UNIVERSITÉLIBRE DEBRUXELLES
Contribution to the developments of rapid acquisition schemes in
Magnetic Resonance Imaging
J
ULIEA
BSILA dissertation submitted for the degree of Doctor of Philosophy
ULB - Faculté des Sciences
Hôpital Erasme - Unité de Résonance Magnétique B
RUXELLES, B
ELGIQUESeptember 2006
Composition of the Graduation Committee President
Prof Dr Pasquale Nardone, Université Libre de Bruxelles Secretary
Prof Dr Pierre Borckmans, Université Libre de Bruxelles Promoter
Prof Dr Thierry Metens, Université Libre de Bruxelles Internal Referees
Prof Dr Kristin Bartik, Université Libre de Bruxelles
Prof Dr Francis Masin, Université Libre de Bruxelles (co-promoter) Prof Dr Thomas Erneux, Université Libre de Bruxelles
External Referees
Prof Dr Klaus Scheffler, Universität Basel
Prof Dr Christoph Segebarth, Université Joseph Fourier de Grenoble
Contents
Acknowledgements vii
List of Figures xiii
List of Tables xvii
1 Introduction 1
1.1 Aims of the thesis . . . 2
1.2 Outline of the thesis . . . 4
2 Magnetic Resonance Imaging : Overview of the Basic Principles 7 2.1 Nuclear Magnetic Resonance of1H . . . 8
2.1.1 Thermal equilibrium . . . 8
2.1.2 Magnetic resonance . . . 10
2.1.3 The Bloch equations . . . 13
2.2 The MRI technique . . . 14
2.2.1 Signal detection . . . 14
2.2.2 Image acquisition . . . 15
2.2.3 Image reconstruction : «k-space» and Fourier Transform . . . 19
2.2.4 MR sequences . . . 22
2.3 Summary . . . 27
3 Balanced-SSFP Imaging 29 3.1 Description of balanced-SSFP sequences . . . 31
3.1.1 Basic sequence . . . 31
3.1.2 The Balanced-FFE sequence . . . 35
3.1.2.1 Steady-state signal calculations . . . 37
3.1.2.2 Off-resonance behavior . . . 38
ix
x CONTENTS
3.2 Steady-state properties . . . 44
3.2.1 Contrast . . . 44
3.2.2 Influence of the relaxation times . . . 46
3.2.3 Influence of the flip angle . . . 51
3.2.4 Influence of the repetition time . . . 52
3.2.5 Conclusion . . . 54
3.3 Transient-state properties . . . 56
3.3.1 Duration of the transient state . . . 56
3.3.2 Shape of the transient state . . . 58
3.3.3 Conclusion . . . 62
3.4 Magnetization manipulations . . . 62
3.4.1 Steady-state magnetization manipulations . . . 63
3.4.1.1 Variable flip angles . . . 63
3.4.1.2 Variable repetition time . . . 64
3.4.1.3 Variable RF phase . . . 65
3.4.2 Transient magnetization manipulations . . . 68
3.4.2.1 Simulations . . . 68
3.4.2.2 Experiments . . . 70
3.4.2.3 Discussion and conclusion . . . 72
3.5 Conclusions . . . 73
4 Fat Suppression Using Variable Flip Angles 75 4.1 Theory . . . 76
4.1.1 The fat chemical shift . . . 76
4.1.2 Modified balanced-FFE scheme . . . 77
4.1.2.1 Magnetization trajectory and signal evolution . . . . 77
4.1.2.2 Off-resonance behavior . . . 79
4.1.2.3 Influence of the parametera . . . 80
4.1.2.4 Influence of the flip angle . . . 84
4.1.2.5 Contrast . . . 86
4.1.2.6 Filter onk-space . . . 86
4.2 Experiments . . . 90
4.2.1 Implementation and post-processing . . . 90
4.2.2 Verification of signal behavior - 1DFT experiments . . . 93
4.2.3 Imaging experiments : fat suppression . . . 95
4.3 Discussion . . . 97
4.3.1 Preparation phase . . . 97
CONTENTS xi
4.3.2 Weak ghosts caused by relaxation . . . 98
4.3.3 SNR and CNR efficiency . . . 101
4.4 Conclusions . . . 102
5 Diffusion-Weighted Balanced-SSFP Imaging 105 5.1 Diffusion imaging : basic concepts and sequences . . . 106
5.1.1 Diffusion physics in biological tissues . . . 106
5.1.2 Diffusion and MRI . . . 108
5.1.3 Diffusion-weighted EPI . . . 110
5.2 Diffusion-weighted balanced-FFE . . . 115
5.2.1 Stejskal-Tanner diffusion preparation . . . 115
5.2.2 Eddy currents generated by the diffusion gradients . . . 119
5.2.2.1 Trapezoidal gradient . . . 119
5.2.2.2 Eddy-current artifacts . . . 120
5.2.2.3 Correction or compensation strategies . . . 122
5.2.3 Eddy currents due to the low-high trajectory . . . 128
5.3 Experiments . . . 130
5.3.1 Methods . . . 130
5.3.2 Phantom experiments . . . 131
5.3.2.1 Low-high trajectory and pairing . . . 131
5.3.2.2 Diffusion imaging : quantitative measurements . . . 134
5.3.2.3 Eddy-current artifacts due to the diffusion gradients 140 5.3.2.4 Comparison with EPI . . . 143
5.3.3 Volunteer experiments . . . 145
5.3.3.1 Diffusion imaging of the brain . . . 145
5.3.3.2 Diffusion imaging of the abdomen . . . 153
5.4 Discussion . . . 158
5.4.1 Low-high trajectory : adjacent pairing and off-resonance . . . 158
5.4.2 T2-weighted images . . . 158
5.4.3 Diffusion-weighted images . . . 158
5.5 Conclusions . . . 161
6 General Conclusions 163 6.1 Future work . . . 164
6.1.1 Balanced-SSFP imaging . . . 165
6.1.2 Fat attenuation . . . 165
6.1.3 Diffusion imaging using balanced-SSFP . . . 166
xii CONTENTS
A Steady-State Calculations 167
A.1 Steady state of the basic balanced-SSFP sequence . . . 167
A.2 Steady state of balanced-FFE . . . 169
A.2.1 Steady-state signal . . . 169
A.2.2 Optimal flip angle . . . 173
A.2.3 Contrast in the approximation TRT1, T2 . . . 173
A.2.4 Locus of the steady states in function of the relaxation times . 175 A.2.4.1 T1=T2 . . . 176
A.2.4.2 T16=T2 . . . 176
A.2.5 Direction of the steady state . . . 180
B Transient-State Calculations 181 B.1 Shape of the on-resonance transient state . . . 181
C Periodically Variable Flip Angles 189 C.1 Dual steady state . . . 189
C.2 Ghost quantification . . . 191
D Diffusion Imaging 195 D.1 Signal attenuation . . . 195
D.2 b-Factor calculations . . . 197
D.2.1 Stejskal-Tanner scheme . . . 197
D.2.2 Multi-Echo scheme . . . 200
D.3 Signal evolution with time . . . 202
Bibliography 205
Publications 215
