magnetic resonance imaging ppt
TRANSCRIPT
-
8/9/2019 magnetic resonance imaging ppt
1/149
MAGNETIC RESONANCE
IMAGING
-
8/9/2019 magnetic resonance imaging ppt
2/149
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$uency• SAR Speci%c a&sorption rate• SE Spin echo• SNR Signa "to"noise ratio• T Tes a• TE Echo time• TOF Time o# 'ight• TR Repetition time
-
8/9/2019 magnetic resonance imaging ppt
3/149
(z"direction) and (*y"p ane) are#re$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+
-
8/9/2019 magnetic resonance imaging ppt
4/149
0
1
2
-
8/9/2019 magnetic resonance imaging ppt
5/149
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+
-
8/9/2019 magnetic resonance imaging ppt
6/149
!ROTON
E7ECTRON
-
8/9/2019 magnetic resonance imaging ppt
7/149
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+
-
8/9/2019 magnetic resonance imaging ppt
8/149
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
-
8/9/2019 magnetic resonance imaging ppt
9/149
:+ 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 +
-
8/9/2019 magnetic resonance imaging ppt
10/149
MAGNETIC MOMENT
-
8/9/2019 magnetic resonance imaging ppt
11/149
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>
-
8/9/2019 magnetic resonance imaging ppt
12/149
6o/ /i a spin &ehave /hen
&rought into a strong magnetic %e d?
-
8/9/2019 magnetic resonance imaging ppt
13/149
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+
-
8/9/2019 magnetic resonance imaging ppt
14/149
-
8/9/2019 magnetic resonance imaging ppt
15/149
-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
-
8/9/2019 magnetic resonance imaging ppt
16/149
NOTE< The 7armor #re$uency is a veryimportant concept that is at the core o#MR imaging+
T e Larmor or precessionfrequency is the rate at /hich spins/o&& e /hen p aced in a magnetic%e d+
-
8/9/2019 magnetic resonance imaging ppt
17/149
The 7armor #re$uency is directly proportional to the strength(B. ) of the
magnetic eld and is given &y the Larmor equation:
-
8/9/2019 magnetic resonance imaging ppt
18/149
9hat happens to the spins precessing and s o/ ya igning inside the
magnetic %e d?
-
8/9/2019 magnetic resonance imaging ppt
19/149
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+
-
8/9/2019 magnetic resonance imaging ppt
20/149
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+
-
8/9/2019 magnetic resonance imaging ppt
21/149
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 +
-
8/9/2019 magnetic resonance imaging ppt
22/149
The re$uired e ectromagnetic /ave isgenerated in a po/er#u radiotransmitter and app ied to the o& ectto &e imaged &y means o# an antennacoi +
-
8/9/2019 magnetic resonance imaging ppt
23/149
The process o# energy a&sorption is 8no/n ase*citation o# the spin system and resu ts in
the ongitudina magnetization &eing moreand more tipped a/ay #rom the z"a*is to/ardthe transverse 3*y"4p ane perpendicu ar tothe direction o# the main magnetic %e d+ A o#
the ongitudina magnetization is rotated intothe transverse p ane &y a radio#re$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+
-
8/9/2019 magnetic resonance imaging ppt
24/149
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
-
8/9/2019 magnetic resonance imaging ppt
25/149
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+
-
8/9/2019 magnetic resonance imaging ppt
26/149
Re"a)ation
-
8/9/2019 magnetic resonance imaging ppt
27/149
9hat happens to the spins a#ter theyhave &een e*cited?
-
8/9/2019 magnetic resonance imaging ppt
28/149
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
-
8/9/2019 magnetic resonance imaging ppt
29/149
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
-
8/9/2019 magnetic resonance imaging ppt
30/149
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, +
-
8/9/2019 magnetic resonance imaging ppt
31/149
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+
-
8/9/2019 magnetic resonance imaging ppt
32/149
• 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+
-
8/9/2019 magnetic resonance imaging ppt
33/149
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
-
8/9/2019 magnetic resonance imaging ppt
34/149
• "!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# " . +
-
8/9/2019 magnetic resonance imaging ppt
35/149
-
8/9/2019 magnetic resonance imaging ppt
36/149
-
8/9/2019 magnetic resonance imaging ppt
37/149
: 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
-
8/9/2019 magnetic resonance imaging ppt
38/149
Transverse Re a*ation is the decay oftrans+erse magnetization ,ecausespins lose coherence (dephasing)
-
8/9/2019 magnetic resonance imaging ppt
39/149
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 +
-
8/9/2019 magnetic resonance imaging ppt
40/149
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+
-
8/9/2019 magnetic resonance imaging ppt
41/149
-
8/9/2019 magnetic resonance imaging ppt
42/149
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+
-
8/9/2019 magnetic resonance imaging ppt
43/149
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#(
-
8/9/2019 magnetic resonance imaging ppt
44/149
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+
-
8/9/2019 magnetic resonance imaging ppt
45/149
Ima#e Contrast
%hree intrinsic features o# a &io ogicatissue contri&ute to its signa intensityor &rightness on an MR image andhence image contrast<
-
8/9/2019 magnetic resonance imaging ppt
46/149
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 +
-
8/9/2019 magnetic resonance imaging ppt
47/149
:+ 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!)
-
8/9/2019 magnetic resonance imaging ppt
48/149
+ 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"!) $
-
8/9/2019 magnetic resonance imaging ppt
49/149
!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+
-
8/9/2019 magnetic resonance imaging ppt
50/149
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(
-
8/9/2019 magnetic resonance imaging ppt
51/149
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+
-
8/9/2019 magnetic resonance imaging ppt
52/149
• 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+
-
8/9/2019 magnetic resonance imaging ppt
53/149
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 +
-
8/9/2019 magnetic resonance imaging ppt
54/149
REMEM-ER<
• S ort TR stron# T* .ei# tin#• %on# TR "o. T* .ei# tin#
The re ationship &et/een the MR
-
8/9/2019 magnetic resonance imaging ppt
55/149
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 $rightbeca!se t e, re#ain most o/ t eir"on#it!dina" ma#netization d!rin# t e
TR interva" and t !s prod!ce astron#er MR si#na"(• Tiss!es .it a long T1 appear dar%
beca!se t e, do not re#ain m!c o/t eir "on#it!dina" ma#netizationd!rin# t e TR interva" and t !sprod!ce a .ea8er MR si#na"(
E Ti &TE' d T1
-
8/9/2019 magnetic resonance imaging ppt
56/149
Ec o Time &TE' and T17ei# tin#
7 at is an ec o4 an,.a,9
-
8/9/2019 magnetic resonance imaging ppt
57/149
For the time &eing it is su cient to 8no/ that
these gradients serve to induce contro edmagnetic %e d inhomogeneities that areneeded to encode the spatia origin o# theMR signa s+ 6o/ever, the gradients a so
contri&ute to spin dephasing+ These e;ectsmust &e reversed &y app ying a re#ocusingpu se &e#ore an ade$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+
-
8/9/2019 magnetic resonance imaging ppt
58/149
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
-
8/9/2019 magnetic resonance imaging ppt
59/149
&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"(
-
8/9/2019 magnetic resonance imaging ppt
60/149
• S ort TE "o. T1 .ei# tin#• %on# TE stron# T1 .ei# tin#
-
8/9/2019 magnetic resonance imaging ppt
61/149
• Tiss!es .it a short T1 appeardar% on T1-.ei# ted ima#es '
• tiss!es .it a long T1 appear$right on T1-.ei# ted ima#es:
-
8/9/2019 magnetic resonance imaging ppt
62/149
• MR images that com&ine T5 and T: e;ects are 8no/nas proton density-1eighted images (20 images) +
• !D se$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$uences are main y used #or imaging o# the &rain,spine, and muscu os8e eta system+
S ! i S R i i
-
8/9/2019 magnetic resonance imaging ppt
63/149
Sat!ration at S ort RepetitionTimes
• SAT RATION" is /hen a series o#e*citation pu ses is app ied, the MRsigna &ecomes /ea8er and /ea8era#ter each repeat pu se+ This processis 8no/n as saturation
-
8/9/2019 magnetic resonance imaging ppt
64/149
Signa intensities o# di;erent tissues on T5" and T:"/eighted images
-
8/9/2019 magnetic resonance imaging ppt
65/149
e ative proton densities 3 4 and intrinsic T5 and T: times 3in msec4 o# di;erentssues
-
8/9/2019 magnetic resonance imaging ppt
66/149
hanism o# saturation+ 9ith a very short TR, the ongitudina magnetization,that /i recover in the interva and &e avai a& e #or su&se$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
-
8/9/2019 magnetic resonance imaging ppt
67/149
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$ui i&rium or steady state4+ In this situation, the indivisigna s that #orm a#ter each e*citation are very /ea8
EP I7I-RI M
TIME
-
8/9/2019 magnetic resonance imaging ppt
68/149
Saturation is an important issue /hen#ast or u tra#ast MR techni$ues areused+ 6ere the MR signa may &ecomevery /ea8 due to the very shortrepetition times+
-
8/9/2019 magnetic resonance imaging ppt
69/149
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+
-
8/9/2019 magnetic resonance imaging ppt
70/149
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$uent e*citation
even i# TR is very short+ 6o/ever, the overa signa is arger thanthe one o&tained /ith a B. 'ip ang e+
-
8/9/2019 magnetic resonance imaging ppt
71/149
/n general. the shorter the %". thesmaller the 3ip angle that is needed toprevent e*cessive saturation+ The 'ipang e ma*imizing the signa #or agiven TR and TE is 8no/n as the&rnst angle (
S"ice Se"ection and Spatia"
-
8/9/2019 magnetic resonance imaging ppt
72/149
S"ice Se"ection and Spatia"Encodin#
For i ustration, /e consider a transverse 3a*ia 4 s ice orcross"section through the &ody+ The magnetic %e dgenerated &y most MR scanners is not directed #rom
top to &ottom, as in the i ustrations /e have used so#ar, &ut a ong the &ody a*is o# the person &eingimaged+ From no/ on, this is the direction that /i &edesignated &y (z) since, as a ready said, z stands forthe direction of the main magnetic eld + The magnetic
%e d gradients that no/ come into p ay are represented&y /edges /ith the thic8 side indicating the higher%e d strength and the tip the o/er %e d strength+
-
8/9/2019 magnetic resonance imaging ppt
73/149
I# a uni#orm %e d o# identica strength /eregenerated throughout the &ody, aprotons /ou d have the same 7armor
#re$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 +
-
8/9/2019 magnetic resonance imaging ppt
74/149
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+
-
8/9/2019 magnetic resonance imaging ppt
75/149
strength o# the gradient app ied de%nes s ice thic8ness+ An RF pu se o# an #re$uency &and/idth produces a thin s ice i# the gradient is strong and a thic8 gradient is /ea8+
-
8/9/2019 magnetic resonance imaging ppt
76/149
spatial encoding , is the mostdi cu t tas8 in generating an MRimage and re$uires the app ication o#
additiona gradients that a ter themagnetic %e d strength a ong the y"and *"a*es+
-
8/9/2019 magnetic resonance imaging ppt
77/149
• Spatia encoding comprises t/osteps 4 phase encoding andfrequency encoding +
-
8/9/2019 magnetic resonance imaging ppt
78/149
!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$uencies o# the spins according totheir ocation a ong the gradient+ As a resu t,the e*cited spins higher up in the scannere*perience a stronger magnetic %e d and thusgain phase re ative to the some/hat s o/erspins #urther do/n+ The resu t is a phaseshift o# the spins re ative to each other
-
8/9/2019 magnetic resonance imaging ppt
79/149
9hen the gradient is s/itched o; a#tersome time, a spins return to theirinitia rate o# precession yet are no/
ahead or &ehind in phase re ative totheir previous state+ !hase no/ variesa ong the y"a*is in a inear #ashion and
each ine /ithin the s ice can thus &eidenti%ed &y its uni$ue phase+
-
8/9/2019 magnetic resonance imaging ppt
80/149
#"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$uencies ma8espins on the e#t side precess s o/erthan the ones on the right side+
-
8/9/2019 magnetic resonance imaging ppt
81/149
9hen /e co ect the MR signa /hi e the#re$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 '(
-
8/9/2019 magnetic resonance imaging ppt
82/149
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$uency
ourier transform
-
8/9/2019 magnetic resonance imaging ppt
83/149
ourier transform &or /re;!enc, ana",sis'
This mathematica operation serves toidenti#y the individua #re$uencies thatma8e up a signa +
-
8/9/2019 magnetic resonance imaging ppt
84/149
-
8/9/2019 magnetic resonance imaging ppt
85/149
Factors A5ectin# t e
Si#na"-to-Noise Ratio
Image noise resu ts #rom a num&er
-
8/9/2019 magnetic resonance imaging ppt
86/149
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+
-
8/9/2019 magnetic resonance imaging ppt
87/149
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
-
8/9/2019 magnetic resonance imaging ppt
88/149
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$uisitions• Scan parameters 3TR, TE, 'ip ang e4• Magnetic %e d strength• Se ection o# the transmit and receive
coi 3RF coi 4
Pi) "4 ) "4 M i)
-
8/9/2019 magnetic resonance imaging ppt
89/149
Pi)e"4 =o)e"4 Matri)• An MR image is digita and consists o# a matri* o#
pi'els or picture e ements+• A matrix is a t/o"dimensiona grid o# ro/s and
co umns+• Each s$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+
-
8/9/2019 magnetic resonance imaging ppt
90/149
A vo*e is the tissue vo ume represented &y a pi*e in the t/o"dimensiona Mimage
-
8/9/2019 magnetic resonance imaging ppt
91/149
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+
-
8/9/2019 magnetic resonance imaging ppt
92/149
• 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% +
-
8/9/2019 magnetic resonance imaging ppt
93/149
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
-
8/9/2019 magnetic resonance imaging ppt
94/149
-
8/9/2019 magnetic resonance imaging ppt
95/149
E;ect o# the FO@ on pi*e size /ith the matri* size he d constant
-
8/9/2019 magnetic resonance imaging ppt
96/149
A sma er matri* size /ith the FO@ he d constant resu ts in arger pi*e s andthus a poorer spatia reso ution
-
8/9/2019 magnetic resonance imaging ppt
97/149
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
-
8/9/2019 magnetic resonance imaging ppt
98/149
)
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$uare rooto# the NE1, improves as the NE1
increases, &ut scan time a so increasesinear y /ith the NE1+
Ima#in# Parameters
-
8/9/2019 magnetic resonance imaging ppt
99/149
Other parameters a;ecting the SNR arethe se$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
-
8/9/2019 magnetic resonance imaging ppt
100/149
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
-
8/9/2019 magnetic resonance imaging ppt
101/149
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
-
8/9/2019 magnetic resonance imaging ppt
102/149
@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
-
8/9/2019 magnetic resonance imaging ppt
103/149
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
-
8/9/2019 magnetic resonance imaging ppt
104/149
Intracavity coi s are sma local recei+ecoils that are inserted into &ody cavitiesto improve image $ua ity as a resu t o#
the c oser vicinity to the target organ+ Inc inica MRI, endorecta coi s are used#or imaging o# the prostate and the anasphincter musc e+ E*perimentaapp ications inc ude endovascu arimaging and imaging o# ho o/ organs+
P ased-Arra, Coi"s
-
8/9/2019 magnetic resonance imaging ppt
105/149
!hased"array coi s serve to recei+e MRsigna s+ A phased"array system consists o#severa independent coi s connected inpara e or series+ Each coi #eeds into aseparate receiver+ The in#ormation #rom theindividua receivers is com&ined to createone image+ !hased"array coi s yie d images
/ith a high spatia reso ution and a o/imaging /ith a arger %e d o# vie/ as theyimprove &oth SNR and signa homogeneity+
E;ects o# di;erent imaging and se$uence
-
8/9/2019 magnetic resonance imaging ppt
106/149
parameters on signa "to"noise ratio 3SNR4
E;ects o# matri* size, s ice thic8ness, and
-
8/9/2019 magnetic resonance imaging ppt
107/149
%e d o# vie/ 3FO@4 on spatia reso ution
E;ects o# di;erent se$uence parameters on
-
8/9/2019 magnetic resonance imaging ppt
108/149
E;ects o# di;erent se$uence parameters onscan time
-
8/9/2019 magnetic resonance imaging ppt
109/149
T e MR Scanner
ma or components o# an MRI system+They are
-
8/9/2019 magnetic resonance imaging ppt
110/149
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+
-
8/9/2019 magnetic resonance imaging ppt
111/149
• 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
-
8/9/2019 magnetic resonance imaging ppt
112/149
-
8/9/2019 magnetic resonance imaging ppt
113/149
T e Ma#net
The main magnetic %e d generated &y the
-
8/9/2019 magnetic resonance imaging ppt
114/149
magnet must have the #o o/ing #eatures<
An ade$uate strength, /hich typica y ranges#rom >(* to ?(> T in medica MR imaging+
A high sta&i ity /ithout 'uctuations in %e dstrength+
The &est homogeneity possi& e /ith a uni#ormstrength throughout the entire %e d and /ithout(ho es)+ Fie d homogeneity is usua y e*pressedin ppm re ative to the main %e d over a certaindistance+ Inhomogeneities throughout the scanvo ume shou d &e &e o/ ppm 3.+... 4+
Three types o# magnets are
-
8/9/2019 magnetic resonance imaging ppt
115/149
distinguished<
#esistive magnets are conventionaelectromagnets that depend on a high and constantpo/er supp y to create a magnetic %e d+" The ma*imum %e d strength generated is a&out >(? T +
ma or disadvantages are the i# operatin# costsdue to the arge amounts o# po/er re$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, +
-
8/9/2019 magnetic resonance imaging ppt
116/149
-
8/9/2019 magnetic resonance imaging ppt
117/149
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$uid he ium4• @ery strong and high y homogeneous
magnetic %e ds o# up to 5 T can &e
generated• B o# a MR systems used today have
superconducting magnets+
-
8/9/2019 magnetic resonance imaging ppt
118/149
• A quench re#ers to a magnet=ssudden oss o# superconductivity /ithsu&se$uent &rea8do/n o# the
magnetic %e d and may &e induced&y very minute movements o# thecoi +
-
8/9/2019 magnetic resonance imaging ppt
119/149
himming the process of ma=ing themagnetic eld homogenous
T e Gradient S stem
-
8/9/2019 magnetic resonance imaging ppt
120/149
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,
-
8/9/2019 magnetic resonance imaging ppt
121/149
S,stem The radiofrequency ("#) systemcomprises a po/er#u "# generator3the 7armor #re$uency at 5+ T is L +
M6z, /hich is in the range o# FMtransmitters4 and a high y sensitiverecei+er +
-
8/9/2019 magnetic resonance imaging ppt
122/149
Farada, ca#e "RF shie ding o# thescanner room to prevent inter#erence#rom e*terna sources+
T e Comp!ter S,stem
-
8/9/2019 magnetic resonance imaging ppt
123/149
The computers o# an MRI systemcontro and coordinate many processesranging #rom turning on and o;
gradients and the RF coi s to datahand ing and image processing+
-asic !u se Se$uences
-
8/9/2019 magnetic resonance imaging ppt
124/149
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
-
8/9/2019 magnetic resonance imaging ppt
125/149
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
-
8/9/2019 magnetic resonance imaging ppt
126/149
Inversion Recover, &IR'S
-
8/9/2019 magnetic resonance imaging ppt
127/149
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$uence+ The inversion pu se 'ips
ongitudina magnetization #rom thepositive z"direction into the negative z"
direction /hich is indicated &y theongitudina magnetization vector no/pointing in the opposite direction+
-
8/9/2019 magnetic resonance imaging ppt
128/149
sion recovery se$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
-
8/9/2019 magnetic resonance imaging ppt
129/149
The time &et/een the 5 . pu se andthe B. RF pu se is the inversiontime (T-) (
T/o IR techni$ues are /ide y used inro tine c inica app ications
-
8/9/2019 magnetic resonance imaging ppt
130/149
routine c inica app ications<• The short TI inversion recovery 3STIR4
se$uence and• the 'uid"attenuated inversion
recovery 3F7AIR4 se$uence+
STIR 3short TI inversion4
-
8/9/2019 magnetic resonance imaging ppt
131/149
recovery4• STIR 3short TI inversion recovery4
se$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
-
8/9/2019 magnetic resonance imaging ppt
132/149
recovery4
F7AIR se$uences a very use#u #ordetecting esions /ith a poor contrastto surrounding &rain tissue+
-
8/9/2019 magnetic resonance imaging ppt
133/149
Fast P!"se Se;!ences
There are severa reasons /hy it is desira& e to speedup scanning+
-
8/9/2019 magnetic resonance imaging ppt
134/149
up scanning
A #ast se$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$uences 3Uu tra"V#ast GRE4+
Samp ing o# mu tip e echoes /ith di;erent phase encodings 3FSE,echo p anar imaging4+
Incomp ete % ing o# 8"space 3#ractiona echo imaging, partia Fourierimaging, rectangu ar %e d o# vie/4+
-
8/9/2019 magnetic resonance imaging ppt
135/149
ingle-shot fast spin echo( # E)
-
8/9/2019 magnetic resonance imaging ppt
136/149
( # E)ingle-shot fast spin echo ( # E) and
half-#ourier acquisition single-shot fastspin echo (> %E) are a ternative
names #or a very #ast MR techni$ue/ith scan times o# 5 sec or ess+
Ec o P"anar Ima#in# &EPI'Se;!ence
-
8/9/2019 magnetic resonance imaging ppt
137/149
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$uency"encoding gradients+
MR Contrast A#ents
-
8/9/2019 magnetic resonance imaging ppt
138/149
-
8/9/2019 magnetic resonance imaging ppt
139/149
• *aramagnetic su$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+
-
8/9/2019 magnetic resonance imaging ppt
140/149
-
8/9/2019 magnetic resonance imaging ppt
141/149
• uperparamagnetic su,stanceshave very strong paramagneticproperties+
-
8/9/2019 magnetic resonance imaging ppt
142/149
• 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'(
-
8/9/2019 magnetic resonance imaging ppt
143/149
-
8/9/2019 magnetic resonance imaging ppt
144/149
-
8/9/2019 magnetic resonance imaging ppt
145/149
-
8/9/2019 magnetic resonance imaging ppt
146/149
-
8/9/2019 magnetic resonance imaging ppt
147/149
-
8/9/2019 magnetic resonance imaging ppt
148/149
-
8/9/2019 magnetic resonance imaging ppt
149/149