Flow Visualization in a Model of a Bifurcated Stent-Graft

A Finite Element Based Analysis of a Hemodynamics Efficient Flow Stent Suitable for Different Abdominal Aneurysm Shapes

This research aimed to examine the impact of a proposed flow stent (PFS) on different abdominal artery shapes. For that purpose, a finite element-based model using the computational fluid dynamics (CFD) method is developed. The effect of PFS intervention on the hemodynamic efficiency is estimated by all of the significant criteria used for the evaluation of aneurysm occlusion and possible rupture; the flow velocity, pressure, wall shear stress (WSS), and WSS-related indices. Results showed that PFS intervention preserves the effects of high flow rate and decreases irregular flow recirculation in the sac of the aneurysm. The flow velocity decreases inside the aneurysm sac in the range of 55% to 80%. The time-averaged wall shear stress (TAWSS) was reduced from 42% to 53% by FPS deployment. The simulation results implies that PFS could heal an aneurysm efficiently with a mechanism that causes the development of thrombus and ultimately leads to aneurysm resorption.

A new approach for the pre-clinical optimization of a spatial configuration of bifurcated endovascular prosthesis placed in abdominal aortic aneurysms

Complexity of the spatial configuration of an aortic implant with bifurcation in the distal part is related to changes in blood hemodynamic in the area of bifurcation which may disturb blood flow and lead to thrombus formation. This study was designed to characterize parameters which define spatial configuration of an aortic implant for which the risk of thrombus formation is the smallest. We used AngioCT data from 74 patients, aged 55 ±10 years, after endovascular procedure to prepare 3D geometries of stent-grafts. Computational Fluid Dynamics (CFD) simulations were used to reconstruct blood hemodynamic and simulate thrombus formation. Next, geometric parameters of stent-grafts included the ratio of volume of upper part to the bifurcations, the relation of inlet and outlet diameters of a stent-graft and deformations in the iliac part of the stent-graft were analyzed. We also analyzed tortuosities (spiral twisting of the flow around the flow direction) and bends (the largest angulation in distal part of a stent-graft). The CFD results were confronted with AngioCT data to verify if computer generated thrombus appeared in particular patient. Additionally, geometric parameters of analyzed stent-grafts were used to propose a mathematical tool for prediction of thrombus appearance. The results showed that tortuosities and bends of a stent-graft had the highest impact on thrombus formation. Formation of thrombi was observed in 22% to 31% of cases (at blood hematocrit Hct = 40%) even for small values of tortuosities and bends indicating that these parameters are dominant in determining blood clotting. Our calculated results overlapped with clinical data in 80% to 91%. Therefore, we conclude that tortuosities and bends have high impact on thrombus formation and should be under special attention during stent-graft recommendation and patients’ follow-ups.

A computational assessment of the hemodynamic effects of crossed and non-crossed bifurcated stent-graft devices for the treatment of abdominal aortic aneurysms

There are several issues attributed with abdominal aortic aneurysm endovascular repair. The positioning of bifurcated stent-grafts (SG) may affect SG hemodynamics. The hemodynamics and geometrical parameters of crossing or non-crossing graft limbs have not being totally accessed. Eight patient-specific SG devices and four pre-operative cases were computationally simulated, assessing the hemodynamic and geometrical effects for crossed (n= 4) and non-crossed (n= 4) configurations. SGs eliminated the occurrence of significant recirculations within the sac prior treatment. Dean's number predicted secondary flow locations with the greatest recirculations occurring at the outlets especially during the deceleration phase. Peak drag force varied from 3.9 to 8.7N, with greatest contribution occurring along the axial and anterior/posterior directions. Average resultant drag force was 20% smaller for the crossed configurations. Maximum drag force orientation varied from 1.4° to 51°. Drag force angle varied from 1° to 5° during one cardiac cycle. 44% to 62% of the resultant force acted along the proximal centerline where SG migration is most likely to occur. The clinician's decision for SG positioning may be a critical parameter, and should be considered prior to surgery. All crossed SG devices had an increased spiral flow effect along the distal legs with reductions in drag forces.

Immediate Change in Suprarenal Neck Angulation After Endovascular Aneurysm Repair

Purpose: To compare the immediate suprarenal neck angulation change between the Ovation stent-graft, with its inflatable sealing rings, and a stent-graft with a conventional sealing mechanism. Methods: A case-control study was conducted in which 30 consecutive patients (mean age 67 years; all men) with abdominal aortic aneurysm (AAA) treated with the Ovation stent-graft (group O) were retrospectively compared with 24 patients (mean age 77 years; all men) contemporaneously treated with the Endurant stent-graft (group E) at 3 high-volume tertiary vascular centers. The variables recorded were the aortic neck length, preoperative and postoperative angulation, minimum and maximum diameters of the infrarenal neck, as well as the maximum AAA diameter. All patients had undergone preoperative and postoperative (within 30 days) computed tomographic angiography. Multiple regression analysis compared the relative contribution to neck angulation change of each geometric parameter and the type of endograft. Data are presented as the mean ± standard deviation. Results: The mean preoperative suprarenal neck angulation in group O was 23.2°±18.0° compared with 23.8°±22.9° in group E ( t test, p=0.91). The neck lengths were 29.2±14.6 and 23.2±11.0 mm in groups O and E, respectively (p=0.1). Similarly, the minimum and maximum neck diameters were 22.4±2.6 and 25±3.5 mm, respectively, in group O vs 23.3±3.6 mm and 27.0±5.7 mm, respectively, in group E (p=0.3 and 0.12, respectively). The maximum transverse diameters of the AAA in the 2 groups were comparable, that is, 57.0±9.0 mm in group O vs 53.2±11.1 mm in group E (p=0.17). The Ovation stent-graft caused greater decrease in the aortic neck angulation postoperatively compared with the Endurant device (13.2°±16.1° vs 6.1°±5.9°, p=0.04). Multiple regression analysis revealed that preoperative neck angulation (β coefficient 0.37, p<0.001) and the type of endograft (β coefficient −7.91, p=0.01) had significant influence on the postoperative neck angulation change. The intraclass correlation coefficient ranged from 0.951 to 0.990 for the preoperative measurements and from 0.911 to 0.999 for the postoperative measurements for each examiner or the total of estimates at the measurement time points. Conclusion: The Ovation stent-graft induces greater postoperative reduction in the AAA neck angulation compared to an endograft with stent-supported graft seal. Expanded research to infrarenal angle as well to greater angles and correlation to clinical events is justified.

Outcomes Following Limb Crossing in Endovascular Aneurysm Repairs

Background:

Crossing the limbs of the stent during endovascular aneurysm repair (EVAR) is often used to aid cannulation of the contralateral limb. This study assessed outcomes following the use of this technique.

Methods:

Retrospective review of crossed (n = 43) and uncrossed (n = 269) EVARs was performed at a tertiary vascular center over 5 years. Primary end points were graft limb occlusion (GLO), endoleak, and sac expansion rates. Indications for limb crossing were also assessed.

Results:

Two-year GLO ( P = .34) and type 1 endoleak ( P = .413) rates were similar between groups. Patients undergoing crossed EVAR experienced more type 2 endoleaks ( P = .002) at 24 months but no increase in sac expansion rates was observed ( P = .275). Thirty-day ( P = .57) and late ( P = .268) mortalities were similar between groups. The main indication for limb crossing was distal aortic angulation (48.8%).

Conclusions:

Crossed EVAR does not increase the risk of GLOs or clinically significant endoleaks. Further studies are needed to determine the effect on type 2 endoleak rates.

Studying the Flow Dynamics in an Aortic Endograft with Crossed-limbs

Purpose

To evaluate the flow phenomena within an aortic endograft with crossed-limbs, comparing to an endograft with the ordinary limb bifurcation.

Methods

An endograft model with crossed-limbs was computationally reconstructed based on Computed Tomography patient-specific data, using commercially available software. Accordingly, its analogue model was reconstructed in the ordinary fashion (ordinary bifurcation). Computational fluid dynamics analysis was performed to determine and compare the flow fields, velocity profiles, pressure and shear stress distribution throughout the different parts of both endograft configurations, in different phases of the cardiac cycle.

Results

The flow patterns between the “Ballerina” and the classic endograft were similar, with flow disturbance near the inlet zone at late diastole and smooth flow patterns during the systolic phase. Both configurations presented similar pressure distribution patterns throughout the cardiac cycle. The highest and lowest pressures were demonstrated in the inlet-main body area and the iliac limbs, respectively. Marked differences were observed in the velocity profiles of the proximal limb segments between the two configurations, mostly in the peak- and end-systolic phase. The regions of lower velocities correlated well to low shear values. Differences in the shear stress distribution were noted between the two configurations in the systolic and, predominantly, in the diastolic phase.

Conclusions

There are differences in the velocity profiles and shear distribution between the limbs of the two endograft configurations. The pathophysiologic implication of our findings and their possible association with clinical events, such as thrombus apposition, deserves further investigation.

A multimodality vascular imaging phantom of an abdominal aortic aneurysm with a visible thrombus

PURPOSE

With the continuous development of new stent grafts and implantation techniques, it has now become technically feasible to treat abdominal aortic aneurysms (AAA) with challenging anatomy using endovascular repair with standard, fenestrated, or branched stent-grafts. In vitro experimentations are very useful to improve stent-graft design and conformability or imaging guidance for stent-graft delivery or follow-up. Vascular replicas also help to better understand the limitation of endovascular approaches in challenging anatomy and possibly improve surgical planning or training by practicing high risk clinical procedures in the laboratory to improve outcomes in the operating room. Most AAA phantoms available have a very basic anatomy, which is not representative of the clinical reality. This paper presents a method of fabrication of a realistic AAA phantom with a visible thrombus, as well as some mechanical properties characterizing such phantom.

METHODS

A realistic AAA geometry replica of a real patient anatomy taken from a multidetector computed tomography (CT) scan was manufactured. To demonstrate the multimodality imaging capability of this new phantom with a thrombus visible in magnetic resonance (MR) angiography, CT angiography (CTA), digital subtraction angiography (DSA), and ultrasound, image acquisitions with all these modalities were performed by using standard clinical protocols. Potential use of this phantom for stent deployment was also tested. A rheometer allowed defining hyperelastic and viscoelastic properties of phantom materials.

RESULTS

MR imaging measurements of SNR and CNR values on T1 and T2-weighted sequences and MR angiography indicated reasonable agreement with published values of AAA thrombus and abdominal components in vivo. X-ray absorption also lay within normal ranges of AAA patients and was representative of findings observed on CTA, fluoroscopy, and DSA. Ultrasound propagation speeds for developed materials were also in concordance with the literature for vascular and abdominal tissues.

CONCLUSIONS

The mimicked abdominal tissues, AAA wall, and surrounding thrombus were developed to match imaging features of in vivo MR, CT, and ultrasound examinations. This phantom should be of value for image calibration, segmentation, and testing of endovascular devices for AAA endovascular repair.

The quantification of hemodynamic parameters downstream of a Gianturco Zenith stent wire using newtonian and non-newtonian analog fluids in a pulsatile flow environment

Although deployed in the vasculature to expand vessel diameter and improve blood flow, protruding stent struts can create complex flow environments associated with flow separation and oscillating shear gradients. Given the association between magnitude and direction of wall shear stress (WSS) and endothelial phenotype expression, accurate representation of stent-induced flow patterns is critical if we are to predict sites susceptible to intimal hyperplasia. Despite the number of stents approved for clinical use, quantification on the alteration of hemodynamic flow parameters associated with the Gianturco Z-stent is limited in the literature. In using experimental and computational models to quantify strut-induced flow, the majority of past work has assumed blood or representative analogs to behave as Newtonian fluids. However, recent studies have challenged the validity of this assumption. We present here the experimental quantification of flow through a Gianturco Z-stent wire in representative Newtonian and non-Newtonian blood analog environments using particle image velocimetry (PIV). Fluid analogs were circulated through a closed flow loop at physiologically appropriate flow rates whereupon PIV snapshots were acquired downstream of the wire housed in an acrylic tube with a diameter characteristic of the carotid artery. Hemodynamic parameters including WSS, oscillatory shear index (OSI), and Reynolds shear stresses (RSS) were measured. Our findings show that the introduction of the stent wire altered downstream hemodynamic parameters through a reduction in WSS and increases in OSI and RSS from nonstented flow. The Newtonian analog solution of glycerol and water underestimated WSS while increasing the spatial coverage of flow reversal and oscillatory shear compared to a non-Newtonian fluid of glycerol, water, and xanthan gum. Peak RSS were increased with the Newtonian fluid, although peak values were similar upon a doubling of flow rate. The introduction of the stent wire promoted the development of flow patterns that are susceptible to intimal hyperplasia using both Newtonian and non-Newtonian analogs, although the magnitude of sites affected downstream was appreciably related to the rheological behavior of the analog. While the assumption of linear viscous behavior is often appropriate in quantifying flow in the largest arteries of the vasculature, the results presented here suggest this assumption overestimates sites susceptible to hyperplasia and restenosis in flow characterized by low and oscillatory shear.

Hemodynamic variations due to spiral blood flow through four patient-specific bifurcated stent graft configurations for the treatment of abdominal aortic aneurysms

Endovascular repair is now a recognised procedure for treating abdominal aortic aneurysms. However, post-operative complications such as stent graft migration and thrombus may still occur. To assess these complications numerically, the correct input boundary conditions, which include the full human aorta with associated branching, should be included. Four patient-specific computed tomography scanned bifurcated stent grafts (SGs) were modelled and attached onto a full human aorta, which included the ascending, aortic arch and descending aortas. Two of the SG geometries had a twisted leg configuration, while the other two had conventional nontwisted leg configurations. Computational fluid dynamics was completed for both geometries and the hemodynamics assessed. The complexity of the flow patterns and secondary flows were influenced by the inclusion of the full human aorta at the SG proximal section. During the decelerating phase significant recirculations occurred along the main body of all SG configurations. The inclusion of the full human aorta did not impact the velocity contours within the distal legs and there was no difference in drag forces with the SG containing the full human aorta and those without. A twisted leg configuration further promoted a spiral flow formation along its distal legs.

Stent graft performance in the treatment of abdominal aortic aneurysms: the influence of compliance and geometry

The long-term success of the endovascular procedure for the treatment of Abdominal Aortic Aneurysms (AAAs ) depends on the secure fixation of the proximal end and the geometry of the stent-graft (SG) device. Variations in SG types can affect proximal fixation and SG hemodynamics. Such hemodynamic variations can have a catastrophic effect on the vascular system and may result from a SG/arterial wall compliance mismatch and the sudden decrease in cross-sectional area at the bifurcation, which may result in decreased distal perfusion, increased pressure wave reflection and increased stress at the interface between the stented and non-stented portion of the vessel. To examine this compliance mismatch, a commercial SG device was tested experimentally under a physiological pressure condition in a silicone AAA model based on computed tomography scans. There was a considerable reduction in compliance of 54% and an increase in the pulse wave velocity of 21%, with a significant amount of the forward pressure wave being reflected. To examine the SG geometrical effects, a commercial bifurcated geometry was compared computationally and experimentally with a geometrical taper in the form of a blended section, which provided a smooth transition from the proximal end to both iliac legs. The sudden contraction of commercial SG at the bifurcation region causes flow separation within the iliac legs, which is known to cause SG occlusion and increased proximal pressure. The blended section along the bifurcation region promotes a greater uniformity of the fluid flow field within the distal legs, especially, during the deceleration phase with reduced boundary layer reversal. In order to reduce the foregoing losses, abrupt changes of cross-section should be avoided. Geometrical tapers could lead to improved clinical outcomes for AAA SGs.

Impact of an aortic nitinol stent graft on flow measurements by time-resolved three-dimensional velocity-encoded MRI

RATIONALE AND OBJECTIVES

Three-dimensional (3D) velocity-encoded cine (VEC) magnetic resonance imaging (MRI) has the potential to quantify 3D hemodynamic aspects known from computational fluid dynamics and to be used to identify hemodynamic risk factors for complications of endovascular aortic repair. The purpose of this study was to investigate the impact of an aortic nickel-titanium (nitinol) stent graft on the accuracy of flow measurements by 3D VEC MRI.

MATERIALS AND METHODS

A pump generated pulsatile aortic flow in an elastic tube phantom mimicking the aorta. Stacked two-dimensional three-directional VEC MRI (stacked-2D-3dir-MRI), 3D three-directional VEC MRI (3D-3dir-MRI), and gold-standard 2D through-plane VEC MRI were applied before and after the insertion of an aortic nitinol stent graft. Six equidistant levels were analyzed twice by the same reader. The percentage difference of the measured flow rate from the gold standard was defined as the parameter of accuracy.

RESULTS

The overall accuracy of in-stent flow measurements related to the gold standard was -5.4% for stacked-2D-3dir-MRI and -4.1% for 3D-3dir-MRI, demonstrating significant overall underestimation compared to the gold standard (P = .016 and P = .013). However, flow measurements with the stent graft were significantly overestimated by 4.1% using stacked-2D-3dir-MRI (P < .001) and by 5.4% using 3D-3dir-MRI (P = .003) compared to identical measurements without the stent graft. In stacked-2D-3dir-MRI, this positive bias was significantly greater at the proximal and distal ends of the stent graft (P = .025). In 3D-3dir-MRI, measurements along the whole length of the stent graft were affected (P = .006). Intraobserver agreement was excellent, with intraclass correlation coefficients of 0.94 for stacked-2D-3dir-MRI (P < .001) and 0.90 for 3D-3dir-MRI (P < .001).

CONCLUSIONS

Flow measurements within an aortic nitinol stent graft by 3D VEC MRI are feasible, but stent grafts may cause a significant positive bias.

In vitro validation of flow measurements in an aortic nitinol stent graft by velocity-encoded MRI

PURPOSE

To validate flow measurements within an aortic nickel-titanium (nitinol) stent graft using velocity-encoded cine magnetic resonance imaging (VEC MRI) and to assess intraobserver agreement of repeated flow measurements.

MATERIALS AND METHODS

An elastic tube phantom mimicking the descending aorta was developed with the possibility to insert an aortic nitinol stent graft. Different flow patterns (constant, sinusoidal and pulsatile aortic flow) were applied by a gear pump. A two-dimensional phase-contrast sequence was used to acquire VEC perpendicular cross-sections at six equidistant levels along the phantom. Each acquisition was performed twice with and without stent graft, and each dataset was analysed twice by the same reader. The percental difference of the measured flow volume to the gold standard (pump setting) was defined as the parameter for accuracy. Furthermore, the intraobserver agreement was assessed.

RESULTS

Mean accuracy of flow volume measurements was -0.45±1.63% without stent graft and -0.18±1.45% with stent graft. Slightly lower accuracy was obtained for aortic flow both without (-2.31%) and with (-1.29%) stent graft. Accuracy was neither influenced by the measurement position nor by repeated acquisitions. There was significant intraobserver agreement with an intraclass correlation coefficient of 0.87 (without stent graft, p<0.001) and 0.80 (with stent graft, p<0.001). The coefficient of variance was 0.25% without stent graft and 0.28% with stent graft.

CONCLUSION

This study demonstrated high accuracy and excellent intraobserver agreement of flow measurements within an aortic nitinol stent graft using VEC MRI. VEC MRI may give new insights into the haemodynamic consequences of endovascular aortic repair.

Engineering silicone rubbers for in vitro studies: creating AAA models and ILT analogues with physiological properties

In vitro studies of abdominal aortic aneurysm (AAA) have been widely reported. Frequently mock artery models with intraluminal thrombus (ILT) analogs are used to mimic the in vivo AAA. While the models used may be physiological, their properties are frequently either not reported or investigated. This study is concerned with the testing and characterization of previously used vessel analog materials and the development of new materials for the manufacture of AAA models. These materials were used in conjunction with a previously validated injection molding technique to manufacture AAA models of ideal geometry. To determine the model properties (stiffness (beta) and compliance), the diameter change of each AAA model was investigated under incrementally increasing internal pressures and compared with published in vivo studies to determine if the models behaved physiologically. A FEA study was implemented to determine if the pressure-diameter change behavior of the models could be predicted numerically. ILT analogs were also manufactured and characterized. Ideal models were manufactured with ILT analog internal to the aneurysm region, and the effect of the ILT analog on the model compliance and stiffness was investigated. The wall materials had similar properties (E(init) 2.22 MPa and 1.57 MPa) to aortic tissue at physiological pressures (1.8 MPa (from literature)). ILT analogs had a similar Young's modulus (0.24 MPa and 0.33 MPa) to the medial layer of ILT (0.28 MPa (from literature)). All models had aneurysm sac compliance (2.62-8.01 x 10(-4)/mm Hg) in the physiological range (1.8-9.4 x 10(-4)/mm Hg (from literature)). The necks of the AAA models had similar stiffness (20.44-29.83) to healthy aortas (17.5+/-5.5 (from literature)). Good agreement was seen between the diameter changes due to pressurization in the experimental and FEA wall models with a maximum difference of 7.3% at 120 mm Hg. It was also determined that the inclusion of ILT analog in the sac of the models could have an effect on the compliance of the model neck. Ideal AAA models with physiological properties were manufactured. The behavior of these models due to pressurization was predicted using finite element analysis, validating this technique for the future design of realistic physiological AAA models. Addition of ILT analogs in the aneurysm sac was shown to affect neck behavior. This could have implications for endovascular AAA repair due to the importance of the neck for stent-graft fixation.

A review of the in vivo and in vitro biomechanical behavior and performance of postoperative abdominal aortic aneurysms and implanted stent-grafts

Endovascular repair of abdominal aortic aneurysms has generated widespread interest since the procedure was first introduced two decades ago. It is frequently performed in patients who suffer from substantial comorbidities that may render them unsuitable for traditional open surgical repair. Although this minimally invasive technique substantially reduces operative risk, recovery time, and anesthesia usage in these patients, the endovascular method has been prone to a number of failure mechanisms not encountered with the open surgical method. Based on long-term results of second- and third-generation devices that are currently becoming available, this study sought to identify the most serious failure mechanisms, which may have a starting point in the morphological changes in the aneurysm and stent-graft. To investigate the "behavior" of the aneurysm after stent-graft repair, i.e., how its length, angulation, and diameter change, we utilized state-of-the-art ex vivo methods, which researchers worldwide are now using to recreate these failure modes.