Basics of Magnetic Resonance Imaging (MRI)

Transcription

1 Basics of Magneic Resonance MRI Shaoing HUANG, PhD Singapore Universi of Technolog and Design

2 OUTLINE Medical imaging modaliies Hisor of MRI Working principles of MRI

3 Medical Modaliies Creae images of human bod non-invasivel X-ra radiograph X-ra compued omograph CT Medical ulrasonograph MRI 3

4 Medical Modaliies X-ra radiograph To use X-ras o view he inernal srucure of a non-uniforml composed and opaque objec Projeced plane X-ra generaor Subjec - 2D images - Radiaion: X-ra is absorbed b he subjec 4

5 Medical Modaliies X-ra Compued Tomograph CT To generae a 3D image of he inernal srucure of an objec from a large series of 2D radiographic images aken around a single ais of roaion D images - X-ra absorbed b he subjec is 100 imes of ha b using -ra radiograph 5

6 Medical Modaliies Medical ulrasonograph Ulrasound-based imaging echnique used for visualiing subcuaneous bod srucures Obseric sonograph blog.healhap.com - I displas 2D cross-secion of he issue Blood flow Moion of he issue over ime The locaion of blood The presen of specific molecules The siff ness of issues Anaom of 3D region - Advanages Provide real-ime images Porable Low cos No harmful radiaion Limiaions on field of view Difficul imaging srucure behind bone The skill of operaors maers 6

7 Medical Modaliies Magneic Resonance MRI billpsudios.blogspo.com - 3D images - Low RF radiaion - Cos: 2-4 millions USD 7

8 MRI Differen names of MRI Magneic resonance imaging MRI Nuclear magneic resonance imaging NMRI Magneic resonance omograph MRT Advanages: Good conras Noninvasive No ioniing radiaion Arbirar scan planes Abdomen Spine Hear / Coronar hp://mrsrl.sanford.edu/~brian/inromr/ 8

9 Hisor of MRI 1952 Herman Carr Harvard Universi 1960 Vladislav Ivanov Sovie Union 1970 Peer Mansfield Universi of Noingham 1971 Ramond Damadian Sae Universi of New York Produced 1D MRI image Filed a documen for a magneic resonance imaging device USSR Sae Commiee for Invenions and Discover a Leningrad Developed a mahemaical echnique ha would allow scans o ake seconds raher han hours and produce clearer images han Lauerbur had. Repored umors and normal issue can be disinguished in vivo b NMR [Science]. This mehod is no effecive and no pracical Ramond Damadian Creaed he world s firs MRI machine & filed a paen 1973 Paul Lauerbur Sae Universi of New York Epended Carr s echnique & generaed and published he firs nuclear magneic resonance 2D and 3D images using gradiens 9

10 Hisor of MRI Ramond Damadian's apparaus and mehod for deecing cancer in issue [1] The Naional Science Foundaion noes "The paen included he idea of using NMR o 'scan' he human bod o locae cancerous issue. However, i did no describe a mehod for generaing picures from such a scan or precisel how such a scan migh be done.[2][3] [1] [2] "Scienis Claims Eclusion From Nobel Prie for MRI". Los Angeles Times Rerieved [3] "Does Dr. Ramond Damadian Deserve he Nobel Prie for Medicine?". The Armenian Reporer Rerieved

11 Hisor of MRI 1974 Paul Lauerbur Generaed he firs cross-secional image of a living mouse 1977 Ramond Damadian Larr Minkoff Michael Goldsmih Performed and published he firs MRI bod scan of a human 1979 Richard S. Likes GE Filed a paen on k-space 1970s John Mallard Universi of Aberdeen Buil he firs full bod MRI scanner a he Universi of Aberdeen 1980 John Mallard Obained he firs clinicall useful image of a paien s inernal issues using MRI using he machine he buil during he 1970s 1980 Paul Boomle GE Buil he firs 1.5T whole-bod MRI/MRS scanner he highes srengh a ha ime 1982 Paul Boomle Johns Hopkins Universi 2003 Paul Lauerbur Peer Mansfield Performed he firs localied MR Specroscop MRS in he human hear and brain Nobel Prie in Phsiolog or Medicine for heir "discoveries concerning magneic resonance imaging" 11

12 Magneic Resonance - Spins A group of aoms wih odd number of proons and/or odd number of neurons Possess a nuclear spin angular momenum Ehibi nuclear MR phenomena e.g. hdrogen 1 H Visualiaion Nucleons Spinning charged spheres Small magneic momens 1 H MR relevan nuclei spins MRI Spin Polariaion Precession Relaaion Signal Recepion 12

13 Magneic Resonance - Spins Eamples of MR-relevan nuclei spins Hdrogen 1 H, single proon - Mos abundan large amoun - Mos sensiive gives large signals - Mos sudied 1 H 31 P MRI Spin Polariaion Precession Relaaion Signal Recepion Phosphorus 31 P Imporan indicaor of meabolism 13

14 Polariaion Spins are aligned o he applied field equilibrium sae Resuls: ne magneiaion No Applied Field Applied Field B 0 MRI MR Polariaion Precession Relaaion Signal Recepion 14

15 Polariaion 15 MRI MR Polariaion Precession Relaaion Signal Recepion

16 Precession Spins precess abou B 0 Angular frequenc & frequenc of he precession B 0 2f B f B or 42 2 MH/Tesla B 0 Equilibrium sae direcion MRI MR Polariaion Precession Relaaion Signal Recepion 16

17 Precession Spins precess abou B 0 Angular frequenc & frequenc of he precession B 0 2f B f B or 42 2 MH/Tesla To obain MR signal: B 1 is uned o o ecie spins OUT OF equilibrium B 1 Ou of equilibrium sae direcion B 0 Equilibrium sae direcion Source: hp://mrsrl.sanford.edu/~bria n/inromr/ MRI MR Polariaion Precession Relaaion Signal Recepion B 1 : radiofrequenc field 17

18 Polariaion 18 MRI MR Polariaion Precession Relaaion Signal Recepion

19 Relaaion Magneiaion reurns eponeniall o equilibrium Longiudinal recover ime consan, T 1 Transverse deca ime consan, T 2 Differen issues have differen T 1 and T 2 M T 1 T 2 M ime Recover ime Deca ms ms B 1 Ou of equilibrium sae direcion MRI MR Polariaion Precession Relaaion Signal Recepion B 0 Equilibrium sae direcion 19

20 Relaaion Magneiaion reurns eponeniall o equilibrium Longiudinal recover ime consan, T 1 Transverse deca ime consan, T 2 Differen issues have differen T 1 and T 2 Source: hp://mrsrl.sanford.edu/~brian/inromr/ MRI MR Polariaion Precession Relaaion Signal Recepion 20

21 Relaaion M T 1 M T 2 ime Recover ime Deca T 1 is deermined b hermal ineracions beween he resonaing proons and oher proons and oher magneic nuclei in he magneic environmen or "laice". - T 2 deca is due o magneic ineracions ha occur beween spinning proons. - T 2 ineracions do no involve a ransfer of energ bu onl a change in phase, which leads o a loss of coherence. MRI MR Polariaion Precession Relaaion Signal Recepion 21

22 Signal Recepion The spin precession causes magneic flu B change in a RF coil The change in flu induces currens/volage The induced currens/volage generaes signal F B RF coil B 0 22 MRI MR Polariaion Precession Relaaion Signal Recepion

23 Signal Recepion The spin precession causes magneic flu B change in a RF coil The change in flu induces currens/volage The induced currens/volage generaes signal Source: hp://mrsrl.sanford.edu/~brian/inromr/ 23 MRI MR Polariaion Precession Relaaion Signal Recepion

24 Signal Recepion The spin precession causes magneic flu B change in a RF coil The change in flu induces currens/volage The induced currens/volage generaes signal 3T 128 MH F B? RF coil B 0 24 MRI MR Polariaion Precession Relaaion Signal Recepion

25 2D Sequence Sep 1: Selecive eciaion: B 1 is applied o he presence of B 0 & G Sep 2: Spaial signal encoding & signal readou Mehod 1 Projecion-reconsrucion mehod -ra CT Mehod 2 2D Fourier ransform mehod popular MRI MR Polariaion Precession Relaaion Signal Recepion 25

26 Nonselecive Eciaions & 3D B 1 is applied o he presence of B 0 onl 3D imaging B 1 Ou of equilibrium sae direcion B 0 Commen: 3D imaging is usuall ime consuming Equilibrium sae direcion B 1 B 0 B 0 MRI MR Polariaion Precession Relaaion Signal Recepion 26

27 Selecive Eciaion & 2D B 1 is applied o he presence of B 0 & a linear gradien field G, G, or G Ecie a plane he -ais 2D imaging B 1 B 0 + G B 0 + G G G e.g. G = 1 Gauss /m MRI MR Polariaion Precession Relaaion Signal Recepion 27

28 Selecive Eciaion & 2D B 1 is applied o he presence of B 0 & a linear gradien field G, G, or G Ecie a plane he -ais 2D imaging 1 B 0 + G 0-1 B 0 B 0 - G B 0 B 1 B 0 + G B 0 + G MRI MR Polariaion Precession Relaaion Signal Recepion 28

29 Selecive Eciaion & 2D B 1 is applied o he presence of B 0 & a linear gradien field G, G, or G Ecie a plane he -ais 2D imaging 1 B 0 + G 0-1 B 0 B 0 - G B 0 B 1 upper B 0 G 2 + G B 0 + G lower B 0 G 2 MRI MR Polariaion Precession Relaaion Signal Recepion 29

30 Selecive Eciaion & 2D B 1 is applied o he presence of B 0 & a linear gradien field G, G, or G Ecie a plane he -ais 2D imaging B 1 G B 0 F.T. + G B 0 G + - The frequenc conen of B 1 mus be a recangular funcion - Ideall, B 1 mus be a sinc funcion - Pracicall, B 1 is a sinc-like funcion MRI MR Polariaion Precession Relaaion Signal Recepion 30

31 2D Sequence Sep 1: Selecive eciaion: B 1 is applied o he presence of B 0 & G B 1 B 0 Preferred siuaion: in phase + G B 0 G + RF G B 1 RF coil eciaion MRI MR Polariaion Precession Relaaion Signal Recepion 31

32 2D Sequence Sep 2: Spaial signal encoding & signal readou G G G G G G B 1 RF G RF sources RF coil recepion G G?? 1, 1 2, MRI MR Polariaion Precession Relaaion Signal Recepion 3, 3 B 0 Eample: G = 1 Gauss /m so B B G G G B G 0 0 G B 0 G G B 2 0 G G 32

33 2D Sequence Sep 2: Spaial signal encoding & signal readou RF sources 3, 3 < 1 > 1 B 0 G B 0 G Mehod 1 Projecion-reconsrucion mehod RF G B 1 RF coil recepion 1, 1 2, 2 A 1, afer eciaion > 1 G G 1 MRI MR Polariaion Precession Relaaion Signal Recepion 33

34 2D Sequence Sep 2: Spaial signal encoding & signal readou B 1 RF sources RF coil recepion 3, 3 1, 1 2, 2 Mehod 1 Projecion-reconsrucion mehod RF G A 1, afer eciaion < 1 > 1 s [ > 1 m, B 0 G B 0 G m, e ig d] e dd ig d G G MRI MR Polariaion Precession Relaaion Signal Recepion 34

35 2D Sequence Sep 2: Spaial signal encoding & signal readou B 1 RF sources RF coil recepion 3, 3 1, 1 2, 2 Mehod 1 Projecion-reconsrucion mehod RF G G G MRI MR Polariaion Precession Relaaion Signal Recepion A 1, afer eciaion < 1 > 1 s [ > 1 m, B 0 G B 0 G m, g e e d] e ig ig d dd ig d g m, d g is he projecion of m, along he -direcion 35

36 2D Sequence Sep 2: Spaial signal encoding & signal readou RF sources 3, 3 < 1 > 1 B 0 G B 0 G G B 1 RF coil recepion 1, 1 2, 2 Mehod 1 Projecion-reconsrucion mehod wih an angle k RF G G Gcos G Gsin G k G MRI MR Polariaion Precession Relaaion Signal Recepion 36

37 2D Sequence Sep 2: Spaial signal encoding & signal readou Mehod 1 Projecion-reconsrucion mehod wih an angle In X-ra CT imaging Each poin in he projecion The sum of he objec disribuion along he appropriae ra pah - A single projec angle DOES NOT provide spaial informaion of he objec disribuion along he ra pah - Muliple angles are needed. MRI MR Polariaion Precession Relaaion Signal Recepion 37

38 2D Sequence Sep 2: Spaial signal encoding & signal readou RF sources B 1 RF coil recepion 3, 3 1, 1 2, 2 Mehod 2 2D Fourier ransform mehod spaial encoding in a smar wa RF G G Phase encoding s ; m, e ig e ig dd G Readou MRI MR Polariaion Precession Relaaion Signal Recepion 38

39 2D Fourier Transform Mehod RF G G G Phase encoding Readou s ; m, e ig e ig dd > 2 39 MRI MR Polariaion Precession Relaaion Signal Recepion

40 2D Fourier Transform Mehod MRI MR Polariaion Precession Relaaion Signal Recepion dd e e m s G i G i, ; d G k 0 2 d G k 0 2 dd e m s k k i ] [ 2, Le Signal equaion dd e m k k M s k k i ] [ 2, ], [ RF G G G Phase encoding Readou 0 1 2

41 2D Fourier Transform Mehod MRI MR Polariaion Precession Relaaion Signal Recepion k k s Measurable, m Objec Fourier Transform dd e e m s G i G i, ; d G k 0 2 d G k 0 2 dd e m s k k i ] [ 2, Le Signal equaion dd e m k k M s k k i ] [ 2, ], [ k-space

42 2D Fourier Transform Mehod Signal equaion s M[ k k-space k s, k ] m, e k-space i2 [ k k ] dd k Measurable Fourier Transform m, Objec Source: MRI MR Polariaion Precession Relaaion Signal Recepion 42

43 2D Sequence Sep 1: Selecive eciaion: B 1 is applied o he presence of B 0 & G Sep 2: Spaial signal encoding & signal readou Mehod 1 Projecion-reconsrucion mehod -ra CT RF G RF G G G G G Mehod 2 2D Fourier ransform mehod popular RF G G G MRI MR Polariaion Precession Relaaion Signal Recepion Phase encoding Readou 43

44 Applicaions of MRI Funcional MRI Diffusion MRI Magneic resonance specroscop Real-ime MRI Inervenional MRI Magneic resonance angiograph Magneic resonance guided focused ulrasound

45 Applicaions of MRI Funcional MRI fmri - fmri measures signal changes in he brain ha are due o changing neural acivi. - Compared o anaomical T1-weighed imaging, he brain is scanned a lower spaial resoluion bu a a higher emporal resoluion picall once ever 2 3 seconds Diffusion MRI - Diffusion MRI measures he diffusion of waer molecules in biological issues. - Clinicall, diffusion MRI Is useful for he diagnoses of condiions e.g., sroke or neurological disorders e.g., muliple sclerosis Helps beer undersand he connecivi of whie maer aons in he cenral nervous ssem Source: hp://en.wikipedia.org/wiki/magneic_resonance_imaging#specialied_applicaions

46 Applicaions of MRI Magneic resonance specroscop MRS - MRS is used o measure he levels of differen meabolies in bod issues. - The MR signal produces a specrum of resonances ha corresponds o differen molecular arrangemens of he isoope being "ecied". - This signaure is used o diagnose cerain meabolic disorders, especiall hose affecing he brain o provide informaion on umor meabolism Source: hp://en.wikipedia.org/wiki/magneic_resonance_imaging#specialied_applicaions

47 * Take Home Message The phsical process of MRI MR Polariaion Precession Relaaion Signal Recepion Three main fields: Main field, B 0 RF field, B 1 Linear gradien field, G sequence Sep 1: eciaion Sep 2: signal encoding & signal readou 2D mehod Signal equaion s M[ k, k ] m, e i2 [ k k ] dd 47

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