The Computational Fluid Dynamics Rupture Challenge 2013--Phase II: Variability of Hemodynamic Simulations in Two Intracranial Aneurysms.

With the increased availability of computational resources, the past decade has seen a rise in the use of computational fluid dynamics (CFD) for medical applications. There has been an increase in the application of CFD to attempt to predict the rupture of intracranial aneurysms, however, while many hemodynamic parameters can be obtained from these computations, to date, no consistent methodology for the prediction of the rupture has been identified. One particular challenge to CFD is that many factors contribute to its accuracy; the mesh resolution and spatial/temporal discretization can alone contribute to a variation in accuracy. This failure to identify the importance of these factors and identify a methodology for the prediction of ruptures has limited the acceptance of CFD among physicians for rupture prediction. The International CFD Rupture Challenge 2013 seeks to comment on the sensitivity of these various CFD assumptions to predict the rupture by undertaking a comparison of the rupture and blood-flow predictions from a wide range of independent participants utilizing a range of CFD approaches. Twenty-six groups from 15 countries took part in the challenge. Participants were provided with surface models of two intracranial aneurysms and asked to carry out the corresponding hemodynamics simulations, free to choose their own mesh, solver, and temporal discretization. They were requested to submit velocity and pressure predictions along the centerline and on specified planes. The first phase of the challenge, described in a separate paper, was aimed at predicting which of the two aneurysms had previously ruptured and where the rupture site was located. The second phase, described in this paper, aims to assess the variability of the solutions and the sensitivity to the modeling assumptions. Participants were free to choose boundary conditions in the first phase, whereas they were prescribed in the second phase but all other CFD modeling parameters were not prescribed. In order to compare the computational results of one representative group with experimental results, steady-flow measurements using particle image velocimetry (PIV) were carried out in a silicone model of one of the provided aneurysms. Approximately 80% of the participating groups generated similar results. Both velocity and pressure computations were in good agreement with each other for cycle-averaged and peak-systolic predictions. Most apparent "outliers" (results that stand out of the collective) were observed to have underestimated velocity levels compared to the majority of solutions, but nevertheless identified comparable flow structures. In only two cases, the results deviate by over 35% from the mean solution of all the participants. Results of steady CFD simulations of the representative group and PIV experiments were in good agreement. The study demonstrated that while a range of numerical schemes, mesh resolution, and solvers was used, similar flow predictions were observed in the majority of cases. To further validate the computational results, it is suggested that time-dependent measurements should be conducted in the future. However, it is recognized that this study does not include the biological aspects of the aneurysm, which needs to be considered to be able to more precisely identify the specific rupture risk of an intracranial aneurysm.

Adaptive higher-order singular value decomposition clutter filter for ultrafast Doppler imaging of coronary flow under non-negligible tissue motion.

BACKGROUND AND OBJECTIVE: With the development of advanced clutter-filtering techniques by singular value decomposition (SVD) and leveraging favorable acquisition settings such as open-chest imaging by a linear high-frequency probe and plane waves, several studies have shown the feasibility of cardiac flow measurements during the entire cardiac cycle, ranging from coronary flow to myocardial perfusion. When applying these techniques in a routine clinical setting, using transthoracic ultrasound imaging, new challenges emerge. Firstly, a smaller aperture is needed that can fit between ribs. Consequently, diverging waves are employed instead of plane waves to achieve an adequate field of view. Secondly, to ensure imaging at a larger depth, the maximum pulse repetition frequency has to be reduced. Lastly, in comparison to the open-chest scenario, tissue motion induced by the heartbeat is significantly stronger. The latter complicates substantially the distinction between clutter and blood signals. METHODS: This study investigates a strategy to overcome these challenges by diverging wave imaging with an optimal number of tilt angles, in combination with dedicated clutter-filtering techniques. In particular, a novel, adaptive, higher-order SVD (HOSVD) clutter filter, which utilizes spatial, temporal, and angular information of the received ultrasound signals, is proposed to enhance clutter and blood separation. RESULTS: When non-negligible tissue motion is present, using fewer tilt angles not only reduces the decorrelation between the received waveforms but also allows for collecting more temporal samples at a given ensemble duration, contributing to improved Doppler performance. The addition of a third angular dimension enables the application of HOSVD, providing greater flexibility in selecting blood separation thresholds from a 3-D tensor. This differs from the conventional threshold selection method in a 2-D spatiotemporal space using SVD. Exhaustive threshold search has shown a significant improvement in Contrast and Contrast-to-Noise ratio for Power Doppler images filtered with HOSVD compared to the SVD-based clutter filter. CONCLUSION: With the improved settings, the obtained Power Doppler images show the feasibility of measuring coronary flow under the influence of non-negligible tissue motion in both in vitro and ex vivo.

Inter-season training effects on cardiovascular health in American-style football players.

BACKGROUND: Recent studies on American-style football (ASF) athletes raised questions about the impact of training on the cardiovascular phenotype, particularly among linemen players who engage mostly in static exercise during competition and who exhibit concentric cardiac remodeling, often considered maladaptive. We aimed to examine the cardiovascular adaptation to the inter-season mixed-team training program among ASF players. METHODS: A prospective, longitudinal, cohort study was conducted among competitive male ASF players from the University of Montreal before and after an inter-season training, which lasted 7 months. This program includes, for all players, combined dynamic and static exercises. Clinical and echocardiographic examinations were performed at both steps. Left atrial (LA) and ventricular (LV) morphological and functional changes were assessed using a multiparametric echocardiographic approach (2D and 3D-echo, Doppler, and speckle tracking). Two-way ANOVA was performed to analyze the impacts of time and field position (linemen versus non-linemen). RESULTS: Fifty-nine players (20 linemen and 39 non-linemen) were included. At baseline, linemen had higher blood pressure (65% were prehypertensive and 10% were hypertensive), thicker LV walls, lower LV systolic and diastolic functions, lower LA-reservoir and conduit functions than non-linemen. After training, linemen significantly reduced weight (Δ-3.4%, P < 0.001) and systolic blood pressure (Δ-4.5%, P < 0.001), whereas non-linemen maintained their weight and significantly increased their systolic (Δ+4.2%, P = 0.037) and diastolic (Δ+16%, P < 0.001) blood pressure ). Mixed training was associated with significant increases in 2D-LA volume (P < 0.001), 3D-LV end-diastolic volume (P < 0.001), 3D-LV mass (P < 0.001), and an improvement in LV systolic function, independently of the field position. Non-linemen remodeled their LV in a more concentric fashion and showed reductions in LV diastolic and LA reservoir functions. CONCLUSIONS: Our study underscored the influence of field position on cardiovascular adaptation among university-level ASF players, and emphasized the potential of inter-season training to modulate cardiovascular risk factors, particularly among linemen.

Quantification of arterial flow using digital subtraction angiography.

PURPOSE: In this paper, a method for the estimation of arterial hemodynamic flow from x-ray video densitometry data is proposed and validated using an in vitro setup. METHODS: The method is based on the acquisition of three-dimensional rotational angiography and digital subtraction angiography sequences. A modest contrast injection rate (between 1 and 4 ml/s) leads to a contrast density that is modulated by the cardiac cycle, which can be measured in the x-ray signal. An optical flow based approach is used to estimate the blood flow velocities from the cyclic phases in the x-ray signal. RESULTS: The authors have validated this method in vitro, and present three clinical cases. The in vitro experiments compared the x-ray video densitometry results with the gold standard delivered by a flow meter. Linear correlation analysis and regression fitting showed that the ideal slope of 1 and intercept of 0 were contained within the 95 percentile confidence interval. The results show that a frame rate higher than 50 Hz allows measuring flows in the range of 2 ml/s to 6 ml/s within an accuracy of 5%. CONCLUSIONS: The in vitro and clinical results indicate that it is feasible to estimate blood flow in routine interventional procedures. The availability of an x-ray based method for quantitative flow estimation is particularly clinically useful for intra-cranial applications, where other methods, such as ultrasound Doppler, are not available.

A Novel Approach for Semiautomated Three-Dimensional Quantification of Mitral Regurgitant Volume Reflects a More Physiologic Approach to Mitral Regurgitation.

BACKGROUND: Quantification of mitral regurgitation (MR) by echocardiography is integral to assessing lesion severity and entails the integration of multiple Doppler-based parameters. These methods are founded primarily upon the principle of proximal isovelocity surface area (PISA), a two-dimensional (2D) method known to involve several assumptions regarding MR jet characteristics. The authors analyzed the results of a semiautomated method of three-dimensional (3D)-based regurgitant volume (RVol) estimation that accounts for jet behavior throughout the cardiac cycle and compared it with conventional 2D PISA methods for MR quantification. METHODS: A total of 50 patients referred for transesophageal echocardiography for evaluation of primary (n = 25) and secondary (n = 25) MR were included for analysis. Three-dimensional full-volume color data sets were acquired, along with standard 2D methods for PISA calculation. A 3D semiautomated MR flow quantification algorithm was applied offline to calculate 3D RVol, with simultaneous temporal curves generated from the 3D data set. Three-dimensional RVol was compared with 2D RVol. Three-dimensional vena contracta area was also performed in all cases. RESULTS: There was a modest correlation between 2D RVol and 3D RVol (r = 0.60). The semiautomated 3D approach resulted in significantly lower values of RVol compared with 2D PISA. Real-time and dynamic flow curve patterns were used for integral estimates of 3D RVol over the cardiac cycle, with a distinct bimodal pattern in functional MR and a brief and solitary peak in primary MR. CONCLUSIONS: Using a semiautomated 3D software for the quantification of MR allows the simultaneous calculation of 3D RVol with an automated generation of dynamic flow curves characteristic of the underlying MR mechanism. The present flow curve pattern results highlight well-known differences between MR flow dynamics in degenerative MR compared with functional MR.

Analysis of inter-system variability of systolic and diastolic intraventricular pressure gradients derived from color Doppler M-mode echocardiography.

Assessment of intraventricular pressure gradients (IVPG) using color Doppler M-mode echocardiography has gained increasing interest in the evaluation of cardiac function. However, standardized analysis tools for IVPG quantification are missing. We aimed to evaluate the feasibility, the test-retest observer reproducibility, and the inter-system variability of a semi-automated IVPG quantification algorithm. The study included forty healthy volunteers (50% were men). All volunteers were examined using two ultrasound systems, the Philips Epiq 7 and the General Electric Vivid 6. Left ventricular diastolic (DIVPG) and systolic (SIVPG) intraventricular pressure gradients were measured from the spatiotemporal distribution of intraventricular propagation flow velocities using color Doppler M-mode in standard apical views. There was good feasibility for both systolic and diastolic IVPG measurements (82.5% and 85%, respectively). Intra and inter-observer test-retest variability measured with the intraclass correlation coefficient were 0.98 and 0.93 for DIVPG respectively, and 0.95 and 0.89 for SIVPG respectively. The inter-system concordance was weak to moderate with Lin's concordance correlation coefficient of 0.59 for DIVPG and 0.25 for SIVPG. In conclusion, it is feasible and reproducible to assess systolic and diastolic IVPG using color Doppler M-mode in healthy volunteers. However, the inter-system variability in IVPG analysis needs to be taken into account, especially when using displayed data.

Validation of Semiautomated Quantification of Mitral Valve Regurgitation by Three-Dimensional Color Doppler Transesophageal Echocardiography.

BACKGROUND: The aim of this study was to evaluate the accuracy of mitral regurgitation (MR) volume quantified on three-dimensional (3D) color Doppler transesophageal echocardiography (TEE) using new semiautomated software compared with conventional two-dimensional (2D) proximal isovelocity surface area (PISA) transthoracic echocardiography (TTE) and TEE and cardiac magnetic resonance imaging (CMR). METHODS: Fifty-one patients (mean age, 63 ± 16 years; 35 men) prospectively underwent TTE, TEE, and CMR for MR evaluation. Regurgitant volume (RVol) by 3D MR flow quantification was compared with 2D TTE, TEE, and CMR, and the accuracy of evaluation of severe MR by 3D MR flow quantification was compared against guideline criteria by TEE. RESULTS: Twenty-nine patients had severe MR, 16 had moderate MR, and six had mild MR. Three-dimensional MR flow quantification was feasible in all patients, including prolapse (n = 37), restriction (n = 9), functional MR (n = 5), and eccentric or multiple jects (n = 41). RVol on 3D MR flow quantification correlated well with RVol on 2D PISA TTE (interclass correlation coefficient [ICC] = 0.75, P < .001), quantitatively estimated RVol (ICC = 0.74, P < .001), and 2D PISA TEE (ICC = 0.79, P < .001). Three-dimensional MR flow quantification agreed better with CMR (ICC = 0.86, P < .001) than did RVol on 2D PISA TTE (ICC = 0.66, P < .001) and 2D PISA TEE (ICC = 0.69, P < .001), with narrower limits of agreement on Bland-Altman analysis. Three-dimensional MR flow quantification had high accuracy for diagnosing severe MR using TEE (area under the curve = 0.85, 95% CI 0.74-0.96, P < .001) or CMR (area under the curve = 0.95; 95% CI, 0.89-1.00; P < .001) as the criterion. CONCLUSIONS: The new software enabled semiautomated 3D MR flow quantification in complex MR with multiple and eccentric jets and showed better agreement with CMR than 2D PISA TTE or TEE, suggesting that this method is more accurate than conventional 2D PISA TTE and TEE.

Unraveling the relationship between arterial flow and intra-aneurysmal hemodynamics.

Arterial flow rate affects intra-aneurysmal hemodynamics but it is not clear how their relationship is. This uncertainty hinders the comparison among studies, including clinical evaluations, like a pre- and post-treatment status, since arterial flow rates may differ at each time acquisition. The purposes of this work are as follows: (1) To study how intra-aneurysmal hemodynamics changes within the full physiological range of arterial flow rates. (2) To provide characteristic curves of intra-aneurysmal velocity, wall shear stress (WSS) and pressure as functions of the arterial flow rate. Fifteen image-based aneurysm models were studied using computational fluid dynamics (CFD) simulations. The full range of physiological arterial flow rates reported in the literature was covered by 11 pulsatile simulations. For each aneurysm, the spatiotemporal-averaged blood flow velocity, WSS and pressure were calculated. Spatiotemporal-averaged velocity inside the aneurysm linearly increases as a function of the mean arterial flow (minimum R(2)>0.963). Spatiotemporal-averaged WSS and pressure at the aneurysm wall can be represented by quadratic functions of the arterial flow rate (minimum R(2)>0.996). Quantitative characterizations of spatiotemporal-averaged velocity, WSS and pressure inside cerebral aneurysms can be obtained with respect to the arterial flow rate. These characteristic curves provide more information of the relationship between arterial flow and aneurysm hemodynamics since the full range of arterial flow rates is considered. Having these curves, it is possible to compare experimental studies and clinical evaluations when different flow conditions are used.

Peak systolic or maximum intra-aneurysmal hemodynamic condition? Implications on normalized flow variables.

BACKGROUND: CFD has been used to assess intra-aneurysmal hemodynamics. Nevertheless, the lack of patient-specific flow information has triggered the possibility of implementing a wide variety of physiological flow conditions. Due to these uncertainties in the patient flow conditions, the normalization of the intra-aneurysmal hemodynamics is generally conducted. PURPOSE: To investigate how intra-aneurysmal and arterial hemodynamics change over time when different physiological flow conditions are imposed. MATERIAL AND METHOD: Eleven image-based aneurysm models were used in this study. CFD simulations were performed under pulsatile flows. Velocity magnitude and wall shear stress (WSS) were calculated during one cardiac cycle. RESULTS: Maximum hemodynamic condition does not necessarily occurred at peak systole. The shifted time from peak systole to the time where the maximum hemodynamic condition occurs inside the aneurysm depends on the aneurysm size, flow rate, surrounding vasculature and the stabilities of flow patterns. Larger shifted times were observed with increasing aneurysm size as well as with reducing the flow rate. Moreover, the maximum hemodynamic condition can occur earlier than peak systole if flow patterns at parent artery change. Differences between peak systolic WSS and maximum WSS can be up to 65%. Moreover, the velocity magnitude and WSS depend on the selected segment of the parent artery, with relatively larger variability near peak systole than the rest of the cardiac cycle. More than 50% of differences were found between two arterial segments arbitrary selected for a given flow rate. CONCLUSIONS: Our results indicate that if the highest intra-aneurysmal stress is calculated, then it is preferable to use the time instance where the maximum WSS occurred instead of the peak systolic WSS. Additionally, the normalization of intra-aneurysmal hemodynamics should be done with variables that do not depend on any arbitrary segment of the parent artery.