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3.SabineIserhardt-Bauer,DiplInf,

4.NielsC.Kö

stner,

5.MiriamSchempershofe,

6.WalterJ.Huk,MD,

7.ThomasErtl,PhD,

8.ChristianStrauss,MDand

9.JohannRomstock,MD

+AuthorAffiliations

1.1FromtheDepartmentsofNeuroradiology（B.F.T.,N.C.K.,M.S.,W.J.H.）andNeurosurgery（P.H.,C.S.,J.R.）,UniversityofErlangen-Nuremberg,Germany;

andtheVisualizationandInteractiveSystemsGroup,UniversityofStuttgart,Germany（S.I-B.,T.E.）.Presentedasascientificexhibitatthe2001RSNAscientificassembly.ReceivedSeptember18,2002,revisionrequestedOctober22,revisionreceivedandacceptedOctober23.AddresscorrespondencetoB.F.T.（e-mail:

tomandl@neuroradiologie-erlangen.de）.

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Abstract

Computedtomography（CT）angiographyisawell-knownimagingtechniquecommonlyappliedtoboththedetectionandtherapyplanningofintracranialaneurysms.Forthispurpose,currentstudiespredominantlyfocusonthree-dimensional（3D）representationsofCTangiographicvolumesobtainedwithvaryingvisualizationapproachesondifferentcomputers.Interactivemanipulationperformedbyusersindividuallyisanimportantprerequisitefordataanalysis.However,thisleadstoinconsistentandbarelyreproducible3Dvisualizationresults.Furthermore,thequalityofany3Drepresentationdependsontheappliedvisualizationstrategy（eg,maximum-intensityprojection,shaded-surfacedisplay,directvolumerendering）.Toovercometheselimitations,theauthorspresentanovelmethodforstandardizedvisualizationofCTangiographicvolumes,consistingofthreesteps:

（a）transferoftheimagedatatoaremotehigh-endgraphicsworkstation,（b）automatic3Dvisualizationwithhigh-resolutiondirectvolumerendering,and（c）consecutivevideogenerationperformedaccordingtoastandardizedprotocol.Therecordedvideosequencesaretransferredforevaluationtoalocaldesktopcomputer.Intheexperimentalsetup,high-qualityvideosbasedon3Dvisualizationswereproducedinlessthan60minutesperpatient.Althoughaneurysmsabovetheskullbaseareusuallyvisualizedwithexcellentquality,theanalysisofaneurysmsattheskullbaseisstilldifficult.

©

RSNA,2002

∙Aneurysm,CT,17.12112,17.12115,17.12116,17.12117

∙Aneurysm,intracranial,17.73

∙Cerebralbloodvessels,CT,17.12112,17.12115,17.12116,17.12117

∙Computedtomography（CT）,three-dimensional,17.12117

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Introduction

Suddenonsetofsevereheadacheisthetypicalleadingsymptominpatientswithsubarachnoidhemorrhage.Thecauseofsubarachnoidhemorrhageisarupturedaneurysmin75%ndash;

85%ofcases,nonaneurysmalperimesencephalichemorrhageinabout10%ofcases,andavarietyofrareconditionssuchasarteriovenousmalformationsinabout5%ofcases

（1）.Computedtomography（CT）isthefirststepintheexaminationofthesepatients.Ifsubarachnoidhemorrhageisconfirmed,itisnecessarytodetectthesourceofbleedingforappropriatetherapy.Whiledigitalsubtractionangiography（DSA）isstillthemostsensitivetoolforthedetectionofintracranialaneurysmsandothervascularmalformations,manystudieshaveshowntheusefulnessofCTangiographyforthispurpose.ThereportedsensitivityofCTangiographyhasbeenreportedtobeintherangeof70%ndash;

96%（2–7）,dependingonthesizeandlocationoftheaneurysm（8）.Inmostofthesestudiesthree-dimensional（3D）visualizationsoftheintracranialvesselswereusedtoassesthevalueofCTangiography.Inthistechnique,thesectionimagesaretransferredtoaworkstationand3Dvisualizationofvascularstructuresisperformed.Theaimof3Dvisualizationistoextractthevascularstructuredatafromthevolume.Thesemethodsalwaysleadtolossofinformation,regardlesoftheappliedalgorithm.Althoughthesuppressionofbraintissuesurroundingtheintracranialvesselsisaprerequisiteforprovidinganopenviewoftheintracranialarteriesina3Drepresentation,itispossiblethatimportantinformationsuchascalcificationorthrombosedpartsofananeurysmisnotseen.Therefore,itismandatorytoaccuratelystudythesectionimagesbefore3Dvisualizationisperformed（Fig1）.

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Figure1a. 

ComparisonofsectionimagesfromCTangiographyto3Drepresentationinapatientwithseveresubarachnoidhemorrhageandaneurysmsofbothinternalcarotidartery（ICA）bifurcations.Theleftaneurysmiscalcified.（a）AxialsectionimageatthelevelofthecircleofWillisshowsananeurysmatthebifurcationoftherightICA（arrow）.Thebasalcisternsarefilledwithsubarachnoidblood（arrowheads）.Forabettercomparisonwiththe3Dimage,theviewisfromabove.（b）AxialsectionimageatthelevelofacalcifiedaneurysmoftheleftICAbifurcation（arrow）.Theextentoftheintramuralcalcificationisclearlydemonstrated（arrowheads）.Again,theviewisfromabove.（c）Superoinferior3Dvisualizationwithdirectvolumerenderingshowsthetwoaneurysms（arrows）inrelationtotheintracranialarteriesandtheskullbase.Theintramuralcalcificationwithintheleftaneurysmisdemonstrated,butitsextentisnotclearlyseen（arrowhead）.Notethatbraintissueandsubarachnoidbloodarenotdemonstratedwithinthe3Drepresentationbecausetheselectedthresholdsdonotincludethevoxelscontainingthisinformation.

Figure1b. 

Figure1c. 

Littleisknownabouttheinfluenceofpostprocessingmethodssuchasmaximum-intensityprojection（MIP）,shaded-surfacedisplay,anddirectvolumerendering（9–11）.However,itcanbeassumedthatthesameCTangiographydatamayresultindifferentdetectionrateswhenvariousvisualizationstrategies,computerplatforms,andgraphicshardwareareused（8,12–15）.Inaddition,theuseroftheworkstationmustbefamiliarwithseveralmanipulationtools.Thesecomprisethetoolsthatpositionobjectssuchasclippingplanes,definethresholds,andadjustso-calledtransferfunctionsforcolorandopacityvalues.RatherthaninvestigatingthevalueofCTangiographyfortheevaluationofintracranialaneurysms,availablestudieshavebeenanalyzingspecificsystems.Ingeneral,thesehaveconsistedoftheinvestigationmodality,suchasCTormagneticresonance（MR）angiography,asthesourceofthedata;

theavailableworkstation,includingrenderingsoftware;

andfinally,aninvestigatorsteeringthe3Dvisualizationprocess.Figure2illustratesthetypicalworkflowinsuchasystem.

Figure2. 

TraditionalworkflowofCTangiography（CTA）.Thedataareanalyzedbyanindividualuseronacommerciallyavailableworkstation.

Reproducibleprotocolscontaininginformationaboutthesectionthickness,theinjectionrateofcontrastmedium,andthereconstructionofthesectionimagesareusuallystandardintheseinvestigations.Astandardizedvisualizationmethodthatprovidesreproducible3DrepresentationsofCTangiographicdatarequirestheuseofaunique