Swirling Flow and Wall Shear: Evaluating the BioMimics 3D Helical Centerline Stent for the Femoropopliteal Segment

Color-coded circulation for visualizing swirling flow with a 3-dimensional helical stent in the superficial femoral artery

Color-coded circulation is a display method that generates dynamic color-coded images based on the time of arrival of contrast agents using parametric imaging to create video displays. By cyclically displaying information in color according to the arrival time of the contrast agent at each pixel, anatomical blood vessel paths and blood flow information can be simultaneously visualized. Three-dimensional (3D) helical stents increase wall shear stress due to swirling flow and prevent intimal hyperplasia. To the best of our knowledge, there are no reports on the visualization of this swirling flow using color-coded circulation. Here, we report the use of color-coded circulation to visualize the swirling flow following the placement of a 3D helical stent in the left superficial femoral artery. Color-coded circulation may facilitate the evaluation of contrast agent distribution and blood flow, which may otherwise go undetected with digital subtraction angiography.

The hemodynamic and geometric characteristics of carotid artery atherosclerotic plaque formation

Background

Ischemic stroke, which has a high incidence, disability, and mortality rate, is mainly caused by carotid atherosclerotic plaque. The difference in the geometric structures of the carotid arteries inevitably leads to the variability in the local hemodynamics, which plays a key role in the formation of carotid atherosclerosis. At present, the combined mechanisms of hemodynamic and geometric in the formation of carotid atherosclerotic plaque are not clear. Thus, this study characterized the geometric and hemodynamic characteristics of carotid atherosclerotic plaque formation using four-dimensional (4D) flow magnetic resonance imaging (MRI).

Methods

Ultimately, 122 carotid arteries from 61 patients were examined in this study. According to the presence of plaques at the bifurcation of the carotid artery on cervical vascular ultrasound (US), carotid arteries were placed into a plaque group (N=69) and nonplaque group (N=53). The ratio of the maximum internal carotid artery (ICA) inner diameter to the maximum common carotid artery (CCA) inner diameter (ICA-CCA diameter ratio), bifurcation angle, and tortuosity were measured using neck three-dimensional time-of-flight magnetic resonance angiography (3D TOF-MRA). Meanwhile, 4D flow MRI was used to obtain the following hemodynamic parameters of the carotid arteries: volume flow rate, velocity, wall shear stress (WSS), and pressure gradient (PG). Independent sample t-tests were used to compare carotid artery geometry and hemodynamic changes between the plaque group and nonplaque group.

Results

The ICA-CCA diameter ratio between the plaque group and the nonplaque group was not significantly different (P=0.124), while there were significant differences in the bifurcation angle (P=0.005) and tortuosity (P=0.032). The bifurcation angle of the plaque group was greater than that of the nonplaque group (60.70°±20.75° vs. 49.32°±22.90°), and the tortuosity was smaller than that of the nonplaque group (1.07±0.04 vs. 1.09±0.05). There were no significant differences between the two groups in terms of volume flow rate (P=0.351) and the maximum value of velocity (velocitymax) (P=0.388), but the axial, circumferential, and 3D WSS values were all significantly different, including their mean values (all P values <0.001) and the maximum value of 3D WSS (P<0.001), with the mean axial, circumferential, 3D WSS values, along with the maximum 3D WSS value, being lower in the plaque group. The two groups also differed significantly in terms of maximum PG value (P=0.030) and mean PG value (P=0.026), with these values being greater in the nonplaque group than in the plaque group.

Conclusions

A large bifurcation angle and a low tortuosity of the carotid artery are geometric risk factors for plaque formation in this area. Low WSS and low PG values are associated with carotid atherosclerotic plaque formation.

Deformed Popliteal Artery Due to Highly Flexed Knee Position Can Cause Kinks, Creating an Unfavorable Hemodynamic State

BACKGROUND

Endovascular treatment devices of the femoropopliteal artery have evolved, improving clinical results. However, the effects of dynamic changes in the popliteal artery during knee flexion have not been sufficiently investigated. In this study we performed a 3-dimensional analysis to clarify the dynamic changes in the popliteal artery during knee flexion and their effects on hemodynamics.

METHODS AND RESULTS

To analyze dynamic changes in the popliteal artery in the knee flexion position, a computed tomography protocol was developed in the right-angled and maximum flexion knee positions. Thirty patients with lower extremity artery disease were recruited. V-Modeler software was used for anatomical and hemodynamic analyses. Various types of deformations of the popliteal artery were revealed, including hinge points and accessory flexions. Kinks can occur in the maximum flexion position; however, they rarely occur in the right-angled flexion position. In addition, hemodynamic analysis revealed a tendency for lower minimum wall shear stress and a higher maximum oscillatory shear index at the maximum curvature of the popliteal artery.

CONCLUSIONS

Kinks in the maximum flexion position suggested that the outcome of endovascular treatment may change in areas such as Japan, where knee flexion is customary. Hemodynamics at the maximum curvature of the popliteal artery indicated that the luminal condition was unfavorable for endovascular treatment.

Carotid Geometry and Wall Shear Stress Independently Predict Increased Wall Thickness-A Longitudinal 3D MRI Study in High-Risk Patients

Introduction: Carotid geometry and wall shear stress (WSS) have been proposed as independent risk factors for the progression of carotid atherosclerosis, but this has not yet been demonstrated in larger longitudinal studies. Therefore, we investigated the impact of these biomarkers on carotid wall thickness in patients with high cardiovascular risk.

Methods: Ninety-seven consecutive patients with hypertension, at least one additional cardiovascular risk factor and internal carotid artery (ICA) plaques (wall thickness ≥ 1.5 mm and degree of stenosis ≤ 50%) were prospectively included. They underwent high-resolution 3D multi-contrast and 4D flow MRI at 3 Tesla both at baseline and follow-up. Geometry (ICA/common carotid artery (CCA)-diameter ratio, bifurcation angle, tortuosity and wall thickness) and hemodynamics [WSS, oscillatory shear index (OSI)] of both carotid bifurcations were measured at baseline. Their predictive value for changes of wall thickness 12 months later was calculated using linear regression analysis for the entire study cohort (group 1, 97 patients) and after excluding patients with ICA stenosis ≥10% to rule out relevant inward remodeling (group 2, 61 patients).

Results: In group 1, only tortuosity at baseline was independently associated with carotid wall thickness at follow-up (regression coefficient = −0.52, p < 0.001). However, after excluding patients with ICA stenosis ≥10% in group 2, both ICA/CCA-ratio (0.49, p < 0.001), bifurcation angle (0.04, p = 0.001), tortuosity (−0.30, p = 0.040), and WSS (−0.03, p = 0.010) at baseline were independently associated with changes of carotid wall thickness at follow-up.

Conclusions: A large ICA bulb and bifurcation angle and low WSS seem to be independent risk factors for the progression of carotid atherosclerosis in the absence of ICA stenosis. By contrast, a high carotid tortuosity seems to be protective both in patients without and with ICA stenosis. These biomarkers may be helpful for the identification of patients who are at particular risk of wall thickness progression and who may benefit from intensified monitoring and treatment.

Biomechanical Forces and Atherosclerosis: From Mechanism to Diagnosis and Treatment

The article provides an overview of current views on the role of biomechanical forces in the pathogenesis of atherosclerosis. The importance of biomechanical forces in maintaining vascular homeostasis is considered. We provide descriptions of mechanosensing and mechanotransduction. The roles of wall shear stress and circumferential wall stress in the initiation, progression and destabilization of atherosclerotic plaque are described. The data on the possibilities of assessing biomechanical factors in clinical practice and the clinical significance of this approach are presented. The article concludes with a discussion on current therapeutic approaches based on the modulation of biomechanical forces.

Mechanical rotational thrombectomy with Rotarex system augmented with drug-eluting balloon angioplasty versus stenting for the treatment of acute thrombotic and critical limb ischaemia in the femoropopliteal segment

Introduction

Mechanical thrombectomy is an alternative to local thrombolysis for the treatment of severe ischaemia in the femoropopliteal segment, but stent implantation is usually required after this procedure. The use of drug-eluting balloons (DEBs) may overcome long-term problems associated with stents, but it remains unclear how often such a treatment is technically feasible and efficient.

Aim

This post hoc single-centre study was aimed at assessment of the feasibility, safety and efficacy of mechanical thrombectomy followed by application of DEBs.

Material and methods

Fifty-one patients, aged 69.1 ±11.6 years, were managed for acute thrombotic or chronic critical ischaemia in the femoropopliteal segment using the Rotarex device. Following mechanical thrombectomy, on condition that there was no significant residual stenosis or dissection, lesions were managed with paclitaxel-coated DEBs, which was a desired strategy (24 patients). The remaining 25 patients underwent stent implantations, which was regarded as bailout treatment. Final follow-up was scheduled 12 months after the procedure.

Results

The primary-assisted patency rate after mechanical rotational thrombectomy with additional balloon angioplasty and/or stenting was 97.1% (49 patients). The early mortality rate was 2.0% (1 patient) and the amputation rate was 4.1% (2 patients). There were no late mortalities or limb amputations at 12-month follow-up, but significant restenoses occurred in 13 (27.1%) patients. These restenoses were more frequent in patients who underwent stent implantation (45.5%) than those managed with DEBs (12.5%), and in patients managed for secondary lesions.

Conclusions

In selected patients mechanical rotational thrombectomy in the femoropopliteal segment followed by application of DEB is a safe, effective and long-lasting method of revascularisation.

Efficiently Generating Mixing by Combining Differing Small Amplitude Helical Geometries

Helical geometries have been used in recent years to form cardiovascular prostheses such as stents and shunts. The helical geometry has been found to induce swirling flow, promoting in-plane mixing. This is hypothesised to reduce the formation of thrombosis and neo-intimal hyperplasia, in turn improving device patency and reducing re-implantation rates. In this paper we investigate whether joining together two helical geometries, of differing helical radii, in a repeating sequence, can produce significant gains in mixing effectiveness, by embodying a ‘streamline crossing’ flow environment. Since the computational cost of calculating particle trajectories over extended domains is high, in this work we devised a procedure for efficiently exploring the large parameter space of possible geometry combinations. Velocity fields for the single geometries were first obtained using the spectral/hp element method. These were then discontinuously concatenated, in series, for the particle tracking based mixing analysis of the combined geometry. Full computations of the most promising combined geometries were then performed. Mixing efficiency was evaluated quantitatively using Poincaré sections, particle residence time data, and information entropy. Excellent agreement was found between the idealised (concatenated flow field) and the full simulations of mixing performance, revealing that a strict discontinuity between velocity fields is not required for mixing enhancement, via streamline crossing, to occur. Optimal mixing was found to occur for the combination R = 0.2 D and R = 0.5 D , producing a 70 % increase in mixing, compared with standard single helical designs. The findings of this work point to the benefits of swirl disruption and suggest concatenation as an efficient means to determine optimal configurations of repeating geometries for future designs of vascular prostheses.