@inproceedings {28451,
	title = {Impact of Flow Rate on Wall Vibration in Intracranial Aneurysms},
	journal = {8th European Congress on Computational Methods in Applied Sciences and Engineer- ing},
	year = {2022},
	address = {Oslo, Norway},
	author = {David A. {Bruneau} and Valen-Sendstad, Kristian and David A. {Steinman}}
}
@inproceedings {27726,
	title = {IMPACT OF FLOW RATE ON HIGH FREQUENCY FLOW INSTABILITIES IN INTRACRANIAL ANEURYSMS, WITH IMPLICATIONS FOR WALL VIBRATION},
	journal = {SB3C2021 Summer Biomechanics, Bioengineering and Biotransport Conference},
	year = {2021},
	author = {David A. {Bruneau} and Najafi, Mehdi and Natarajan, Thangam and Valen-Sendstad, Kristian and David A. {Steinman}},
	editor = {, }
}
@article {26490,
	title = {Automated and Objective Removal of Bifurcation Aneurysms: Incremental Improvements, and Validation Against Healthy Controls},
	journal = {Journal of Biomechanics},
	volume = {96},
	year = {2019},
	pages = {109342},
	publisher = {Elsevier},
	doi = {https://doi.org/10.1016/j.jbiomech.2019.109342},
	url = {https://www.sciencedirect.com/science/article/abs/pii/S0021929019305640},
	author = {Bergersen, Aslak and Chnafa, Christophe and Gallo, Diego and Piccinelli, Marina and David A. {Steinman} and Valen-Sendstad, Kristian}
}
@article {26227,
	title = {Multiple Aneurysms AnaTomy CHallenge 2018 (MATCH) - Phase II: Rupture Risk Assessment},
	journal = {International Journal of Computer Assisted Radiology and Surgery},
	volume = {14},
	number = {10},
	year = {2019},
	month = {05/2019},
	pages = {1795-1804},
	publisher = {British Medical Journal Publishing Group},
	doi = {https://doi.org/10.1007/s11548-019-01986-2},
	url = {https://link.springer.com/article/10.1007/s11548-019-01986-2},
	author = {Berg, Philipp and Vo{\ss}, Samuel and Saalfeld, Sylvia and Janiga, G{\'a}bor and Bergersen, Aslak and Valen-Sendstad, Kristian and Bruening, Jan and Goubergrits, Leonid and Spuler, Andreas and Nicole M. {Cancelliere} and David A. {Steinman} and Vitor M. {Pereira} and Tin Lok {Chiu} and Anderson Chun On {Tsang} and Bong Jae {Chung} and Juan R. {Cebral} and Cito, Salvatore and Pallares, Jordi and Copelli, Gabriele and Csippa, Benjamin and Pa{\'a}l, Gy{\"o}rgy and Fujimura, Soichiro and Takao, Hiroyuki and Hodis, Simona and Hille, Georg and Karmonik, Christof and Elias, Saba and Kellermann, Kerstin and Muhammad Owais {Khan} and Alison L. {Marsden} and Hern{\'a}n G. {Morales} and Piskin, Senol and Ender A. {Finol} and Pravdivtseva, Mariya and Rajabzadeh-Oghaz, Hamidreza and Paliwal, Nikhil and Meng, Hui and Seshadhri, Santhosh and Howard, Matthew and Shojima, Masaaki and Sugiyama, Shin-ichiro and Niizuma, Kuniyasu and Sindeev, Sergey and Frolov, Sergey and Wagner, Thomas and Brawanski, Alexander and Qian, Yi and Wu, Yu-An and Kent D. {Carlson} and Dragomir-Daescu, Dan and Beuing, Oliver}
}
@article {26195,
	title = {Multiple Aneurysms AnaTomy CHallenge 2018 (MATCH): Phase I: Segmentation},
	journal = {Cardiovascular Engineering and Technology},
	volume = {9},
	year = {2018},
	month = {09/2018},
	pages = {565{\textendash}581},
	publisher = {Springer US},
	issn = {1869-408X},
	doi = {10.1007/s13239-018-00376-0},
	url = {http://link.springer.com/10.1007/s13239-018-00376-0},
	author = {Berg, Philipp and Vo{\ss}, Samuel and Saalfeld, Sylvia and Janiga, G{\'a}bor and Bergersen, Aslak and Valen-Sendstad, Kristian and Bruening, Jan and Goubergrits, Leonid and Spuler, Andreas and Nicole M. {Cancelliere} and David A. {Steinman} and Vitor M. {Pereira} and Tin Lok {Chiu} and Anderson Chun On {Tsang} and Bong Jae {Chung} and Juan R. {Cebral} and Cito, Salvatore and Pallares, Jordi and Copelli, Gabriele and Csippa, Benjamin and Pa{\'a}l, Gy{\"o}rgy and Fujimura, Soichiro and Takao, Hiroyuki and Hodis, Simona and Hille, Georg and Karmonik, Christof and Elias, Saba and Kellermann, Kerstin and Owais Mohammad {Khan} and Alison L. {Marsden} and Hern{\'a}n G. {Morales} and Piskin, Senol and Ender A. {Finol} and Pravdivtseva, Mariya and Rajabzadeh-Oghaz, Hamidreza and Paliwal, Nikhil and Meng, Hui and Seshadhri, Santhosh and Howard, Matthew and Shojima, Masaaki and Sugiyama, Shin-ichiro and Niizuma, Kuniyasu and Sindeev, Sergey and Frolov, Sergey and Wagner, Thomas and Brawanski, Alexander and Qian, Yi and Wu, Yu-An and Kent D. {Carlson} and Dragomir-Daescu, Dan and Beuing, Oliver}
}
@article {25537,
	title = {Cerebral aneurysm blood flow simulations: There{\textquoteright}s solver settings and then there{\textquoteright}s solver settings},
	journal = {Journal of Biomechanics},
	volume = {61},
	year = {2017},
	pages = {280-280},
	publisher = {Journal of Biomechanics},
	doi = {10.1016/j.jbiomech.2017.04.039},
	author = {Owais Mohammad {Khan} and Valen-Sendstad, Kristian and David A. {Steinman}}
}
@inproceedings {25051,
	title = {Freezing   Turbulence   in   Brain   Aneurysm   Flow   Visualizations:   An   Applied   Art-Science   Collaboration.},
	journal = {Special Interest Group on GRAPHics and Interactive Techniques},
	year = {2016},
	author = {Coppin, Peter and Julian, Maxwell and Harvey, John and Valen-Sendstad, Kristian and David A. {Steinman}}
}
@inproceedings {25050,
	title = {Impact of Outflow Boundary Conditions on Flow Rates in Cerebrovascular CFD Models.},
	journal = {22nd Congress of the European Society of Biomechanics},
	year = {2016},
	author = {Chnafa, Christophe and Valen-Sendstad, Kristian and Brina, Olivier and Pereira, Vitor and David A. {Steinman}}
}
@article {25037,
	title = {Improved Reduced-Order Modelling of Cerebrovascular Flow Distribution by Accounting for Arterial Bifurcation Pressure Drops.},
	journal = {Journal of Biomechanics},
	volume = {51},
	year = {2016},
	pages = {83-88},
	publisher = {Journal of Biomechanics},
	author = {Chnafa, Christophe and Valen-Sendstad, Kristian and Brina, Olivier and Pereira, Vitor and David A. {Steinman}}
}
@inproceedings {25053,
	title = {Non-Newtonian versus Numerical Rheology: Practical Impact Of Shear-thinning On The Prediction         Of Stable And Unstable Flows In Intracranial Aneurysms.},
	journal = {American Society of Mechanical Engineers Summer Biomechanics, Bioengineering and Biotransport Conference},
	year = {2016},
	author = {Owais Mohammad {Khan} and David A. {Steinman} and Valen-Sendstad, Kristian}
}
@article {25036,
	title = {Non-Newtonian versus Numerical Rheology: Practical Impact Of Shear-thinning On The Prediction  Of Stable And Unstable Flows In Intracranial Aneurysms.},
	journal = {International Journal for Numerical Methods in Biomedical Engineering},
	volume = {33},
	number = {7},
	year = {2016},
	month = {10/16},
	publisher = {Int J Numer Method Biomed Eng},
	author = {Owais Mohammad {Khan} and David A. {Steinman} and Valen-Sendstad, Kristian}
}
@inproceedings {25048,
	title = {Proper Orthogonal Decomposition  Analyses of High-Frequency Transient Flow Instabilities in Intracranial Aneurysms},
	journal = {American Physical Society Division of Fluid Dynamics},
	year = {2016},
	author = {Owais Mohammad {Khan} and Chnafa, Christophe and Valen-Sendstad, Kristian and David A. {Steinman}}
}
@article {25038,
	title = {On the Quantification and Visualization of Transient Periodic Instabilities in Pulsatile Flows},
	journal = {Journal of Biomechanics},
	number = {52},
	year = {2016},
	month = {12/2016},
	pages = {179-182},
	publisher = {Journal of Biomechanics},
	author = {Owais Mohammad {Khan} and Chnafa, Christophe and Gallo, Diego and Molinari, Filippo and Morbiducci, Umberto and David A. {Steinman} and Valen-Sendstad, Kristian}
}
@inproceedings {25052,
	title = {A Simple and Rational Approach to Outflow Conditions in Cerebrovascular CFD Models.},
	journal = {American Society of Mechanical Engineers Summer Biomechanics, Bioengineering and Biotransport Conference},
	year = {2016},
	author = {Chnafa, Christophe and Valen-Sendstad, Kristian and Brina, Olivier and Pereira, Vitor and David A. {Steinman}}
}
@inproceedings {23967,
	title = {CFD Simulation of Transition to Turbulence for Newtonian vs. Non-Newtonian Flow Through a Stenosis},
	journal = {The Summer Biomechanics, Bioengineering \& Biotransport Conference},
	year = {2015},
	note = {\&nbsp;Winner of the Best PhD Student Paper\&nbsp; competition},
	month = {06/2015},
	author = {Owais Mohammad {Khan} and Valen-Sendstad, Kristian and Biswas, D. and Casey, D. and Loth, F. and David A. {Steinman}}
}
@inproceedings {23970,
	title = {Characterization of Transition to Turbulence for Blood in a S-Shaped Pipe Under Steady Flow Conditions},
	journal = {The Summer Biomechanics, Bioengineering \& Biotransport Conference},
	year = {2015},
	month = {06/2015},
	author = {Biswas, D. and Casey, D. and D. C. {Crowder} and Valen-Sendstad, Kristian and David A. {Steinman} and Y. H. {Yun} and Loth, F.}
}
@article {berg2015computational,
	title = {The Computational Fluid Dynamics Rupture Challenge 2013{\textemdash}Phase II: variability of hemodynamic simulations in two intracranial aneurysms},
	journal = {Journal of Biomechanical Engineering},
	volume = {137},
	year = {2015},
	month = {2015},
	pages = {121008/1-121008/13},
	publisher = {American Society of Mechanical Engineers},
	author = {Berg, Philipp and Roloff, Christoph and Beuing, Oliver and Voss, Samuel and Sugiyama, Shin-Ichiro and Aristokleous, Nicolas and Andreas S. {Anayiotos} and Ashton, Neil and Revell, Alistair and Neil W. {Bressloff} and Alistair G. {Brown} and Bong Jae {Chung} and Juan R. {Cebral} and Copelli, Gabriele and Fu, Wenyu and Qiao, Aike and Arjan J. {Geers} and Hodis, Simona and Dragomir-Daescu, Dan and Nordahl, Emily and Yildirim Bora {Suzen} and Owais Mohammad {Khan} and Valen-Sendstad, Kristian and Kono, Kenichi and Prahlad G. {Menon} and Priti G. {Albal} and Mierka, Otto and M{\"u}nster, Raphael and Hern{\'a}n G. {Morales} and Bonnefous, Odile and Osman, Jan and Goubergrits, Leonid and Pallares, Jordi and Cito, Salvatore and Passalacqua, Alberto and Piskin, Senol and Pekkan, Kerem and Ramalho, Susana and Marques, Nelson and Sanchi, St{\'e}phane and Kristopher R. {Schumacher} and Sturgeon, Jess and Svihlov{\'a}, Helena and Hron, Jaroslav and Usera, Gabriel and Mendina, Mariana and Xiang, Jianping and David A. {Steinman} and Janiga, G{\'a}bor}
}
@inproceedings {23977,
	title = {Effects of non-Newtonian rheology on transition to turbulence},
	journal = {The 2015 AMMCS-CAIMS Congress},
	year = {2015},
	month = {06/2015},
	author = {Owais Mohammad {Khan} and Valen-Sendstad, Kristian and David A. {Steinman}}
}
@article {23408,
	title = {Estimation of Inlet Flow Rates for Image-Based Aneurysm CFD Models: Where and How to Begin?},
	journal = {Annals of Biomedical Engineering},
	volume = {43},
	year = {2015},
	pages = {1422-1431},
	publisher = {Springer},
	author = {Valen-Sendstad, Kristian and Piccinelli, Marina and Krishnankuttyrema, Resmi and David A. {Steinman}}
}
@inproceedings {23982,
	title = {Flow Instabilities in Volume-Matched Sidewall ICA Aneurysms: A Possible Association with Rupture Status?},
	journal = {Congress of Neurological Surgeons},
	year = {2015},
	month = {09/2015},
	author = {Valen-Sendstad, Kristian and Lauric, Alexandra and David A. {Steinman} and Adel M. {Malek}}
}
@inproceedings {23975,
	title = {Impact of Non-Newtonian Rheology in Transition to Turbulence in Artery Models.},
	journal = {4th International Conference on Computational and Mathematical Biomedical Engineering},
	year = {2015},
	month = {06/2015},
	author = {David A. {Steinman} and Owais Mohammad {Khan} and Valen-Sendstad, Kristian and Biswas, D. and Casey, D. and Loth, F.}
}
@inproceedings {23968,
	title = {Inlet Flow Rate Variation and Onset of Flow Instabilities in the Carotid Siphon},
	journal = {The Summer Biomechanics, Bioengineering \& Biotransport Conference},
	year = {2015},
	month = {06/2015},
	author = {Krishnankuttyrema, Resmi and Valen-Sendstad, Kristian and David A. {Steinman}}
}
@article {23444,
	title = {Narrowing the Expertise Gap for Predicting Intracranial Aneurysm Hemodynamics: Impact of Solver Numerics versus Mesh and Time-Step Resolution},
	journal = {American Journal of Neuroradiology},
	volume = {36},
	number = {7},
	year = {2015},
	publisher = {American Society of Neuroradiology},
	abstract = {BACKGROUND AND PURPOSE:\&nbsp;Recent high-resolution computational fluid dynamics studies have uncovered the presence of laminar flow instabilities and possible transitional or turbulent flow in some intracranial aneurysms. The purpose of this study was to elucidate requirements for computational fluid dynamics to detect these complex flows, and, in particular, to discriminate the impact of solver numerics versus mesh and time-step resolution.MATERIALS AND METHODS:\&nbsp;We focused on 3 MCA aneurysms, exemplifying highly unstable, mildly unstable, or stable flow phenotypes, respectively. For each, the number of mesh elements was varied by 320\&times; and the number of time-steps by 25\&times;. Computational fluid dynamics simulations were performed by using an optimized second-order, minimally dissipative solver, and a more typical first-order, stabilized solver.RESULTS:\&nbsp;With the optimized solver and settings, qualitative differences in flow and wall shear stress patterns were negligible for models down to \&sim;800,000 tetrahedra and \&sim;5000 time-steps per cardiac cycle and could be solved within clinically acceptable timeframes. At the same model resolutions, however, the stabilized solver had poorer accuracy and completely suppressed flow instabilities for the 2 unstable flow cases. These findings were verified by using the popular commercial computational fluid dynamics solver, Fluent.CONCLUSIONS:\&nbsp;Solver numerics must be considered at least as important as mesh and time-step resolution in determining the quality of aneurysm computational fluid dynamics simulations. Proper computational fluid dynamics verification studies, and not just superficial grid refinements, are therefore required to avoid overlooking potentially clinically and biologically relevant flow features.},
	author = {Owais Mohammad {Khan} and Valen-Sendstad, Kristian and David A. {Steinman}}
}
@misc {Simula.simula.2588,
	title = {CFD Modelling of Cerebrovascular Hemodynamics: Breaking News},
	howpublished = {9th international symposium on Biomechanics in Vascular Biology and Cardiovascular Disease, Montreal, Canada, April, 2014},
	year = {2014},
	month = {04/2014},
	keywords = {Conference},
	author = {Valen-Sendstad, Kristian and Owais Mohammad {Khan} and Piccinelli, Marina and David A. {Steinman}}
}
@misc {Simula.simula.2592,
	title = {Flow Instabilities in Sidewall Aneurysms: a Possible Association With Rupture Status?},
	howpublished = {World Congress of Biomechanics},
	year = {2014},
	author = {Valen-Sendstad, Kristian and Lauric, Alexandra and Malek, Adel and David A. {Steinman}}
}
@article {Simula.simula.2594,
	title = {High-Resolution CFD Detects Flow Instabilities in the Carotid Siphon: Implications for Aneurysm Initiation and Rupture?},
	journal = {Journal of Biomechanics},
	number = {47},
	year = {2014},
	pages = {3210-3216},
	publisher = {Elsevier},
	author = {Valen-Sendstad, Kristian and Piccinelli, Marina and David A. {Steinman}}
}
@inproceedings {Simula.simula.2896,
	title = {Illustration-Inspired Visualization of Blood Flow Dynamics},
	journal = {World Congress of Biomechanics Proceedings},
	year = {2014},
	publisher = {JBioMech},
	type = {Conference},
	abstract = {Image-based computational fluid dynamics (CFD) has emerged as a central tool in the evaluation of hemodynamic factors in cardiovascular disease development and treatment planning, to the point where major vendors are now seeking to deploy CFD solvers on their medical imaging platforms. Detailed hemodynamic data available from CFD reveal complicated flows that are difficult to render clearly - and thus communicate to clinical stakeholders - using conventional engineering flow visualization techniques. This is especially challenging considering the four-dimensional nature of the flow patterns (i.e., varying in space and time), as well as the clinical need for generating static reports rather than cumbersome digital animations. Taking a cue from the rich history of biomedical illustration, the goal of our multi-disciplinary collaboration between the University of Toronto and the Ontario College of Art and Design University is to develop new data-driven paradigms for visualizing blood flow, based on the principles of illustration, sequential art and caricature, and informed by the visual vocabularies and conventions of radiology and vascular surgery. Our first investigations have involved the depiction of transitional flows in cerebral aneurysms. The top row of the included figure shows selected frames from an animation of vortex cores (i.e., positive and negative Q-criterion), created using the open-source scientific visualization package, Paraview. Note how difficult it is to infer the dynamics from these static images. For the bottom row, vortex cores were outlined in black to better discriminate them from each other. Surface shading was enhanced, and colouring was used to highlight selected cores, the dynamic motion of which can be followed easily across the individual frames. The original translucent lumen surface rendering was also replaced by a simple outline drawing that nevertheless retains, and perhaps even heightens, the impression of a 3D aneurysm. Although this example was composed by a professional biomedical illustrator (JH) using graphics editing tools applied to the individual static frames, we are presently working towards automating these steps in collaboration with Toronto-based Side Effects Software, developers of the Houdini 3D Animation suite widely used in the motion picture industry.},
	keywords = {Conference},
	author = {Coppin, Peter and Harvey, John and Valen-Sendstad, Kristian and David A. {Steinman}},
	editor = {Jay D. {Humphrey}}
}
@inproceedings {Simula.simula.2892,
	title = {Inlet Flow Rates for Image-Based Aneurysm CFD Models: Where (and How) to Begin?},
	journal = {World Congress of Biomechanics Proceedings},
	year = {2014},
	publisher = {JBioMech},
	type = {Conference},
	author = {Valen-Sendstad, Kristian and Piccinelli, Marina and Krishnankuttyrema, Resmi and David A. {Steinman}},
	editor = {Jay D. {Humphrey}}
}
@misc {23407,
	title = {It{\textquoteright}s not the size, but what you do with it: CFD solver settings trump model resolution.},
	howpublished = {Zurich},
	year = {2014},
	publisher = {Interdisciplinary Cerebrovascular Symposium / Intracranial Stent Meeting},
	author = {Valen-Sendstad, Kristian and Owais Mohammad {Khan} and Piccinelli, Marina and David A. {Steinman}}
}
@article {Simula.simula.1978,
	title = {Mind the Gap: Impact of Computational Fluid Dynamics Solution Strategy on Prediction of Intracranial Aneurysm Hemodynamics and Rupture Status Indicators},
	journal = {American Journal of Neuroradiology},
	volume = {35},
	number = {3},
	year = {2014},
	note = {Commentary:AJNR, 2014 Mar;35(3):544-5},
	month = {March},
	pages = {536-543},
	publisher = {American Journal of Neuroradiology},
	author = {Valen-Sendstad, Kristian and David A. {Steinman}}
}
@inproceedings {Simula.simula.2894,
	title = {A Novel Framework for Classifying Wall Shear Stress Phenotypes in Arterial Disturbed Blood Flow},
	journal = {World Congress of Biomechanics Proceedings},
	year = {2014},
	publisher = {JBioMech},
	type = {Conference},
	abstract = {Background and Purpose: Recent high-resolution (HR) computational fluid dynamics (CFD) simulations have revealed that flow instabilities with high-frequency fluctuations can occur at relatively low Reynolds numbers (Valen-Sendstad and Steinman, 2014). We have assumed that the highly disturbed wall shear stress (WSS) caused by such flow instabilities impact the (dys)functional nature of arterial endothelial cells, believed to initiate arterial diseases. Current indices designed to quantify the disturbed WSS, such as oscillatory shear index (OSI) and transverse wall shear stress (transWSS), however, sometimes fail to distinguish different types of highly disturbed WSS. We propose a method for quantifying and distinguishing the WSS disturbances adequately and demonstrate its capability. Materials and Methods: We used an anatomically realistic carotid siphon derived from the open-source Aneurisk data-set. The aneurysm was removed with previously developed and verified tools. Pulsatile flow was simulated using 30,000 time-steps per cycle and the equivalent of 24M linear tetrahedrons. Changes in instantaneous WSS were decomposed into magnitude and angle variations, respectively, with the angle defined relative to the cycle-averaged WSS vector direction. The proposed classification of WSS phenotypes therefore consists of a permutation (from low to high values) of WSS, WSS magnitude change, and WSS angle change; a total of 9. Results: The attached figure shows the distributions of OSI, transWSS, and the currently proposed phenotypes. Despite highly different stimuli at the wall (shown in the polar plots, where the WSS magnitude is plotted on a common logarithmic scale), both OSI and transWSS calculations return the same numerical value for large regions. E.g., the WSS phenotype in polar plots OSI=.211},
	author = {Shimogonya, Yuji and Valen-Sendstad, Kristian and David A. {Steinman}},
	editor = {Jay D. {Humphrey}}
}
@inproceedings {Simula.simula.2890,
	title = {Numerical Resolution Requirements for Converging Wall Shear Stresses in Intracranial Aneurysms},
	journal = {World Congress of Biomechanics Proceedings},
	year = {2014},
	publisher = {JBioMech},
	type = {Conference},
	abstract = {Abstract: Background: CFD can simulate the hemodynamics forces like wall shear stress (WSS) that are known to play an important role in rupture of intracranial aneurysms. Every stone has been turned striving for more {{\textquoteleft}}patient-specific\&$\#$39; simulations, except for numerical accuracy itself. The purpose of this study is to quantify such errors, particularly for WSS. Methods: Five refinement levels were simulated for three previously reported cases based on flow phenotype (Valen-Sendstad and Steinman, 2014): stable-sidewall (case 8), quasi-unstable- bifurcation (case 9), and unstable-bifurcation (case 16). Pulsatile simulations were performed using second-order-accurate P2-P1 (Taylor-Hood) tetrahedral elements. Finest meshes had a sac resolution of 0.055mm, 0.061mm, and 0.10mm for cases 8, 9 and 16, respectively, resulting in 4-million elements (\~32M linear tetrahedra). These finest meshes were successively coarsened by a factor of sqrt(2) with respect to side length, resulting in coarsest meshes having \~100,000 elements (\~800,000 linear tetrahedra). All meshes had four boundary layers. We used 35,000 time- steps/cycle and second-order time-stepping scheme for three reasons: 1) to avoid adding artificial numerical viscosity to stabilize the solution, 2) to eliminate temporal discretization errors, and 3) to isolate and quantify spatial discretization errors alone. Results: In contrast to conventional mesh refinement studies, we show a 3-fold decrease in effective node spacing, not simply mesh size. As a consequence, mesh sizes grew cubically. In the figure, we show normalized L2 errors for domain-averaged velocity and dome-averaged WSS at peak systole relative to the finest mesh. Although the velocity errors, a standard metric for monitoring grid convergence, appear to be within 2\% error for the second-finest mesh (equivalent to \~13M linear tetrahedra), the WSS errors are an order of magnitude away from convergence. Having said this, patterns of WSS were qualitatively similar. Conclusions: High spatial resolution is required to converge WSS, even when velocity appears to be well-converged: on average, an effective mesh resolution of \~0.05mm is needed to converge WSS below \~10\%. Work to be presented will also include temporal refinement study with high spatial resolution to clarify the requirements on temporal resolution, as well as the impact on popular scalar hemodynamic indices.},
	author = {Owais Mohammad {Khan} and Valen-Sendstad, Kristian and David A. {Steinman}},
	editor = {Jay D. {Humphrey}}
}
@misc {Simula.simula.2590,
	title = {Unstable Flow in the Carotid Siphon: Implications for Aneurysm Initiation and Rupture},
	howpublished = {World Congress of Biomechanics},
	year = {2014},
	keywords = {Conference},
	author = {Valen-Sendstad, Kristian and Piccinelli, Marina and David A. {Steinman}}
}
@misc {Simula.simula.2254,
	title = {{{\textquoteleft}}Freezing Time{\textquoteright} to Show 4D Evolution of Transitional Vortex Cores in an Aneurysm That Is Undergoing Highly Unstable Flow},
	year = {2013},
	author = {Coppin, Peter and Harvey, John and Valen-Sendstad, Kristian and David A. {Steinman}}
}
@article {Simula.simula.1698,
	title = {High-Resolution Computational Fluid Dynamics Detects High-Frequency Velocity Fluctuations in Bifurcation, But Not Sidewall, Aneurysms of the Middle Cerebral Artery},
	journal = {Journal of Biomechanics},
	volume = {46},
	year = {2013},
	pages = {402-407},
	publisher = {Elsevier},
	abstract = {High-frequency flow fluctuations in intracranial aneurysms have previously been reported in vitro and in vivo. On the other hand, the vast majority of image-based computational fluid dynamics (CFD) studies of cerebral aneurysms report periodic, laminar flow. We have previously demonstrated that transitional flow, consistent with in vivo reports, can occur in a middle cerebral artery (MCA) bifurcation aneurysm when ultra-high-resolution direct numerical simulation methods are applied. The object of the present study was to investigate if such high-frequency flow fluctuations might be more widespread in adequately-resolved CFD models. A sample of N=12 anatomically realistic MCA aneurysms (five unruptured, seven ruptured), was digitally segmented from CT angiograms. Four were classified as sidewall aneurysms, the other eight as bifurcation aneurysms. Transient CFD simulations were carried out assuming a steady inflow velocity of 0.5 m/s, corresponding to typical peak systolic conditions at the MCA. To allow for detection of clinically-reported high-frequency flow fluctuations and resulting flow structures, temporal and spatial resolutions of the CFD simulations were in the order of 0.1 ms and 0.1 mm, respectively. A transient flow response to the stationary inflow conditions was found in five of the 12 aneurysms, with energetic fluctuations up to 100 Hz, and in one case up to 900 Hz. Incidentally, all five were ruptured bifurcation aneurysms, whereas all four sidewall aneurysms, including one ruptured case, quickly reached a stable, steady state solution. Energetic, rapid fluctuations may be overlooked in CFD models of bifurcation aneurysms unless adequate temporal and spatial resolutions are used. Such fluctuations may be relevant to the mechanobiology of aneurysm rupture, and to a recently reported dichotomy between predictors of rupture likelihood for bifurcation vs. sidewall aneurysms.},
	author = {Valen-Sendstad, Kristian and Mardal, Kent-Andre and David A. {Steinman}}
}
@inproceedings {Simula.simula.1980,
	title = {Impact of CFD Solution Strategy on Predicted Aneurysm Hemodynamics: Good News and Bad News},
	journal = {Proceedings of 8th international symposium on Biomechanics in Vascular Biology and Cardiovascular Disease},
	year = {2013},
	publisher = {Proceedings},
	type = {Conference},
	abstract = {Last year at this meeting we demonstrated that high-resolution (HR) CFD simulations could detect a highly transient response to steady inflow conditions in 5 of 12 MCA aneurysm cases [1]. We also showed preliminary results suggesting that normal resolution (NR) simulations, representative of those from the clinical literature (e.g, [2,3]), might be misleading about the nature of aneurysm flow patterns and wall shear stress (WSS), and hence derived predictors of rupture risk. We now test this hypothesis in a controlled manner, by comparing NR vs. HR simulations of physiologically pulsatile flow in the 12 MCA aneurysm cases from [1].},
	author = {Valen-Sendstad, Kristian and David A. {Steinman}},
	editor = {Gijsen et {al.}}
}
@misc {Simula.simula.1982,
	title = {Impact of CFD Solution Strategy on Predicted Aneurysm Hemodynamics: Good News and Bad News},
	howpublished = {8th International Symposium on Biomechanics in Vascular Biology and Cardiovascular Disease, Rotterdam},
	year = {2013},
	month = {April},
	keywords = {Conference},
	author = {Valen-Sendstad, Kristian and David A. {Steinman}}
}
@article {Simula.simula.1746,
	title = {Variability of Computational Fluid Dynamics Solutions for Pressure and Flow in a Giant Aneurysm: the ASME 2012 Summer Bioengineering Conference CFD Challenge},
	journal = {Journal of Biomedical Engineering},
	volume = {135},
	number = {2},
	year = {2013},
	publisher = {J Biomech Eng},
	author = {David A. {Steinman} and Hoi, Yiemeng and Fahy, P. and Morris, L. and Walsh, MT and Aristokleous, N. and Anayiotos, AS and Papaharilaou, Y. and Arzani, A. and Shadden, SC and Berg, Philipp and Janiga, G. and Bols, Joris and Segers, Patrick and Neil W. {Bressloff} and Cibis, M. and Gijsen, FH and Cito, S. and Pallareacutes, J. and Joan Heller {Brown} and Costelloe, JA and Lynch, AG and Degroote, Joris and Vierendeels, Jan and Fu, Wenyu and Qiao, Aike and Hodis, S. and Kallmes, DF and Kalsi, H. and Long, Quan and Kheyfets, VO and Fino, EA and Kono, K. and Malek, AM and Lauric, Alexandra and Prahlad G. {Menon} and Pekkan, K. and Moghadam, Esmaily and Marsden, AL and Oshima, M. and Katagiri, K. and Peiffer, V. and Mohamied, Y. and Sherwin, SJ and Schaller, J. and Goubergrits, L. and Usera, G. and Mendina, M. and Habets, DF and Valen-Sendstad, Kristian and Xiang, Jianping and Meng, Hui and Karniadakis, GE and Shaffer, N. and Loth, F.}
}
@inproceedings {Simula.simula.1292,
	title = {CFD CHALLENGE: SOLUTIONS USING an IN-HOUSE FINITE ELEMENT METHOD FLOW SOLVER IMPLEMENTED in FEniCS},
	journal = {ASME 2012 Summer Bioengineering Conference, Fajardo, Puerto Rico},
	year = {2012},
	type = {Conference},
	abstract = {FEniCS is a finite element library for {{\textquoteleft}}automated, efficient solution of differential equations\&$\#$39; (www.fenicsproject.org). The focus has been to develop high level software interface that allows the user to type in the equations nearly as they are read. Recent development has involved extensions for parallell computing and building efficient pipeline for patient specific geometries based on Vascular Modelling Toolkit (VMTK, www.vmtk.org) meshes. The aim is therefore to evaluate the accuracy and efficiency of one of the developed computational fluid mechanics (CFD) solvers. The simulations were perfored according to the given instructions and information is not repeated. This abstract is therefore not complete and must be seen in the light of the Challenge.},
	author = {Valen-Sendstad, Kristian and David A. {Steinman}}
}
@misc {Simula.simula.1286,
	title = {Is CFD Misleading Us About the Nature of Wall Shear Stresses in Cerebral Aneurysms?},
	howpublished = {The 7th International Symposium on Biomechanics in Vascular Biology \& Cardiovascular Disease, Atlanta, Georgia, USA},
	year = {2012},
	abstract = {Computational fluid dynamics (CFD) simulations are increasingly being used to model cerebral aneurysm hemodynamics and wall shear stress (WSS) in order to infer aneurysm wall strength and therefore risk of rupture. Numerous recent computational studies have been successful in retrospectively classifying aneurysms according to their rupture status, but the link between the proposed hemodynamic indices and the precise mechanisms leading to rupture remains unclear. One presumption common to virtually all of these CFD studies is that blood flow inside an aneurysm is laminar and stable, even though it is well known that many aneurysms produce sounds or bruits with energy peaks at hundreds of Hz. Clinicians have reported this invasively and non-invasively, and turbulence has been shown to occur in intracranial aneurysms in several experiments. These phenomena seem have raised little interest in the computational biomechanics community, and there is therefore a gap between the clinically reported frequencies and the resolution at which the simulations are performed. We have previously shown, using direct numerical simulation of pulsatile flow in two patient-specific middle cerebral artery (MCA) aneurysms, that transitional flow was present, leading to strong wall shear stress gradients. More recently, we have demonstrated the presence of unstable flow patterns in 5 of 12 MCA aneurysm cases simulated under high resolution, transient steady flow conditions. These findings would suggest that in many cases the aneurysmal wall may be subject to more violent forces than have been predicted by the vast majority of CFD model studies. To illustrate this, we carried out transient steady flow simulations for the two MCA cases we had previously simulated with DNS, using a first order accurate solver with roughly 600,000 linear tetrahedral and boundary layer elements and 10 msec temporal resolution, and a second order solver with the same number of elements (i.e. more than double the spatial resolution) and 0.03 msec temporal resolution. The latter simulations exhibited strong velocity and WSS fluctuations, as anticipated from the previous DNS studies, whereas the first order simulations stabilized quickly. These preliminary head-to-head comparisons are currently being extended to more cases, and under pulsatile flow conditions. In summary, the presumption of laminar, stable flow underlying most CFD models of cerebral aneurysms may be a self-fulfilling prophecy that leads to a misinterpretation of the biomechanical forces at play in aneurysm progression and rupture.},
	author = {Valen-Sendstad, Kristian and David A. {Steinman}}
}
@inproceedings {Simula.simula.1290,
	title = {Is CFD Misleading Us About the Nature of Wall Shear Stresses in Cerebral Aneurysms?},
	journal = {The 7th International Symposium on Biomechanics in Vascular Biology \& Cardiovascular Disease, Atlanta, Georgia, USA},
	year = {2012},
	publisher = {Internal},
	type = {Conference},
	abstract = {Computational fluid dynamics (CFD) simulations are increasingly being used to model cerebral aneurysm hemodynamics and wall shear stress (WSS) in order to infer aneurysm wall strength and therefore risk of rupture. Numerous recent computational studies have been successful in retrospectively classifying aneurysms according to their rupture status, but the link between the proposed hemodynamic indices and the precise mechanisms leading to rupture remains unclear. One presumption common to virtually all of these CFD studies is that blood flow inside an aneurysm is laminar and stable, even though it is well known that many aneurysms produce sounds or bruits with energy peaks at hundreds of Hz. Clinicians have reported this invasively and non-invasively, and turbulence has been shown to occur in intracranial aneurysms in several experiments. These phenomena seem have raised little interest in the computational biomechanics community, and there is therefore a gap between the clinically reported frequencies and the resolution at which the simulations are performed. We have previously shown, using direct numerical simulation of pulsatile flow in two patient-specific middle cerebral artery (MCA) aneurysms, that transitional flow was present, leading to strong wall shear stress gradients. More recently, we have demonstrated the presence of unstable flow patterns in 5 of 12 MCA aneurysm cases simulated under high resolution, transient steady flow conditions. These findings would suggest that in many cases the aneurysmal wall may be subject to more violent forces than have been predicted by the vast majority of CFD model studies. To illustrate this, we carried out transient steady flow simulations for the two MCA cases we had previously simulated with DNS, using a first order accurate solver with roughly 600,000 linear tetrahedral and boundary layer elements and 10 msec temporal resolution, and a second order solver with the same number of elements (i.e. more than double the spatial resolution) and 0.03 msec temporal resolution. The latter simulations exhibited strong velocity and WSS fluctuations, as anticipated from the previous DNS studies, whereas the first order simulations stabilized quickly. These preliminary head-to-head comparisons are currently being extended to more cases, and under pulsatile flow conditions. In summary, the presumption of laminar, stable flow underlying most CFD models of cerebral aneurysms may be a self-fulfilling prophecy that leads to a misinterpretation of the biomechanical forces at play in aneurysm progression and rupture.},
	author = {Valen-Sendstad, Kristian and David A. {Steinman}},
	editor = {Oshinski, John}
}
@misc {Simula.simula.1288,
	title = {Revisiting {\textquoteright}Turbulence{\textquoteright} in Cerebral Aneurysms},
	year = {2012},
	abstract = {One of the commonly made assumptions in Computational Fluid Dynamics (CFD) is that cerebral blood flow is laminar, i.e., viscous forces are dominating and the flow is smooth. The numerical methods that are used, are chosen or tailored accordingly, i.e., to converge to such a laminar flow solution with the minimal amount of work1-2. The flow is modeled with a temporal resolution far below the clinically reported frequencies3 and thus any potential flow disturbances, if physically present, are being numerically suppressed. Since no potential disturbances are resolved, no disturbances are observed and the assumption of laminar flow is therefore a self-fulfilling prophecy. Even though recent laminar CFD studies can discriminate between ruptured and unruptured aneurysms better than, e.g. aneurysm size, the link between the proposed hemodynamic agonists and the mechanisms of rupture is unclear. However, the literature contains much evidence which supports that certain aneurysms exhibit energetic high frequency flow fluctuations. For example, high frequency flow fluctuations are known to produce sound, and clinicians have reported such sound or {\textquoteright}bruits{\textquoteright} from aneurysms during craniotomy.3 The predominant frequencies were at 460 Hz, which is consistent with the energy peaks at higher frequencies recorded acoustically on the eyes in patients with aneurysms.4 Unstable flow has also been reported in glass model studies of aneurysms.5 This evidence seems to have been ignored, or at least has not received much attention in computational modeling studies. The goal of the present study was to investigate if such high-frequency flow fluctuations in intracranial aneurysms might be a common occurrence, by using thousands of time steps in contrast to, e.g., one hundred2.},
	keywords = {Conference},
	author = {Valen-Sendstad, Kristian and Mardal, Kent-Andre and David A. {Steinman}}
}
@misc {Simula.simula.846,
	title = {{\textquoteright}Turbulence{\textquoteright} in Cerebral Aneurysms},
	howpublished = {CBC Workshop on Biomechanics, 17-18 November},
	year = {2011},
	author = {Valen-Sendstad, Kristian and Mardal, Kent-Andre and Mortensen, Mikael and Hans Petter {Langtangen} and Bj{\o}rn Anders Petterson {Reif} and David A. {Steinman}}
}
