Helicity Intensity Research Articles

On the role of aortic valve architecture for physiological hemodynamics and valve replacement, Part II: Spectral analysis and anisotropy

Severe aortic valve stenosis can lead to heart failure and aortic valve replacement (AVR) is the primary treatment. However, increasing prevalence of aortic stenosis cases reveal limitations in current replacement options, necessitating improved prosthetic aortic valves. We investigate flow disturbances downstream of severe aortic stenosis and two bioprosthetic aortic valve (BioAV) designs using advanced energy-based analyses. Three-dimensional high-fidelity fluid–structure interaction simulations have been conducted and a dedicated and novel spectral analysis has been developed to characterise the kinetic energy (KE) carried by eddies in the wavenumber space. In addition, new field quantities, i.e. modal KE anisotropy intensity as well as normalised helicity intensity, are introduced. Spectral analysis shows kinetic energy (KE) decay variations, with the stenotic case aligning with Kolmogorov’s theory, while BioAV cases differing. We explore the impact of flow helicity on KE transfer and decay in BioAVs. Probability distributions of modal KE anisotropy unveil flow asymmetries in the stenotic and one BioAV cases. Moreover, an inverse correlation between temporally averaged modal KE anisotropy and normalised instantaneous helicity intensity is noted, with the coefficient of determination varying among the valve configurations. Leaflet dynamics analysis highlights a stronger correlation between flow and biomechanical KE anisotropy in one BioAV due to higher leaflet displacement magnitude. These findings emphasise the role of valve architecture in aortic turbulence as well as its importance for BioAV performance and energy-based design enhancement.

Design of experiment approach to identify the dominant geometrical feature of left coronary artery influencing atherosclerosis

Background and Objective. Coronary artery geometry heavily influences local hemodynamics, potentially leading to atherosclerosis. Consequently, the unique geometrical configuration of an individual by birth can be associated with future risk of atherosclerosis. Although current researches focus on exploring the relationship between local hemodynamics and coronary artery geometry, this study aims to identify the order of influence of the geometrical features through systematic experiments, which can reveal the dominant geometrical feature for future risk assessment. Methods. According to Taguchi’s method of design of experiment (DoE), the left main stem (LMS) length (l LMS), curvature (k LMS), diameter (d LMS) and the bifurcation angle between left anterior descending (LAD) and left circumflex (LCx) artery (α LAD-LCx) of two reconstructed patient-specific left coronary arteries (LCA) were varied in three levels to create L9 orthogonal array. Computational fluid dynamic (CFD) simulations with physiological boundary conditions were performed on the resulting eighteen LCA models. Average helicity intensity (h 2) and relative atheroprone area (RAA) of near-wall hemodynamic descriptors were analyzed. Results. The proximal LAD (LAD proximal) was identified to be the most atheroprone region of the left coronary artery due to higher h 2, large RAA of time averaged wall shear stress (TAWSS < 0.4 Pa), oscillatory shear index (OSI ∼ 0.5) and relative residence time (RRT > 4.17 Pa−1). In both patient-specific cases, based on h 2 and TAWSS, d lms is the dominant geometric parameter while based on OSI and RRT, α LAD-LCx is the dominant one influencing hemodynamic condition in proximal LAD (p < 0.05). Based on RRT, the rank of the geometrical factors is: α LAD-LCx > d LMS > l LMS > k LMS, indicating that α LAD-LCx is the most dominant geometrical factor affecting hemodynamics at proximal LAD which may influence atherosclerosis. Conclusion. The proposed identification of the rank of geometrical features of LCA and the dominant feature may assist clinicians in predicting the possibility of atherosclerosis, of an individual, long before it will occur. This study can further be translated to be used to rank the influence of several arterial geometrical features at different arterial locations to explore detailed relationships between the arterial geometrical features and local hemodynamics.

The effect of splenic vein diameter on the diagnosis of portal vein thrombosis.

It was still controversial that whether the increase of splenic vein (SV) diameter increased the risk of portal vein thrombosis (PVT), which was a severe disease with high mortality, in the clinic. This study, using computational fluid dynamics method, aimed to investigate how the increase of SV diameter affects the portal vein hemodynamics under different anatomical and geometric features of the portal venous system, thus how it induced to PVT. The ideal models of the portal system, including different anatomical structures according to the location of left gastric vein (LGV) and inferior mesenteric vein (IMV), and different geometric morphological parameters models were established to carry out numerical simulation in this study. In addition, the morphological parameters of real patients were measured to verify the numerical simulation results. First, the wall shear stress (WSS) and helicity intensity, which were closely related to the occurrence of thrombosis, gradually decreased with the increase of SV diameter in all models. However, the degree of decrease was bigger in following models: (1) the models with LGV and IMV linking to SV compared with them linking to PV; (2) the models with big angle of PV and SV compared with small angle. In addition, the morbidity of PVT was higher when LGV and IMV linked to SV rather than them linked to PV in the real patients. Moreover, the angle of PV and SV was also different between PVT and non-PVT patients (125.53°±16.90° vs. 115.03°±16.10°, p=0.01). Whether the increase of SV diameter will result in PVT is depended on the anatomical structure of portal system and the angle between PV and SV, this is also the reason leading to the clinical controversy that the increase of SV diameter is the risk factor of PVT.

Impact of wall displacements on the large-scale flow coherence in ascending aorta

In the context of aortic hemodynamics, uncertainties affecting blood flow simulations hamper their translational potential as supportive technology in clinics. Computational fluid dynamics (CFD) simulations under rigid-walls assumption are largely adopted, even though the aorta contributes markedly to the systemic compliance and is characterized by a complex motion. To account for personalized wall displacements in aortic hemodynamics simulations, the moving-boundary method (MBM) has been recently proposed as a computationally convenient strategy, although its implementation requires dynamic imaging acquisitions not always available in clinics.In this study we aim to clarify the real need for introducing aortic wall displacements in CFD simulations to accurately capture the large-scale flow structures in the healthy human ascending aorta (AAo). To do that, the impact of wall displacements is analyzed using subject-specific models where two CFD simulations are performed imposing (1) rigid walls, and (2) personalized wall displacements adopting a MBM, integrating dynamic CT imaging and a mesh morphing technique based on radial basis functions. The impact of wall displacements on AAo hemodynamics is analyzed in terms of large-scale flow patterns of physiological significance, namely axial blood flow coherence (quantified applying the Complex Networks theory), secondary flows, helical flow and wall shear stress (WSS).From the comparison with rigid-wall simulations, it emerges that wall displacements have a minor impact on the AAo large-scale axial flow, but they can affect secondary flows and WSS directional changes. Overall, helical flow topology is moderately affected by aortic wall displacements, whereas helicity intensity remains almost unchanged. We conclude that CFD simulations with rigid-wall assumption can be a valid approach to study large-scale aortic flows of physiological significance.

The effects of plaque morphological characteristics on the post-stenotic flow in left main coronary artery bifurcation

Local post-stenotic hemodynamics has critical influence in the atherosclerotic plaque progression occurring in susceptible arterial sites, in particular the left main coronary artery (LMCA) bifurcation. Understanding the effects of plaque morphological characteristics: stenosis severity (SS), eccentricity index (EI) and lesion length (LL) on the post-stenotic flow behavior can significantly improve treatment planning. In order to investigate these effects, we have employed computational fluid dynamics (CFD) simulations in twenty computer-generated and five patient-specific LMCA models and the hemodynamic parameters: velocity, pressure (P), wall pressure gradient (WPG), wall shear stress (WSS), time averaged wall shear stress (TAWSS), oscillatory shear index (OSI), relative residence time (RRT) and helicity intensity (h2) were analyzed. Our results revealed that the effect of stenosis eccentricity varied significantly for different values of stenosis severity and lesion length. Regions with low WSS, low TAWSS and high RRT were more prominent in models having higher stenosis severity. For smaller lesion length, at low and moderate stenosis severity, surface area with low TAWSS and high RRT decreased with increasing eccentricity index, whereas for high stenosis severity models, low TAWSS region and average RRT values increased with eccentricity. However, for models with longer lesion length, regions with high OSI and RRT overall increased gradually with eccentricity. The helicity intensity (h2) of all models remained very low except at the most eccentric model with longer lesion length. The presence of very high helical flow in this model suggests the possibility of atheroprotective flow. It can be concluded that all plaque morphological characteristics covered under this investigation play an important role in plaque progression.

Computational analysis of the coronary artery hemodynamics with different anatomical variations

Abstract Hemodynamic parameters have been identified as a significant determinant in the development and progression of plaque, which leads to coronary artery disease (CAD). This study aims to use computational modeling to investigate how geometrical variations influence intravascular and near-wall hemodynamics. Branch angles and tortuosities were varied in idealized models of both bifurcations and trifurcations. Four patient-specific models with bifurcation (70°, 95°, and 135°) and trifurcation (90°) geometries were also computed. Computational fluid dynamics (CFD) analysis was performed for idealized and patient-specific geometries to simulate physiological conditions, enabling the quantification of local hemodynamics, including threshold analysis. The branch angle has a significant impact on the locations of adverse hemodynamics in the idealized and patient-specific cases. The angulation of different geometries influences the formation of low-velocity regions, which is more prominent in the trifurcation geometries opposite to the carina. Helicity intensity demonstrates a significant positive correlation with bifurcation (r = 0.94, P 0.2, and relative residence time (RRT) > 4.17 Pa-1 is observed with increasing branch angles, and the magnitude is more prominent for trifurcation arteries. Arterial tortuosity influences the formation of helicity intensity and promotes atheroprotectivity by decreasing the area exposed with near-wall descriptors. In conclusion, geometrical changes, including the branch angle and tortuosity, have a significant impact on local hemodynamics of the left coronary artery (LCA) and its correlation with atherosclerosis formation.

In-stent graft helical flow intensity reduces the risk of migration after endovascular aortic repair

During the last years endovascular aneurysm repair (EVAR) became the elective treatment for abdominal aortic aneurysms (AAAs) thanks to lower mortality and morbidity rates than open surgery. In face of these advantages, stent-graft performances are still clinically suboptimal. In particular, post-surgical complications derive from device migration as a consequence of the hemodynamic forces acting on the endograft. In this regard, while the importance of hemodynamic surface forces is well recognized, the role of the in-stent flow is still unclear. Here we hypothesize that in-stent helical blood flow patterns might influence the distribution of the displacement forces (DFs) acting on the stent-graft and, ultimately, the risk of stent migration. To test this hypothesis, the hemodynamics of 20 post-EVAR models of patients treated with two different commercial endografts was analyzed using computational hemodynamics.The main findings of the study indicate that: (1) helical flow intensity decreases the risk of endograft migration, as given by an inverse correlation between helicity intensity (h2) and time-averaged displacement forces (TADFs) (p < 0.05); (2) unbalanced counter-rotating helical structures in the legs of the device contribute, in particular along the systole, to significantly suppress TADFs (p < 0.01); (3) as expected, helical flow intensity is positively correlated with pressure drop and resistance to flow (p < 0.001). The findings of this study suggest that a design strategy promoting in-stent helical flow structures could contribute to minimize the risk of migration of implanted EVAR devices.

Relationship Between Ascending Thoracic Aortic Aneurysms Hemodynamics and Biomechanical Properties.

Ascending thoracic aortic aneu-rysm (aTAA) is a major cause of human deaths. Despite important recent progress to better understand its pathogenesis and development, the role played by deranged hemodynamics on aTAA risk of rupture is still partially unknown. Our aim was to develop and apply a novel methodology to assess the correlation between aTAA rupture risk and hemodynamic biomarkers combining for the first time in vivo, in vitro, and in silico analyses. Computational fluid dynamic analyses were performed and validated on ten patients using patient-specific data derived from CT scan and four-dimensional MRI. Systolic wall shear stress, time-averaged wall shear stress (TAWSS), flow eccentricity (Floweccentricity), and helicity intensity (h2) were assessed. A bulge inflation test was carried out in vitro on the ten aTAA samples resected during surgical repair. The biomechanical and rupture properties of these samples were derived: the burst pressure, the physiological tangent elastic modulus (Ephysio), the Cauchy stress at rupture (σrupt), the rupture stretch (λrupt), and the rupture stretch criterion (Υstretch). Statistical analysis was performed to determine correlation between all variables. Statistically highly significant (p < 0.01) positive correlation between λrupt and the TAWSS (r = 0.867 and p = 0.001) was found. This study shows that relatively low TAWSS significantly correlates with reduced rupture properties in aTAAs. Understanding the pathogenesis of aTAA remains crucial to reduce morbidity and mortality. Our aim is to establish possible correlations between aTAA rupture risk and hemodynamic biomarkers by combining for the first time in vivo, in vitro, and in silico analyses.

Processes Associated with Convection Initiation in the North American Mesoscale Forecast System, Version 3 (NAMv3)

Abstract In support of the Next Generation Global Prediction System (NGGPS) project, processes leading to convection initiation in the North American Mesoscale Forecast System, version 3 (NAMv3) are explored. Two severe weather outbreaks—occurring over the southeastern United States on 28 April 2014 and the central Great Plains on 6 May 2015—are forecast retrospectively using the NAMv3 CONUS (4 km) and Fire Weather (1.33 km) nests, each with 5-min output. Points of convection initiation are identified, and patterns leading to convection initiation in the model forecasts are determined. Results indicate that in the 30 min preceding convection initiation at a grid point, upward motion at low levels of the atmosphere enables a parcel to rise to its level of free convection, above which it is accelerated by the buoyancy force. A moist absolutely unstable layer (MAUL) typically is produced at the top of the updraft. However, when strong updrafts are collocated with large vertical gradients of potential temperature and moisture, noisy vertical profiles of temperature, moisture, and hydrometeor concentration develop beneath the rising MAUL. The noisy profiles found in this study are qualitatively similar to those that resulted in NAMv3 failures during simulations of Hurricane Joaquin in 2015. The CM1 cloud model is used to reproduce these noisy profiles, and results indicate that the noise can be mitigated by including explicit vertical diffusion in the model. Left unchecked, the noisy profiles are shown to impact convective storm features such as cold pools, precipitation, updraft helicity intensity and tracks, and the initiation of spurious convection.

The Atheroprotective Nature of Helical Flow in Coronary Arteries.

Arterial hemodynamics is markedly characterized by the presence of helical flow patterns. Previous observations suggest that arterial helical blood flow is of physiological significance, and that its quantitative analysis holds promise for clinical applications. In particular, it has been reported that distinguishable helical flow patterns are potentially atheroprotective in the carotid bifurcation as they suppress flow disturbances. In this context, there is a knowledge gap about the physiological significance of helical flow in coronary arteries, a prominent site of atherosclerotic plaque formation. This study aimed at the quantitative assessment of helical blood flow in coronary arteries, and to investigate its possible associations with vascular geometry and with atherogenic wall shear stress (WSS) phenotypes in a representative sample of 30 swine coronary arteries. This study demonstrates that in coronary arteries: (1) the hemodynamics is characterized by counter-rotating bi-helical flow structures; (2) unfavorable conditions of WSS are strongly and inversely associated with helicity intensity (r = -0.91; p < 0.001), suggesting an atheroprotective role for helical flow in the coronary tree; (3) vascular torsion dictates helical flow features (r = 0.64; p < 0.001). The findings of this work support future studies on the role of helical flow in atherogenesis in coronary arteries.

Steady Flow in a Patient-Averaged Inferior Vena Cava-Part II: Computational Fluid Dynamics Verification and Validation.

The embolus trapping performance of inferior vena cava (IVC) filters critically depends on how emboli flow through the IVC and, thereby, on the underlying hemodynamics. Most previous studies of IVC hemodynamics have used computational fluid dynamics (CFD), but few have validated their results by comparing with quantitative experimental measurements of the flow field and none have validated in an anatomical model of the IVC that includes the primary morphological features that influence the hemodynamics (iliac veins, infrarenal curvature, and non-circular vessel cross-section). In this study, we perform verification and validation of CFD simulations in a patient-averaged anatomical model of the IVC. Because we are most interested in the fluid dynamics that influence embolus transport and IVC filter embolus trapping, we focus our analyses on the velocity distribution and the amount of swirl and mixing in the infrarenal IVC. A rigorous mesh refinement study is first conducted at the highest flow rate condition to verify the computed solutions. To validate the CFD predictions of the flow patterns, we then compare with particle image velocimetry (PIV) data acquired in the same model in two planes (coronal and sagittal) within the infrarenal IVC at two flow rates corresponding to rest and exercise conditions. Using unstructured hexahedral meshes ranging in size from 800,000 to 102.5 million computational cells, we demonstrate that a coarse mesh may be used to resolve the gross flow patterns and velocity distribution in the IVC. A finer mesh is, however, required to obtain asymptotic mesh convergence of swirl and mixing in the IVC, as quantified by the local normalized helicity, LNH, and the volume-averaged helicity intensity, [Formula: see text]. Based on the results of the mesh refinement study, we use a moderately fine mesh containing approximately 26 million cells for comparison with experimental data. The validation study demonstrates excellent qualitative agreement between CFD predictions and PIV measurements of the velocity field at both conditions. Quantitatively, we show that the global relative comparison error, E, between CFD and PIV ranges from 3 to 11%. By performing sensitivity studies, we demonstrate that the quantitative discrepancy is attributable to a combination of uncertainty in the inlet flow rates and uncertainty associated with precisely aligning the PIV data with the CFD geometry. Overall, the study demonstrates mesh-convergent CFD simulations that predict IVC flow patterns that agree reasonably well with PIV data, even at exercise conditions where the flow in the IVC is extremely complex.

Ascending thoracic aorta aneurysm repair induces positive hemodynamic outcomes in a patient with unchanged bicuspid aortic valve

We report a patient-specific case of bicuspid aortic valve with fusion of right and left coronary leaflets (R-L type I BAV), moderate aortic valve deficiency and ascending thoracic aortic aneurysms (ATAA) who was treated by only ascending aorta replacement preserving the BAV. The flow eccentricity, the helicity intensity (h2), the circumferential time averaged wall shear stress (TAWSScircumferential), the cumulative viscous energy loss at the systolic peak (EL') and the pulse wave velocity (PWV) were calculated by combining 4D flow MRI and CFD analysis before (Stage I) and after (Stage II) the surgical procedure. CFD analyses assumed rigid walls, a non-Newtonian behavior for the blood and MRI measured patient-specific blood flow profiles as inlet boundary conditions. Stage II results showed suppression of recirculation in the ascending aorta, loss of jet flow impingement onto the aortic wall, maximum TAWSScircumferential decrease (from 6.69 Pa in Stage I to 6 Pa in Stage II), reduction of flow helicity (from 10.97 in Stage I to 8.47 in Stage II) and EL' (from 15.8 mW in Stage I to 11.2 mW in Stage II). However, Floweccentricity and PWV were found higher in Stage II due to the diameter reduction (Floweccentricity = 0.60 in Stage I and Floweccentricity = 0.91 in Stage II; PWV = 3.80 m/s in Stage I and PWV = 9.37 m/s in Stage II).Our work has permitted to compute for the first time the hemodynamic alterations obtained after restoration of normal ascending aorta and sinotubular junction geometry even preserving an R-L type I BAV with still acceptable function.

Healthy and diseased coronary bifurcation geometries influence near-wall and intravascular flow: A computational exploration of the hemodynamic risk

Local hemodynamics has been identified as one main determinant in the onset and progression of atherosclerotic lesions at coronary bifurcations. Starting from the observation that atherosensitive hemodynamic conditions in arterial bifurcation are majorly determined by the underlying anatomy, the aim of the present study is to investigate how peculiar coronary bifurcation anatomical features influence near-wall and intravascular flow patterns. Different bifurcation angles and cardiac curvatures were varied in population-based, idealized models of both stenosed and unstenosed bifurcations, representing the left anterior descending (LAD) coronary artery with its diagonal branch. Local hemodynamics was analyzed in terms of helical flow and exposure to low/oscillatory shear stress by performing computational fluid dynamics simulations.Results show that bifurcation angle impacts lowly hemodynamics in both stenosed and unstenosed cases. Instead, curvature radius influences the generation and transport of helical flow structures, with smaller cardiac curvature radius associated to higher helicity intensity. Stenosed bifurcation models exhibit helicity intensity values one order of magnitude higher than the corresponding unstenosed cases. Cardiac curvature radius moderately affects near-wall hemodynamics of the stenosed cases, with smaller curvature radius leading to higher exposure to low shear stress and lower exposure to oscillatory shear stress. In conclusion, the proposed controlled benchmark allows investigating the effect of various geometrical features on local hemodynamics at the LAD/diagonal bifurcation, highlighting that cardiac curvature influences near wall and intravascular hemodynamics, while bifurcation angle has a minor effect.

Coronary fractional flow reserve measurements of a stenosed side branch: a computational study investigating the influence of the bifurcation angle.

BackgroundCoronary hemodynamics and physiology specific for bifurcation lesions was not well understood. To investigate the influence of the bifurcation angle on the intracoronary hemodynamics of side branch (SB) lesions computational fluid dynamics simulations were performed.MethodsA parametric model representing a left anterior descending—first diagonal coronary bifurcation lesion was created according to the literature. Diameters obeyed fractal branching laws. Proximal and distal main branch (DMB) stenoses were both set at 60 %. We varied the distal bifurcation angles (40°, 55°, and 70°), the flow splits to the DMB and SB (55 %:45 %, 65 %:35 %, and 75 %:25 %), and the SB stenoses (40, 60, and 80 %), resulting in 27 simulations. Fractional flow reserve, defined as the ratio between the mean distal stenosis and mean aortic pressure during maximal hyperemia, was calculated for the DMB and SB (FFRSB) for all simulations.ResultsThe largest differences in FFRSB comparing the largest and smallest bifurcation angles were 0.02 (in cases with 40 % SB stenosis, irrespective of the assumed flow split) and 0.05 (in cases with 60 % SB stenosis, flow split 55 %:45 %). When the SB stenosis was 80 %, the difference in FFRSB between the largest and smallest bifurcation angle was 0.33 (flow split 55 %:45 %). By describing the ΔPSB−QSB relationship using a quadratic curve for cases with 80 % SB stenosis, we found that the curve was steeper (i.e. higher flow resistance) when bifurcation angle increases (ΔP = 0.451*Q + 0.010*Q2 and ΔP = 0.687*Q + 0.017*Q2 for 40° and 70° bifurcation angle, respectively). Our analyses revealed complex hemodynamics in all cases with evident counter-rotating helical flow structures. Larger bifurcation angles resulted in more pronounced helical flow structures (i.e. higher helicity intensity), when 60 or 80 % SB stenoses were present. A good correlation (R2 = 0.80) between the SB pressure drop and helicity intensity was also found.ConclusionsOur analyses showed that, in bifurcation lesions with 60 % MB stenosis and 80 % SB stenosis, SB pressure drop is higher for larger bifurcation angles suggesting higher flow resistance (i.e. curves describing the ΔPSB−QSB relationship being steeper). When the SB stenosis is mild (40 %) or moderate (60 %), SB resistance is minimally influenced by the bifurcation angle, with differences not being clinically meaningful. Our findings also highlighted the complex interplay between anatomy, pressure drops, and blood flow helicity in bifurcations.

Effects of Vessel Tortuosity on Coronary Hemodynamics: An Idealized and Patient-Specific Computational Study.

Although coronary tortuosity can influence the hemodynamics of coronary arteries, the relationship between tortuosity and flow has not been thoroughly investigated partly due to the absence of a widely accepted definition of tortuosity and the lack of patient-specific studies that analyze complete coronary trees. Using a computational approach we investigated the effects of tortuosity on coronary flow parameters including pressure drop, wall shear stress, and helical flow strength as measured by helicity intensity. Our analysis considered idealized and patient-specific geometries. Overall results indicate that perfusion pressure decreases with increased tortuosity, but the patient-specific results show that more tortuous vessels have higher physiological wall shear stress values. Differences between the idealized and patient-specific results reveal that an accurate representation of coronary tortuosity must account for all relevant geometric aspects, including curvature imposed by the heart shape. The patient-specific results exhibit a strong correlation between tortuosity and helicity intensity, and the corresponding helical flow contributes directly to the observed increase in wall shear stress. Therefore, helicity intensity may prove helpful in developing a universal parameter to describe tortuosity and assess its impact on patient health. Our data suggest that increased tortuosity could have a deleterious impact via a reduction in coronary perfusion pressure, but the attendant increase in wall shear stress could afford protection against atherosclerosis.

Some New Features of Photon Statistics in a Fully Quantized Parametric Amplification Process

An exact quantum treatment reveals that signal and idler photon number operators are not well-behaved dynamical operators for studying photon statistics in parametric amplification/down-conversion processes. Contrary to expectations, the mean signal-idler photon number difference varies with time, while the corresponding signal-idler photon number cross-correlation function is complex and experiences an interference phenomenon driven by the interaction parameters. The intensity operators and related polarization operators of the polarized signal-idler photon pair in positive and negative helicity states are identified as the appropriate operators specifying a conservation law and the dynamical symmetry group (SU(1, 1)) of the parametric amplification process. The conservation of the mean positive and negative helicity photon intensity difference and the purely real positive-negative helicity intensity cross-correlation function correctly account for the simultaneous production of polarized signal and idler photons in positive and negative helicity states.

A computational exploration of helical arterio-venous graft designs

Although arterio-venous grafts (AVGs) are the second best option as long-term vascular access for hemodialysis, they suffer from complications caused by intimal hyperplasia, mainly located in vessel regions of low and oscillating wall shear stress. However, certain flow patterns in the bulk may reduce these unfavorable hemodynamic conditions. We therefore studied, with computational fluid dynamics (CFD), the impact of a helical AVG design on the occurrence of (un)favorable hemodynamic conditions at the venous anastomosis.Six CFD-models of an AVG in closed-loop configuration were constructed: one conventional straight graft, and five helical designed grafts with a pitch of 105mm down to 35mm. At the venous anastomosis, disturbed shear was assessed by quantifying the area with unfavorable conditions, and by analyzing averaged values in a case-specific patch. The bulk hemodynamics were assessed by analyzing the kinetic helicity in and the pressure drop over the graft.The most helical design scores best, being instrumental to suppress disturbed shear in the venous segment. There is, however, no trivial relationship between the number of helix turns of the graft and disturbed shear in the venous segment, when a realistic closed-loop AVG model is investigated. Bulk flow investigation showed a marked increase of helicity intensity in, and a moderate pressure drop over the AVG by introducing a lower pitch.At the venous anastomosis, unfavorable hemodynamic conditions can be reduced by introducing a helical design. However, due to the complex flow conditions, the optimal helical design for an AVG cannot be derived without studying case by case.

Helical flow in carotid bifurcation as surrogate marker of exposure to disturbed shear

The main purpose of the study is to verify if helical flow might be a surrogate marker of the exposure to disturbed shear in normal carotid bifurcations. Based on hemodynamic simulations data of 50 carotid bifurcations, we defined appropriate helicity-based bulk flow descriptors and multivariate linear regression analysis was performed to assess the potential for a combination of helicity-based indicators in describing the exposure to disturbed shear. To select the optimal combination of helicity-based descriptors in the regression model, the Akaike information criterion was applied. The identified statistical model is composed of two bulk flow descriptors proposed here to quantify helicity intensity and the balance between counter-rotating helical flow patterns in the flow field. The model revealed the existence of a significant relationship with adjusted squared Pearson's correlation coefficient in the range 0.4–0.7 (P<0.0001). In detail, while a high helicity intensity is instrumental in suppressing flow disturbances, this protective effect could be moderated when one direction of rotation is dominant in the flow field. The in vivo quantification of the bulk flow features emerging in this study would offer a practical way to infer the presence of disturbed shear in large-scale in vivo studies of local risk factors in atherosclerosis.