@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}}
}
@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}}
}
