dependence of the apparent t1 on magnetization transfer€¦ · increase of the shorter component...
TRANSCRIPT
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DependenceoftheapparentT1onMagnetizationTransfer
PetervanGelderen,Xu Jiang,andJeffHDuynAMRI,LFMI,NINDS,NationalInstitutesofHealth,
Bethesda,MD,UnitedStates
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Two-PoolModelTissuehastwopoolsof1H:- free1Hinwater(visible)- bound1Hinmacro-molecules(notvisible)
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MTClassicMTexperiment:steadystatesaturationofboundpool,measurelevelofMz infreepool
RF
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InversionClassicT1experimentsignoreboundpool
RF
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Inversion:2-poolmodelNon-invertedboundpoolchangesIR-recovery
RF RF
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2-PoolEffects• Inversionrecoveryisdouble
exponential• ShapeofIRdependsonRFlevel
saturatingtheboundpool• ApparentT1 dependsonTI• ApparentT1 dependsonbound
poolsize
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Poster#0015:• DependenceofapparentT1 onTI• IRdataon11moresubjects• Comparison3Tand7T• Resultsoftwopoolanalysis• RelationbetweenT1 andboundpoolsize
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DependenceoftheapparentT1onMagnetizationTransfer
PetervanGelderen,Xu Jiang,andJeffHDuynAMRI,LFMI,NINDS,NationalInstitutesofHealth,
Bethesda,MD,UnitedStates
Poster0015
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IntroductionInthispresentation,wediscusshowthelessmobile1Hnucleiinlargermoleculesinfluenceboththelongitudinalrelaxation(T1)propertiesofwaterandtheirmeasurement.Itisshowntheinversionrecoverysignalisbi-exponential,andthattheamplitudeofthesecondexponentdependsontheparametersoftheappliedinversionpulses.Itisalsoshownthemacro-molecularcontentmayexplainmostofobservableT1-contrastinthebrain.
Poster0015MTdependentT1
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Background• Mosttissues,inparticularbraintissues,have2distinctclassesof1H
nuclei: mobile1Hinwater,andless-mobile1Hinlargermolecules(lipids,proteinsetc.).
• Themobileatoms,formingthefreepool,contributetotheMRIsignal;thelessmobileones,makinguptheboundpool,havea10-100μsT2 andsoarenotdirectlyMRI-visibleinmostexperiments.
• Thetwopoolsarecoupledbychemicalandmagneticexchangeprocesses,theboundpoolcanthereforestillinfluencetheMRIsignalgeneratedbythefreepool.
Poster0015MTdependentT1
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Background• MT(magnetizationtransfer)experimentsaimtosaturatetheboundpool
andcharacterizethispoolbyexplicitmodelingoftheexchangewiththefreepool.
• MostT1 measurements,basedoneitherInversionRecovery(IR)orsaturationexperiments,implicitlyassumeasinglepoolmodel,ignoringeffectsoftheboundpool.
• BecauseoftheirshortT2,theboundpool1Harenotinverted,butonly(party)saturated.ThisdifferenceinMz statebetweenthetwopoolsbecomesMRIvisiblethroughexchangebetweenthepools,creatingaMTeffectinIRmeasurements.
Poster0015MTdependentT1
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TheoryEquationforMz intwopoolswithexchangeandT1 relaxation[1,2]:
whereSf andSb arethesaturationlevelsinthetwopools(1-Mz),theR1f andR1b theirrelaxationrates(1/T1),k theexchangeratefromfreetoboundpool,andv theboundpoolvolumerelativetothefreepool.Thesolutionforthefreepoolstateis:
ie.theinversionrecoveryisbi-exponentialduetotheexchangewiththeboundpool.
Poster0015
ddt
SfSb
!
"
##
$
%
&&=
−R1 f − k k
k / v −R1b − k / v
!
"
##
$
%
&&
SfSb
!
"
##
$
%
&&
Sf (t) = a1e−λ1t + a2e
−λ2t
1.Henkelman et.al.,MRM29:759(1993).2Gochberg,Kennan&Gore,MRM38:224(1997).
MTdependentT1
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TheoryTheexponentsinthesolutionareapproximatelyequalto:
Thisapproximationholdsiftheexchangeisfastcomparedtothedifferenceinrelaxationratesbetweenthepool.Thefirstexponentdecayswithvolumeaveragedrelaxationrates(R1)ofthetwopools,thesecondoneisdominatedbythesumoftheforward(k)andreverse(k/v)exchangerates.
Poster0015
λ1 ≈R1 f + vR1b1+ v
,λ2 ≈R1b + vR1 f1+ v
+ k + kv
MTdependentT1
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ExperimentsAllexperimentsusedEPI,withoneinversionpulsefor5slicesat5differentTItimes,andshufflingthesliceordertoacquireallTI’sforeveryslice[1].Otherrelevantparameters:TR4s,TE30ms,resolution1.7x1.7x2mm,senserate2,14averageswithinversion,4withouttoserveasreference.
Poster0015
EPI EPIEPI EPI EPI
Inv Slice1 Slice5Slice4Slice3Slice2
…..
Slice2 Slice1Slice5Slice4Slice3… ……………
10 600ms25813872time: 0
Inv
1.Clare&Jezzard,MRM45:630(2001)
MTdependentT1
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ExperimentsFourinversionpulseswerecompared:1) Hyperbolicsecantadiabaticpulse,830HzmaxB1,5mslongat3T,7msat7T.2-4) A90x-180y-90x compositehardpulsetrain,atB1 levelsof2) 830Hz,3) 278Hzand4) 145Hz(totallength:1.2ms,3.6msand6.9ms).
Threesetsofexperimentswereexecuted:1) 5subjectsat3T(SiemensSkyra),10TI’sfrom8-600ms(2setsof5acquiredininterleavedfashion),withinversionpulses1-32)11subjectsat3T(SiemensSkyra),5TI’sfrom8-1200ms,withpulses1-43) thesame11subjectsat7T(SiemensMagnetom),5TI’s8-1200ms,pulses1-4
Poster0015MTdependentT1
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Results Poster0015TheeffectiveT1,ascalculatedfromthedifferencebetweenimagesattwoTI’s(3T,Exp.1),showingthattheinitialpartoftheIR(toprow)isfasterthanthelatertimes(bottomrow).TheeffectismorepronouncedforthelowerB1 inversionpulse(rightcolumn),asthatpulseresultsinlessboundpoolsaturationandathereforeabiggerdifferencebetweenboundandfreepoolstates.
MTdependentT1
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Results Poster0015TheIR-curvesinanROIinthespleniumofthecorpuscallosum(SCC),averagedover5subjects(3T,Exp.1).ThetopgraphshowstheMz forthe3inversionpulsesused,withthedoubleexponentialfitsplottedontop.Theshortcompositepulseandtheadiabaticpulseproducenearlyidenticaldata.Thebottomgraphshowsthesamedataplottedasalogofthesaturation,withstraightlinesfittedtothelastthreepoints.Thisshowsasingleexponentialfunctionisinsufficienttofitthisdata,whilethedouble-exponentialfitisnearlyperfect(topgraph).
MTdependentT1
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Results Poster0015AveragedataofSCC-ROI’son11subjectsscannedat7T(Exp.3).Thelogofthe1-Mz isplottedasfunctionofTI,for4inversionpulses,withthedoubleexponentialfitinsolidandasingleexponentialfitindottedlines.Confirmingthe3Tresults,theIRisnotmono-exponentialandthedeviationdependsontheRFamplitude.Theoverallverticalshiftofthelinesisduetothedecreasinginversionefficiencywithlongerpulses.
MTdependentT1
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Results Poster0015
SimulationofMz asfunctionofT2(*).Theboundpool,withaT2 intherangeindicatedbytheturquoisbar,willbesaturatedtoavaryinglevel,duetothedifferentlevelsofRFenergyinthepulsesapplied.Thelongerpulsealsoshowslessinversionefficiencyforthefreepool(grayband);especiallyrelevantfor7T,wheretheT2* willbeshorter.
MTdependentT1
100 101 102 103 104 105T2 [µs]
−1.0
−0.5
0.0
0.5
1.0
Mz
100 101 102 103 104 105
Adiabatic 7.0msComposite 1.2msComposite 3.6msComposite 6.9ms
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Results Poster0015
ThedeviationoftheimagesatTi=8mscomparedtoasingleexponentialfittothelasttwoTi’2(258&1200ms),onesliceforfourinversionpulsesat7T.Thedeviationisbiggerfortheinversionpulseswithaloweramplitude,resultinginlessboundpoolsaturation.Whitemattershowsabiggereffectthangreymatter,asthathasahigherconcentrationofnon-mobile1H.
hbs cmp 1.2 cmp 3.6 cmp 6.9
MTdependentT1
Pulse:
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Results Poster0015
Exampleoffittingresultsforonesubject,scannedat3Tand7T,showingtheamplitudeofbothcomponents(top:a1 andbottom:a2)foronesliceforthefourinversionpulses.Thenormalizedsignalisconvertedtosaturation(S=1- Mz),forcompleteinversionthesumofthecomponentswouldbe2.Theincreaseoftheshortercomponent(bottomrow)withdecreasingMTsaturationisclear,especiallyat7T.Thedecreasinginamplitude(at7T)anteriorandposteriorinthebrainisduetoRFinhomogeneity.
3T 7T
hbs cmp 1.2 cmp 3.6 cmp 6.9 hbs cmp 1.2 cmp 3.6 cmp 6.9
MTdependentT1a 1
a 2
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Results Poster0015
Examplefitresultsfromonesubject(continuedfrompreviousslide).Shownaretheslowerdecayrate(λ1)andthederivedrelativeboundpoolsize(v)andexchangerate(thisrateiswater->bound);allratesarein1/s.Althoughthedecayisfielddependent(notethescaledifference),bothλ1-mapsshowsimilarcontrast,andbotharesimilartothederivedboundpoolsize.Thecalculatedpoolsizeandexchangeratesarealsosimilarforthetwofieldstrength,althoughthe3Tk-valuesareslightlyhigherandhavemorenoise.
3T 7T
λ1 v k λ1 v k
MTdependentT1
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Results Poster0015Furtherillustrationofrelationbetweentheboundpoolsize(v)andtheapparentR1-rate(theslowcomponent,λ1),forallvoxelsforonesubjectattwofieldstrength.TheredlinesarethepredictedmodelsbasedonafixedR1f of0.4/s(bothfields)andaR1b of2/sfor7Tand3.8/sfor3T.TheplotsconfirmthenotionthatexchangewiththeboundpoolmaybethedominantcontributiontoT1 contrastinthebrain[1].
7T
3T
1.Rooneyet.al.,MRM57:308(2007)
MTdependentT1
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Results Poster0015AverageresultsofSCC-ROIin11subjectsat3&7T(Exp.2and3).Toptableshowsthefittedamplitudesoftheslowandfastcomponentsandthederivedsaturationlevelsforthetwopools.Thebottomtableshowsthefitteddecayrates,theassumedboundpoolrelaxationrateandthederivedexchangerateandrelativeboundpoolsize(allratesarein1/s).Thefittedparameters(a’sandλ’s)andSf followdirectlyfromthedata,SbdependsonR1bandv whichwerederivedinconjunctionwithfurtherMTexperimentsnotdescribedhere.Theamplitudeofthefastcomponent(a2)clearlydependsontheRFamplitudeoftheinversion,reflectingthedifferencesinboundpoolsaturation.
B0 Inv-RF a1 a2 Sf Sb7T HBS 1.77 0.17 1.94 0.87
Cmp 1.2 1.70 0.18 1.88 0.81
Cmp 3.6 1.61 0.22 1.84 0.58
Cmp 6.9 1.54 0.24 1.78 0.44
3T HBS 1.85 0.12 1.97 0.86
Cmp 1.2 1.86 0.11 1.97 0.88
Cmp 3.6 1.78 0.17 1.95 0.60
Cmp 6.9 1.72 0.19 1.92 0.47
B0 λ1 λ2 R1b k v
7T 0.77 9.24 2.0 2.03 0.37
3T 1.10 12.14 3.8 2.39 0.36
MTdependentT1
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Conclusion Poster0015
• Inversionrecoveryinbraintissueappearstobebi-exponential,whichcanbeexplainedbya2-poolexchangemodelforT1 (andMT)effects,confirmingearlierreports(e.g.[1,2]).
• Inwhitematter,exchangewiththeboundpoolcontributesafastcomponenttotheIRsignalwitharelativeamplitudeof0.08and0.29at3T,andfrom0.16to0.40at7T,dependingonthelevelofsaturation.
• IgnoringexchangewiththeboundpoolresultsinadependencyofthecalculatedT1 ontheexperimentalparameters(RFamplitude,TI’s,TR).
• ExchangewiththeboundpoolislikelythedominantsourceofT1-contrastinthebrain.
1.Pranter et.al.,MRM60 555(2008).2.Labadieet.al.,MRM71:375(2014).
MTdependentT1