Morphology and mechanical properties of Abdominal Aortic

111 Pages·2007·6.06 MB·English

Morphology and mechanical
properties of Abdominal Aortic
Aneurysms
AcataloguerecordisavailablefromtheEindhovenUniversityofTechnologyLibrary
ISBN:978-90-386-1057-3
Coverdesign: EvelynevanDam&JorritvanRijt,OranjeVormgevers
PrintedbyUniversiteitsdrukkerijTUEindhoven,Eindhoven,TheNetherlands.
Thisresearchwas performedinthescopeofthehemodynproject,acooperationbe-
tween Philips Medical systems (Healthcare IT - Advanced Development), Best, the
TechnischeUniversiteitEindhoven(BiomedicalEngineeringdepartment),Eindhoven
andtheErasmusUniversity(Thoraxcenter,BiomedicalEngineering),Rotterdam. The
Hemodyn project is partly funded by SenterNovem (Dutch Ministry of economic af-
fairs).
Morphology and mechanical
properties of Abdominal Aortic
Aneurysms
PROEFSCHRIFT
terverkrijgingvandegraadvandoctor
aandeTechnischeUniversiteitEindhoven,
opgezagvandeRectorMagni(cid:2)cus,prof.dr.ir. C.J.vanDuijn,
vooreencommissieaangewezendoorhetCollegevoorPromoties
inhetopenbaarteverdedigenop
dinsdag3juli2007om16.00uur
door
Evelyne Andrea van Dam
geborenteMaastricht
Ditproefschriftisgoedgekeurddoordepromotor:
prof.dr.ir. F.N.vandeVosse
Copromotoren:
dr.ir. M.C.M.Rutten
en
dr.ir. G.W.M.Peters
v
-neetzeure,begin,‘thetgennezin
gewachttevergeafsop‘nwonder-
Vechte,valleenopstoan-RowwenHeze
vi
Contents
Summary ix
1 Introduction 1
1.1 AbdominalAorticAneurysms . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Ruptureriskprediction . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4 Objectiveandoutline . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2 DeterminationofLinearViscoelasticBehaviourofAbdominalAorticAneurysm
Thrombus 9
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.2 MethodsandMaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2.1 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2.2 Discretetransitiongroup . . . . . . . . . . . . . . . . . . . . . . 13
2.2.3 Continuoustransitiongroup . . . . . . . . . . . . . . . . . . . . 14
2.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.3.1 Discretetransition . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.3.2 Continuoustransition . . . . . . . . . . . . . . . . . . . . . . . 17
2.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3 Non-linear ViscoelasticBehaviourofAbdominal AorticAneurysmThrom-
bus 23
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.2.1 Kinematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.2.2 Theconstitutivemodel . . . . . . . . . . . . . . . . . . . . . . . 27
3.2.3 Determinationofmaterialparameters . . . . . . . . . . . . . . 29
3.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.3.1 Smallstrainexperiments. . . . . . . . . . . . . . . . . . . . . . 33
3.3.2 Largestrainexperiments . . . . . . . . . . . . . . . . . . . . . . 34
3.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
vii
viii Contents
4 Discrimination ofComponentsofAAAVesselWallbyMulti ContrastMRI 43
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.2 MethodsandMaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.2.1 MultiContrastMRI . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.2.2 (cid:22)CT&histology . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4.2.3 Imageanalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
5 AMixedNumericalExperimentalMethodforMechanicalCharacterisation
ofAAAVesselWall 55
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
5.2 Mixednumericalexperimentalmethod . . . . . . . . . . . . . . . . . . 57
5.2.1 LoadprotocolandMRimaging . . . . . . . . . . . . . . . . . . 58
5.2.2 DigitalImageCorrelation . . . . . . . . . . . . . . . . . . . . . 59
5.2.3 MeshgenerationandFiniteElementModelling . . . . . . . . . 61
5.2.4 Parameterestimation. . . . . . . . . . . . . . . . . . . . . . . . 61
5.3 Validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
5.3.1 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
5.3.2 Resultssimulation . . . . . . . . . . . . . . . . . . . . . . . . . 64
5.3.3 Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5.3.4 Resultsexperiment . . . . . . . . . . . . . . . . . . . . . . . . . 65
5.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
6 LocalMechanicalPropertiesofAbdominal AorticAneurysms 69
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
6.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
6.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
7 GeneralDiscussion: TowardsRuptureRiskAnalysis 81
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
7.2 Implicationforwallstressanalyses . . . . . . . . . . . . . . . . . . . . 83
7.3 Limitationsforclinicalapplication . . . . . . . . . . . . . . . . . . . . 87
7.4 Futureaspects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
References 89
Samenvatting 97
Dankwoord 99
Samenvatting 99
CurricilumVitae 101
Morphology and
mechanical properties of
Abdominal Aortic Aneurysms:
Summary
AbdominalAorticAneurysm(AAA)ruptureisalife-threateningevent,butcan,when
diagnosed timely, be avoided by either endovascular or conventional repair. Since
these methods are accompanied by a high mortality rate, the decision to operate
should only be made when the rupture risk exceeds the risks of repair. To date,
the maximum diameter of the aneurysm is used as a measure for rupture risk and
hence as a criterion for surgical intervention. It has, however, been shown already
that small aneurysms do rupture sometimes, while some large aneurysms have not
rupturedyet,andthereforethediametercriterionalonemaynotbesuf(cid:2)cient. Since
rupture of an AAA occurs when locally the wall stress exceeds the strength of the
vessel wall, it is generally believed that wall stress distribution could help to better
assessAAArupture.
Patientspeci(cid:2)cmodelsforwallstresscomputationsdonotonlyrequirethegeometry
oftheaneurysmbutalsothemechanicalpropertiesoftheaneurymtissue. Studieson
the material propertiesof vessel wall have mainly focused on describing AAA vessel
wallandthrombusasahomogeneousmaterial. Localinhomogeneitiescanhavelarge
in(cid:3)uences on the stress magnitude and distributions. AAA vessel wall may contain
inhomogeneitiessuchascalci(cid:2)cationsandatheroscleroticplaques. Furthermore,the
thrombus,whichis presentinthemajority oftheaneurysms mayin(cid:3)uence thelocal
mechanics considerably. Itisa layered (cid:2)brinstructure,thatshowsdifferentlevelsof
degeneration. Thelevel ofdegenerationof the(cid:2)brin structuremight havein(cid:3)uence
onthemechanicalpropertiesofthethrombus.
To improve future wall rupture risk prediction based on wall stress, the objective of
thisstudyistoobtainthelocalmechanicalpropertiesofboththrombusandtheAAA
vesselwall.
AAA vessel wall and thrombusare obtainedfrompatients treated with conventional
surgery. Theviscoelasticbehaviourofthrombusisdeterminedusingplate-platerheom-
ix
x Summary
etry. To study the changes in mechanical propertiesthroughout the thickness of the
thrombus a radially oriented stack of samples was used. In the small strain regime
frequency sweep test are peformed and the elastic and viscous moduli are found to
beintherangeof1.7(cid:6)1.3kPaand0.2(cid:6)0.1kParespectively. Sincelargedeforma-
tionsoccurinthrombusthenon-linearpropertiesaredeterminedbystressrelaxation
experiments. Todescribethephenomenaobservedexperimentally,anon-linearmulti
modemodelisused. Theparametersforthismodelareobtainedby(cid:2)ttingthismodel
successfullytotheexperimentsinboththelinearandnon-linearregime.
TodeterminethemorpholgyoftheAAAvesselwall,theapplicabilityofmulticontrast
MRItodiscriminate thecomponentsofthevessel wall is studied. Multi contrastMR
resultsarecomparedtothegoldenstandards,histologyand(cid:22)CT.Componentslikethe
media, calci(cid:2)ed deposits, areas containing cholesterol, thrombus and the adventitia
containingfatcellsandvasavasorumcanberecognisedintheMRimages. Theresults
obtained so far are not suitable for an automatic classi(cid:2)cation by an unsupervised
clusteringalgorithm.
To obtain the mechanical properties of the components present in the vessel wall a
mixed numerical experimental method is proposed. This method is a robust way to
determinethemechanicalpropertiesoftheindividualcomponentswithouthavingto
isolatethem.
ThemethodisappliedtoAAAvesselwallsamples. Themorphologyisobtainedwith
multi contrast MR. The Young’s moduli of media, adventitia, calci(cid:2)cation, an area
with cholesterol crystals and thrombus are determined. Although the media is less
stiffthantheadventitiathemoduliareinsametheorderofmagnitude((cid:25)1.0MPa).
Thestiffnessofthrombusisfoundtobeanorderofmagnitudelower((cid:25)40kPa). An
area with a lot of cholesterol is stiffer than the rest of the wall ((cid:25) 7.5 MPa) and a
calci(cid:2)edareaisevenstiffer((cid:25)50MPa).
Byimplementingthe(cid:2)ndingsforthrombusinapatientspeci(cid:2)cmodelitisshownthat
thrombusdoeschangethestressdistributionandpeakwallstresses,buttheeffectis
muchlowerthanreportedinliteraturebyothers. Thepresenceofcalci(cid:2)cation hasa
largeeffectonthewallstressdistribution. Itisshownthatthewallstressdistribution
isin(cid:3)uencedbytheaccuracywithwhichthegeometryofthecalci(cid:2)cationisdecribed.
Thelimitedresolutionavailableintheclinicalsetttingresultsinanon-realisticstress
distribution. For correct implementation of inhomogeneities of the vessel wall the
spatialresolutionofbothimagingtechniquesandcomputationsshouldincrease.
The contribution of this work consists not only of the reported morphologies and
mechanicalproperties. Theobservationsreportedinthisworkalsosuggestthateffort
should be put into development of clinical methods to include the morphology and
mechanicalpropertiesoftheaneurysmintotheruptureriskanalysis.