DependenceoftheapparentT1onMagnetizationTransfer

PetervanGelderen,Xu Jiang,andJeffHDuynAMRI,LFMI,NINDS,NationalInstitutesofHealth,

Bethesda,MD,UnitedStates

Two-PoolModelTissuehastwopoolsof1H:- free1Hinwater(visible)- bound1Hinmacro-molecules(notvisible)

MTClassicMTexperiment:steadystatesaturationofboundpool,measurelevelofMz infreepool

RF

InversionClassicT1experimentsignoreboundpool

RF

Inversion:2-poolmodelNon-invertedboundpoolchangesIR-recovery

RF RF

2-PoolEffects• Inversionrecoveryisdouble

exponential• ShapeofIRdependsonRFlevel

saturatingtheboundpool• ApparentT1 dependsonTI• ApparentT1 dependsonbound

poolsize

Poster#0015:• DependenceofapparentT1 onTI• IRdataon11moresubjects• Comparison3Tand7T• Resultsoftwopoolanalysis• RelationbetweenT1 andboundpoolsize

DependenceoftheapparentT1onMagnetizationTransfer

PetervanGelderen,Xu Jiang,andJeffHDuynAMRI,LFMI,NINDS,NationalInstitutesofHealth,

Bethesda,MD,UnitedStates

Poster0015

IntroductionInthispresentation,wediscusshowthelessmobile1Hnucleiinlargermoleculesinfluenceboththelongitudinalrelaxation(T1)propertiesofwaterandtheirmeasurement.Itisshowntheinversionrecoverysignalisbi-exponential,andthattheamplitudeofthesecondexponentdependsontheparametersoftheappliedinversionpulses.Itisalsoshownthemacro-molecularcontentmayexplainmostofobservableT1-contrastinthebrain.

Poster0015MTdependentT1

Background• Mosttissues,inparticularbraintissues,have2distinctclassesof1H

nuclei: mobile1Hinwater,andless-mobile1Hinlargermolecules(lipids,proteinsetc.).

• Themobileatoms,formingthefreepool,contributetotheMRIsignal;thelessmobileones,makinguptheboundpool,havea10-100μsT2 andsoarenotdirectlyMRI-visibleinmostexperiments.

• Thetwopoolsarecoupledbychemicalandmagneticexchangeprocesses,theboundpoolcanthereforestillinfluencetheMRIsignalgeneratedbythefreepool.

Poster0015MTdependentT1

Background• MT(magnetizationtransfer)experimentsaimtosaturatetheboundpool

andcharacterizethispoolbyexplicitmodelingoftheexchangewiththefreepool.

• MostT1 measurements,basedoneitherInversionRecovery(IR)orsaturationexperiments,implicitlyassumeasinglepoolmodel,ignoringeffectsoftheboundpool.

• BecauseoftheirshortT2,theboundpool1Harenotinverted,butonly(party)saturated.ThisdifferenceinMz statebetweenthetwopoolsbecomesMRIvisiblethroughexchangebetweenthepools,creatingaMTeffectinIRmeasurements.

Poster0015MTdependentT1

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

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

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

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

Results Poster0015TheeffectiveT1,ascalculatedfromthedifferencebetweenimagesattwoTI’s(3T,Exp.1),showingthattheinitialpartoftheIR(toprow)isfasterthanthelatertimes(bottomrow).TheeffectismorepronouncedforthelowerB1 inversionpulse(rightcolumn),asthatpulseresultsinlessboundpoolsaturationandathereforeabiggerdifferencebetweenboundandfreepoolstates.

MTdependentT1

Results Poster0015TheIR-curvesinanROIinthespleniumofthecorpuscallosum(SCC),averagedover5subjects(3T,Exp.1).ThetopgraphshowstheMz forthe3inversionpulsesused,withthedoubleexponentialfitsplottedontop.Theshortcompositepulseandtheadiabaticpulseproducenearlyidenticaldata.Thebottomgraphshowsthesamedataplottedasalogofthesaturation,withstraightlinesfittedtothelastthreepoints.Thisshowsasingleexponentialfunctionisinsufficienttofitthisdata,whilethedouble-exponentialfitisnearlyperfect(topgraph).

MTdependentT1

Results Poster0015AveragedataofSCC-ROI’son11subjectsscannedat7T(Exp.3).Thelogofthe1-Mz isplottedasfunctionofTI,for4inversionpulses,withthedoubleexponentialfitinsolidandasingleexponentialfitindottedlines.Confirmingthe3Tresults,theIRisnotmono-exponentialandthedeviationdependsontheRFamplitude.Theoverallverticalshiftofthelinesisduetothedecreasinginversionefficiencywithlongerpulses.

MTdependentT1

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

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:

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

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

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

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

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