RESEARCHARTICLE View-Angle Tilting and Slice-Encoding Metal Artifact Correction for Artifact Reduction in MRI: Experimental Sequence Optimization for Orthopaedic Tumor Endoprostheses and Clinical Application PiaM.Jungmann1*,CarlGanter1,ChristophJ.Schaeffeler1,2,JanS.Bauer1,3, a11111 ThomasBaum1,ReinhardMeier1,MathiasNittka4,FlorianPohlig5,HansRechl5, RuedigervonEisenhart-Rothe5,ErnstJ.Rummeny1,KlausWoertler1 1 DepartmentofRadiology,KlinikumrechtsderIsar,TechnischeUniversitaetMuenchen,Ismaninger Strasse22,81675,Munich,Germany,2 MusculoskeletalImaging,KantonsspitalGraubuenden,Loestrasse 170,CH-7000,Chur,Switzerland,3 DepartmentofNeuroradiology,KlinikumrechtsderIsar,Technische UniversitaetMuenchen,IsmaningerStrasse22,81675,Munich,Germany,4 SiemensAG,Healthcare Sector,AlleeamRoethelheimpark2,91052,Erlangen,Germany,5 DepartmentofOrthopaedicSurgery, KlinikumrechtsderIsar,TechnischeUniversitaetMuenchen,IsmaningerStrasse22,81675,Munich, OPENACCESS Germany Citation:JungmannPM,GanterC,SchaeffelerCJ, * pia.jungmann@tum.de BauerJS,BaumT,MeierR,etal.(2015)View-Angle TiltingandSlice-EncodingMetalArtifactCorrection forArtifactReductioninMRI:ExperimentalSequence OptimizationforOrthopaedicTumorEndoprostheses Abstract andClinicalApplication.PLoSONE10(4):e0124922. doi:10.1371/journal.pone.0124922 AcademicEditor:HeyeZhang,Shenzheninstitutes Background ofadvancedtechnology,CHINA MRIplaysamajorroleinfollow-upofpatientswithmalignantbonetumors.However,after Received:January4,2015 limbsalvagesurgery,orthopaedictumorendoprosthesesmightcausesignificantmetal- Accepted:March10,2015 inducedsusceptibilityartifacts. Published:April24,2015 Purposes Copyright:©2015Jungmannetal.Thisisanopen accessarticledistributedunderthetermsofthe Toevaluatethebenefitofview-angletilting(VAT)andslice-encodingmetalartifactcorrec- CreativeCommonsAttributionLicense,whichpermits tion(SEMAC)forMRIoflarge-sizedorthopaedictumorendoprosthesesinanexperimental unrestricteduse,distribution,andreproductioninany modelandtodemonstrateclinicalbenefitsforassessmentofperiprostheticsofttissue medium,providedtheoriginalauthorandsourceare credited. abnormalities. DataAvailabilityStatement:Allrelevantdataare Methods withinthepaper. Funding:ThisworkwassupportedbytheGerman Inanexperimentalsetting,tumorendoprostheses(n=4)werescannedat1.5Twiththree ResearchFoundation(DFG)andtheTechnische versionsofoptimizedhigh-bandwidthturbo-spin-echopulsesequences:(i)standard,(ii) UniversitätMünchenwithinthefundingprogramme VATand(iii)combinedVATandSEMAC(VAT&SEMAC).Pulsesequencesincludedcoro- OpenAccessPublishing.Thefundershadnorolein nalshort-tau-inversion-recovery(STIR),coronalT1-weighted(w),transverseT1-wandT2- studydesign,datacollectionandanalysis,decisionto publish,orpreparationofthemanuscript. wTSEsequences.Forclinicalevaluation,VAT&SEMACwascomparedtoconventional metalartifact-reducingMRsequences(conventionalMR)inn=25patientswithmetalim- CompetingInterests:Theco-authorM.N.isan employeeofSiemensandwasinvolvedinthe plantsandclinicalsuspicionoftumorrecurrenceorinfection.Diametersofartifactswere PLOSONE|DOI:10.1371/journal.pone.0124922 April24,2015 1/18 SEMACforArtifactReductioninMRI technicaldevelopmentofthemetal-artifactreducing measuredquantitatively.Qualitativeparameterswereassessedonafive-pointscale pulsesequences.Hedidnotcontributeto,accessor (1=best,5=worst):“imagedistortion”,“artificialsignalchangesattheedges”and“diagnostic influencethedesign,performanceoranalysisofthis confidence”.Imagingfindingswerecorrelatedwithpathology.T-testsandWilcoxon-signed study.Thisdoesnotaltertheauthors'adherenceto PLOSONEpoliciesonsharingdataandmaterials. ranktestswereusedforstatisticalanalyses. Results ThetruesizeoftheprostheseswasoverestimatedonMRI(P<0.05).Asignificantreduction ofartifactswasachievedbyVAT(P<0.001)andVAT&SEMAC(P=0.003)comparedto thestandardgroup.QuantitativescoresimprovedintheVATandVAT&SEMACgroup (P<0.05).OnclinicalMRimages,artifactdiametersweresignificantlyreducedintheVAT&- SEMAC-groupascomparedwiththeconventional-group(P<0.001).Distortionandartificial signalchangeswerereducedanddiagnosticconfidenceimproved(P<0.05).Intwocases, tumor-recurrence,intencasesinfectionandinthirteencasesotherpathologieswere diagnosed. Conclusions SignificantreductionofmetallicartifactswasachievedbyVATandSEMAC.Clinicalresults suggest,thatthesenewtechniqueswillbebeneficialfordetectingperiprostheticpatholo- giesduringpostoperativefollow-up. Introduction Preoperativechemotherapyandthedevelopmentofspecificendoprostheseswithstrongerma- terialsandindividualimplantshaveenabledlimb-sparingsurgerywithagoodfunctionalout- comeinsteadofamputation[1–5].However,therearemorepotentialcomplicationswithlimb salvagethanwithamputationorrotationplasty[6].Fordetectionoftumorrecurrence,infec- tion,woundnecrosis,nervepalsyorprostheses-relatedpathologiessuchasloosening,foreign- bodyreactionorimplantfailure[7],MRimagingplaysamajorroleafterlimbsalvagesurgery [8–11].However,implantscancausesignificantmetal-induceddistortionofmagnetic-fieldho- mogeneity,leadingtoerrorsinsliceexcitationandspatialencoding.Whilesignallossdueto (cid:1) T2 shorteningcanbesuccessfullycompensatedbyusingspinechoes(SE),spatialdistortions duringslice-selectiveexcitationandsignalreadoutremainafundamentalproblem.Withincer- tainlimitations,definedbytheavailablehardware,theallowedspecificabsorptionrate(SAR) andminimumrequirementsforthesignal-to-noiseratio(SNR),spatialdistortioncanbead- dressedbyemployinghighbandwidthsforexcitationandreadout.Byusingstrongergradients, susceptibilityduetofieldinhomogeneitiesisreduced.Inthevicinityofmetallicimplants,however,thesegenericmethodsmostoftenfailtoprovidesufficientartifactsuppression[12–15]. Fortunately,morespecifictechniqueshavebecomeavailable,whicharecompatiblewiththe methodsdiscussedabove. In-planedisplacementinthereadout-encodingdirectionwithinanexcitedslicecanbecor- rectedbythe‘viewangletilting’(VAT)technique[16],whichappliesanadditionalgradientin slice-encodingdirectionduringsignalreadout.Toreduceblurring,whichisincreasedbyVAT, thinslicesandhighreadoutbandwidthsaremandatory,causingatrade-offwithspatialcover- ageandSNR,respectively[17]. PLOSONE|DOI:10.1371/journal.pone.0124922 April24,2015 2/18 SEMACforArtifactReductioninMRI Through-planedistortions,i.e.non-planarsliceexcitations,canbecorrectedwithatech- nique,dubbed‘slice-encodingmetalartifactcorrection’(SEMAC)[18],byapplyingadditional phaseencodinggradientsperpendiculartotheslice(z-direction)priortosignalreadout.Assum- ingthatthrough-planedistortionsoftheexcitedslicescanbelimitedtoablockofwidthSESx slicethickness(SES=numberofsliceencodingsteps)centeredonthenominalslicelocation,a numberofSESscanrepetitionsisneededinordertoperformtheSEMACphaseencoding.Un- likeVAT,SEMACincreasestheacquisitiontimeandrequiresadditionalpost-processing. AlthoughVATandSEMACappeartobeusefulformetalartifactreductiononMRimages aftertotalhipreplacement[19]ortotalkneereplacement[20],applicabilityforlargetumor endoprostheseswithmassivematerialhasnotbeenevaluatedsofar.Amajorchallengeisthe needofhighnumbersofslices,largefieldsofview(FOV)andindividualadjustmentforeach patient.Further,clinicalbenefitsofthenewsequencesinpatientswithmetalimplantsandsus- picionoftumor-recurrenceorperiprostheticinfectionhavenotbeendemonstratedsofar. Thepurposeofthisstudywas(i)toshow,thatthenewmetalreducingsequencesareappli- cablewithreducednumbersofsliceencodingsteps(SES),alargeFOVandahighnumberof slicesforMRimagingoforthopaedictumorendoprostheses,(ii)toevaluatequantitativelyand qualitativelythebenefitofVATandSEMACforMRimagingoflarge-sizedorthopaedictumor endoprosthesesinanexperimentalmodeland(iii)todemonstratethebenefitofVATand SEMACincomparisontopreviouslyappliedmetalartifactreducingMRsequencesintheeval- uationofperiprostheticsofttissueabnormalities. MaterialandMethods Experimentalsetting Inanexperimentalmodel,fourdifferentcommerciallyavailableorthopaedictumorendo- prosthesesforreplacementoflargeboneandjointdefectsinthelowerlimbswereassessed (MUTARS,implantcastGmbH,Buxtehude,Germany;Fig1):(i)Silver-coatedtitanium (TiAl V )proximalfemurreplacementwithacementedintraosseousshaft;(ii)Titanium 6 4 proximalfemurreplacementwithacementlessintraosseousshaft;(iii)Cobalt-chromium (CoCrMo)distalfemurreplacementattachedtoacementedproximaltibiareplacement(Co- balt-Chromium)withaninterposedpolyethylenecomponent;(iv)Titaniumnitride(TiN)- coated,CoCrModistalfemurreplacementattachedtoasilvercoatedtitaniumproximaltibia implantwithacementlessintraosseousshaftandaninterposedpolyethylenecomponent. Tumorendoprostheseswereindividuallyembeddedinwater,whichsurroundedtheprostheses >10mmineverydirection. Magneticresonance(MR)Sequences MRimagingwasperformedona1.5Tsystem(MagnetomAvanto,SiemensAG,Erlangen,Ger- many).Experimentalscanswereperformedwithasix-channelbodyphasedarraycoilanterior- lyandtwospinecoilclusters(threechannelseach)posteriorly.Forclinicalscans,depending onthelocalizationofthetumorthebodycoilordedicatedsurfacecoilswereused.Awork-in- progresssoftwarepackageincludingturbospinecho(TSE)pulsesequences,allowingformore flexibleadjustmentofRFandreadoutbandwidthsandoptionalapplicationofVATand SEMAC,wasprovidedbySiemensAG(SiemensAG,HealthcareSector,Erlangen,Germany). ExperimentalMRscans Pulsesequencescomprisingshort-tau-inversion-recovery(STIR),T1-weighted(w)andT2-w contrastwereadaptedforourpurposesbyincreasingthenumberofslicesandenlargingthe PLOSONE|DOI:10.1371/journal.pone.0124922 April24,2015 3/18 SEMACforArtifactReductioninMRI Fig1.Experimentalmodel.Tumorendoprostheseswereembeddedinwater.Specificationsandmaterials areindicatedontheleft.CentralslicesofcoronalSTIRandcoronalT1-wMRpulsesequencesarepresented foreachtechnique(standard;VAT,view-angletilting;VAT&SEMAC,view-angletiltingandslice-encoding metalartifactcorrection). doi:10.1371/journal.pone.0124922.g001 PLOSONE|DOI:10.1371/journal.pone.0124922 April24,2015 4/18 SEMACforArtifactReductioninMRI Table1. MRpulsesequences,experimental. Pulsesequence CoronalSTIR CoronalT1 TransverseT2 TransverseT1 Repetitiontime(RT;msec) 7600 546 6810 606 Echotime(ET;msec) 40 6.3 576 5.4 Slicethickness(mm) 4 4 4 4 Refocusingflipangle 150° 140° 136° 120° Fieldofview(FOV;mm) 500 500 220 220 Matrix 256x265 256x256 256x256 256x256 Bandwidth(Hz/Pixel) 651 651 574 610 Echotrainlength 23 7 21 7 Inversiontime(msec) 145 - - - Numberofslices 25 27 40 40 Slice-encodingsteps(SES) SEMACgroup 6 6 VATgroup,Standardgroup 0 0 0 0 Acquisitiontime(min) SEMACgroup 4:43 3:49 4:41 4:55 VATgroup,Standardgroup 0:55 0:41 VAT,view-angletilting.SEMAC,slice-encodingmetalartifactcorrection.1cm2=100mm2. doi:10.1371/journal.pone.0124922.t001 FOV.ReductionofSESfrom10to8,6,4,2and0(0isequivalenttotheVATgroup)wasevalu- atedforcoronalT1-wpulsesequencesandfindingswereconfirmedforotherprosthesesand pulsesequences.MRscanswereacquiredinthreedifferentstudygroups(Table1).Inthefirst group,imageswereacquiredbyusingstandardhigh-bandwidthtechniqueswithoutapplica- tionofVATorSEMAC(otherwiseidenticalsequences;standardgroup).Inthesecondgroup VATwasapplied(VATgroup).Inthethirdgroupboth,VATandSEMACwith6SESwere used(VAT&SEMACgroup).WhileSEMACprofitsfromincreasingthenumberofSES,the directproportionalitybetweenacquisitiontimeandSESisanontrivialissue.Toarriveatclini- callyreasonablescandurations,theavailablerangeofconventionalaccelerationstrategies(in- creasedreadoutandRFbandwidths,highturbofactors,parallelimaging)hastobeexploited. Coronalimageswereacquiredinallthreegroups,whereastransverseimageswereacquiredin theVATgroupandthestandardgrouponly.Withtheexceptionoftransversecoverageof large-sizedtumorprostheses,whichrequireaparticularlylargenumberofslices,clinicallyac- ceptablescantimescouldbereachedincombinationwithSEMAC. Clinicalexample Exemplarily,VAT&SEMACwasappliedinapatientwithalargetumorendoprosthesisduring routinefollow-upMRimaging.MRimageswereperformedinayoung,femalepatient(15 yearsold)afterresectionofanosteosarcomagradeIIIattheleftdistalfemurandimplantation ofacombinedfemoralandtibialreplacement.PulsesequencesincludedacoronalSTIRse- quence(RT,9000msec;ET,39msec;slicethickness,3.5mm;refocusingflipangle,150°;FOV, 48cm2;matrix,448;bandwidth,657Hz/Pixel;inversiontime,145msec;numberofslices,30; SES,6;acquisitiontime,6:29min)andacoronalT1-wsequencewithoutandwithcontrastap- plication(RT,716msec;ET,5.7msec;slicethickness,3.5mm;refocusingflipangle,140°;FOV, 48cm2;Matrix,512;Bandwidth,651Hz/Pixel;Echo-spacing,5.72msec;numberofslices,30; SES,6;acquisitiontime,6:56min). PLOSONE|DOI:10.1371/journal.pone.0124922 April24,2015 5/18 SEMACforArtifactReductioninMRI Studypopulation Inthefollowing,comparisonofVAT&SEMACandconventionalmetalartifactreducingMR sequences(conventional)wasperformed.Twenty-fiveconsecutivepatients,referredfromthe orthopaedicdepartmentwithmetalimplantsandwithclinicalsuspicionoflocaltumorrecur- renceorperiprostheticinfection,wereprospectivelyincludedinthepresentstudy.Writtenin- formedconsentwasobtainedfromallpatients.Thestudywasreviewedandapprovedbythe localinstitutionalreviewboards(EthikkommissionderFakultaetfuerMedizinderTech- nischenUniversitaetMuenchen,Germany).Thestudyhasbeenconductedaccordingtothe principlesexpressedintheDeclarationofHelsinki.Exclusioncriteriaincludedgeneralcontra- indicationsforMRimaging. ClinicalMRexaminations VAT&SEMAC-techniquesandconventionaltechniqueswerecomparedforcoronalSTIR (n=19)andcoronalcontrastenhancedT1-wsequences(n=19).AllMRsequencesweread- justedforeachindividualpatient,basedontheexperimentalMRprotocol.MRparametersare presentedinTable2. Histopathology Bymeansofdigitalmedicalrecordshistopathologicreportsandmicrobiologyreportswereret- rospectivelyanalyzedinthosepatientsthathadundergonebiopsyorsurgeryfollowingtheMR examinationduetosuspicionoftumorrecurrenceorperiprostheticinfection. QuantitativeAnalysis Intheexperimentalmodel,foreachendoprosthesis,objectivequantitativemeasurementsof artifactdiameterswereperformedbytwoinvestigatorsinconsensus(P.M.J.andC.J.S.).Atsix standardizedlevels,asindicatedinFig2,artifactdiameters(includingsignallossandsignal pile-up)werecomparedtotruediametersoftheprosthesesusingaverniercapiler.Thesemea- surementswereconfirmedonoriginalsketchesofthetumorendoprosthesesprovidedbythe manufacturer(implantcastGmbH,Buxtehude,Germany;Fig2).Atthreelevels(markedwith asterisksinFig2),realdiameterswereidenticalfordifferentimplantmaterialsanddifferent Table2. ExemplaryclinicallyappliedMRpulsesequences. Pulsesequence CoronalSTIRConventional CoronalSTIRVAT&SEMAC CoronalT1Conventional CoronalT1VAT&SEMAC Repetitiontime(RT;msec) 5350 9150 500 716 Echotime(ET;msec) 60 43 15 5.7 Slicethickness(mm) 5 3.5 5. 3 Refocusingflipangle 180 150 180 140 Fieldofview(FOV;mm) 400 400 400 400 Matrix 384x384 238x384 517x640 410x512 Bandwidth(Hz/Pixel) 766 651 710 651 Echotrainlength 7 23 5 7 Inversiontime(msec) 155 145 - - Numberofslices 20 28 20 28 Slice-encodingsteps(SES) - 6 - 6 Acquisitiontime(min) 4:13 5:40 5:13 6:56 VAT,view-angletilting.SEMAC,slice-encodingmetalartifactcorrection.1cm2=100mm2. doi:10.1371/journal.pone.0124922.t002 PLOSONE|DOI:10.1371/journal.pone.0124922 April24,2015 6/18 SEMACforArtifactReductioninMRI Fig2.Truediametersoforthopaedictumorendoprostheses.Truediametersweremeasuredatsix standardizedspotsasindicatedandconfirmedonoriginalsketches(continuouslines).Asterisksindicate threestandardizedspotsthatwereusedforcomparisonsofdifferentimplantmaterials;brokenlinesindicate diametersthatwereadditionallymeasuredforthispurpose. doi:10.1371/journal.pone.0124922.g002 coatings,allowingacomparisonofartifactsizesforthese.OnclinicalMRimagesof patientsquantitativeartifactdiametersweremeasuredatfourrandomlyselectedspotsfor eachimplant. QualitativeMRAnalysis QualitativeevaluationofMRimageswasperformedonpicturearchivingcommunicationsys- tem(PACS)workstations(EasyVision,Philips,Best,TheNetherlands).ForMRimagesfrom theexperimentalmodel,evaluationwasperformedseparatelybytworadiologists(P.M.J.and C.J.S.,5and10yearsofexperience).Incaseofdiscordance,consensusreadingsbybothradiol- ogistsandanadditionalindependentmusculoskeletalradiologist(J.S.B.,12yearsofexperi- ence)werecommenced.Clinicalscanswerereadoutinconsensusbytworadiologists(P.M.J. andR.M.5and8yearsofexperience).Incaseofdiscordance,consensusreadingsbybothradi- ologistsandanadditionalindependentmusculoskeletalradiologist(C.J.S.,10yearsofexperi- ence)werecommenced. Spatial-frequencyinhomogeneity-inducedsusceptibilityartifactswereassessedsubjectively onafive-pointscale(1=minimalartifacts;5=maximumartifacts)[12]: 1. “Geometricimagedistortion”withanatomicdisplacement,describinghowaccuratethe truegeometricshapeoftheprosthesiswasdepictedontheMRimage(geometricdistortion).Thisparametermainlydescribedthrough-planeimagedistortion,butwasalsoaffect- edbyin-planedistortion. 2. “Artificialsignalchanges”withsignallossorsignalpile-upintheimmediatevicinityofthe prostheses,especiallyattheedges,intersectionsandinterfacesbetweendifferentimplant components(signalchanges).Theparameter”artificialsignalchanges”implicatedbothin- planeandthrough-planedistortion. Forclinicalscans,theimageswerereadoutdiagnostically.Additionally,theparameters“di- agnosticconfidence”and“spacialblurring"weredocumentedonafive-pointscale(1=best, 5=worst).Tumorrecurrencewasdiagnosed,ifanewlydevelopedmasswasidentified,thatex- hibitedthesameMRcharacteristicsastheprimarylesion.Periprostheticinfectionwasdiag- nosed,ifintenseedemateoussofttissueswellingwithmarkedcontrastenhancementwas depicted;additionallooseningoftheimplantwassuspectedincaseofperiprostheticfluid collections.Abscessformationwasdiagnosed,ifawelldefinedfluidcollectionwithathick PLOSONE|DOI:10.1371/journal.pone.0124922 April24,2015 7/18 SEMACforArtifactReductioninMRI contrast-enhancingwallandsurroundingsoft-tissueedemawasseen.Incontrast,seromawas definedasalocalizedfluidcollectionwithoutorwithinternalstructuressurroundedbya smoothlining,whichexhibitedlowT2-weightedsignalintensityandlittletomoderatecontrast enhancementintheabsenceofadjacentsofttissueedema.Bursitiswassuspected,ifalobulated fluidcollectionwasseeninatypicalanatomiclocation.Boneinfarctionwasdefinedasaserpin- giousareawithinbonemarrow,whichexhibitedadouble-linesignandinternalfatsignal. Reproducibilitymeasurements Forqualitativemeasurements,allexperimentalMRscansweregradedtwiceforeachparameter bytworadiologists(P.M.J.andC.J.S.)ontwoseparateoccasionswithanintervaloffourweeks in-betweenthetwotimepoints.Inter-andintra-observeragreementwascalculatedusing linear-weightedCohensKappa’svalues[21].Reproducibilityforquantitativediametermea- surementswasdeterminedinasampleof18randomlyselectedmeasurements.Eachlevelwas measuredthreetimesbytwoinvestigators(P.M.J.;C.J.S.).Reproducibilityerrorswerecalculat- edastherootmeansquareerror(ms)andastherootmeansquareerrorcoefficientofvariation (%)[22]. Statisticalanalysis StatisticalprocessingwasperformedwithSPSSversion17.0(SPSSInstitute,Chicago,IL,USA; P.M.J.,T.B.).DifferencesbetweengroupswereassessedviaT-tests(quantitativeassessment)and Wilcoxon-signedranktests(qualitativeassessment).Resultsarereportedasmean±standard errorofthemean(SEM).Meandifferences±SEMandlower95%ConfidenceInterval(CI)and upper95%CIwerecalculatedforintergroupcomparisons.DifferenceswithaP<0.05werecon- sideredasstatisticallysignificant. Results MRSequences ThenumberofSESforSEMACwasreducedwithrespecttotheoriginalMRprotocolwith12 SES(Fig3).Theartifactdiameterwassignificantlylargerinthestandardgroupthaninall othergroups(meandifference±SEMfrom1.8±0.7mmto2.6±1.0mm;P<0.05).Diametersin thegroupswithdifferentSESwerenotstatisticallysignificance.Subjectivequalitativeevalua- tionshowedacontinuousreductionofartifactswithincreasingSESuntil6SES(Fig3). ExperimentalquantitativeMRanalysis Meantruediameter±SEMatthemeasuredlevelsofthetumorprostheseswas34.7±4.0mm, whichwassignificantlysmallerthanartifactdiametersonallMRimages(P<0.05;Fig4; Table3).ThelargestartifactdiametersweremeasuredonstandardcoronalSTIRimages(42.2 ±4.7mm;P<0.001).ArtifactsintheVATandintheVAT&SEMACgrouphadadiameterof 39.9±4.6mm(P<0.001)and39.5±4.4mm(P=0.003).Artifactsweresignificantlysmalleron T1-wimagescomparedtoSTIRimages(P<0.05).OnT1-wimagesthemeanartifactdiameter was39.4±4.7mminthestandardgroup,37.4±4.5mmintheVATgroup(P=0.003)and37.6 ±4.5mmintheVAT&SEMACgroup(P=0.011).OntransverseT2-wimages,meanartifactdi- ameterwas48.1±4.0mminthestandardgroupand45.6±3.0mmintheVATgroup(P=0.016; T1-wimages,45.2±3.6mmversus42.0±3.2mm,P=0.031).OncoronalSTIRimages,artifacts producedbyimplantsmadefromTiAl V weresmallerthanartifactsproducedbyCoCrMo 6 4 implants(29.4±0.4mmversus30.6±0.7mm;P=0.002).ForcoronalT1-wimagesthisdiffer- encewassmaller(TiAl V versusCoCrMo,28.7±0.3mmversus29.5±0.7mm;P=0.047). 6 4 PLOSONE|DOI:10.1371/journal.pone.0124922 April24,2015 8/18 SEMACforArtifactReductioninMRI Fig3.Reductionofslice-encodingsteps(SES)forslice-encodingmetalartifactcorrection(SEMAC).SESwerereducedfrom10to0andassessed quantitativelyandqualitatively.NoSEMACisequivalentto0SES.AcquisitiontimeforallSESvaluesisshownintheimages.Forqualitativeevaluation, artifactdiameters(mm)weremeasuredonallMRimages.VAT,view-angletilting.SEMAC;slice-encodingmetalartifactcorrection. doi:10.1371/journal.pone.0124922.g003 Therewasnosignificantdifferencebetweenartifactsproducedbyproximalfemurprostheses (TiAl V )withandwithoutsilvercoating(P>0.05;STIR,P=0.957)orbetweenartifactspro- 6 4 ducedbydistalfemurprostheses(CoCrMo)withandwithoutTiNcoating(P>0.05;STIR, P=0.464). ExperimentalqualitativeMRanalysis Withrespecttothequalitativeparameter“geometricdistortion”combinedapplicationofVAT andSEMAC(VAT&SEMACgroup)showedthesmallestartifacts(STIR,1.50±0.25,P=0.002; Table4).TheVATgroupshowedameanvalueof3.25±0.25andthestandardgroupshoweda meanvalueof4.25±0.25.LessdistortionwasobservedoncoronalT1-wimagesascomparedto STIRimages.Ontransverseimages,distortionwasreducedbyapplicationofVAT.Distortion wasseenattheinterfacebetweenthecollarofthestemandthestemoftheprosthesis(Fig5A) andbetweenthemetalandthepolyethylenecomponent(“B“).“Artificialsignalchanges”were reducedbytrendonMRimagesintheVATgroupascomparedtothestandardgroup(STIR; 3.50±0.25versus4.25±0.50P=0.058;Table4).Significantreductionofsignalchangeswas PLOSONE|DOI:10.1371/journal.pone.0124922 April24,2015 9/18 SEMACforArtifactReductioninMRI Fig4.QuantitativeresultsforcoronalMRpulsesequences(STIRandT1-wsequences).Diametermeasurementswereperformedatsixstandardized levelsonallMRimagesandcomparedtotruediametersofthetumorendoprostheses.Studygroupswereastandardgroup(Standard)withoutapplicationof VAT(view-angletilting)orSEMAC(slice-encodingmetalartifactcorrection)butotherwiseidenticalpulsesequences,agroupwhereVATwasapplied(VAT; noSEMAC)andagroupwherebothtechniques,VATandSEMAC,wereapplied(VAT&SEMAC).Meanvalues±SEMarepresented.Truediameterswere significantlysmallerthanartifactdiametersinallgroups(P<0.05). doi:10.1371/journal.pone.0124922.g004 achievedbyadditionalapplicationofSEMAC(2.75±0.25;Fig5C).Similarresultswerefound forcoronalT1-wimages. Clinicalexample MRscansofafemalepatientafterresectionofanosteosarcomaarepresentedinFig6.Postop- erativesignalchangesofthesurroundingtissue,butnosuspiciouspathologiesweredetected. Clinicalresults Twenty-fiveconsecutivepatients(13male,12female)withn=30metalimplantsandamean age±standarddeviation(SD)of58.5±20.9years(range,15–80years)wereprospectively Table3. Quantitativemeasurements,experimental. CoronalSTIR CoronalSTIRVAT& Coronal Coronal TransverseT2 TransverseT1 VAT SEMAC T1VAT T1VAT&SEMAC VAT VAT MeanDifferencevs. -2.3 -2.7 -2.0 -1.9 -2.5 -3.2 "Standard”(mm) SEM 0.5 0.8 0.6 0.7 1.0 1.4 Lower95%CI -1.3 -1.0 -0.8 -0.5 -0.5 -0.3 Upper95%CI -3.3 -4.3 -3.2 -3.2 -4.5 -6.1 P-value <0.001 0.003 0.003 0.011 0.016 0.031 ReductionofartifactsintheVATgroupandtheVAT&SEMACgroupascomparedtothestandardgroupforeachpulsesequence.SEM,standarderrorof themean;CI,confidenceinterval;P,P(t-test). doi:10.1371/journal.pone.0124922.t003 PLOSONE|DOI:10.1371/journal.pone.0124922 April24,2015 10/18

View-Angle Tilting and Slice-Encoding Metal Artifact Correction for Artifact Reduction in MRI

Similar View-Angle Tilting and Slice-Encoding Metal Artifact Correction for Artifact Reduction in MRI