery. Wall shear stress was investigated using both stented and non-stented models to assess the simple stent characteristics. The flow behavior inside the stented aneurysm sac was unusual and wall shear stress was much smaller inside the aneurysm sac. Stent placement effectively repressed the temporal and spatial variations and the magnitude of wall shear stress. Hence, there is an effective possibility that would retard the progress of cerebral aneurysms by even simple stent.

Hybrid stage I palliation for hypoplastic left heart syndrome has no advantage on ventricular energetics: a theoretical analysis

A hybrid procedure combining bilateral pulmonary artery banding with ductal stenting has recently been used as stage I palliation for hypoplastic left heart syndrome. However, the advantage of the hybrid procedure over the Norwood procedure on ventricular energetics remains unclear. To clarify this, we performed a computational analysis with a combination of time-varying elastance chamber model and modified three-element Windkessel vascular model. Although mean pulmonary artery (PA) pressure, pulmonary flow, and oxygen saturation were almost equivalent with the Norwood procedure, the hybrid procedure delivered higher systolic and lower diastolic systemic arterial pressures compared to the Norwood procedure with right ventricle (RV) to PA shunt. As a result, the hybrid procedure yielded increased systolic pressure-volume area and impaired mechanical efficiency. Therefore, the hybrid procedure has probably no advantage on ventricular energetics compared to the Norwood procedure with a RV-PA shunt.