Robustness of common hemodynamic indicators with respect to numerical resolution in 38 middle cerebral artery aneurysms

Background: Using computational fluid dynamics (CFD) to compute the hemodynamics in cerebral aneurysms has received much attention in the last decade. The usability of these methods depends on the quality of the computations, highlighted in recent discussions. The purpose of this study is to investigate the convergence of common hemodynamic indicators with respect to numerical resolution. Methods: 38 middle cerebral artery bifurcation aneurysms were studied at two different resolutions (one comparable to most studies, and one finer). Relevant hemodynamic indicators were collected from two of the most cited studies, and were compared at the two refinements. In addition, correlation to rupture was investigated. Results: Most of the hemodynamic indicators were very well resolved at the coarser resolutions, correlating with the finest resolution with a correlation coefficient >0.95. The oscillatory shear index (OSI) had the lowest correlation coefficient of 0.83. A logarithmic Bland-Altman plot revealed noticeable variations in the proportion of the aneurysm under low shear, as well as in spatial and temporal gradients not captured by the correlation alone. Conclusion: Statistically, hemodynamic indicators agree well across the different resolutions studied here. However, there are clear outliers visible in several of the hemodynamic indicators, which suggests that special care should be taken when considering individual assessment

Sensitivity Analysis of Simulated Blood Flow in Cerebral Aneurysms

Bakgrunn: En cerebral aneurysme er en dilatasjon av en blodåre, som kan utvide seg og briste, og derved forårsake blødninger i og rundt hjernen. En slik tilstand er alvorlig og medfører ofte alvorlig hjerneskade eller død. For å bedre forstå hvorfor enkelte aneurysmer utvikler seg og brister, gjør man datasimuleringer av blodstrømningen i disse aneurysmene for å få en økt forståelse, og på sikt bidra til diagnoser og behandlingsmetoder. Slike simuleringer fører med seg mange usikkerhetsmomenter og kilder til feil. I denne oppgaven er feilkilder knyttet til usikkerhet rundt geometri, blodmodellering og grensebetingelser undersøkt. Metode: En eksisterende Navier-Stokes løser (skrevet i Python og bygget på FEniCS) ble modifisert til å ta inn ikke-Newtonske modeller for blod og beregne veggskjærstress (VSS). Denne løseren ble deretter verifisert ved å sjekke konvergens mot en eksakt løsning på et enkelt problem. En pasientspesifikk geometri ble studert i en kvalitativ analyse: effekter av geometri, ikke-Newtonske effekter og effektene av en endring i hematokritt-nivået ble undersøkt. Deretter ble en kvantitativ analyse gjennomført på 12 forskjellige pasientspesifikke aneurysmer: ikke-Newtonske effekter, effekten av økt hematokrittnivå, effekt av økt innstrømsfluks og effekten av å endre grensebetingelser på utløp ble undersøkt. Resultater: Ved bruk av enkle statistiske metoder fant vi noen signifikante endringer (P<0.05). Ved å anta blod til å oppføre seg som en Newtonsk væske overestimeres maks VSS (gj.sn. 2.305%). Ved å øke hematokrittnivået fra 38% til 40% øker gjennomsnittlig VSS i aneurysmen (gj.sn. 1.555%). Ved å øke innstrømsfluksen med 33% øker VSS betydelig mer (maks VSS: 72.766%, gjennomsnittlig VSS: 73.565%). Det vises stor varians ved å endre grensebetingelser på utløp fra en resistanse- til en nulltrykks-betingelse. Konklusjon: For å nøyaktig simulere blodstrømning i cerebrale aneurysmer, synes det absolutt nødvendig å bruke nøyaktige, pasientspesifikke grensebetingelser. De ikke-Newtonske effektene av blod kan neglisjeres dersom disse grensebetingelsene ikke er tilgjengelige. En økning i hematokrittnivået kan være med å forklare utviklingen av aneurysmer

Variational data assimilation for transient blood flow simulations - Cerebral aneurysms as an illustrative example

Several cardiovascular diseases are caused from localised abnormal blood flow such as in the case of stenosis or aneurysms. Prevailing theories propose that the development is caused by abnormal wall shear stress in focused areas. Computational fluid mechanics have arisen as a promising tool for a more precise and quantitative analysis, in particular because the anatomy is often readily available even by standard imaging techniques such as magnetic resonance and computed tomography angiography. However, computational fluid mechanics rely on accurate initial and boundary conditions, which are difficult to obtain. In this paper, we address the problem of recovering high‐resolution information from noisy and low‐resolution physical measurements of blood flow (for example, from phase‐contrast magnetic resonance imaging [PC‐MRI]) using variational data assimilation based on a transient Navier‐Stokes model. Numerical experiments are performed in both 3D (2D space and time) and 4D (3D space and time) and with pulsatile flow relevant for physiological flow in cerebral aneurysms. The results demonstrate that, with suitable regularisation, the model accurately reconstructs flow, even in the presence of significant noise

Correlation coefficient between all hemodynamic and morphological indicators.

<p>Correlation coefficient between all hemodynamic and morphological indicators.</p

The figure illustrates the differences between the coarsest resolution on the left, and the finest resolution on the right.

<p>The clip is done just upstream from the aneurysm of model 1. The black dots represent points at which the velocity is computed.</p

Flow profile used in the simulations.

<p>Flow profile used in the simulations.</p

Variability in selected indicators, representative of worst case.

<p>Left figures represent the coarse resolution, right figures represent the fine. (A) Shows the WSS fields at different refinements of case 21. (Coarse/fine: AWSS = 9.7/8.9 Pa, MWSS = 43.7/51.8 Pa.) (B) OSI fields at different refinements for case 48 (coarse: 0.028, fine: 0.012). (C) LSA fields (|<i>τ</i>| < 0.1 <i>in red</i>) at different refinements of case 9 (coarse: 0.033, fine: 0.076). (D) ICI fields at different refinements for case 24 at t = 0.2 (coarse: 1.20, fine: 1.14).</p

Comparison between the ruptured and unruptured aneurysms for all indicators at both resolutions.

<p>Comparison between the ruptured and unruptured aneurysms for all indicators at both resolutions.</p

Definition of computed hemodynamic indicators added for completion.

<p>Definition of computed hemodynamic indicators added for completion.</p