artefacts lecture part1

8/3/2019 Artefacts Lecture Part1

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Artefacts

InClinic

alMRI

KrisA

rmoogumMS

c

Departmen

tofMedicalPhysics,

Nine

wellsHospital

Dundee

DD21QW

[email protected]

Stephen.Ga

[email protected]

14thScottis

hMRISemina

r,Wednesday

19thNovembe

r2003


2/30

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


11/30

F

lowArtefactII-

Brain

Velocityprofilelaminarflow

(Re

artefacts lecture part1

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