7 BIOMEKANIKK 7.1 Biomechanics of the heart: Numerical analysis of the influence of the mitral valve on left ventricle hemodynamics This project addresses use of e.g. FLUENT to determine hemodynamics (velocity and pressure) during pulsatile blood flow around the mitral valve. The main result from the simulations is local pressure distributions on the leaflets of the valve during the heart cycle. Type of task: theoretical/numerical Contact: Prof B Skallerud, Prof. L.R. Hellevik, PhD-student S. Dahl, 7.2 Numerical simulation of pressure and flow wave propagation in blood vessel networks Pressure and flow pulses originating in the heart, propagate and are reflected in the arterial tree. The propagation may be estimated by hyperbolic one-dimensional differential equations accounting for mass and momentum transport. The objective in this thesis will be to investigate and implement adequate numerical methods for such hyperbolic differential equations. In particular, the methods for the boundary conditions will be a focus. Further, the topology of the network must be described, and the geometry and material properties of each blood vessel must be accounted for. Type of task: theoretical/numerical Contact: LR Hellevik Collaborators: Frans van de Vosse, Eindhoven University of Technology (http://www.mate.tue.nl/mate/showemp.php/9) HP Langtangen/GK Pedersen (http://simula.no/research/scientific/cbc)
7.3 Modellering av mekanisk interaksjon mellom syke og friske regioner av hjertemuskelen For effektiv pumping av blod fra Normal forkortning og forlengelse Moderat dysfunksjonell region Totalt dysfunksjonell hjertets venstre hovedkammer (venstre ventrikkel) trekker muskelfibrene rundt hjertekammeret seg sammen synkront og forlenger seg synkront når kammeret fylles med blod. Ved enkelte sykdommer er synkronisiteten dårlig, og det kan forekomme strekning av syke regioner mens de friske forkorter seg og motsatt. Vår hypotese er at dette er forårsakes av balansen av aktive og passive krefter mellom friske og syke regioner. Muskelfibrene genererer de aktive kreftene etter fibrene stimuleres elektrisk ved start av hjertesyklus mens passive krefter hovedsakelig skyldes deformasjon av bindevevet i hjerteveggen. Oppgaven går ut på å sette opp en matematisk modell som representerer de aktive og passive kreftene i friske og syke regioner og hvordan disse endres gjennom hjertesyklus og undersøke hva som skjer ved dyssynkron aktivering og ved forskjeller i aktiv styrke i de forskjellige regionene. En litteraturstudie av tidligere publiserte matematiske modeller av aktive og passive krefter vil inngå i oppgaven. I prosjektet vil det også inngå å sammenlikne simulerte resultater med målinger fra virkelige hjerter i normal tilstand, ved iskemi (forhindret blodtilførsel) og ved regionalt forsinket elektrisk aktivering av hjertemuskelen. Figuren viser målte strain-kurver fra tre områder av hjertemuskelen rundt venstre hjertekammer med varierende grad av iskemi (forhindret blodtilførsel). Type oppgave: teoretisk/numerisk Antall studenter: 1 Kontakt: LR Hellevik Samarbeidspartner: Espen Remme, PhD, Institutt for kirurgisk forskning, Rikshospitalet Universitetssykehus, Oslo 7.4 Non-invasive pressure estimation in the human fetal descending aorta The human fetal circulation has been extensively studied by Doppler ultrasound with emphasis on Doppler flow velocity waveforms and blood flow estimates. Both pressure and blood flow waveforms are needed to describe a circulatory system in terms of peripheral resistance and compliance. In this thesis a
method to estimate blood pressure waveforms and flow volume information in the human fetal circulation system using ultrasound measurements, should be developed. The method may be based on a previously published model using a 2 element Windkessel and a Hookean wall. An assessment should be made of whether a 3 element Windkessel, and/or a better wall model, will improve the predictions. Type of task: theoretical/numerical Contact: LR Hellevik Collaborators: Prof P Segers (http://navier.ugent.be/public/biomed), Univ. Ghent, Belgium. Prof T Kiserud (Haukeland University Hospital)/(SH Eik-Nes) 7.5 Numerical modeling and analysis of intracranial aneurysms Modeling the material behaviour of soft biological tissues is a challenging topic, both with respect to defining the best model and to find representative material parameters. A key question is whether in vitro tests represent the in vivo behavior. Biomechanical modeling of arteries has been under extensive study over the last decade due to the large mortality rate connected to pathologies in such tissues (aneurysms and stenoses). The project will be based on a previous project which addressed the modeling and simulation of aneurysms in the brain. The soft tissue in a healty artery is composed of three layers with different material behavior, albeit with a more or less isotropic global response. However, the surrounding vessels will be anisotropic. The loading is also a challenge, as the problem at hand is a fluid structure interaction (FSI) problem. Local hemodynamics inside the aneurysm will cause local pressure fluctuations on the wall that give rise to inhomogenous loading and stresses. In order to analyze this problem, a coupled FSI-approach will be taken, utilizing both Fluent (a cfd solver) and ABAQUS (structural solver). Typical global response data such as pressure versus diameter change is of interest. Also stress and strain distribution in the vessel layers should be compared and discussed. A main goal with this research is to discuss wall stress based criteria that can be used as indicators for criticality of the aneurysm. With this a neurosurgeon will have a much more quantitative assessment of need for treatment than the present criteria provide (such as size). In summary the project contains: Literature study Set up Fluent/Abaqus simulations, with different constitutive material models of increasing refinement
Find representative numerical cases to compare with own analyses etc. Compare results Conclusion Type of task: theoretical/numerical Contact: LR Hellevik/B Skallerud/PR Leinan 7.6 Numerical investigation of the hemodynamics in the human fetal umbilical vein/ ductus venosus bifurcation Knowledge about the central venous pressure in the human fetus is generally accepted as a key to understand central blood circulation and hemodynamic changes in disease. In this thesis a numerical investigation of the hemodynamics in the human fetal umbilical vein/ductus venosus bifurcation will be conducted. In particular two aspects will be of interest: 1) the pressure drop from the umbilical vein to the fetal heart via the ductus venosus and 2) how the pulsations in pressure and flow in the ductus venosus affect the umbilical flow pattern. FLUENT and ABAQUS will be used for the fluid structure interaction simulations, but user defined functions need to be programmed to account for blood vessel wall motion. The simulations should be validated against results from experimental investigations. Type of task: theoretical/numerical Contact: LR Hellevik Collaborators: PR Leinan, Prof J Vierendeels/J Degroote (http://users.ugent.be/~jjdgroot/), Univ. Ghent, Belgium. Prof T Kiserud, Haukeland University Hospital, Bergen. 7.7 Experimental investigation of the hemodynamics in the human fetal umbilical vein/ ductus venosus bifurcation Knowledge about the central venous pressure in the human fetus is generally accepted as a key to understand central blood circulation and hemodynamic changes in disease. In this thesis an in vitro model of the human fetal umbilical vein/ductus venosus bifurcation should be developed for hemodynamic investigations. In particular two aspects will be of interest: 1) the pressure drop from the umbilical vein to the fetal heart via the ductus venosus and 2) how the pulsations in pressure and flow in the ductus venosus affect the umbilical flow
pattern. The experiments will be conducted at the premises of Institute of Biomedical Technology (IBITECH) of the Ghent University, Belgium. The experimental results are intended to be used for validation of numerical simulations. Type of task: experimental/theoretical Contact: LR Hellevik Collaborators: PR Leinan, Prof P. Segers (http://navier.ugent.be/public/biomed), Univ. Ghent, Belgium. Prof T Kiserud, Haukeland University Hospital, Bergen. 7.8 Numerical analysis of the mitral valve dynamics during diastolic filling The mitral valve is located between the left atrium and the left ventricle in the heart. It controls blood flow into the ventricle during diastole and prevents backflow into the atrium during systole. The objective of this thesis is further development of previous work on simulation of the diastolic filling phase. The motion of the arterial and ventricular walls will be prescribed based on ultrasound speckle tracking, whereas the motion of the mitral valves and the blood flow will be simulated. FLUENT will be used for the fluid structure interaction simulations, but user defined functions need to be modified to account for valve motion. The simulations should be validated against ultrasound recordings. The focus will be on improvement on numerical schemes, parallelization, and possible extension to 3D. Type of task: theoretical/numerical Contact: LR Hellvik/B Skallerud/SK Dahl 7.9 Sammenheng mellom kardiameter og ultralydresonansfrekvens til mikro- gassbobler i små kar Mikrobobler av gass (3 micron diam) benyttes til å forsterke visualiseringen av små blodkar (diam 10-100 micron) ved ultralyd, for eksempel for å detektere kreftsvulster eller redusert blodstrøm i hjerteveggen ved infarkt. Mikroboblene er mye mindre enn ultralydbølgelengde (1:100) slik at væsken (blod,vann) rundt mikroboblene i det vesentlig beveger seg med skjærdeformasjon, og relativt liten volumkompresjon. Dette gir en medsvingende masse av den omliggende væsken som er ca 3 ganger boblens volum i uendelig væske. Medsvingende masse vekselvirker med gassen og skallet elastisitet, som er ulineær, og skaper resonans. Når boblen kommer i nærheten av grenseflater vil disse påvirke strømningsmønsteret rundt boblen, og derved den medsvingende massen og resonansfrekvensen.
Det er publisert resultater fra FEM simuleringer som viser at resonansfrekvensen begynner å falle for kardiametre under 70 micron, fra 3.5 MHz for bobler i uendelig væske ned til under 1MHz for bobler i kappilærer med 10 micron diameter,. Det stilles en del kritiske spørsmål til forutsetningene for disse simuleringene og det er et stort behov for nærmere studier av fenomenet. Resonansfrekvensen benyttes aktivt i deteksjon av bobler ved ultralyd og et slikt fall vil bety at dagens deteksjonsmetoder er langt fra optimale for å detektere boblene i de små karene. Det er derfor av stor interresse å undersøke dette fenomenet nærmere. Arbeidet kan gjøres teoretisk eller eksperimentelt, eller som en kombinasjon, eventuelt også ved besøk ved Erasmus University, Rotterdam. Type oppgave: teoretisk/eksperimentelt Antall studenter: 1 Kontakt: LR Hellevik Medveiledere: Prof BA Angelsen, Medisinsk Fakultet, Sirkulasjon og bildediagnostikk, NTNU. Seniorforsker Harald Laux, Prosessteknologi SINTEF. Samarbeidspartner: Erasmus University, Rotterdam. 7.10 FEM simulering av deformasjon i vev ved trykk og skjærbølger Bløtt vev består av 60-70% vann, med resterende del som cellestrukturer av store molekyler. Den store vannmengden gjør at volumelastisiteten er som i vann og gir opphav til ultralyd kompresjonsbølger (trykkbølger) med forplantningshastighet som i vann ~ 1500 m/s. Væsker har ingen skjærelastisitet ved null hastighet (ingen form stivhet), men cellestrukturene gir bløtt vev en viss skjærelastistet som gir vevet form. Forplantningshastigeten for skjærbølger er lav ~ 1-10 m/sec. Svulster har høyere skjærstivhet enn normalt vev og de kjennes da som en "kul" når man trykker (palperer) på vevet. For små svulster som ligger dypt i vevet kan det være vanskelig å kjenne dette. Ultralyd trykkbølger kan benyttes til å måle strain når man trykker vevet sammen, og kan derved benyttes til å observere økning i skjærstivhet dypt i vevet. Vi har utviklet nye ultralydmetoder som måler strain med større nøyaktiget: Metodenes nøyaktighet testes best ved simuleringer, og det er i den sammenheng nyttig å kunne foreta FEM simuleringer av skjærdeformasjon i bløtt vev med en kuleformet svulst, som utsettes for et ytre trykk. Type oppgave: teoretisk/eksperimentelt Antall studenter: 1 Kontakt: LR Hellevik Medveiledere: Prof BA Angelsen, Medisinsk Fakultet, Sirkulasjon og bildediagnostikk, NTNU.
7.11 Nano-indentation of anisotropic material: numerical approaches to extract elasticities from nano-indentation This project relates to the increased use of nano-indentation of bone tissue in order to establish elastic properties bone. The stiffness measured from nano-indentation is a stiffness describing the average response of the material beneath the tip of the indenter. Som analytical or numerical (FEM) tool must be used in order to find the actual Young s moduli. For anisotropic materials this can be a challenge. The project will utilize (nonlinear ) finite element tools, e.g. ABAQUS, to analyse materials of different levels of anisotropies (starting with isotropic material) and relate anisotropy effects to the global indentation stiffness measured in tests. Experimental and numerical results found in the literature, if available., should be considered also. Type of task: numerical/analytical No. students: 1-2 Supervisors: Prof B Skallerud, Dr J He, Prof ZL Zhang 7.12 Nano-indentation of microbubbles coated with nano-particles: stiffness and strength This project relates to targeted drug delivery, where the drug is inside the nano-particles attached to the surface of gas bubbles. The micro-bubble is the vehicle to transport the medicine to the intended location (e.g. a tumor) via the blood stream. This work is a collaboration with NT- and Medical Faculty. It is very important to characterize mechanical properties of the bubble/nano-particle system in order to understand response to different loading scenarios. The microbubbles coated with nano-particles will be provided by SINTEF Materials and Chemistry. The nano-indentation tests are carried out in our own lab. The measured stiffness of the bubbles should be analysed by means of some numerical scheme in order to extract mechanical stiffness and strength for the bubble shell surface. Type of task: experimental/numerical/maybe some analytical No. students: 1-2 Supervisors: Dr J He, Prof B Skallerud, Prof ZL Zhang 7.13 Effects of scatter in material parameters on the global response of the left ventricle during diastolic filling This project addresses the global motion of the left ventricle in the filling phase of the heart cycle (diastole). In this phase the heart muscle mainly is passive, i.e. no active muscle contraction. The heart muscle (myocardium) can be modeled as an orthotropic hyperelastic material in this phase. Several material parameters appear in the material models, and these parameters have significant variability between persons and due to different diseases. The
project will apply recently implemented material models in ABAQUS, and run nonlinear simulations with different material parameters in order to investigate how a given variability in material parameters is correlated with corresponding variability in global ventricular response. Type of task: numerical No. students: 1-2 Supervisors: Assoc Prof V Prot, Prof B Skallerud 7.14 Finite element studies on composite materials as an alternative for steel/titanium hip prostheses In total hip replacement it is a goal to have as long as possible durability of the prosthesis. Loosening of prosthesis with need for re-operation should be minimized. Some studies show that prostheses made of composite materials may provide a smoother load transfer from upper body to the leg via the prosthesis compared to prostheses made of traditional materials (e.g. titanium). The project will employ extensive finite element analyses in order to investigate the load transfer between the prosthesis and the femur. Both linear and nonlinear finite element simulations (accounting for contact) should be employed. Discussion of results in relation to findings in the literature should be included. Type of task: numerical No. students: 1 Supervisors: Prof B Skallerud