case reviews in mriamos3.aapm.org/abstracts/pdf/155-54031-1531640-157798-104127069.pdf · •...
TRANSCRIPT
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CaseReviewsinMRI
Nathan Yanasak, PhDDepartment of Radiology and ImagingAugusta University
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Ihavenothingtodisclose.
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OverarchingThemeHowmightamedicalphysicistsolveproblemsintheMRIworld?
NathanYanasak
Fourrealms:• AcuteClinicalCare• ResearchProject• Accreditation/QualityAssurance• ”Fixingrecurrentproblems”intheclinic
Whatstrategies/cluesdidIusetoidentify/solvetheseproblems?
“Mywayoflearningistoheaveawildandunpredictablemonkey-wrenchintothemachinery.”
―DashiellHammett,TheMalteseFalcon
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Case#1
Trackingdownasubtleartifactinaresearchproject
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ScenarioDate:August2019– February2020
Settinguparesearchprotocolforagrant• Protocolonournewestscanner.• Imagequalityisnotverygoodonvariousscans,afterrepeatedtries.
• Settingupaprotocolonanolderscannerasabackup.• Newartifactisdiscoveredonstructuralscansontheolderscanner.
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RelevantFacts• Keyprotocolelements:DTI(~15min),ASL,“hi-res”T1structuralimage.• Originalplatform:PhilipsIngenia scanner+32-channelheadcoil.• NumerousartifactsonPhilipssystem.Serviceproblemswithgradients.• Settingupolder3TGEHDxT (8-channelheadcoil)asbackupsystem.• Aftermyinitialcheckforquality,datasenttouniversityresearchteamfor
scrutiny.• UGAQCvs.AUQCmethods:
“it’snotasgoodasithasbeen”vs.metrics/numbersareinspec.Prosvs.Cons
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What’sGoingOnHere?
UGAlooksatimagesusingresearch-specificsoftware(AFNI,SPM,andFSL).
Interfaceshowsreconstructionfrom3differentorientations(axialacquisition).
Wehadn’texpectedslice-to-slicedifferences,sothiswasasurprise.
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Verify
LoadtheimageintoImageJandverifywithacoronalreformatthatIseethesamething.
Artifactpresent?Yes– actualartifact
à NotUGAsoftware/methodologyissue.
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Howcanslicesbedifferent?Brainstorming…
Axialacquisitionà S-Islice-selectdirection.Looksveryperiodic…
Alittletimebymyselftoconsidertheoptions:Whataresomepossiblesources?Acquisitionordering?Slicegap?DifferentwindowandlevelsettingsinDICOMforimageslices?
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Howcanslicesbedifferent?
LoadtheimageontoOsirix andverifywithacoronalreformatthatIseethesamething.Stillseetheartifact?
Yes– likelynotaDICOMissue(window/levelperslice)
Waitaminute!Thisisa3Dscan,so… noslicegaporacquisitionordereffectspossible.
Remainingpossiblities:herringboneartifact(spikeink-space)forour3Dscan?whypurelyalongtheS-Idirection?
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Howcanslicesbedifferent?UGAhastwosamplesfromus,anditoccurredinbothsetsofscans.Differentstrength…sameperiodicity(e.g.,everyXslices?)Periodicitythesame,butfainter.
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Characterizethesizeandlookatprotocol.Measurethe“periodicity”intheimageusingImageJ:
Repeatsaboutevery5slices(@1mmslicethk,effectis5mm).Eureka!(andtextingwithJasonStaffordtothinkthisthrough)• 3Dscanisnon-obliqueaxiallyprescribed• pMRI turnedon(image-based…SENSE-like)• pMRI calibrationscanpurelyinaxialorientation…5mmslicethickness.
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Speed:lessphaseencodes=smallerFOV(withsameresolution)
13
aliasing
SmallerFOV
a
ba ”+”b
ReviewofpMRI andcalibrationscans#1
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pMRI calibrationscans• Lo-resdata(eitherimagesorraw)thatallowsyouquantifythesensitivityofthecoil,to“unwrap”theundersampled image.
• Separateacquisitionperpatient.I’1=s1,aIa +s1,bIba
b
b
I’2=s2,aIa +s2,bIb
a
Ia
Ib
I’2
I’1
element1
element2s2
s1
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FollowingUp
Slice8(T2,TE=80msec)
ChangecalibrationscanthicknessPhantomtests
Solution:• Slightobliquityofcalibrationscan• Forcecalibrationslicestobenon-
multipleofimagingslicethickness.
Whydidweneverseethisclinically?
Intersectionofpureaxial3Dscansandcal scansofcertainthickness.
5mmcalibration
10mmcalibration
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Case#2
“ShouldIbelievewhatIseeinthispatient’shead?”
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ScenarioJune27-28th 2015
Quickmeeting(firstthinginAM)inareadingroombasedonaclinicalimage
• Neuroradiologist callsmyofficetoconsultaboutimagefeature– artifactornot?
• Patienttohavesurgeryshortlythereafter,soneedtodecidequickly/efficiently.
• Featureisweird/potentiallydisturbing.
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RelevantFacts• 41-yroldmalewithParkinson’s,pre-op
scanbeforeinstallingadeepbrainstimulator.
• Surgeryscheduledfornextday…but,toinstallaDBSortoextractthispathology?
• FeatureonlyseenincoronalT2FLAIR.• Doesn’tlooklikeanytypicalartifact– no
periodicity,ghosting…• NothingobviousinFOV.
… mygutisalreadytellingmesomething.
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Frustration&ParallelImagingArtifacts
pMRI artifactshaveacertainappearance/behaviorforwhichyoumaydevelopagutinstinct…andthenyoustarttoseethemeverywhere.
Inparticular:• periodicities• well-definedghostsinseeminglyrandomplaces• stuffintheMIDDLEofyourFOV
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SensitivityMapMismatchArtifacts
Thin,brightstructuresintheperipheryofsensitivitymap—mismatchbetweensensitivityandanatomy.
Acceleration=2inthisexample.
Duplicatespacing=½xFOV
Acceleration=Nà Duplicatespacing=1/NxFOV
Yanasak &Kelly,Radiographics,2014
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SensitivityMapMismatchArtifacts
Thin,brightstructuresintheperipheryofsensitivitymap—mismatchbetweensensitivityandanatomy.
Acceleration=2inthisexample.
Duplicatespacing=½xFOV
Acceleration=Nà Duplicatespacing=1/NxFOV
Yanasak &Kelly,Radiographics,2014
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SensitivityMapMismatchArtifactsYanasak &Kelly,Radiographics,2014
Thin,brightstructuresintheperipheryofsensitivitymap—mismatchbetweensensitivityandanatomy.
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2DAcceleration(3Dscan)andSensitivityMapMismatchArtifacts
Cerebellarlesion?
No…sensitivitymapmismatch,projectedinslicedirection(PE#1,accfactor=2),
90SlicesinseriesSlice#58
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2DAcceleration(3Dscan)andSensitivityMapMismatchArtifacts
Cerebellarlesion?
No…sensitivitymapmismatch,projectedinslicedirection(PE#1,accfactor=2),
90SlicesinseriesSlice#58Slice#12
58-12=46~90/2
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2DAcceleration(3Dscan)andSensitivityMapMismatchArtifacts
Cerebellarlesion?
No…sensitivitymapmismatch,projectedinslicedirection(PE#1,accfactor=2),
...andin-planephase-encodingdirection(PE#2,accfactor=2)
90SlicesinseriesSlice#58Slice#12
58-12=46~90/2
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2DAcceleration(3Dscan)andSensitivityMapMismatchArtifacts
Cerebellarlesion?
No…sensitivitymapmismatch,projectedinslicedirection(PE#1,accfactor=2),
...andin-planephase-encodingdirection(PE#2,accfactor=2)
Don’tgetmyopicaboutartifactorigins…lookawayfromthesource.
90SlicesinseriesSlice#58Slice#12
58-12=46~90/2
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Case#3
ACRtestingdifficultiesthataresointractable,theyaretrivial.
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ScenarioJuly2012Acceptancetestingofbrandnewmagnet• AfewkeyACRtestskeepfailingrepeatedly(multipletrials,multipledays).
• Philips3TIngenia +15-channelneurovascularheadcoil.• ACRphantom:associatedwithanold1.5TSiemensthatwedecommissioned
(sameMRIsuite).• Lowcontrastdetectability– failingonslice9(asIrecall)
à (onmany3Tsystems,all40spokesvisible)• Uniformity– failingjustbarely,severaltimes.• Finallypassedafterusingspecificimagecorrectionroutine(PIU=83.6%>82%)
RelevantFacts
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VerifyScanthephantommultipletimes.Sameproblem?
Yes– nosingle-scananomaly(although…whatifitpasses2ndtime?)
Repositionandrescan.Sameproblem?Yes– somethingfundamentallywrong(nopositioningissues).
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CompareSpeakwithphysicistatMissionhospitalsinAsheville,NC(newIngenia).“How’syourACRperformance?”
Capableofpassinglowcontrastdetectability?Yes (~39spokes…notprettyforanew
3T,butsobeit)– problemwithourmagnet?
WhatelsecanwedotodeterminewhetheritisOURmachine/institutionvs.asystematicPhilipsissue?
Slice8(T2,TE=80msec)
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TrySomethingDifferent(HailMary)Aftertwodaysofscanning,wetriedourACRphantomfortheGE3Tdownstairs.
Didwereproducefailure?NO!40spokesvisible
Repeatscans…success?YES!!!à Phantomperformanceissues?
Slice8(T2,TE=80msec)
Morefacts:ACRphantomfor1.5TSiemens:purchased2001ACRphantomfor3TGE:purchased2003-4Solution:Weboughtanewphantom(Philips)in2012
Slice8(T2,TE=80msec)Slice8(T2,TE=80msec)
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FollowingUp
Slice8(T2,TE=80msec)
Ihaveheardabout“potatochip”-ing ofLCDinserts(seeslidefromDonnaReeve).
Couldsomethingsimilarbehappeningforus?
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Test#1:Scannedtheoldphantomandthenewer(GE)phantomashi-ressagittal.Reformatasaxial.Overlayimages(inMatlab):older(Siemens)ACRphtm (red),newer(GE)ACRphtm (blue)– colormismatch=offset.Findoffsetof>2mmbetweendisksandwedge/phantomhousing.
FollowingUp
Discaligned Wedge/Phtm aligned
Othercomments:Grid/wedgealittlemisaligned,variousstructuresonslc 1alignedwithwedge.
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Alsoscannedtheold(Siemens)phantomandournewest(Philips)phantomw/ACRprotocolusinga32-channelheadcoil.
FollowingUp
Bothpass(muchhigherSNRcoil)…
And,stillseeoffset~2mm indiskhousing.
Slice8(T2,TE=80msec)
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Case#4
Usingmedicalphysics“intangibles”tomaintainclinicalpractice
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ScenarioCirca2011-2
LiverIronQuantificationforSickle-CellPatientisinError• Beganapartnershipwithoursickle-cellclinic,scanningpatientswhomight
benefitfromchelationtherapy(Feoverload).
• Westartedperformingtheexams~2011;concurrently,akeyservicecontractwaseliminated(hadsoftwaresince2003-ish).
• Afewyearslater,theFequantificationsoftwarestoppedperformingasexpected.
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RelevantFacts• GEworkstationwithReportCard software(forFequantification).
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RelevantFacts• GEworkstationwithReportCard software(forFequantification).
TIME(TE)
INTENSITY
I j (ti ) = Aje−ti /T 2
*j +Cj
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RelevantFacts• GEworkstationwithReportCard software(forFequantification).• Discoveryofabug– clearlyapoorfittothedata.• Institutionalcontractswithanothervendorà nonewsupportforworkstation
(I.e.,nonewreleasesforus)
FitreportedasT2*=7.5±1.0msec
Problem:longerT2*=lessFe=healthier.
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Problem:• FDAclearedsoftware(methodtested),butfittingalgorithmitselfisfailing.
Solution:• Writesomesoftwarethatdoesexactlythesamething(methodology)buthasa
morerobustfitter(andbaselineasymptoticintensity=fluid).
Validation:• Validateaccordingtoseveralpreviouscaseswithgoodfits.• Samefitvaluesof[Fe]?
Yes!!
RestatementofProblem
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Problem:• FDAclearedsoftware(methodtested),butfittingalgorithmitselfisfailing.
Solution:• Writesomesoftwarethatdoesexactlythesamething(methodology)buthasamorerobust
fitter(andbaselineasymptoticintensity=fluid).
Validation:• Validateaccordingtoseveralpreviouscaseswithgoodfits.• Samefitvaluesof[Fe]?
Yes!!
T2*=2.6msec vs.7.5msec – bigclinicaldifference.
RestatementofProblem
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WhatHaveWeLearned?42
• Brainstormissues,thencomeupwitha(informal)decisiontreetodiagnoseyourproblem.
(e.g.,canyouseparatesoftwarefromscanner/user/phantomissues?)• Beingabletorunseriesofscanswithslightdifferencesishelpfulfroma
validationstandpoint.• HavingahandfulofquickanddirtytoolslikeImageJcanhelp(programming
canbeveryuseful).Tryandmeasure/overlaystuff– messaround.• DON’TGIVEUPPREMATURELYWITHA½-ANSWERINHAND.• pMRI canintroducemanysubtleartifacts.Learntheircharacteristics/quirks.• Donotdiscountthe“impossible”(e.g.,ACRphantomissues).
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Questions?