Wednesday, December 3, 2014

Minding the Gap

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Apologies for the long absence, but professional and personal commitments keep me distracted from (other) writing. Nevertheless, I wanted to share some very good news about some recent work published in AJNR.

First, people are sitting up and taking notice of our "Mind the Gap" paper.  It has been featured in the December 2014 AJNR Digest, and also a special collection of articles called "Aneurysms and the American Journal of Neuroradiology: The Next 20 Years", which as its eye candy features a figure from our paper (see right).

Congratulations to Kristian Valen-Sendstad, whose tireless (and often, er, turbulent) efforts to promote good practice in aneurysm CFD appear to be paying off.

Second, congratulations to PhD student Owais Khan, whose first journal article, a follow-up to "Mind the Gap", was recently accepted by AJNR. Here's a preview:

Narrowing the Expertise Gap for Predicting Intracranial Aneurysm Hemodynamics: Impact of Solver Numerics vs. Mesh and Time-step Resolution
M. Owais Khan, Kristian Valen-Sendstad, David A. Steinman

Background: Recent high-resolution computational fluid dynamics (CFD) 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 CFD to be able to detect these complex flows, and in particular to discriminate the impact of solver numerics vs. mesh and time-step resolution.
Methods: We focused on three MCA aneurysms, exemplifying highly unstable, mildly unstable, or stable flow phenotypes, respectively. For each, the number of mesh elements was varied by 320x, and the number of time steps by 25x. CFD simulations were carried out 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 two unstable flow cases. These findings were verified using the popular commercial CFD solver, Fluent.
Conclusions:  Solver  numerics must be considered at least as important as mesh and time-step resolution in determining the quality of aneurysm CFD simulations. Proper CFD verification studies, and not just superficial grid refinements, are therefore required in order to avoid overlooking potentially clinically- and biologically-relevant flow features.


Friday, February 21, 2014

(Re)Mind the Gap

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We were pleasantly surprised to find out that BSL postdoc Kristian Valen-Sendstad's paper in press with AJNR, "Mind the gap: Impact of CFD solution strategy on prediction of intracranial aneurysm hemodynamics and rupture status indicators", is the subject of a commentary -- all the more surprising because  AJNR wasn't too keen on the paper when it was first submitted :-)  Way to go Kristian!

Monday, December 30, 2013

Monday, December 23, 2013

Paper Published: Carotid Bifurcation Geometry Is an Independent Predictor of Early Wall Thickening at the Carotid Bulb

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This paper, just published online by Stroke, is the culmination of nearly a decade's worth of effort (e.g., see herehere, here, and here) trying to answer a deceptively simple question: can carotid bifurcation geometry be considered an additional risk factor for atherosclerosis.

As it turns out, the answer is "yes, but".  "Yes", because we did tease out a significant and independent association between early wall thickening and bifurcation flare & curvature geometric parameters; "but", because the association was relatively weak.

An equally interesting aspect of the study was the care we had to take to exclude secondary effects of thickening on geometry.  Had we not done so -- and as we showed -- the associations would have been even weaker or, worse, significantly stronger but inverse to what they should have been.