MRI in Practice

Foreword ix Preface to the Fourth Edition xi Acknowledgments xiii Chapter 1 Basic principles 1 Introduction 1 Atomic structure 1 Motion in the atom 2 MR active nuclei 2 The hydrogen nucleus 4 Alignment 4 Precession 8 The Larmor equation 9 Resonance 11 The MR signal 15 The free induction decay signal (FID) 16 Relaxation 16 T1 recovery 16 T2 decay 16 Pulse timing parameters 19 Chapter 2 Image weighting and contrast 21 Introduction 21 Image contrast 21 Contrast mechanisms 22 Relaxation in diff erent tissues 23 T1 contrast 25 T2 contrast 27 Proton density contrast 27 Weighting 29 T2* decay 31 Introduction to pulse sequences 34 Chapter 3 Encoding and image formation 59 Encoding 59 Introduction 59 Gradients 60 Slice selection 62 Frequency encoding 65 Phase encoding 69 Sampling 73 Data collection and image formation 79 Introduction 79 K space description 80 K space fi lling 81 Fast Fourier transform (FFT) 86 Important facts about K space 90 K space traversal and gradients 96 Options that fill K space 98 Types of acquisition 101 Chapter 4 Parameters and trade-offs 103 Introduction 103 Signal to noise ratio (SNR) 104 Contrast to noise ratio (CNR) 123 Spatial resolution 126 Scan time 131 Trade-offs 134 Decision making 134 Volume imaging 137 Chapter 5 Pulse sequences 140 Introduction 140 Spin echo pulse sequences 141 Conventional spin echo 141 Fast or turbo spin echo 143 Inversion recovery 151 Fast inversion recovery 157 STIR (short tau inversion recovery) 157 FLAIR (fluid attenuated inversion recovery) 159 IR prep sequences 163 Gradient echo pulse sequences 164 Conventional gradient echo 164 The steady state and echo formation 166 Coherent gradient echo 169 Incoherent gradient echo (spoiled) 172 Steady state free precession (SSFP) 175 Balanced gradient echo 179 Fast gradient echo 185 Single shot imaging techniques 186 Parallel imaging techniques 193 Chapter 6 Flow phenomena 198 Introduction 198 The mechanisms of flow 198 Flow phenomena 200 Time of flight phenomenon 200 Entry slice phenomenon 203 Intra-voxel dephasing 206 Flow phenomena compensation 207 Introduction 207 Even echo rephasing 207 Gradient moment rephasing (nulling) 207 Spatial pre-saturation 210 Chapter 7 Artefacts and their compensation 225 Introduction 225 Phase mismapping 225 Aliasing or wrap around 234 Chemical shift artefact 243 Out of phase artefact (chemical misregistration) 244 Truncation artefact 249 Magnetic susceptibility artefact 250 Cross-excitation and cross-talk 252 Zipper artefact 255 Shading artefact 256 Moiré artefact 256 Magic angle 257 Chapter 8 Vascular and cardiac imaging 261 Introduction 261 Conventional MRI vascular imaging techniques 262 Magnetic resonance angiography (MRA) 269 Cardiac MRI 290 Cardiac gating 291 Peripheral gating 298 Pseudo-gating 300 Multiphase cardiac imaging 300 Ciné 301 SPAMM 304 Chapter 9 Instrumentation and equipment 307 Introduction 307 Magnetism 309 Permanent magnets 312 Electromagnets 314 Superconducting electromagnets 317 Fringe fields 321 Shim coils 322 Gradient coils 323 Radio frequency (RF) 330 Patient transportation system 337 MR computer systems and the user interface 337 Chapter 10 MRI safety 341 Introduction 341 Government guidelines 342 Safety terminology 343 Hardware and magnetic field considerations 345 Radio frequency fields 346 Gradient magnetic fields 349 The main magnetic field 351 Projectiles 355 Siting considerations 357 MRI facility zones 358 Safety education 360 Protecting the general public from the fringe field 360 Implants and prostheses 361 Devices and monitors in MRI 367 Pacemakers 367 Patient conditions 368 Safety policy 369 Safety tips 370 Reference 371 Chapter 11 Contrast agents in MRI 372 Introduction 372 Mechanism of action of contrast agents 373 Molecular tumbling 373 Dipole-dipole interactions 375 Magnetic susceptibility 376 Relaxivity 378 Gadolinium safety 380 Other contrast agents 383 Current applications of gadolinium contrast agents 385 Conclusion 393 Chapter 12 Functional imaging techniques 396 Introduction 396 Diff usion weighted imaging (DWI) 397 Perfusion imaging 400 Susceptibility weighting (SWI) 404 Functional imaging (fMRI) 404 Interventional MRI 405 MR spectroscopy (MRS) 407 Whole body imaging 410 MR microscopy (MRM) 411 Glossary 413 Index 427