Eurographics Italian Chapter July 11-12- 2002 OPEN CASCADE AND RAPID PROTOTYPING IN HUMAN CAROTID LUMEN RECONSTRUCTION Fabrizio Murgia CRS4 - EIP - GEMMS VI Strada OVEST Z.I. Macchiareddu, C.P. 94, 09010 UTA (CA - Italy) ph +39 0702796321, fax +39 0702796216, e-mail: [email protected]Gabriella Pusceddu CRS4 - EIP - GEMMS VI Strada OVEST Z.I. Macchiareddu, C.P. 94, 09010 UTA (CA - Italy) ph +39 0702796321, fax +39 0702796216, e-mail: [email protected]Gregorio Franzoni CRS4 - EIP - GEMMS VI Strada OVEST Z.I. Macchiareddu, C.P. 94, 09010 UTA (CA - Italy) ph +39 0702796347, fax +39 0702796216, e-mail: [email protected]Abstract: Image processing algorithms, CAD-CAM tools and rapid prototyping (RP) techniques are able to produce complex lumen artery replicas. This work presents a system for manufacturing the lumen of human carotid from computed tomography acquisition. The pipe-line of manufacturing process of a human carotid lumen replication is presented. Each stage of the pipe-line is briefly discussed. Technical details of the 3D surface reconstruction phase, based on the Open Cascade geometric modelling software, and the RP manufaturing process based on Fused Deposition Modelling are presented. Keywords: Rapid Prototyping, Geometric Modelling, Medical Imag- ing 1 Introduction Rapid Prototyping (RP) is an emerging technique used in industry for manufacturing prototypes [11]. RP can be considered a new imaging technique. Its capability to physically reproduce geometrical complex shapes is getting increasing interest in medicine [5]. Stereolithographic biomodelling has already been used in craniofacial surgery for management of deformities, trau- mas and tumours. Accurate analysis of Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) data of a patient is a common practice in vascular surgery. Surgeons try to men- tally reconstruct a virtual 3D surgical anatomy model. Physical models can help in this important task providing the physician with a visual and tactile support [8], [4]. Physical replicas can also facilitate experimental studies of computational vascular fluid dynamics [3]; they also permit in vitro reproductions of flows in living subjects before and after surgery. The CT data are transmitted from the acquisition clinical apparatus to the graphics worksta- tion. The replica manufacturing process starts with making a segmentation step. It is necessary to extract the set of points (curves) which represents the lumen. This step is a semi-automatic Center for Advanced Studies, Research and Development in Sardinia Energy and Process Engineering Geometric Modelling and Monte Carlo Simulations 1
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EurographicsItalian Chapter July 11-12- 2002
OPEN CASCADE AND RAPID PROTOTYPINGIN HUMAN CAROTID LUMEN RECONSTRUCTION
Fabrizio MurgiaCRS4
�- EIP
�- GEMMS
�VI StradaOVESTZ.I. Macchiareddu,C.P. 94,09010UTA (CA - Italy)
Keywords: Rapid Prototyping, Geometric Modelling, Medical Imag-ing
1 Introduction
RapidPrototyping(RP) is anemerging techniqueusedin industryfor manufacturingprototypes[11]. RP can be considereda new imaging technique. Its capability to physically reproducegeometricalcomplex shapesis getting increasinginterestin medicine[5]. Stereolithographicbiomodellinghasalreadybeenusedin craniofacialsurgeryfor managementof deformities,trau-masandtumours. Accurateanalysisof ComputedTomography(CT) andMagneticResonanceImaging(MRI) dataof a patientis a commonpracticein vascularsurgery. Surgeonstry to men-tally reconstructavirtual 3D surgicalanatomymodel.Physicalmodelscanhelpin this importanttaskproviding thephysicianwith a visualandtactilesupport[8], [4]. Physicalreplicascanalsofacilitateexperimentalstudiesof computationalvascularfluid dynamics[3]; they alsopermit invitro reproductionsof flows in living subjectsbeforeandaftersurgery.
TheCT dataaretransmittedfrom theacquisitionclinical apparatusto thegraphicsworksta-tion. Thereplicamanufacturingprocessstartswith makinga segmentationstep. It is necessaryto extract the setof points(curves)which representsthe lumen. This stepis a semi-automatic�Centerfor AdvancedStudies,ResearchandDevelopmentin Sardinia�Energy andProcessEngineering�GeometricModellingandMonteCarloSimulations
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processaidedby an imageprocessingtool. Thesepointsarelinked together, by usingcommer-cial CAD-CAM systemsto build 3D surfaces.This stepis calledthe geometricreconstruction.BSplineareusedto definethe geometryof curvesandsurfaces. The last stageof the pipelineis RapidPrototyping. At this point the surfacesare tessellatedto obtain the meshof trianglesin STL format which is the input format for theRP device; RP device builds the objectusingalayer-by-layermanufacturingprocess.
Pentecostgroup[10] usedadigital photomicrograph(10micrometerssectionthickness)appa-ratusto reconstructthecardiacbloodspaceof humanembryonicheart(approximatelythegreatestdimensionof the embryois ����� � ). The curvesrepresentingthe lumenweremanuallytracedfromthephotosandreconstructedin surfacesbyusingMayasoftware(http://www.aliaswavefront.-com/). Thereplicaswerebuilt usingaStereoLithographicApparatus(SLA) with layersthicknessof ������������ � .
Lermusiauxwork [7] reportsthe useof CT scanacquisition(the reconstructioninterval of����� � every ������ � andapitchof ����� ) with contrastliquid andwithoutcardiacsynchronisationduring the acquisitionprocedure.An unspecifiedsemi-automaticimageprocessingtool, drivenby the operator, is usedto generatea structuredpoints cloud and to computethe interpolatedsurfacesfrom adjacentsections.TheSTL modelis slicedin ������� � crosssections.SLA 250(3DSystemsCorp. Valencia,CA) is usedto build the physicalreplicasin epoxyresin. The systemwasdevelopedto reconstructabdominalaortic aneurysm(AAA). Threedayswere requiredtoproducethefinal replica.
Renaudingroup[2] usedMRI apparatus(a 3D MR angiograph����� andstaticMR imagingechosettingwith ��� � pixel sizedimension)to develop a completephantomof the coronaryarteriesdedicatedto 2D or 3D angiographicimaging. Thesystemcanalsobeusedto constructa realisticphantomof stenosesof the coronaryarteries.The CAD Euclid system(from Matra-Renaulthttp://www.matra-datavision.com/)wasusedto build 3-D anthropomorphicphantoms.The segmentationwasdoneby usingparametriccurves(B-Spline)startingfrom centerlines ofthevesselsmanuallycomputedby anexpertoperator. Phantomswerebuilt usingaSLA technique(manufacturingprecisionis under ������� � thatcanbeimprovedtill ��������� � ). Thephantomcanalsobeusedfor testing3D reconstructionof vessels.
Friedman[1] useda slightly differentapproachto manufacturingvascularreplicasthat canbeusedfor testingin vitro reproductionof flows in living subjects:a MRI acquisitionapparatus(1.5 T GE). 3D time-of-flight imagingsequenceis usedto producesixty sections����� � thickwith a pixel sizeof ���� �!���"�� � . PointswereFourier interpolatedto ���� �#������ � hasbeenused.He usedanad-hocimageprocessingroutinemanuallydrivenby theoperatorto define120pointsfor eachlumencurves.Thesepointswereequallyspacedaroundtheperimeterof eachaxialsection.A CAD systemwasusedto interpolatethesepointsin BSplinesurfaces.Thesesurfacesweretransferred(by usingIGES format) to the powerful commercialCAD-CAM systemCatia(http://www.catia.com). Both, two moldpartingplanesandthetool pathfor numericalcontrolledprocesswerecreatedin CATIA. A replicaof realarterywasmilled out by a machinableplasticdevice. The distancebetweeneachfinal stepof the cutter was �������$��� � producingscallops����������� � high. Therun time was ��� h.
Our studyfollows a previous work madeat CRS4on the developmentof the ViVa system[3]. In this work we have investigatedtheapplicability of RP techniquein the vascularsurgeryfield. WefocusedonapplyingRPtechniqueto thecarotidlumenreconstructionof ahumanarteryacquiredthroughComputedTomography(CT).
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2 Materials and methods
2.1 Data acquisition
Digital datawereobtainedby spiralCT on a patientaffectedby carotidaneurysm.Theexamina-tion wasperformedusingaPickerscannerat theBrotzuHospitalin Cagliari.Datawastransferredto our workstationby usingDICOM3 (http://medical.nema.org) standardprotocol. We consid-ered48 CT imagesfor this case.The distancebetweenslides(derived from CT acquisition)is��� � .
2.2 Segmentation
The picture segmentationis asemi-automaticprocessdrivenby theenduser(operator).In this stageeachCT imageis fil-teredto extract pointsbelongingtothelumenboundary.Figure 1 shows a typical CT im-ageafter filtering. The lumento bereconstructedis the boundarylayerwhich separatesdark pixels (insidelumen)from grey pixels(intima).
For the segmentationwe usedgsnake(http://www.cs.wisc.edu/com-puter-vision/projects/gsnake.html) li-brary. The segmentationprocess
Figure 1: Acquisitionand segmentationsteps.Theleftimage showsthe filtered CT image of a carotid (2.1).The right image showsthe resultsobtainedapplyingthesegmentationstepandinterpolatingthepointsbyaBSplinecurve(see2.2).
startsopeninga CT image,selectingthe region of interest(ROI) on the imageandpositioninga contourof 3-D points insidethe lumenregion. At this point, an iteration loop startsmovingpointsto lumenregions.Whenall pointsreachthelumenboundarytheevolvingprocessstopsandthecartesiancoordinatesof eachpointis storedin anindexedvectorof 3D vertices.Thisstageis costantlymonitoredby theoperatorthatfixesthenumberof iterationsandsomeotherparameters.Thisvectorrepresentsthegeometricconstraintsfor requestedinterpolationcurve. This segmentationprocedureis repeatedsemi-automaticallyfor eachCT image.This proceduregenerates24 slides,eachonestoring �$���3D vertices(Fig. 1).
2.3 Geometric reconstruction
Thelumenreconstructionis realisedusingtheOpenCASCADE(OC)(http://www.opencascade.-com) software. OC is a powerful 3D modellingapplicationdevelopmentplatform. It consistsinreusableC++ objectlibrariesanddevelopmenttoolsthatareavailableasOpenSource.
OC is usedto createdomainspecific3D graphicapplicationssuchas:ComputerAidedEngi-neering(CAE), ArchitectureEngineeringConstruction(AEC) andGeographicInformationSys-tem(GIS),CAD-CAM.
We use the OC’s BoundaryRepresentation(BRep) scheme[6] to managethe model re-
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constructionof the lumen and to convert it in STL format. In fact, the topological li-brary supportedby this programallows us to build the valid topologicaldatastructureof theartery.
The setof parameterizedpointsproducedby the segmentationandstoredin a file, aredividedin horizontalplanes.Thesepointsarestoredinto anarrayof gp Pnt (OC datastructure).We in-terpolatepointsbelongingto thesameplaneby a BSplinecurve usingtheGeomApi Interpolateclass of OC. We use Bspline becausesuch a representationpresentsa local control fea-ture. This meansthat modifying one control point (vertex) only affects the part of the curvenear that control point (Figure 1).In order to obtaina valid Brep ob-ject of the OC classTopoDSSolid,we computethe previous interpo-lation algorithm for eachslide byBRepBuilder API MakeShellandBRepBuilder API MakeSolid clas-ses.Theleft imagein Figure2 showstheBspline surfacesobtainedinterpo-lating two adjacentBsplinecurves.The innersurfacerepresentsthe re-questedartery lumen. This surface
Figure 2: Reconstructionstep. The image on the leftshowstwoBsplinecurvesobtainedin thegeometricre-constructionandsegmentationsteps(2.3).
hascontinuityC2. Theoutersurfacerepresentsanoffsetsurfacenecessaryto give theright man-ufacturingthicknessto thephysicalreplica.Theinnerandtheoutersurfacesarecappedonthetop(right image)andon thebottomby holedplanedishesin orderto computea valid BRepschemewith OpenCascade(2.3)Thissolutionis obtainedovercomingboth,numericalproblemsdueto theBSplinecurves/surfacecomputationandproblemsdueto topologicalaspectsin thedefinitionof theBRep(ex: orientationof computedsurfaces).At thispoint, thesolidmodelis completedandits BReprepresentsavalid3D solid [6]. Last stepis the tessellationof the BRepwith trianglesby Stl API Writer classofOC. This classproducesthemeshof trianglethat is storedin a outputfile usingtheSTL format(Fig. 3)
The numberof trianglesproducedby Stl API Writer classis kept low (settingtessellationparametersof OC) becausethe FDM apparatusdefinesan upperboundlimit on the numberoftriangles.BecausetheSTL file hasbeengeneratedfrom a valid BReprepresentationno manualoperationis neededfrom the RP operatorto fix it. Surfacemodelof the carotidlumenin eachof theabove specimenswasconvertedto STL format in thepreviousstepandtransmittedto ourprototypingcenterProto21. Proto21usesa RP machinewith FDM technology. FDM meansFusedDepositionModelling, andis a techniquewherebydigital surfacemodelsareconvertedtoscalemodelsof resinor wax. It worksby meansof amovingheadwhichis drivenontheXY planeby thecoordinatesdatain the3D wireframesurfacemodel. While moving, this headextrudesafusedwire of materialwhichsolidifiesproducinga thin layer(0.178to 0.5mm)of theobject.
2.4 Rapid Prototyping
Thenthe workplanemovesdown alongZ-axis of exactly the thicknessof the layer created,
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andanotherlayer is built up over theprevious. The processgoeson layerby layer, andcreatesa pretty accuratesolid resinmodel.In our case,resinwasABS andthemachinewasaStratasysFDM 2000[9]. The manufacturingorientationis shown in Figure3. This positionhasbeenchosenin orderto havethemaximummanufacturingresolutionalongthe lumenaxis. This meshisobtainedfrom OpenCASCADEbythetessellationprocessandstoredinSTL formatduringthegeometricre-constructionstep(see2.3).
Figure 3: RapidPrototyping. Themeshof trianglespo-sitionedin themanufacturingStratasysFDM 2000po-sition is shownon the left. Theright image showsthemachining pathof theFDM disposal(SSLformat)ona manufacturingcrosssection(see2.4).
3 Discussion and conclusions
Lumenreplicascanhelpvascularsurgeonsin diagnosyandtherapy of particularcarotiddiseases.In orderto beableto planetheoperation,vascularsurgeonsneedto know, with greataccuracy, theanatomyof thepart. Thephysicalmodelprovidesa visualanda tactilesupportthatcanimprovethecommunicationqualitybetweensurgeonandsurgicalequipebetweensurgeonandthepatient.Replicasarealsoimportantfor trainingscope.Lumenreplicasof humancarotidoffersameantovisualiseinternalanatomicfeaturesdifficult to seeotherwise.
Thefirst productionof the lumenreplicais shown in Figure4. TheFDM techniqueis fairlycheap. Its cost is continuouslydecreasinganddependsof the heightof the artery to be manu-factured.Thebuilding processfor the lumenreplicawasratherfast, it took four hoursto builda complete3D lumenreplica. For future modelswe areplanningto usetransparentmaterials(silicone)insteadof ABS. We arealsoplanningto includemarkingsto distinguishpathologicalareasfrom healthyones.
Figure 4: This image showthe physicalre-constructionof the carotid in FDM. Theprinting stage is about30 minutes.
The systemdevelopedso far is a prototype.It is a partof undergoingresearchproject: Lab-oratory for AdvancedPlanningandSimulations(http://www.crs4.it/% laps/).
Preliminaryresultspresentedhereindemon-strate mainly how to use the Open Cascadelibrary and FDM techniqueto build a lumenreplica. It provides a software environmentwherewe canverify different interpolationandblending surface algorithmssupportedby OC.We have also checked the capability of OC togeneratecorrecttessellatedgeometrieswhichcanautomaticallybesentto theFDM devicewithout
any further interventionof theRPoperator. Next phaseof this studywill bedevotedto segmentcomplex carotid CT datato automaticallyextract complex geometricand topologicalfeatures(reconstructionof the carotidbifurcation). This technique,developedin collaborationwith the
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RadiologyDept. and the VascularSurgery Dept. of the OspedaleBrotzu of Cagliari, will beused,in thenext stageof theproject,to reconstructthecarotidreplicasof 10patientswith differ-entcarotidpathologiesin orderto studyapplicabilityof this techniqueto a minimumnumberofcases.
4 Acknowledgements
This researchwassupportedby theItalian Government- MIUR - insidetheprojectLAPS (Lab-oratory for AdvancedPlanningand Simulation). The medicaldatasetswere provided by theRadiologyandVascularSurgery Dept. of the OspedaleBrotzu of Cagliari, Italy. The authorsalsoacknowledgeDr. AndreaGiachettiandDr. Alan Scheininefor their technicalassistanceinsegmentationtechniquesandMD. MassimilianoTuveri (AB/MIDA-CRS4)andDr. PieroPili fortheir support.
References
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[2] C.P. RENAUDIN — B. BARBIER — R. RORIZ — D. REVEL — M. AMIEL. Coronaryarteries:New designfor three-dimensionalarterialphantom. Radiology, pages579–582,Mars1994.
[3] G. ABDULAEEV et al. Viva: the virtual vascularproject. Information Technology inBiomedicine, pages268–273,December1988.
[4] M.H. FRIEDMAN. Arteriosclerosisresearchusingvacularflow models:From2-dbranchesto compliantreplicas. ASMEJournal of BIOMECHANICSENGINEERING, 3:595–601,1993.
