MRI in Practice. Catherine Westbrook MSc, DCRR, CTC Senior Lecturer Anglia Polytechnic University Cambridge UK. John Talbot MSc, DCRR

Transcription

1 MRI in Practice Third edition Catherine Westbrook MSc, DCRR, CTC Senior Lecturer Anglia Polytechnic University Cambridge UK and Carolyn Kaut RothRT(R) (MR) (CT) (M) (CV) Fellow SMRT (Section for Magnetic Resonance Technologists) Director Technologist Continuing Education and MRI Internship Programs for Technologists University of Pennsylvania Heal Systems, Philadelphia, Pennsylvania, USA with John Talbot MSc, DCRR Senior Lecturer Anglia Polytechnic University Cambridge UK Blackwell Publishing

2 Contents Foreword Preface to the third edition Acknowledgments Chapter 1 Chapter 2 Basic principles Atomic structure Motion in the atom MR active nuclei The hydrogen nucleus Alignment Precession The Larmor equation Resonance The MR signal The free induction decay signal (FID) Relaxation Tl recovery T2 decay Pulse timing parameters Questions Image weighting and contrast Image contrast Contrast mechanisms Relaxation in different tissues Tl contrast T2 contrast Proton density contrast Weighting T2* decay Pulse sequences Questions χ xi xii

3 vi Contents c h a p t e r 3 Encoding and image formation 61 Encoding Gradients Slice selection Frequency encoding Phase encoding Sampling Data collection and image formation Κ space description Κ space filling 87 Fast Fourier transform (FFT) Important facts about Κ space Κ space traversal and gradients Options that fill Κ space Types of acquisition Questions Chapter 4 Parameters and trade-offs Signal to noise ratio (SNR) 105 Contrast to noise ratio (CNR) 125 Spatial resolution 128 Scan time 135 Trade-offs 137 Decisionmaking 137 Volume imaging 139 Questions 142 Chapter 5 Pulse sequences Spin echo pulse sequences 145 Conventional spin echo 145 Fast or turbo spin echo 146 Inversion recovery 156 Fast inversion recovery 162 STIR (short tau inversion recovery) 162 FLAIR (fluid attenuated inversion recovery) 165 Gradient echo pulse sequences 168 Conventional gradient echo 168 The steady state and echo formation 170 Coherent gradient echo 173 Incoherent gradient echo (spoiled) 176 Steady state free precession (SSFP) 179 Balanced gradient echo 184 Fast gradient echo 189 Echo planar imaging (EPI) 191

4 Contents vii Parallel imaging techniques 198 Questions 201 Chapter 6 Flow phenomena The mechanisms of flow 202 Flow phenomena 204 Time of flight phenomenon 204 Entry slice phenomenon 207 Intra-voxel dephasing 212 Flow phenomena compensation Even echo rephasing 214 Gradient moment rephasing (nulling) 214 Spatial pre-saturation 217 Questions 228 Chapter 7 Artefacts and their compensation Phase mismapping 229 Aliasing or wrap around 238 Chemical shift artefact 246 Chemical misregistration 249 Truncation artefact 251 Magnetic susceptibility artefact 251 Cross excitation and cross talk 255 Zipper artefact 257 Shading artefact 258 Moires artefact 259 Magic angle 260 Questions 262 Chapter 8 Vascular and cardiac imaging Conventional MRI vascular imaging techniques 263 Magnetic resonance angiography (MRA) 269 Perfusion and diffusion imaging 285 Cardiac gating 286 Peripheral gating 292 Pseudo-gating 294 Multi-phase cardiac imaging 294 Ciné 295 SPAMM 298 Questions 300 Chapter 9 Instrumentation and equipment Magnetism 302

5 viii Contents Permanent magnets 306 Electromagnets 306 Superconducting electromagnets 309 Fringe fields 313 Shim coils 313 Gradient coils 314 Radio frequency coils 321 The pulse control unit 326 Patient transportation system 326 Operator interface 327 Questions 328 Chapter 10 MRI safety The main magnetic field 330 Projectiles 335 Medical emergencies 336 Implants and prostheses 337 Pacemakers 342 Gradient magnetic fields 342 Radio frequency fields 344 Claustrophobia 346 Quenching 346 Safety education 347 Patient monitoring 347 Monitors and devices in MRI 348 Site planning 350 Questions 351 Chapter 11 Contrast agents in MRI Review of weighting 353 Mechanism of action 354 Dipole-dipole interactions 355 Magnetic susceptibility 356 Relaxivity 358 Gadolinium safety 359 Iron oxide safety 361 Current applications of contrast agents 362 Conclusion 371 Questions 371 Chapter 12 Functional imaging techniques Diffusion weighted imaging (DWI) 373 Per fusion imaging 377 Functional imaging (fmri) 380 Interventional MRI 382

6 Contents ix MR spectroscopy (MRS) 383 Whole body imaging 386 MR microscopy (MRM) 387 Questions 388 Answers to questions 389 Glossary 393 Index 403