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Artefacts
InClinic
alMRI
KrisA
rmoogumMS
c
Departmen
tofMedicalPhysics,
Nine
wellsHospital
Dundee
DD21QW
Stephen.Ga
14thScottis
hMRISemina
r,Wednesday
19thNovembe
r2003
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Artefactsarepartsofrec
onstructedimage
sthatarenotpre
sentin
thetrue
anatomy.
Artefactsaredependen
tonavariety
offactorsfrom
patient
movementtomagneticfieldinhomogeneities.
Artefactscanleadtom
isdiagnosisifth
eyarenotreco
gnised
and/orr
emoved.
Ideally,
wewantallimageartefactstobebelowthelevelofuser's
perception.
Mainclassifications-
1.Movem
entArtefacts
2.GeometricalArtefacts
3.Resolu
tion/Sequence
Artefacts
4.BoArtefacts
5.RFArt
efacts
6.Noise
Introduction
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1.Movement
Artefacts
Mo
tionArtefactspatient
Flo
wartefact
inflow/washouteffect,
dia
stole,systole,arterialflow
Re
ducingflowartefactsG
radient
Mo
mentNulling
Re
spiratorycompensation,
triggering,
RO
PE,navigatorechoes
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M
otionAr
tefact-Patient
Patientm
ovementasouter
areasofk-spaceacquired
Maymim
ictruncationartefact
Differenc
etruncationartefact
diminishe
swithdistancefr
om
thehighcontrastboundary
Ifrelated
topulsationofve
ssels,
thiscanb
ereducedbyapplying
ananteriorsatband
T2WS
EThoracicSp
ine
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PatientMo
vement
Artefact
Smearing
ofimage
Particularlyinphasedirection
Solutions
Immobilisethepatientmoreeffectively
Reduceth
escantimereduceNSA,breathold,
shorterTR,lessk-space
lines
Reduceth
escantime(reducedk-spaceacquisition)e.g.HASTE,TSEwith
largeturbofactor
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T1WG
Esequence
RFpulses
aturatesthebloodmomentarilyintheslice(yellow)
Ifbloodisstationary,longT1ofbloodmeans
thatnosignalavailable
forsuccessiveRFpulsesto
excitehypointe
nsesignal
Ifvelocityofinflowingblood
>z/TRthenfullinflowoccursandthenext
RFpulseseesunsaturated
spinsintheslice
BrightBloods
ignal
Othertissu
eswithintheslicearesaturated,andthereforesupp
ressed
FlowArtefactInflo
wEffec
t
90opulse
Brightsignal
90opulse
Darksig
nal
NO
FLOW
FLO
W
NextTR
NextTR
Image
z
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Flow
ArtefactWashoutEffe
ct
Spin-echosequence-90opulseexcitesspinsintheslice
Inthea
bsenceofflow,b
rightsignalissee
nbecausethesp
ins
experie
nceboththe90o
and180opulses
Inthep
resenceofflow,bloodflowsoutof
thesliceanddoe
snot
experie
ncethe180opuls
enorephasing,BlackBloodsig
nal
D.G.N
ishimura"Time-of-FlightMRAngiogr
aphy."
Magn.R
eson.Med.
14:194
-201(1990)
Image
90op
ulse
18
0opulse
Nosignal
90opulse
180opulse
Signal
NOFLOW
FLOW
TE/2
TE
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FlowArt
efactI-
Body
Diasto
le(fillingphase),sy
stole(emptyingphase).
Aortic
ghostsinPEdirection[becauseFEste
p(msec)takesmuchless
timeth
anaPE(sec)step]
Physio
logicalmodulation
Grad
DS
Phase
Shift
-1
+1
T
T
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Wh
yonlyth
ePEDirection?
Whyismotionartefact
onlyseeninthePEdirection?
AFEsteptakesmuch
lesstime(oftheorderofmsec)
thanaP
Estep(ofthe
orderofsecon
ds)
MostmotionthatoccursduringclinicalMRIismuch
slowerthantherapidsamplingproce
ssalongtheF
Eaxis
Howeve
r,eachPEline
isseparatedbythetimeinterval
TRwhic
hislongenou
ghforbloodto
flow/move/dephase
betweensuccessivephaseencoding
s
!
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PhaseShiftEffects
Systolic
-diastolicswitchin
goftheflowvelo
city(atfrequency
m
)
modulatestheMRsignal(withfrequency
)
Twofre
quencysidebands(upperandlowe
rfrequencysideb
and
compon
ents)appearasghostseitherside
oftheprimaryim
age
S
D
o
m
FT
o
o+
m
o-m
M
RSignal
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F
lowArtefactII-
Brain
Velocityprofilelaminarflow
(Re
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