Magnetic Resonance Imaging (MRI)

Transkript

1 Magnetic Resonance Imaging (MRI) Volker Rasche

2 History of MRI 1946 Nuclear Magnetic Resonance (NMR) Effect reported by Purcell and Bloch 1950 Spin-Echo, E. Hahn E. Purcell F. Bloch P. Lauterbur R. Ernst 1952 Nobel Prize: Purcell, Bloch 1966 Fourier transform technique for MR-Spectroscopy, R. Ernst 1971 Medical application, Damadian 1973 MR-Tomography (PR), Lauterbur 1974 MR-Tomography (FT) Kumar, Welti, Ernst 2

3 History of MRI 1977 Point Imaging, Damadian 1982 first industrial systems (Technicare, Picker) 1983 first 0.5T system at Philips first 1,5 T - Head (GE, PHilips) 1991 Nobel Prize: Ernst 1993 Real-Time MRI (Stanford, Hamburg) 1998 Parallel Imaging 2003 Nobel Prize: Lauterbur, Mansfield 3

4 Underlying Physics

5 Zeeman-Effekt E m mh 5

6 Basics Classical Picture!!

7 Spins in Static Magnetic Fields: Classical Nucleus magnetic moment Nucleus is charged (Neutrons & Protons) Spin In classical picture just a rotation of the nucleus around central axis Angular momentum J Rotating charge Causes a microscopic magnetic moment µ = J Arbitrary orientated 7

8 Spins in Static Magnetic Fields: Classical Possible orientations Nucleus in external magnetic field B0 Spin UP Alignment with magnetic field Only two orientations possible (QM!!!) Precession of µ around B0 (gyroscope) Spin Down 8

9 Spins in Static Magnetic Fields: Classical Possible orientations Nucleus in external magnetic field B0 Spin UP Alignment with magnetic field Only two orientations possible (QM!!!) Precession of µ around B0 (gyroscope) Spin Down Spin UP Spin Down 9

10 Spins in Static Magnetic Fields: Classical Precession of nucleus magnetic moment g = = 42.6 MHz / T 10

11 Spins in Static Magnetic Fields: Classical Precession of nucleus magnetic moment Precession frequency depends on local magnetic field B0!!! 11

12 Spins in Static Magnetic Fields: Classical Ensemble magnetic moment M Difference between Spin UP and Spin Down Net magnetization M M i i 12

13 Spins in Static Magnetic Fields: Classical Ensemble magnetic moment M Orientation of spins arbitrary after introduction into magnetic field Projected xy-magnetization has random phase No net resulting signal due to interferences M z = M0 = const M xy = 0 Phase: angle between x-axis and magnetic moment of the spin 13

14 Spins in Static Magnetic Fields: Classical Ensemble magnetic moment M Boltzmann distribution: Relates the energy of a small system to the energy of a large surrounding system with energy E therm. E therm k B T 1.38x10 23 T The propability of the small system being in energy state is: P exp exp i / E / E i therm therm 14

15 Spins in Static Magnetic Fields: Classical Ensemble magnetic moment M 0.5h Nh P( up down) 300K 2k T B B0 P 42.6MHz / Tesla 2 10 ppm Tesla B = 0.1 tesla T = 0 K M B = 0 tesla T = 300 K B = 0.1 tesla T = 300 K M B = 0.5 tesla T = 300 K M Spins aligned 100% Spins aligned 0% Spins aligned 1 ppm Spins aligned 5 ppm 15

16 Spins in Static Magnetic Fields: Classical Ensemble magnetic moment M P( up down) Nh 2k B T B 0 h M 0 Nh 2k T B 0 2V 2 2 h 4k T B B 0 16

17 Some Nuclei and their Concentration within Biologic Systems Isotope Concentration (MHz/T) with Spin (mol/l) 1 H 90, P 0, C 0, Na 0, F 0, % fat (CH 2, CH 3 ) 65 % water (H 2 0) Larmor-Frequency: = B 0 : gyromagnetic Ratio (MHz/T) B 0 : magnetic Field (Tesla = A/m) 18

18 Nuclear Magnetic Resonance Effect

19 Equation of motion Torque N X B Current distribution in a constant magnetic field Change of angular momentum dj dt N Equation of motion µ = J d dt X B Bloch Equation 20

20 Quantummechanical Analysis Ensemble of spins behaves like a classical magnetic moment dm dt = M x B dm x dt dm y dt dm z dt = M y B z - M z B y = M z B x - M x B z = M x B y - M y B x 21

21 Equations of Motion dm dt = M x B B = (0,0,B Z ) B = (B x,0,0) dm x dt dm y dt dm z dt = M y B z - M z B y = M z B x - M x B z = M x B y - M y B x dm x dt dm y dt dm z dt = M y B Z = - M x B Z = 0 Solution: M x = cos ( B z t) M y = - sin ( B z t) dm x dt dm y dt dm z dt = 0 = M z B x = - M y B x Solution: M z = cos ( B x t) M y = - sin ( B x t) M xy rotates around z-axis M z is constant M rotates around x-axis 22

22 Equations of Motion dm dt = M x B B = (0,0,B Z ) Free induction decay B = (B x,0,0) Excitation dm x dt dm y dt dm z dt = M y B z - M z B y = M z B x - M x B z = M x B y - M y B x dm x dt dm y dt dm z dt = M y B Z = - M x B Z = 0 Solution: M x = cos ( B z t) M y = - sin ( B z t) dm x dt dm y dt dm z dt = 0 = M z B x = - M y B x Solution: M z = cos ( B x t) M y = - sin ( B x t) M xy rotates around z-axis M z is constant M rotates around x-axis 23

23 Excitation External B1 Larmor Frequency z B 0 M Resonance Condition : HF = 0 RF-Coil RF-Coil sinus cosine rotating RF-Field B 1 24

24 Excitation External B1 Lamour Frequency 25

25 Excitation External B1 Larmor Frequency B 0 M Rotating frame: M B 0 = B 0-0 B 1 constant direction! B 1 -Field Laboratory System Rotating Frame 26

26 Excitation Longitudinal and Transversal Magnetisation B 0 z M z M M z longitudinal component x M xy y M xy transversal component rotates alround z-axis 27

27 Faraday s Law z RF-Coil Voltage time rotating magnet Rotating magnetization induces current in coil! 28

28 MR-Signal Reception B 0 RF-Coil Signal FID time rotating M xy Signaldetection Signal in RF-Coil 29

29 Relaxation

30 Longitudinal Relaxation (Spin Lattice) H H H H H H O O O H O H H O H H O H Rotation Vibration Translation 31

31 Longitudinal Relaxation (Spin Lattice) z x y Longitudinal Relaxation -t T 1 M z = M 0 (1 - e ) M z M 0 T1 63% 95% 0 T1 2 T1 3 T1 32

32 Longitudinal Relaxation (Spin Lattice) M z 63 % Fat Muscle Liquor t 33

33 Transversal (Spin-Spin) Relaxation z x y M x y Transversal Relaxation M 0 -t M xy = M 0 e T 2 T2 37% 5% 0 T2 2 T 2 3 T 2 34

34 Transversal (Spin-Spin) Relaxation M xy Liquor 37 % Muskel Fett t 35

35 T1 vs. T2 Relaxation M z Fat Muscle M xy 63 % Liquor Liquor 37 % Muskel Fett t -t T 1 M z = M 0 (1 - e ) -t M xy = M 0 e T 2 t 36

36 Transversal (Spin-Spin) Relaxation Two components Spin spin interaction Intrinsic property of tissue Fluctuating magnetizations (off-resonances) Local off-resonances Property of the environment Shortens apparent T2 of tissue Static off-resonances Not separable by conventional NMR experiment 37

37 Spin Echo Sequenz 90 -Puls 180 -Puls Spin-Echo Dephasing Rephasing Compensation of off-resonance-induced dephasing by inversion of the magnetization by additional 180 pulse. 38

38 100 m race U-turn! (corresponds to 180 -puls) 50 m start 39

39 100 m race Fastest is at the back Slowest is in front 50 m start 40

40 100 m race T 2 *-Decay T 2 -Decay 180 Echo FID Echo time 50 m start 41

41 Comparison Gradient/Spin Echo TE = 5 TE = 20 Gradient Echo TE = 40 ms TE = 70ms Spin-Echo 42

42 Basic Principles of NMR Visualization of Distribution of a Certain Nucleus Relaxation constants are Tissue dependend (T1, T2(*)) Field strength dependend T1/T2 for 1H and 3T 43

43 T 1 - und T 2 -Values of Tissue Gewebe T 1 bei 1,5 T T 1 bei 0,5 T T 2 (ms) (ms) (ms) Muscle Liver Kidney Spleen Fat Gray Matter White Matter Liquor (ca.) Lung

44 Contrast Manipulation Mz Mxy Mz Mxy T R >> T 1 ; T E << T 2 spin density weighting T R >> T 1 ; T E ~ T 2 T 2 - weighting T R < T 1 ; T E << T 2 T 1 - weighting T R < T 1 ; T E >= T 2 mixture of all 45

45 46 Equation of Motion y y x x z z e t M e t M T e T t M M B X M dt dm y L x L z o e t e t B B e t B sin cos 1 t M t M t M t M M T t B t B t B T B t B B T t M t M t M dt d z y x z L L L L z y x 1 1 cos sin cos 1 sin with

46 Summary The resulting image contrast varies dramatically with the acquisition parameters chosen. 47

47 The Basic MRI Experiment Transmit Coil B 0 Relaxation t sample Receive Coil Excitation Reception FT v v ) v 0 ( 0 48

48 The Basic MRI Experiment B 0 Transmit Coil Relaxation sample Receive Coil t BLUR!!! Excitation FT Reception FT e t * T 2 49

49 MR Spectroscopy Chemical Shift Broadening due to signal decay Multiple resonances due to magnetic schielding of electron cloud S H 2 O S H 2 O CH 2 Frequency Frequency 50

50 Spatial Encoding

51 B Spatial Encoding B 0 10 mt 1 Tesla B 0 + G x *x S(t) z B 0 S(t) t S(t) 0 t S(t) + + = t x F(S) t Fouriertransform = 0 = B 0 = 0 + G x x f 52

52 Encoding of Volume z Encoding in three spatial directions required y Selection of volume similar to slice selection Encoding of volume X frequency encoding Y phase encoding Z phase encoding x 53

53 Frequency Encoding RF-Puls Excitation G Z Slice Selection G x Frequency encoding Signal time 54

54 Frequency Encoding z Local main magnetic field depends on the x-position of the magnetic moment y B 0 (x) = B 0 + xg x x G x 55

55 Frequency Encoding Same frequency for all M along a line orthogonal to x z z y y x Magnetic moments at different x-positions precess at different frequencies x 56

56 Frequency Encoding (1D) Assumption Simultaneous start of frequency encoding gradient and sampling! All magnetization in plane! i S(t,x) = M(x) e i (B0 + xgx)t = M(x) e i x Received signal from all M: S(t) = M(x) e i x dx S(k x )= e i M(x) e i xkx dx k x = G x t Fourier Tranform of M(x) along the x-axis!!!!!! 57

57 General S k M r e ikr dr M r FT 1 S k 58

58 Frequency Encoding 2D Ortsraum Not covered Frequency encoding 2D FT Not covered Frequenzraum P(x) x 1D Projection of Object - each point represents the integral of all M in plane orthogonal to x 59

59 Phase Encoding (2D) Goal Make local B0 (Lamor frequency, phase) dependent on x and y direction. Apply gradient in x and y direction! RF-Puls Excitation G Z Slice Selection G y Phase encoding G x Frequency encoding Signal time 60

60 Phase Encoding (2D) y Simultaneous application of gradients cause change in direction of gradient! x Still 1D projection, but acquired with different projection direction 61

61 Phase Encoding (2D) Apply gradient in x and y direction! RF-Puls Excitation G Z Slice Selection G y Phase-encoding Gy G x Frequency encoding Signal TR time Nx times 62

62 Phase Encoding (2D) y y During application of y-gradient x During application of x-gradient x k y = G y t y No sampling k x = G x t y Sampling 63

63 Phase Encoding (2D) k x = G x t y k y = G y t y 2D FT G y Phase-encoding Gy G x Frequency encoding One line in k-space: position in y-direction defined by k y! 64

64 Phase Encoding (2D) k x = G x t y k y = G y t y 2D FT G y Phase-encoding Gy Frequency encoding G x Multiple lines in k-space: subsequent filling of 2D k-space 65

65 Ny 2D Pulse Sequence RF Gy Measurement is repeated with different values of phase-encoding gradient k-raum Gx k y Signal Samples Nx Samples k x 66

66 Phase Encoding (3D) Apply gradient in x, y and z direction! RF-Puls Excitation G Z Slice Selection G y Phase-encoding Gy G x Frequency encoding Signal TR time ny x nz times 67

67 K-Space Encoding on Arbitrary Trajectories K Y G X G Y t G X G Y K X t 68

68 Slice Selection (2D Technique) Principle and Alteration of the Slice Thickness = B bandwidth position rf-pulse

69 Slice Selection (2D Technique) Displacement of the Slice = B position rf-pulse

70 Slice Selection (2D Technique) Excitation pulse shape f(x) f ( x) A 0 0 x sonst x 0 x0 x -1/x01/x0 To avoid side-bands Multiplication with Gauss or Hamming window 72

71 Acquisitiontechniques

72 Ny Lines Gradient Echo 2D RF Gy k-space b Gx k y Signal TE 0 a a b c d e c d k x e Nx Points 74

73 Gradient Echo T2 RF G r Signal Gradient-Echo without field inhomogeneties 75

74 Gradient Echo T2* RF G r Signal Gradient Echo with field inhomogeneties 76

75 T2* - Relaxation B 0 -Field inhomogenity frequency spatial distribution of the precession Dephasing of the transversal magnetization T2 77

76 Gradient Echo Local Dephasing Effects Signalvoid caused by local field inhomogeneties TE = 5 TE = 20 Gradient-Echos TE = 40 ms 78

77 Gradient Echo 3D RF Gz Gy Add phase encoding gradient into z-direction Weightings No, T1, T2, balanced Gx Signal TE Different approaches to handle remaining transversal magnetization 79

78 Comparison Gradient/Spin Echo 90 Gradient-Echo: Dephasing by T2 and T2* relaxation Spin-Echo: Dephasing by T2 relaxation T2 Signal T2* Echotime TE Gradient-Echo Spin-Echo 80

79 Comparison Gradient/Spin Echo TE = 5 TE = 20 Gradient Echo TE = 40 ms TE = 70ms Spin-Echo 81

80 Spin Echo T1-Weighting TR = 500 ms TE = 15 ms TR = 3000 ms TE = 15 ms 82

81 Multi-Echo Imaging 83

82 Ny lines Conventionel Spin-Echo RF k-space Gy k y Gx TE Signal samplepoints Nx points k x 84

83 Turbo Spin-Echo (TSE): RF Turbofactor (TF) = 4 Gy Gx Signal k-space 85

84 Effects in TSE Effective echo time TE is assumed as the echo time at which the center of k-space is acquire Blur S( k ) S t / T 0 ( k ) e M ( r) FT e 2 t / T 2 86

85 Echo Planar Imaging (EPI): RF EPI-Factor = 4 Gy Gx Signal k-space 87

86 Acquisition Times Spin-Echo: T aq = N y x TR typically: 256 x 400 ms = approx. 2 min. (T1-weighting) 256 x 2000 ms = approx. 10 min. (T2-weighting) Turbo Spin-Echo: T aq = N y x TR / TF EPI: typically: 256 x 400 ms /3 = approx. 1 min. (T1-weighting) 256 x 2000 ms /10 = approx. 1 min. (T2-weighting) T aq = N y x TR / EF typically: 256 x 400 ms / 10 = approx. 10 sec. (T1-weighting) Single-Shot TSE (N y Spin-Echos) typically: 500 ms (T2-weighting) Single-Shot EPI (N y Gadient-Echos) typically: 50 ms (T2*-weighting) 88

87 Hardware 89

88 Prototype (1981) 0,15 T System 90

89 Open Designs B 0 B 0 91

90 Permanent Magnets B 0 N S Magnetisiertes Eisen 92

91 Superconducting Magnets Cryostat Liquid Helium 4 Kelvin = C 93

92 Magnet-Concepts Typ B0 Mass Costs (Tesla) (tons) He(l) kw /year Permanent 0,19-0,25 > Resitive Magn. 0,15-0,35 > Superconducting 0,5-3,

93 Magnets 95

94 Active Shielding 96

95 Gradient Coils x y z 97

96 Gradients Actively shielded gradient coil: 98

97 RF-Coils Send/Receive Coils Receive Coil Send Coil 99

98 Signal-to-Noise Noise Volume Reduced Noise Volume 100

99 SNR-Gain Body Coil Dedicated Knee Coil SNR-Gain =10 101

100 Dedicated Receive Coils 102

101 Phased-Array-Coils Small Noise Volume Synergy (= phased array)-coils: Large FOV, less noise 103

102 System Components Gradientamplifier Computer 200 V = 50 A RF-Amp 25 kw RF Oscillator switch Computer Amplifier MIxer Filter ADC Reconstructor 104

103 Signal to Noise

104 Bildqualitätsparameter Kontrast Auflösung Signal-zu- Rausch Aufnahmezeit geringe Artefakte 106

105 Signal-zu-Rausch pro Zeiteinheit Einfluß der Feldstärke Feldstärke (Tesla) Signal-zu-Rausch-Verhältnis ungefähr proportional zur Feldstärke Faster Acquisition at same SNR Linear increase of SNR with B0 Sense/Smash/... decrease of SNR proportional to square root of reduction factor SNR (3T, R=4) = SNR (1.5T, R=1)!!!!! Improved spatial resolution at same SNR Reduction of spatial resolution in all three dimensions by 0.8 Reduction of in-plane resolution by

106 SNR und Feldstärke Signal-zu-Rausch- Verhältnis pro Zeiteinheit 4 3,5 3 ohne T1-Wichtung mit T1-Wichtung 2,5 2 1,5 1 0, ,5 1 1,5 2 2,5 3 3,5 4 4,5 Feldstärke (Tesla) 108

107 SNR und Feldstärke S/R B 0 2 (a B 0 1/2 + b B 0 2 ) 1/2 Rauschbeitrag von Spulen und Elektronik Rauschbeitrag vom menschl. Körper T 1 -Relaxationzeit wird länger wenn B 0 größer. Deswegen: = B Leitfähigkeit des Körpers wird größer wenn Frequenz größer. Deswegen: = B Das Verhältnis a/b ist ungefähr 0.3 (abhängig von Spulengröße usw.) 109

108 SNR und Feldstärke S/N=100 S/N=40 S/N=90 3.0T NSA=1 1.5T NSA=1 1.5T NSA=4 110

109 Empfangsspule und Rauschen Rauschen empfangen mit der Körperspule Rauschen empfangen mit der Oberflächenspule 111

110 Empfangsspule und Rauschen wenig Rauschen pro Spule Synergy (= phased array)-spulen: großes Messfeld, wenig Rauschen 112

111 Bildqualität Spin-Echo TE = 20 TR = 300 Matrix 256 x 256 FOV 140 mm Schichtdicke 2.5 mm NSA = 2 Aufnahmezeit 2:34 Min. Voxelgröße 0.55 x 0.55 x 2.50 mm 113

112 Einfluß der Empfangsspule Kniespule Körperspule 114

113 Definition der Voxelgröße Voxel Y-Achse: Richtung der Phasenkodierung Matrixgröße (Zahl der Voxel) w w = Schichtdicke FOV (mm) Matrixgröße (64,128,256, etc.) Schichtdicke (selektive Anregung) X-Achse: Richtung der Frequenzkodierung 115

114 Einfluss der Schichtdicke Voxel Schichtdicke Schichtdicke halbiert: S/R halbiert 116

115 Einfluss der Schichtdicke Schichtdicke 2.5 mm Schichtdicke 1.25 mm 117

116 Summe 1 / 4 Signal Rauschen Bei N = 4 Messungen: Meßzeit vervierfacht Signal vervierfacht Rauschen verdoppelt S/R verdoppelt Allgemein: S/R N 118

117 SNR und Mittelung NSA = 2 NSA = 1 119

118 SNR und Mittelung NSA = 2 NSA = 4 120

119 Field of View Voxel w Schichtdicke Meßfeld halbiert: S/R vier mal kleiner 121

120 Einfluß des Meßfeldes (Field of View) FOV 140 mm FOV 70 mm 122

121 Einfluß der Matrix Voxel Zahl der Phasenkodierschritte w Schichtdicke Matrix verdoppelt: Matrixgröße (Zahl der Voxel) Signal: ein Viertel Meßzeit: verdoppelt S/R 1 / 4 2 (=0,35) 123

122 Einfluß der Matrix Matrix 256 x 256 Matrix 512 x 512 Aufnahmezeit 5:08 Min. 124

123 Frequenzkodiergrad. und Bandbreite Frequenzkodgrad. groß Frequenzkodgrad. klein Frequenzdifferenz groß Frequenzdifferenz klein -Puls -Puls Abtastzeit kurz Abtastfrequenz hoch Abtastzeit lang Abtastfrequenz klein 125

124 Methoden zur Meßzeitverkürzung Volle Matrix Scan Percentage < 100%: Datenmatrix (k-raum) Bild - Verzicht auf äußeren Teil des k-raums - Auflösung in Phasenkodierrichtung verringert - S/R besser 126

125 Methoden zur Meßzeitverkürzung Halfscan - ca 60% des k-raums asymmetrisch abgetastet - Auflösung unverändert - S/R 20% schlechter Rechteck. FOV < 100%: Datenmatrix (k-raum) Bild - Größere Schritte im k-raum - Auflösung unverändert - Meßfeld in einer Richtung kleiner - S/R schlechter 127

126 Einfluß von "Scan Percentage" Voxel Y-Achse: Richtung der Phasenkodierung w Schichtdicke Bei 50% Reduktion: X-Achse: Richtung der Frequenzkodierung Signal verdoppelt Meßzeit halbiert S/R 2 2 (= 1,4) 128

127 Einfluß von "Scan Percentage" Scan Percentage 100 % Matrix 256 x 256 Scan Percentage 50 % Matrix 128 x

128 Einfluß von Halfscan Ohne Halfscan Mit Halfscan 130

129 Rechteckiges Messfeld Volles Messfeld Rechteck. Messfeld 131

130 Frequenzkodiergrad. und Bandbreite Frequenzkodgrad. groß: Signalbandbreite groß Frequenzkodgrad. klein: Signalbandbreite klein Signal Signal Freq Freq Rauschen Rauschen Freq Freq S/R 1 Bandbreite 132

131 Frequenzkodiergrad. und Bandbreite Wasser-Fett-Verschiebung 2.0 Pixel WF-Versch. 1.0 Pixel 133

132 Exkurs FFE

133 Phase of M Phase over FFE acquisition Superposition of xy magnetization generated at different points in time Various T2* weightings Phase errors 1st RF 2nd RF 3rd RF 4th RF t 135

134 Gradient Echo Sequence Different Weightings No Sequence as depicted, normally plus spoiler gradients after readout gradient Reduction of residual xy magnetization T1 Additional phase variations by RF pulse Complete supresseion of residual xy magnetization T2 Spoiler gradient directly after excitation Complete supression of fresh xy magnetization Balanced All gradients (phases) completely balanced (net phase zero) Mixed signal from fresh and old xy magnetization 136

135 Präpärationspulse WECHSELN!!!!!

136 Präpärationspulse Fettunterdrückung Fat STIR 180 Selective 90 + crusher SPIR selective + IR Binominal... Blood-Enhancement T2 Präparation Black-Blood SPAMM Kontrastverstärkend Inversion Sättigung Magnetic Transfer... Stabilisierend Sättigung in EKGgetriggerten Sequenzen 138

137 Vorbereitungspulse Inversion Recovery 180 Preparation Pulse for increased T 1 weighting +M 0 Longitudinal magnetization Short T1 Cessation of 180opulse Long T 1 Time TI = 750ms TI = 500ms -M 0 o 180 TI o 90 I T1 M T T M 1 2e M z TI, T1 0, TI ln 2 T1 z i, R 0 T 139

138 Vorbereitungspulse Saturation Recovery 30, 25ms 30, 200ms Major Difference: no 180 pulse T 2* relaxation instead of T 2 relaxation S t c 1 e T T R 1 1 sin cos e T T e E * 2 T T R 1 140

139 Vorbereitungspulse Black-Blood - DIR measurement slice non selective slice selective M z 1 Myocardium M z blood Blood t ln 500ms 1 2 T 1 141

140 Vorbereitungspulse Black-Blood 142

141 Vorbereitungspulse Black-Blood Spatial Presaturation Sättigungspuls t=0 t!=0 143

142 Vorbereitungspulse T2 Präpäration 90 o180o 180 o -180 o -180 o -9 o 0 RF langes T2 (Blut) kurzes T2 (Muskel) Abfall der transversalen Magnetisierung mit T2 Nach Puls ist die transversale Magnetisierung stark von der T2 Relaxation abhängig 144

143 Beispiel Prepulse Abbildung 3: Die Abgrenzung des Blutes im Vergleich zum Muskel (siehe Pfeil) kann durch die Verwendung von steady-state-free-precision (ssfp) Techniken (b) im Vergleich zu konventionellen Flash-Techniken (a) signifikant verbessert werden. Eine weitere Verbesserung des Kontrastes kann durch die Verwendung eines speziellen Präparationspulses (T2prep) erreicht werden. In allen Beispielen erfolgte eine Fettunterdrückung zur Verbeserung des Kontrastes zwischen der RCA und dem epikardialen Fetts (siehe Kreis). 145

144 Vorbereitungspulse Fettunterdrückung: S Water hydrogens Fat hydrogens H 2 O CH 2 Unterschiedliche Resonanzfrequenz Unterschied in Resonanzfrequenz zwischen Fett und Wasserprotonen: 3.5ppm 1.5T) Saturation: Sättigung mit frequenzselektiven Anregungspuls Binominal Puls: Aufteilung des Anregungspulses in 2 (oder mehr) Teile (z.b.: 2x45 ) mit Wartezeit (1,5T = 2.3ms) zwischen den beiden Pulsen, so dass Fettspins +/- 45 sehen. Frequency M z 1.5T 3T M z M z water fat Unterschiedliche Relaxationsparameter Short T1 Inversion Recovery: Inversion recovery Technik mit Kombination Spectral Presaturation with Inversion Recovery (SPIR) 45 M xy 45 M xy M xy 146

145 Example: Fettunterdrückung 147

146 Vorbereitungspulse 90 in Kombination mit EKG Trigger pp-delay pp-delay KM (kurzes T1) nativ (langes T1) 90 -Puls 90 -Puls Vorteil eines 90 -Pulses: Kontrast ist unabhängig von Herzfrequenzvariationen 148

147 MR Angio (MRA)

148 MRA Time of flight Contrast Agents Phase-Contrast Angio Black-blood 150

149 Time-of-Flight Contrast Saturation Excitatio n No Flow Acquisitio n No Signal Medium Flow High Flow Mediu m Signal Vessel Vessel Vessel High Signal 151

150 HR TOF 3T MTC 152

151 Contrast Agents Near Field Agents (T1) - GD Far Field Agents (T2*) (U)SPIO

152 signal Contrast Agents T1=50ms Gd enhanced blood T1=1000ms background # of RFs (TR=10 ms) T1-weighted FFE 154

153 Contrast Agents Magnevist Arterial phase No venous enhancement 155

154 Phase Contrast Angio QFlow,

155 Phasenkontrast-Angiographie: Wirkung eines bipolaren Gradienten 0 Dephasing Rephasing Blut Statisches Gewebe - Phase verhält sich linear zur Blutgeschwindigkeit: ~ v - stationäre Spins: = 0 - Es sind zwei Messungen mit unterschiedlichen Gradienten nötig, um Differenzbilder zu berechnen 157

156 Flowing Spins Introduce Phase Error t, v T G x vt dt 2T G x vt dt t, v 0 GvT 2 T 158

157 Phasenkontrast-Angiographie: Wirkung eines bipolaren Gradienten 1.Messung 2.Messung Gradient Gradient Phasenwinkel Phasenwinkel 0 0 Blut Statisches Gewebe Dephasing Rephasing Statisches Gewebe Dephasing Rephasing Blut 159

158 Phasenkontrast-Angiographie Berechnung des Flußbildes 160

159 Phasenkontrast-Angiographie Phase Contrast MRA Images 2 acquisitions with different flow sensitivities FFE/M anatomical PCA/P quantitative flow 161

160 Black-Blood measurement non slice sliceselective selective 162

161 Blck-Blood: Coronary Vessel Wall Proximal bifurkation 4.1mm a) b) c) d) < 1mm e) f) g) h) 163

162 Bewegungskompensation Herzbewegung Atmungsbewegung Daten: M. Stuber, Johns Hopkins University, Baltimore

163 Bewegungskompensation Atemstillhaltetechnik Single Breathhold Einfache Technik Limitiert für kurze Akquisitionen < 15s? Multiple Breathhold Einfache Technik Reproduzierbarkeit der Atemposition 165 Daten: M. Stuber, Johns Hopkins University, Baltimore

164 Bewegungskompensation Monitoren der Atembewegung / Navigator Lokale Anregung eines Zylinders der lokal die Lungen/Leber-Grenze abbildet. Position der Lungen/Leber-Grenze über die Zeit liefert Indikation für den Bewegungszustand. Navigator Segmentierung der Position der Lungen / Leber Grenze. t Definition eines Bereiches, der einen akzeptablen Atemzustand beschreibt. 166 Daten: M. Stuber, Johns Hopkins University, Baltimore

165 Navigatortechnik - 2D-Anregungspuls Navigator Kernel : Anregung B 1 G x G y 167

166 Bewegungskompensation Monitoren der Atembewegung / Navigator Gating Definition eines Bereiches der für die Datenakquisition genutzt wird Freie Atmung, relative einfache und zuverlässige Technik Geringe Effizienz, lange Akquisitionszeit, residuale Bewegung Navigatorpulse benötigen Zeit 168

167 Current Navigator Implementation Navigator-based real-time slice-tracking 1 Limitations of current approach: Limited correlation diaphragm-heart Patient specificity ignored Only correction of FH-translation 0.6 2) image slice small gating window required prolonged scan time 1. McConnel et al., MRM, 37: ; Wang et al., MRM, 33:713, navigator Courtesy Kay Nerke 169

168 Whole-Heart Coronary MRA Volume Rendering 170 Courtesy, O. Weber, UCSF

169 Whole-Heart Coronary MRA Volume Rendering 171 Courtesy, O. Weber, UCSF