Kevin Steinman

CIDREE: Norway's 2024 Generative AI Journey in Schools and Assessments: Responding to a Call for Guidance on Newer Digital Technologies in Norway's Secondary Schools https://www.cidree.org/wp-content/uploads/2024/11/cidree_yearbook-2024.pdf 

Indigenous Poetry and Sustainability: Troubling Anthropocene Logic Through Kinship and Holistic Care, in Poetry and Sustainability in Education, Kleppe, S. L, & Sorby, A., eds. (2022)

• Bergersen, Aslak; Chnafa, Christophe; Steinman, David & Valen-Sendstad, Kristian (2019). Automated and Objective Removal of Bifurcation Aneurysms: Incremental Improvements, and Validation Against Healthy Controls. Journal of Biomechanics. ISSN 0021-9290. 96, p. 109342–109342. doi: 10.1016/j.jbiomech.2019.109342. Full text in Research Archive
• Berg, Philipp; Bergersen, Aslak; Valen-Sendstad, Kristian; Steinman, David; Pereira, Vitor & Chiu, Tin [Show all 9 contributors for this article] (2019). Multiple Aneurysms AnaTomy CHallenge 2018 (MATCH) - Phase II: Rupture Risk Assessment. International Journal of Computer Assisted Radiology and Surgery. ISSN 1861-6410. 14, p. 1795–1804. doi: 10.1007/s11548-019-01986-2. Full text in Research Archive
• Berg, Philipp; Bergersen, Aslak; Valen-Sendstad, Kristian; Steinman, David; Pereira, Vitor & Chiu, Tin [Show all 9 contributors for this article] (2018). Multiple Aneurysms AnaTomy CHallenge 2018 (MATCH): Phase I: Segmentation. Cardiovascular Engineering and Technology. ISSN 1869-408X. 9(4). doi: 10.1007/s13239-018-00376-0. Full text in Research Archive
• Khan, Owais; Valen-Sendstad, Kristian & Steinman, David (2017). Cerebral aneurysm blood flow simulations: There’s solver settings and then there’s solver settings. Journal of Biomechanics. ISSN 0021-9290. 61, p. 280–280. doi: 10.1016/j.jbiomech.2017.04.039. Full text in Research Archive
• Khan, Owais; Chnafa, Christophe; Steinman, David & Valen-Sendstad, Kristian (2016). On the Quantification and Visualization of Transient Periodic Instabilities in Pulsatile Flows. Journal of Biomechanics. ISSN 0021-9290. p. 179–182. Full text in Research Archive
• Chnafa, Christophe; Valen-Sendstad, Kristian; Pereira, Vitor & Steinman, David (2016). Improved Reduced-Order Modelling of Cerebrovascular Flow Distribution by Accounting for Arterial Bifurcation Pressure Drops. Journal of Biomechanics. ISSN 0021-9290. 51, p. 83–88. Full text in Research Archive
• Khan, Owais; Steinman, David & Valen-Sendstad, Kristian (2016). Non-Newtonian versus Numerical Rheology: Practical Impact Of Shear-thinning On The Prediction Of Stable And Unstable Flows In Intracranial Aneurysms. International Journal for Numerical Methods in Biomedical Engineering. ISSN 2040-7939. 33. Full text in Research Archive
• Berg, Philipp; Khan, Owais; Valen-Sendstad, Kristian & Steinman, David (2015). The Computational Fluid Dynamics Rupture Challenge 2013—Phase II: variability of hemodynamic simulations in two intracranial aneurysms. Journal of Biomechanical Engineering. ISSN 0148-0731. 137(12). Full text in Research Archive
• Khan, Owais; Valen-Sendstad, Kristian & Steinman, David (2015). Narrowing the Expertise Gap for Predicting Intracranial Aneurysm Hemodynamics: Impact of Solver Numerics versus Mesh and Time-Step Resolution. American Journal of Neuroradiology. ISSN 0195-6108. 36. Full text in Research Archive Show summary

  BACKGROUND AND PURPOSE: 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: 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× and the number of time-steps by 25×. Computational fluid dynamics simulations were performed by using an optimized second-order, minimally dissipative solver, and a more typical first-order, stabilized solver.RESULTS: With the optimized solver and settings, qualitative differences in flow and wall shear stress patterns were negligible for models down to ∼800,000 tetrahedra and ∼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: 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.

• Valen-Sendstad, Kristian; Piccinelli, Marina & Steinman, David (2015). Estimation of Inlet Flow Rates for Image-Based Aneurysm CFD Models: Where and How to Begin? Annals of Biomedical Engineering. ISSN 0090-6964. 43(6), p. 1422–1431. Full text in Research Archive
• Valen-Sendstad, Kristian & Steinman, David (2014). Mind the Gap: Impact of Computational Fluid Dynamics Solution Strategy on Prediction of Intracranial Aneurysm Hemodynamics and Rupture Status Indicators. American Journal of Neuroradiology. ISSN 0195-6108. 35, p. 536–543. Full text in Research Archive
• Valen-Sendstad, Kristian; Piccinelli, Marina & Steinman, David (2014). High-Resolution CFD Detects Flow Instabilities in the Carotid Siphon: Implications for Aneurysm Initiation and Rupture? Journal of Biomechanics. ISSN 0021-9290. p. 3210–3216. Full text in Research Archive
• Steinman, David; Hoi, Yiemeng; Morris, L.; Kono, K.; Valen-Sendstad, Kristian & Loth, F. (2013). Variability of Computational Fluid Dynamics Solutions for Pressure and Flow in a Giant Aneurysm: the ASME 2012 Summer Bioengineering Conference CFD Challenge. American Journal of Biomedical Engineering. ISSN 2163-1050. 135. Full text in Research Archive

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• Burner, Tony; Carlsen, Christian & Steinman, Kevin David (2020). Echo. Texts in English 8-10. Fagbokforlaget. ISBN 9788211034298. 320 p. Full text in Research Archive

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• Khan, Owais; Steinman, David & Valen-Sendstad, Kristian (2016). Non-Newtonian versus Numerical Rheology: Practical Impact Of Shear-thinning On The Prediction Of Stable And Unstable Flows In Intracranial Aneurysms. Full text in Research Archive
• Chnafa, Christophe; Valen-Sendstad, Kristian; Pereira, Vitor & Steinman, David (2016). A Simple and Rational Approach to Outflow Conditions in Cerebrovascular CFD Models. Full text in Research Archive
• Coppin, Peter; Valen-Sendstad, Kristian & Steinman, David (2016). Freezing Turbulence in Brain Aneurysm Flow Visualizations: An Applied Art-Science Collaboration. Full text in Research Archive
• Chnafa, Christophe; Valen-Sendstad, Kristian; Pereira, Vitor & Steinman, David (2016). Impact of Outflow Boundary Conditions on Flow Rates in Cerebrovascular CFD Models. Full text in Research Archive
• Khan, Owais; Chnafa, Christophe; Valen-Sendstad, Kristian & Steinman, David (2016). Proper Orthogonal Decomposition Analyses of High-Frequency Transient Flow Instabilities in Intracranial Aneurysms. Full text in Research Archive
• Valen-Sendstad, Kristian; Lauric, Alexandra & Steinman, David (2015). Flow Instabilities in Volume-Matched Sidewall ICA Aneurysms: A Possible Association with Rupture Status? Full text in Research Archive
• Khan, Owais; Valen-Sendstad, Kristian & Steinman, David (2015). Effects of non-Newtonian rheology on transition to turbulence. Full text in Research Archive
• Steinman, David; Khan, Owais; Valen-Sendstad, Kristian; Biswas, D.; Casey, D. & Loth, F. (2015). Impact of Non-Newtonian Rheology in Transition to Turbulence in Artery Models. Full text in Research Archive
• Biswas, D.; Casey, D.; Valen-Sendstad, Kristian; Steinman, David; Yun, Y. & Loth, F. (2015). Characterization of Transition to Turbulence for Blood in a S-Shaped Pipe Under Steady Flow Conditions. Full text in Research Archive
• Krishnankuttyrema, Resmi; Valen-Sendstad, Kristian & Steinman, David (2015). Inlet Flow Rate Variation and Onset of Flow Instabilities in the Carotid Siphon. Full text in Research Archive
• Khan, Owais; Valen-Sendstad, Kristian; Biswas, D.; Casey, D.; Loth, F. & Steinman, David (2015). CFD Simulation of Transition to Turbulence for Newtonian vs. Non-Newtonian Flow Through a Stenosis. Full text in Research Archive
• Valen-Sendstad, Kristian; Khan, Owais; Piccinelli, Marina & Steinman, David (2014). It's not the size, but what you do with it: CFD solver settings trump model resolution. Full text in Research Archive
• Coppin, Peter; Valen-Sendstad, Kristian & Steinman, David (2014). Illustration-Inspired Visualization of Blood Flow Dynamics. Full text in Research Archive Show summary

  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.

• Shimogonya, Yuji; Valen-Sendstad, Kristian & Steinman, David (2014). A Novel Framework for Classifying Wall Shear Stress Phenotypes in Arterial Disturbed Blood Flow. Full text in Research Archive Show summary

  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

• Valen-Sendstad, Kristian; Piccinelli, Marina & Steinman, David (2014). Inlet Flow Rates for Image-Based Aneurysm CFD Models: Where (and How) to Begin? Full text in Research Archive
• Khan, Owais; Valen-Sendstad, Kristian; Steinman, David & Humphrey, Jay (2014). Numerical Resolution Requirements for Converging Wall Shear Stresses in Intracranial Aneurysms. Full text in Research Archive Show summary

  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 {`}patient-specific' 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.

• Valen-Sendstad, Kristian; Lauric, Alexandra & Steinman, David (2014). Flow Instabilities in Sidewall Aneurysms: a Possible Association With Rupture Status? Full text in Research Archive
• Valen-Sendstad, Kristian; Piccinelli, Marina & Steinman, David (2014). Unstable Flow in the Carotid Siphon: Implications for Aneurysm Initiation and Rupture. Full text in Research Archive
• Valen-Sendstad, Kristian; Khan, Owais; Piccinelli, Marina & Steinman, David (2014). CFD Modelling of Cerebrovascular Hemodynamics: Breaking News. Full text in Research Archive
• Valen-Sendstad, Kristian & Steinman, David (2013). Impact of CFD Solution Strategy on Predicted Aneurysm Hemodynamics: Good News and Bad News. Full text in Research Archive
• Valen-Sendstad, Kristian; Steinman, David & al., Gijsen (2013). Impact of CFD Solution Strategy on Predicted Aneurysm Hemodynamics: Good News and Bad News. Full text in Research Archive Show summary

  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].

• Valen-Sendstad, Kristian; Mardal, Kent-Andre & Steinman, David (2012). Revisiting 'Turbulence' in Cerebral Aneurysms. Full text in Research Archive Show summary

  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 'bruits' 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.

• Valen-Sendstad, Kristian & Steinman, David (2012). Is CFD Misleading Us About the Nature of Wall Shear Stresses in Cerebral Aneurysms? Full text in Research Archive Show summary

  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.

• Valen-Sendstad, Kristian; Steinman, David & Oshinski, John (2012). Is CFD Misleading Us About the Nature of Wall Shear Stresses in Cerebral Aneurysms? Full text in Research Archive Show summary

  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.

• Valen-Sendstad, Kristian & Steinman, David (2012). CFD CHALLENGE: SOLUTIONS USING an IN-HOUSE FINITE ELEMENT METHOD FLOW SOLVER IMPLEMENTED in FEniCS. Full text in Research Archive Show summary

  FEniCS is a finite element library for {`}automated, efficient solution of differential equations' (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.

• Valen-Sendstad, Kristian; Mardal, Kent-Andre; Mortensen, Mikael; Langtangen, Hans; Reif, Bjørn & Steinman, David (2011). 'Turbulence' in Cerebral Aneurysms. Full text in Research Archive

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