magnetic resonance imaging ppt

8/9/2019 magnetic resonance imaging ppt

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MAGNETIC RESONANCE

IMAGING


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Abbreviations

• FID Free induction decay• FSE Fast spin echo• GRE Gradient echo• IR Inversion recovery• MHz Megahertz• MR Magnetic resonance• MRA Magnetic resonance angiography• MRI Magnetic resonance imaging• msec Mi iseconds• NMR Net magnetization vector• PC MRA !hase"contrast MR angiography• PD !roton density• ppm !arts per mi ion• RF Radio#re$uency• SAR Speci%c a&sorption rate• SE Spin echo• SNR Signa "to"noise ratio• T Tes a• TE Echo time• TOF Time o# 'ight• TR Repetition time


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(z"direction) and (*y"p ane) are#re$uent y used+ In a %gures, the mainmagnetic %e d, -., is represented #rom&ottom to top and its direction isdesignated &y z+ The other t/odimensions o# the magnetic %e d aredenoted &y * and y+ The *y"p ane isperpendicu ar to the z"a*is and is thusrepresented horizonta y in the %gures+


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0

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Spin and t e N!c"ear Ma#neticResonance P enomenon

Medica magnetic resonance 3MR4 imaginguses the signa #rom the nuc ei o# hydrogenatoms 3564 #or image generation+ A

hydrogen atom consists o# a nuc euscontaining a sing e proton and o# a sing ee ectron or&iting the nuc eus + The protonhaving a positive charge and the e ectron anegative charge, the hydrogen atom as a/ho e is e ectrica y neutra + The proton iso# interest here+


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!ROTON

E7ECTRON


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Apart #rom its positive charge, theproton possesses spin , an intrinsicproperty o# near y a e ementarypartic es+ This means that the protonrotates a&out its a*is i8e a spinningtop+


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T9O IM!ORTANT !RO!ERTIES OF!ROTON

5+ As a rotating mass 3m4, the protonhas angular momentum and acts i8e aspinning top that strives to retain thespatia orientation o# its rotation a*is

ANG$%AR MOMENT$M


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:+ As a rotating mass /ith an e ectricacharge, the proton additiona y hasmagnetic moment (B) and &ehaves i8ea sma magnet+ There#ore, the protonis a;ected &y e*terna magnetic %e dsand e ectromagnetic /aves and, /hen

it moves, induces a vo tage in areceiver coi +


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MAGNETIC MOMENT


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There is another, very importantdi;erence< /hi e a spinning top can &es o/ed do/n and thus %na y comes toa standsti , a proton=s spin a /ays hasthe same magnitude and can neither&e acce erated nor dece erated,

precise y &ecause it is a #undamentaproperty o# e ementary partic es+ Spinis simp y there a the time>


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6o/ /i a spin &ehave /hen

&rought into a strong magnetic %e d?


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9hen an e*terna #orce 3typica y the

earth=s gravitationa %e d G4 acts on aspinning top and tries to a ter theorientation o# its rotationa a*is, the top&egins to /o&& e, a process ca ed

precession + At the same time,#riction at the point o# contact /ithdra/senergy #rom the spinning top and s o/s

do/n its rotation+ As a resu t, its a*is&ecomes more and more inc ined and thetop %na y #a s over+


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-ac8 to our hydrogen nuc ei< /hen these aree*posed to ane*terna magnetic %e d, -., themagnetic moments, or spins, a ign /ith thedirection o# the %e d i8e compass need es+ Themagnetic moments do not on y a ign /ith the%e d &ut, i8e spinning tops, undergoprecession+ !recession o# the nuc ei occurs at acharacteristic speed that is proportiona to thestrength o# the app ied magnetic %e d and isca ed Larmor frequency + A ignment o# thespins para e to the magnetic %e d is a graduaprocess and, as /ith spinning tops, isassociated /ith the dissipation o# energy


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NOTE< The 7armor #re$uency is a veryimportant concept that is at the core o#MR imaging+

T e Larmor or precessionfrequency is the rate at /hich spins/o&& e /hen p aced in a magnetic%e d+


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The 7armor #re$uency is directly proportional to the strength(B. ) of the

magnetic eld and is given &y the Larmor equation:


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9hat happens to the spins precessing and s o/ ya igning inside the

magnetic %e d?


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9hi e the spin system re a*es andsett es into a sta& e state, longitudinalmagnetization M z is &ui ding up in thez"direction &ecause the magneticvectors representing the individuamagnetic moments add together+


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NM@" /hen a s ight y arger #raction o#the spins a ign /ith the main magnetic%e d+ It is this sma di;erence thatactua y produces the measura& e netmagnetization Mz and is represented&y the net magnetization vector(NMV)" Mz increases /ith the %e d strength+


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Energy can &e introduced into such asta& e spin system &y app ying ane ectromagnetic /ave o# the same#re$uency as the 7armor #re$uency+

This is ca ed the resonancecondition +


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The re$uired e ectromagnetic /ave isgenerated in a po/er#u radiotransmitter and app ied to the o& ectto &e imaged &y means o# an antennacoi +


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The process o# energy a&sorption is 8no/n ase*citation o# the spin system and resu ts in

the ongitudina magnetization &eing moreand more tipped a/ay #rom the z"a*is to/ardthe transverse 3*y"4p ane perpendicu ar tothe direction o# the main magnetic %e d+ A o#

the ongitudina magnetization is rotated intothe transverse p ane &y a radio#re$uency 3RF4pu se that is strong enough and app ied ongenough to tip the magnetization &y e*act yB. 3 9 ! "# pulse 4+ The resu tingmagnetization is no/ denoted &y M*y ratherthan Mz &ecause it no/ ies in the *y"p ane+


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no e*terna magnetic %e d present, spins rotate a&out their a*es in randomtion &a'( In the presence o# a magnetic %e d, s ight y more spins a ign para ee main magnetic %e d, -., and thus produce ongitudina magnetization, Mz &b'u se &c' tips the magnetization vector &y e*act y B. , causing the entire ongitunetization to 'ip over and rotate into transverse magnetization, M*y &d'

a &

c d


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9henever transverse magnetization ispresent, it rotates or precesses a&outthe z"a*is, /hich has the e;ect o# ane ectrica generator and induces ana ternating vo tage o# the same#re$uency as the 7armor #re$uency in

a receiver coi < the M" signal$ Thissigna is co ected and processed /ithsensitive receivers and computers to

generate the MR image+


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Re"a)ation


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9hat happens to the spins a#ter theyhave &een e*cited?


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T9O INDE!ENDENT !ROCESSES T6AT RED CE TRANS@ERSE MAGNETI0ATION AND T6 S CA SE A

RET RN TO T6E STA-7E STATE !RESENT-EFORE E1CITATION<

5+spin" attice interaction:+ spin"spin interaction+

Theset/o processes cause %&rela'ation and % rela'ation ,respective y


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T*+ %on#it!dina"Re"a)ation

5 re a*ation+ Decay o# transverse magnetization and regro/th o# magnetizationong the z"a*is re$uire an e*change o# energy


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The transverse magnetization remaining/ithin the *y"p ane strict y spea8ing thepro ection o# the ma#netization vector onto

t e ),-p"ane decreases s"o.", and the MRsi#na" /ades in proportion + As transversemagnetization decays, the ongitudinamagnetization, Mz the pro ection o# the

magnetization vector onto the z"a*is iss o/ y restored+ This process is 8no/n aslongitudinal relaxation or T* recover, +


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The nuc ei can return to the groundstate on y &y dissipating their e*cessenergy to their surroundings 3the( attice), /hich is /hy this 8ind o#re a*ation is a so ca ed spin" atticere a*ation4+


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• The time constant #or this recovery isT1 and is dependent on the strengtho# the e*terna magnetic %e d,-.,and the interna motion o# themo ecu es &0ro.nian motion'(

• -io ogica tissues have T5 va ues o#ha # a second to severa seconds at5+ T+


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T12T13+ TransverseRe"a)ation

e+ @ector B has a phase o# H5. re ative to /hi e * has a phase o# . + that a vectors rotate a&out the z"a*is /hi e their phases di;er &y the respecties


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• "!6ASE "re#ers to the position o# amagnetic moment on its circu arprecessiona path and is e*pressed as an

ang e+ Consider t/o spins, A and -,precessing at the same speed in the *y"p ane+ I# - is ahead o# A in its angu armotion &y 5. , then /e can say that -has a phase o# H5. re ative to A+Converse y, a spin C that is &ehind A &y

. has a phase o# " . +


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: and T:J re a*ation+ Spins get out o# phase 3 ose phase coherence4, resu ting ie oss o# transverse magnetization /ithout energy dissipation


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Transverse Re a*ation is the decay oftrans+erse magnetization ,ecausespins lose coherence (dephasing)


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Transverse re a*ation di;ers #romongitudina re a*ation in that the spins

do not dissipate ener#, to t eirs!rro!ndin#s &ut instead e)c an#eener#, .it eac ot er +


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Coherence is ost in t/o /ays<

• 5+ Energy transfer &et/een spins as aresu t o# local changes in the magnetic

eld +

Such 'uctuations are due to the #act thatthe spins are associated /ith sma magnet%e ds that random y interact /ith eachother+ Spins precess #aster or s o/eraccording to the magnetic %e d variationsthey e*perience+


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Most o# the inhomogeneities thatproduce the T:J e;ect occur at tissue&orders, particu ar y at airKtissueinter#aces, or are induced &y ocamagnetic %e ds 3e+g+ iron partic es4+

The oss o# the MR signa due to T:J

e;ects is ca ed free induction decay(#/0) + T:J e;ects can &e avoided &yusing spin echo se$uences+


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Di;erence o# T: and T:J• T1 denotes t e process o/

ener#, trans/er bet.een spins4. i"e T13 re/ers to t e e5ects o/additiona" 6e"d in omo#eneitiescontrib!tin# to dep asin#(


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T5 and T: re a*ation are comp ete yindependent o# each other &ut occurmore or ess simultaneously > Thedecrease in the MR signa due to T:re a*ation occurs /ithin the %rst 5..

.. msec, /hich is ong &e#ore there

has &een comp ete recovery o#ongitudina magnetization Mz due to

T5 re a*ation 3.+ sec4+


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Ima#e Contrast

%hree intrinsic features o# a &io ogicatissue contri&ute to its signa intensityor &rightness on an MR image andhence image contrast<


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5+ The proton density , i+e+ thenum&er o# e*cita& e spins per unitvo ume, determines the ma*imumsigna that can &e o&tained #rom a giventissue+ !roton density can &eemphasized &y minimizing the other t/o

parameters, T5 and T:+ Such images areca ed proton density !eighted orsimp", proton density images +


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:+ The T1 time o# a tissue is the timeit ta8es #or the e*cited spins to recoverand &e avai a& e #or the ne*te*citation+ T5 a;ects signa intensityindirect y and can &e varied atrandom+ Images /ith contrast that is

main y determined b, T* are ca""edT1 !eighted images (T1!)


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+ The T" time most y determinesho/ $uic8 y an MR signa #ades a#tere*citation+ The T: contrast o# an MRimage can &e contro ed &y theoperator as /e + Images /ith contrastthat is main y determined &y T: are

ca ed T" !eighted images (T"!) $


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!roton density and T5 and T: times are intrinsic#eatures o# &io ogica tissues and may vary

/ide y #rom one tissue to the ne*t+ Dependingon /hich o# these parameters is emphasized inan MR se$uence, the resu ting images di;er intheir tissue"tissue contrast+ This provides the

&asis #or the e*$uisite so#t"tissue discriminationand diagnostic potentia o# MR imaging< &asedon their speci%c di;erences in terms o# thesethree parameters, tissues that are virtua y

indistinct on computed tomography 3CT4 scanscan &e di;erentiated &y MRI /ithout contrastmedium administration+


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Repetition Time &TR' and T*7ei# tin#

In order to generate an MR image, a s icemust &e e*cited and the resu ting signa

recorded many times+

• #epetition time (T#) is t e interva"

bet.een t.o s!ccessive e)citationso/ t e same s"ice(


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Repetition time &TR' is the ength o# there a*ation period &et/een t/o e*citationpu ses and is there#ore crucia #or T5

contrast+ 9hen TR is ong, more e*citedspins rotate &ac8 into the z"p ane andcontri&ute to the regro/th o# ongitudinamagnetization+ The more ongitudinamagnetization can &e e*cited /ith the ne*tRF pu se, the arger the MR signa that can&e co ected+


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• I# a short repetition time 3 ess than a&out L.. msec4is se ected, image contrast is strong y a;ected &y

T5 nder this condition, tissues /ith a short T5 re a*$uic8 y and give a arge signa a#ter the ne*t RFpu se 3and hence appear bri# t on the image4+

Tissues /ith a ong T5, on the other hand, undergoon y itt e re a*ation &et/een t/o RF pu ses andhence ess ongitudina magnetization is avai a& e/hen the ne*t e*citation pu se is app ied+ Thesetissues there#ore emit ess signa than tissues /ith a

short T5 and appear dar8 + An image ac$uired /ith ashort TR is %&-1eighted &ecause it contains most y

T5 in#ormation+


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I# a #air y long repetition time3typica y over 5 .. msec4 is se ected,a tissues inc uding those /ith a ong

T5 have enough time to return toe$ui i&rium and hence they a givesimi ar signa s As a resu t, there is less

%& 1eighting &ecause the e;ect o# T5on image contrast is on y sma +


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REMEM-ER<

• S ort TR stron# T* .ei# tin#• %on# TR "o. T* .ei# tin#

The re ationship &et/een the MR


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The re ationship &et/een the MRsigna o# a tissue and its appearanceon T5"/eighted images is as #o o/s<

• Tiss!es .it a short T1 appear $rightbeca!se t e, re#ain most o/ t eir"on#it!dina" ma#netization d!rin# t e

TR interva" and t !s prod!ce astron#er MR si#na"(• Tiss!es .it a long T1 appear dar%

beca!se t e, do not re#ain m!c o/t eir "on#it!dina" ma#netizationd!rin# t e TR interva" and t !sprod!ce a .ea8er MR si#na"(

E Ti &TE' d T1


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Ec o Time &TE' and T17ei# tin#

7 at is an ec o4 an,.a,9


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For the time &eing it is su cient to 8no/ that

these gradients serve to induce contro edmagnetic %e d inhomogeneities that areneeded to encode the spatia origin o# theMR signa s+ 6o/ever, the gradients a so

contri&ute to spin dephasing+ These e;ectsmust &e reversed &y app ying a re#ocusingpu se &e#ore an ade$uate MR signa iso&tained+ The signa induced in the receiver

coi a#ter phase coherence has &een restoredis 8no/n as a spin echo and can &emeasured+


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hip &et/een TE and T: contrast+ 9hen TE is very short 3A4, there isno si#na" di5erence bet.een t.o tiss!es .it di5erent T1 times /hereces become apparent . en TE is "on#er 3-4< a tissue /ith a short T: rapidd &ecomes dar8 /hi e a tissue /ith a ong T: retains its &righter signa #or a

me


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&cho time (T&) is t e interva"

bet.een app"ication o/ t ee)citation p!"se and co""ection o/t e MR si#na"(


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• S ort TE "o. T1 .ei# tin#• %on# TE stron# T1 .ei# tin#


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• Tiss!es .it a short T1 appeardar% on T1-.ei# ted ima#es '

• tiss!es .it a long T1 appear$right on T1-.ei# ted ima#es:


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• MR images that com&ine T5 and T: e;ects are 8no/nas proton density-1eighted images (20 images) +

• !D se$uences are especia y use#u #or eva uating

structures /ith o/ signa intensities such as the &onesor connective tissue structures such as igaments andtendons+ !roton density /eighting is o#ten used #orhigh"reso ution imaging+ SE se$uences are pre#erredover FSE se$uences #or !D imaging &ecause SE images

are ess prone to distortion+ In the c inica setting, !Dse$uences are main y used #or imaging o# the &rain,spine, and muscu os8e eta system+

S ! i S R i i


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Sat!ration at S ort RepetitionTimes

• SAT RATION" is /hen a series o#e*citation pu ses is app ied, the MRsigna &ecomes /ea8er and /ea8era#ter each repeat pu se+ This processis 8no/n as saturation


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Signa intensities o# di;erent tissues on T5" and T:"/eighted images


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e ative proton densities 3 4 and intrinsic T5 and T: times 3in msec4 o# di;erentssues


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hanism o# saturation+ 9ith a very short TR, the ongitudina magnetization,that /i recover in the interva and &e avai a& e #or su&se$uent e*citation decrr each RF pu se+ In the e*amp e sho/n, the TR is so short that s ight y ess thane origina ongitudina magnetization can regro/ &e#ore the ne*t e*citation puered


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itudina magnetization at short repetition time+ A#ter repeat e*citation at short interva s, the amount o# ongitudina magnetization, Mz, restored a#ter ee sett es at a o/ eve 3e$ui i&rium or steady state4+ In this situation, the indivisigna s that #orm a#ter each e*citation are very /ea8

EP I7I-RI M

TIME


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Saturation is an important issue /hen#ast or u tra#ast MR techni$ues areused+ 6ere the MR signa may &ecomevery /ea8 due to the very shortrepetition times+


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F"ip An#"e &Tip An#"e'

2artial 3ip angle imaging is atechni$ue that can &e used tominimize saturation and o&tain anade$uate MR signa despite a veryshort repetition time+


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A sma er 'ip ang e does not de'ect the magnetization a the /aythrough ( Mechanism o# saturation+ 9ith a very short TR, the

ongitudina magnetization, Mz, that /i recover in the interva and&e avai a& e #or su&se$uent e*citation decreases a#ter each RFpu se+ In the e*amp e sho/n, the TR is so short that s ight y essthan ha # o# the origina ongitudina magnetization can regro/&e#ore the ne*t e*citation pu se is de ivered F( 7ongitudina

magnetization at short repetition time+ A#ter repeat e*citation atvery short interva s, the amount o# ongitudina magnetization, Mz,restored a#ter each pu se sett es at a o/ eve 3e$ui i&rium orsteady state4+ In this situation, the individua MR signa s that #orma#ter each e*citation are very /ea8 B. &ut on y &y some#raction o# B. 3e+g+ . 4+ As a resu t there is ess transverse

magnetization and the individua MR signa s are sma er /hi e moreongitudina magnetization is avai a& e #or su&se$uent e*citation

even i# TR is very short+ 6o/ever, the overa signa is arger thanthe one o&tained /ith a B. 'ip ang e+


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/n general. the shorter the %". thesmaller the 3ip angle that is needed toprevent e*cessive saturation+ The 'ipang e ma*imizing the signa #or agiven TR and TE is 8no/n as the&rnst angle (

S"ice Se"ection and Spatia"


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S"ice Se"ection and Spatia"Encodin#

For i ustration, /e consider a transverse 3a*ia 4 s ice orcross"section through the &ody+ The magnetic %e dgenerated &y most MR scanners is not directed #rom

top to &ottom, as in the i ustrations /e have used so#ar, &ut a ong the &ody a*is o# the person &eingimaged+ From no/ on, this is the direction that /i &edesignated &y (z) since, as a ready said, z stands forthe direction of the main magnetic eld + The magnetic

%e d gradients that no/ come into p ay are represented&y /edges /ith the thic8 side indicating the higher%e d strength and the tip the o/er %e d strength+


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I# a uni#orm %e d o# identica strength /eregenerated throughout the &ody, aprotons /ou d have the same 7armor

#re$uency and /ou d &e e*citedsimu taneous y &y a sing e RF pu se+ To ena& e se ective e*citation o# a desireds ice, the magnetic %e d is there#ore madeinhomogeneous in a inear #ashion a ongthe z"direction &y means o# a gradient coi +


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Gradients are additiona magnetic %e ds thatare generated &y gradient coi s and add to orsu&tract #rom the main magnetic %e d+Depending on their position a ong the gradient,protons are temporari y e*posed to magnetic%e ds o# di;erent strength and hence di;er intheir precessiona #re$uencies+ A sha o/gradient generates a thic8er s ice /hi e a steepgradient generates a thinner s ice+ S ice positionis de%ned &y changing the center #re$uency o#the RF pu se app ied+


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strength o# the gradient app ied de%nes s ice thic8ness+ An RF pu se o# an #re$uency &and/idth produces a thin s ice i# the gradient is strong and a thic8 gradient is /ea8+


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spatial encoding , is the mostdi cu t tas8 in generating an MRimage and re$uires the app ication o#

additiona gradients that a ter themagnetic %e d strength a ong the y"and *"a*es+


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• Spatia encoding comprises t/osteps 4 phase encoding andfrequency encoding +


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!6ASE ENCODING• a gradient in the y"direction 3#rom top to

&ottom4 is s/itched on a#ter the spins have&een e*cited and precess in the *y"p ane+Such a phase-encoding gradient a ters the7armor #re$uencies o# the spins according totheir ocation a ong the gradient+ As a resu t,the e*cited spins higher up in the scannere*perience a stronger magnetic %e d and thusgain phase re ative to the some/hat s o/erspins #urther do/n+ The resu t is a phaseshift o# the spins re ative to each other


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9hen the gradient is s/itched o; a#tersome time, a spins return to theirinitia rate o# precession yet are no/

ahead or &ehind in phase re ative totheir previous state+ !hase no/ variesa ong the y"a*is in a inear #ashion and

each ine /ithin the s ice can thus &eidenti%ed &y its uni$ue phase+


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#"E45E6*7- E6*80/6G G" 0/E6%

This gradient generates a magnetic%e d that increases in strength #romright to e#t+ The corresponding

changes in 7armor #re$uencies ma8espins on the e#t side precess s o/erthan the ones on the right side+


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9hen /e co ect the MR signa /hi e the#re$uency"encoding gradient is s/itched on,/e do not o&tain a sing e #re$uency &ut a/ho e frequency spectrum comprising high#re$uencies #rom the right edge o# the s iceand o/ #re$uencies #rom the e#t edge+ Eachco umn o# the s ice is thus characterized &y aspeci%c #re$uency+ Fre$uency and phasetogether ena& e uni$ue spatia identi%cationo# each vo ume e ement &voxel '(


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uency encoding &y means o# the *"gradient+ 9ith the gradient s/itched o; 4, on y a sing e #re$uency is received, the 7armor #re$uency Q.+ 9ith the gradiched on (right) , a #re$uency spectrum is received /ith each co umn &eing idens uni$ue #re$uency

ourier transform


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ourier transform &or /re;!enc, ana",sis'

This mathematica operation serves toidenti#y the individua #re$uencies thatma8e up a signa +


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Factors A5ectin# t e

Si#na"-to-Noise Ratio

Image noise resu ts #rom a num&er


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Image noise resu ts #rom a num&ero# di;erent #actors<

• Imper#ections o# the MR system suchas magnetic %e d inhomogeneities,therma noise #rom the RF coi s, or

non inearity o# signa amp i%ers+• Factors associated /ith image

processing itse #+•

!atient"re ated #actors resu ting #rom&ody movement or respiratory motion+


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signal to noise ratio ( N#) There ationship &et/een the MR signaand the amount o# image noise

present (

A high SNR is desira& e in MRI+ The SNR is


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A high SNR is desira& e in MRI+ The SNR isdependent on the #o o/ing parameters<

• S ice thic8ness and receiver &and/idth• Fie d o# vie/• Size o# the 3image4 matri*• Num&er o# ac$uisitions• Scan parameters 3TR, TE, 'ip ang e4• Magnetic %e d strength• Se ection o# the transmit and receive

coi 3RF coi 4

Pi) "4 ) "4 M i)


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Pi)e"4 =o)e"4 Matri)• An MR image is digita and consists o# a matri* o#

pi'els or picture e ements+• A matrix is a t/o"dimensiona grid o# ro/s and

co umns+• Each s$uare o# the grid is a pi)e" /hich is assigned

a va ue that corresponds to a signa intensity+• Each pi*e o# an MR image provides in#ormation on

a corresponding•

three"dimensiona vo ume e ement, termed a +o'el• The vo*e size determines the spatia reso ution o#

an MR image+


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A vo*e is the tissue vo ume represented &y a pi*e in the t/o"dimensiona Mimage


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The receiver $and!idth is the rangeo# #re$uencies co ected &y an MRsystem during #re$uency encoding+ Is

either set automatica y or can &echanged &y the operator+A /ide receiver &and/idth ena& es

#aster data ac$uisition and minimizeschemica shi#t arti#acts &ut a soreduces SNR as more noise is inc uded+


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• An interslice gap is a sma space&et/een t/o ad acent s ices+

• t/o ad acent s ices over ap at theiredges /hen c ose y spaced+ nderthese conditions, the RF pu se #orone s ice a so e*cites protons in

ad acent s ices+ Such inter#erence is8no/n as cross tal% +


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a Idea s ice pro% e+ bDistorted,nonrectangu ars ice pro% e in SEimaging /ithinadvertent e*citation o#ad acent s ices reducesSNR+ c 9ith inters icegaps, the drop in SNR isminimized


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E;ect o# the FO@ on pi*e size /ith the matri* size he d constant


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A sma er matri* size /ith the FO@ he d constant resu ts in arger pi*e s andthus a poorer spatia reso ution


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ct o# matri* size on spatia reso ution+ Consider /e are imaging a smi ey /ith a %ne matri* 3 top 4 and a coarse matri* 3 ,ottom 4+ The pi*e s representingare & ac8+ The t/o depictions o# the #ace i ustrate the much poorer detai res

n a coarser matri* 3 ,ottom right 4 is used< pupi and eye cannot &e distinguisheopen mouth appears to &e c osed

N!mber o/ E)citations


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)

The num,er of e'citations (6E ) ornum,er of signal a+erages (6 )denotes ho/ many times a signa #rom

a given s ice is measured+ The SNR,/hich is proportiona to the s$uare rooto# the NE1, improves as the NE1

increases, &ut scan time a so increasesinear y /ith the NE1+

Ima#in# Parameters


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Other parameters a;ecting the SNR arethe se$uence used, echo time 3TE4,repetition time 3TR4, and the 'ip ang e+ The

SNR increases /ith the TR &ut the T5 e;ectis a so ost at onger TRs+ Converse y, theSNR decreases as the TE increases+ 9ith ashort TE, the T: contrast is ost+ For this

reason, the option o# shortening TE toimprove SNR is avai a& e on y #or T5"/eighted se$uences+

Ma#netic Fie"d Stren#t


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App ying a higher magnetic eld strengthincreases longitudinal magnetization&ecause more protons a ign a ong the

main a*is o# the magnetic %e d, resu tingin an increase in SNR+ The improved SNRachieved /ith high"%e d systems can &euti ized to generate images /ith animproved spatia reso ution or to per#orm#ast imaging+

Coi"s


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An e;ective means to improve SNR, /ithoutincreasing vo*e size or engthening scan time, isse ecting an appropriate radiofrequency ("#) coil + Ingenera , an RF coi shou d &e as c ose as possi& e to

the anatomy &eing imaged and surround the targetorgan+ The nearer the coi can &e p aced to theorgan under e*amination, the &etter the resu tingsigna + RF coi s can &e used either to transmit RFand receive the MR signa or to act as receiver coi son y+ In the atter case, e*citation pu ses arede ivered &y the &ody coi + The &asic coi types thatare distinguished are &rie'y descri&ed &e o/+

=o"!me Coi"s


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@o ume coi s may &e used e*c usive y as recei+ecoils or as com,ined transmit; recei+e coils +@o ume coi s comp ete y surround the anatomy to&e imaged+ T/o /ide y used vo ume coi

con%gurations are the saddle coil and the ,irdcagecoil + @o ume coi s are characterized &y ahomogeneous signa $ua ity+ Another type o#vo ume coi is the ,ody coil , /hich is an integra

part o# an MR scanner and is usua y ocated /ithinthe &ore o# the magnet itse #+ 6ead and e*tremitycoi s are #urther e*amp es o# vo ume coi s+

S!r/ace Coi"s


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Most sur#ace coi s can on y receive theMR signa and re y on the &ody coi #orde ivery o# RF pu ses+ Com&ined

transmitKreceive sur#ace coi s are a soavai a& e+ Sur#ace coi s are used #orspina MRI and imaging o# sma

anatomic structures+

Intracavit, Coi"s


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Intracavity coi s are sma local recei+ecoils that are inserted into &ody cavitiesto improve image $ua ity as a resu t o#

the c oser vicinity to the target organ+ Inc inica MRI, endorecta coi s are used#or imaging o# the prostate and the anasphincter musc e+ E*perimentaapp ications inc ude endovascu arimaging and imaging o# ho o/ organs+

P ased-Arra, Coi"s


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!hased"array coi s serve to recei+e MRsigna s+ A phased"array system consists o#severa independent coi s connected inpara e or series+ Each coi #eeds into aseparate receiver+ The in#ormation #rom theindividua receivers is com&ined to createone image+ !hased"array coi s yie d images

/ith a high spatia reso ution and a o/imaging /ith a arger %e d o# vie/ as theyimprove &oth SNR and signa homogeneity+

E;ects o# di;erent imaging and se$uence


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parameters on signa "to"noise ratio 3SNR4

E;ects o# matri* size, s ice thic8ness, and


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%e d o# vie/ 3FO@4 on spatia reso ution

E;ects o# di;erent se$uence parameters on


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E;ects o# di;erent se$uence parameters onscan time


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T e MR Scanner

ma or components o# an MRI system+They are


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They are<• A strong magnet to generate the static magnetic

%e d 3-.4+• A gradient system consisting o# three coi s toproduce inear %e d

• distortions in the *", y", and z"directions and thecorresponding ampli ers +

• A radiofrequency ("#) transmitter /ith a transmitcoi &ui t into the scanner+

• A high y sensitive "# recei+er to pic8 up andamp i#y the MR signa +

• A ternative y, imagers may use a sing e RF cois/itched &et/een the transmit and receive modes+


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• Additiona coi s, either receive coi s or transmitKreceivecoi s+

• @arious computers #or contro ing the scanner and thegradients 3 control computer 4, #or creation o# the MR images(array processor) , and #or coordinating a processes 3 main

or host computer , to /hich are connected the operator


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T e Ma#net

The main magnetic %e d generated &y the


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magnet must have the #o o/ing #eatures<

An ade$uate strength, /hich typica y ranges#rom >(* to ?(> T in medica MR imaging+

A high sta&i ity /ithout 'uctuations in %e dstrength+

The &est homogeneity possi& e /ith a uni#ormstrength throughout the entire %e d and /ithout(ho es)+ Fie d homogeneity is usua y e*pressedin ppm re ative to the main %e d over a certaindistance+ Inhomogeneities throughout the scanvo ume shou d &e &e o/ ppm 3.+... 4+

Three types o# magnets are


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distinguished<

#esistive magnets are conventionaelectromagnets that depend on a high and constantpo/er supp y to create a magnetic %e d+" The ma*imum %e d strength generated is a&out >(? T +

ma or disadvantages are the i# operatin# costsdue to the arge amounts o# po/er re$uired and a6e"d omo#eneit, t at is o/ten poor +An advantage is the sa#ety o# the system as the %e d

can be t!rned o5 instant", in an emer#enc, +


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uperconducting magnets• consist o# a coi made o# a nio,ium-titanium

(6,-%i) alloy • using coo ants 8no/n as cryogens 3usua y

i$uid he ium4• @ery strong and high y homogeneous

magnetic %e ds o# up to 5 T can &e

generated• B o# a MR systems used today have

superconducting magnets+


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• A quench re#ers to a magnet=ssudden oss o# superconductivity /ithsu&se$uent &rea8do/n o# the

magnetic %e d and may &e induced&y very minute movements o# thecoi +


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himming the process of ma=ing themagnetic eld homogenous

T e Gradient S stem


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T e Gradient S,stem

A set o# three separate gradient coi s, each /ithits o/n amp i%er, is needed to a ter themagnetic %e d strength a ong the *", y", and z"a*es+

The changing magnetic %e ds generated /henthe gradients are s/itched ead to the typica&anging sound heard during an MR scan+Simi ar to a oudspea8er, /hich is nothing &ut acoi inside a magnetic %e d, the gradient coi s(try to move) /hen the current is s/itched onand o;, /hich causes a noisy c anging+

T e Radio/re;!enc,


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S,stem The radiofrequency ("#) systemcomprises a po/er#u "# generator3the 7armor #re$uency at 5+ T is L +

M6z, /hich is in the range o# FMtransmitters4 and a high y sensitiverecei+er +


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Farada, ca#e "RF shie ding o# thescanner room to prevent inter#erence#rom e*terna sources+

T e Comp!ter S,stem


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The computers o# an MRI systemcontro and coordinate many processesranging #rom turning on and o;

gradients and the RF coi s to datahand ing and image processing+

-asic !u se Se$uences


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asic !u se Se$uences

e$uence+ The e*citation pu se a /ays has a 'ip ang e o# B. the dephaseds are re#ocused into the spin echo &y the 5 . pu se+ The dashed ines indicatee"encoding steps

0"ac8 0"ood E5ect


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0 ac8 0 ood E5ect

"The $lac% $lood e+ect 4 or out,o!e+ect , re#ers to a natura highcontrast &et/een 'o/ing & ood and

tissue+"a speci%c #eature o# SE se$uences dueto the ong echo time


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Inversion Recover, &IR'S


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Se;!encesAn IR se$uence is an SE se$uence /ith anadditiona 5 . inversion pu se thatprecedes the usua B. e*citation pu seand 5 . rephasing pu se o# a conventionaSE se$uence+ The inversion pu se 'ips

ongitudina magnetization #rom thepositive z"direction into the negative z"

direction /hich is indicated &y theongitudina magnetization vector no/pointing in the opposite direction+


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sion recovery se$uence /ith T5 re a*ation+ Fo o/ing the 5 . inversion &a' , the ongitudina magnetization vector points in the opposite direction &b' +a*ation ta8es p ace #rom z to Hz &c4 d' + No signa #orms as ong as there is no

onent in the transverse p ane 3the nu point o# a tissue4


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The time &et/een the 5 . pu se andthe B. RF pu se is the inversiontime (T-) (

T/o IR techni$ues are /ide y used inro tine c inica app ications


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routine c inica app ications<• The short TI inversion recovery 3STIR4

se$uence and• the 'uid"attenuated inversion

recovery 3F7AIR4 se$uence+

STIR 3short TI inversion4


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recovery4• STIR 3short TI inversion recovery4

se$uences are /ide y used #or #atsuppression &ecause they re ia& y

e iminate the signa #rom #at at amagnetic %e d strengths+

F7AIR 3'uid"attenuated inversionrecovery4


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recovery4

F7AIR se$uences a very use#u #ordetecting esions /ith a poor contrastto surrounding &rain tissue+


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Fast P!"se Se;!ences

There are severa reasons /hy it is desira& e to speedup scanning+


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up scanning

A #ast se$uence a o/s one to per#orm dynamic studies, e+g+ to trac8a contrast medium &o us+ Shorter ac$uisition is ess prone to motion arti#acts, /hich isespecia y important in uncooperative patients+

A se$uence that is #ast enough can &e ac$uired during &reath"ho dand thus yie ds images /ithout respiratory arti#acts+@arious techni$ues are avai a& e to shorten scan time<

se o# state"o#"the"art gradient and RF systems to #u capacity andmore e;ective timing o# conventiona se$uences 3Uu tra"V#ast GRE4+

Samp ing o# mu tip e echoes /ith di;erent phase encodings 3FSE,echo p anar imaging4+

Incomp ete % ing o# 8"space 3#ractiona echo imaging, partia Fourierimaging, rectangu ar %e d o# vie/4+


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ingle-shot fast spin echo( # E)


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( # E)ingle-shot fast spin echo ( # E) and

half-#ourier acquisition single-shot fastspin echo (> %E) are a ternative

names #or a very #ast MR techni$ue/ith scan times o# 5 sec or ess+

Ec o P"anar Ima#in# &EPI'Se;!ence


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Se;!ence

Echo p anar imaging 3E!I4 ena& esu tra#ast data ac$uisition, ma8ing it ane*ce ent candidate #or dynamic and

#unctiona MR imaging+ This methodre$uires strong and rapid y s/itched#re$uency"encoding gradients+

MR Contrast A#ents


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• *aramagnetic su$stances havemagnetic moment 3resu ting #romindividua spins4 &ecause they

consist o# atoms or mo ecu es thathave magnetic moment due tounpaired e ectron or&its in their outere ectron she s or unpaired nuc eonsin their atomic nuc ei+


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• uperparamagnetic su,stanceshave very strong paramagneticproperties+


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• erromagnetic su$stances a so consisto# arge groups o# atoms /hose unpairede ectron spins are strong y ent/ined &ye*change coup ing 3so id state4+

• These su&stances retain theirmagnetization even a#ter the e*ternamagnetic %e d has &een removed andsu&se$uent y &ecome permanent magnets+

• The &est 8no/n e*amp e is iron &Fe'(


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magnetic resonance imaging ppt