CaseReviewsinMRI

Nathan Yanasak, PhDDepartment of Radiology and ImagingAugusta University

Ihavenothingtodisclose.

OverarchingThemeHowmightamedicalphysicistsolveproblemsintheMRIworld?

NathanYanasak

Fourrealms:• AcuteClinicalCare• ResearchProject• Accreditation/QualityAssurance• ”Fixingrecurrentproblems”intheclinic

Whatstrategies/cluesdidIusetoidentify/solvetheseproblems?

“Mywayoflearningistoheaveawildandunpredictablemonkey-wrenchintothemachinery.”

―DashiellHammett,TheMalteseFalcon

Case#1

Trackingdownasubtleartifactinaresearchproject

ScenarioDate:August2019– February2020

Settinguparesearchprotocolforagrant• Protocolonournewestscanner.• Imagequalityisnotverygoodonvariousscans,afterrepeatedtries.

• Settingupaprotocolonanolderscannerasabackup.• Newartifactisdiscoveredonstructuralscansontheolderscanner.

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

What’sGoingOnHere?

UGAlooksatimagesusingresearch-specificsoftware(AFNI,SPM,andFSL).

Interfaceshowsreconstructionfrom3differentorientations(axialacquisition).

Wehadn’texpectedslice-to-slicedifferences,sothiswasasurprise.

Verify

LoadtheimageintoImageJandverifywithacoronalreformatthatIseethesamething.

Artifactpresent?Yes– actualartifact

à NotUGAsoftware/methodologyissue.

Howcanslicesbedifferent?Brainstorming…

Axialacquisitionà S-Islice-selectdirection.Looksveryperiodic…

Alittletimebymyselftoconsidertheoptions:Whataresomepossiblesources?Acquisitionordering?Slicegap?DifferentwindowandlevelsettingsinDICOMforimageslices?

Howcanslicesbedifferent?

LoadtheimageontoOsirix andverifywithacoronalreformatthatIseethesamething.Stillseetheartifact?

Yes– likelynotaDICOMissue(window/levelperslice)

Waitaminute!Thisisa3Dscan,so… noslicegaporacquisitionordereffectspossible.

Remainingpossiblities:herringboneartifact(spikeink-space)forour3Dscan?whypurelyalongtheS-Idirection?

Howcanslicesbedifferent?UGAhastwosamplesfromus,anditoccurredinbothsetsofscans.Differentstrength…sameperiodicity(e.g.,everyXslices?)Periodicitythesame,butfainter.

Characterizethesizeandlookatprotocol.Measurethe“periodicity”intheimageusingImageJ:

Repeatsaboutevery5slices(@1mmslicethk,effectis5mm).Eureka!(andtextingwithJasonStaffordtothinkthisthrough)• 3Dscanisnon-obliqueaxiallyprescribed• pMRI turnedon(image-based…SENSE-like)• pMRI calibrationscanpurelyinaxialorientation…5mmslicethickness.

Speed:lessphaseencodes=smallerFOV(withsameresolution)

13

aliasing

SmallerFOV

a

ba ”+”b

ReviewofpMRI andcalibrationscans#1

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

FollowingUp

Slice8(T2,TE=80msec)

ChangecalibrationscanthicknessPhantomtests

Solution:• Slightobliquityofcalibrationscan• Forcecalibrationslicestobenon-

multipleofimagingslicethickness.

Whydidweneverseethisclinically?

Intersectionofpureaxial3Dscansandcal scansofcertainthickness.

5mmcalibration

10mmcalibration

Case#2

“ShouldIbelievewhatIseeinthispatient’shead?”

ScenarioJune27-28th 2015

Quickmeeting(firstthinginAM)inareadingroombasedonaclinicalimage

• Neuroradiologist callsmyofficetoconsultaboutimagefeature– artifactornot?

• Patienttohavesurgeryshortlythereafter,soneedtodecidequickly/efficiently.

• Featureisweird/potentiallydisturbing.

RelevantFacts• 41-yroldmalewithParkinson’s,pre-op

scanbeforeinstallingadeepbrainstimulator.

• Surgeryscheduledfornextday…but,toinstallaDBSortoextractthispathology?

• FeatureonlyseenincoronalT2FLAIR.• Doesn’tlooklikeanytypicalartifact– no

periodicity,ghosting…• NothingobviousinFOV.

… mygutisalreadytellingmesomething.

Frustration&ParallelImagingArtifacts

pMRI artifactshaveacertainappearance/behaviorforwhichyoumaydevelopagutinstinct…andthenyoustarttoseethemeverywhere.

Inparticular:• periodicities• well-definedghostsinseeminglyrandomplaces• stuffintheMIDDLEofyourFOV

SensitivityMapMismatchArtifacts

Thin,brightstructuresintheperipheryofsensitivitymap—mismatchbetweensensitivityandanatomy.

Acceleration=2inthisexample.

Duplicatespacing=½xFOV

Acceleration=Nà Duplicatespacing=1/NxFOV

Yanasak &Kelly,Radiographics,2014

SensitivityMapMismatchArtifacts

Thin,brightstructuresintheperipheryofsensitivitymap—mismatchbetweensensitivityandanatomy.

Acceleration=2inthisexample.

Duplicatespacing=½xFOV

Acceleration=Nà Duplicatespacing=1/NxFOV

Yanasak &Kelly,Radiographics,2014

SensitivityMapMismatchArtifactsYanasak &Kelly,Radiographics,2014

Thin,brightstructuresintheperipheryofsensitivitymap—mismatchbetweensensitivityandanatomy.

2DAcceleration(3Dscan)andSensitivityMapMismatchArtifacts

Cerebellarlesion?

No…sensitivitymapmismatch,projectedinslicedirection(PE#1,accfactor=2),

90SlicesinseriesSlice#58

2DAcceleration(3Dscan)andSensitivityMapMismatchArtifacts

Cerebellarlesion?

No…sensitivitymapmismatch,projectedinslicedirection(PE#1,accfactor=2),

90SlicesinseriesSlice#58Slice#12

58-12=46~90/2

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

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

Case#3

ACRtestingdifficultiesthataresointractable,theyaretrivial.

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

VerifyScanthephantommultipletimes.Sameproblem?

Yes– nosingle-scananomaly(although…whatifitpasses2ndtime?)

Repositionandrescan.Sameproblem?Yes– somethingfundamentallywrong(nopositioningissues).

CompareSpeakwithphysicistatMissionhospitalsinAsheville,NC(newIngenia).“How’syourACRperformance?”

Capableofpassinglowcontrastdetectability?Yes (~39spokes…notprettyforanew

3T,butsobeit)– problemwithourmagnet?

WhatelsecanwedotodeterminewhetheritisOURmachine/institutionvs.asystematicPhilipsissue?

Slice8(T2,TE=80msec)

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)

FollowingUp

Slice8(T2,TE=80msec)

Ihaveheardabout“potatochip”-ing ofLCDinserts(seeslidefromDonnaReeve).

Couldsomethingsimilarbehappeningforus?

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.

Alsoscannedtheold(Siemens)phantomandournewest(Philips)phantomw/ACRprotocolusinga32-channelheadcoil.

FollowingUp

Bothpass(muchhigherSNRcoil)…

And,stillseeoffset~2mm indiskhousing.

Slice8(T2,TE=80msec)

Case#4

Usingmedicalphysics“intangibles”tomaintainclinicalpractice

ScenarioCirca2011-2

LiverIronQuantificationforSickle-CellPatientisinError• Beganapartnershipwithoursickle-cellclinic,scanningpatientswhomight

benefitfromchelationtherapy(Feoverload).

• Westartedperformingtheexams~2011;concurrently,akeyservicecontractwaseliminated(hadsoftwaresince2003-ish).

• Afewyearslater,theFequantificationsoftwarestoppedperformingasexpected.

RelevantFacts• GEworkstationwithReportCard software(forFequantification).

RelevantFacts• GEworkstationwithReportCard software(forFequantification).

TIME(TE)

INTENSITY

I j (ti ) = Aje−ti /T 2

*j +Cj

RelevantFacts• GEworkstationwithReportCard software(forFequantification).• Discoveryofabug– clearlyapoorfittothedata.• Institutionalcontractswithanothervendorà nonewsupportforworkstation

(I.e.,nonewreleasesforus)

FitreportedasT2*=7.5±1.0msec

Problem:longerT2*=lessFe=healthier.

Problem:• FDAclearedsoftware(methodtested),butfittingalgorithmitselfisfailing.

Solution:• Writesomesoftwarethatdoesexactlythesamething(methodology)buthasa

morerobustfitter(andbaselineasymptoticintensity=fluid).

Validation:• Validateaccordingtoseveralpreviouscaseswithgoodfits.• Samefitvaluesof[Fe]?

Yes!!

RestatementofProblem

Problem:• FDAclearedsoftware(methodtested),butfittingalgorithmitselfisfailing.

Solution:• Writesomesoftwarethatdoesexactlythesamething(methodology)buthasamorerobust

fitter(andbaselineasymptoticintensity=fluid).

Validation:• Validateaccordingtoseveralpreviouscaseswithgoodfits.• Samefitvaluesof[Fe]?

Yes!!

T2*=2.6msec vs.7.5msec – bigclinicaldifference.

RestatementofProblem

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).

Questions?