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

18 Pages·2015·2.01 MB·English

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,
* [email protected]
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
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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,how-
ever,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].
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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
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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
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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).
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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
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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(geometricdistor-
tion).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
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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
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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
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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