How does MRI work An Introduction to the Physics and Function of Magnetic Resonance Imaging

172 Pages·2006·2.73 MB·English

by  Dominik Weishaupt

Dominik Weishaupt · Victor D. Köchli · Borut Marincek
How Does MRI Work ?
Dominik Weishaupt
Victor D. Köchli
Borut Marincek
How Does MRI Work ?
An Introduction to the Physics and Function
of Magnetic Resonance Imaging
Second Edition
Contributors:
J. M. Froehlich, D. Nanz, K. P. Pruessmann
With 57 Figures and 9 Tables
123
Dominik Weishaupt, MD Contributors:
Associate Professor Klaas P. Pruessmann, PhD
Institute of Diagnostic Radiology Assistant Professor
Raemistrasse 100, 8091 Zurich, Switzerland Institute of Biomedical Engineering
Swiss Federal Institute of Technology
Victor D. Köchli, MD Gloriastrasse 35, 8092 Zurich, Switzerland
Rötelstrasse 30, 8006 Zurich, Switzerland
Johannes M. Froehlich, PhD
Borut Marincek, MD Guerbet AG
Professor and Chairman Winterthurerstrasse 92, 8006 Zurich,
Institute of Diagnostic Radiology Switzerland
Raemistrasse 100, 8091 Zurich, Switzerland
Daniel Nanz, PhD
Department of Medical Radiology
Raemistrasse 100, 8091 Zurich, Switzerland
Translator:
Bettina Herwig
Hauptstraße 4 H, 10317 Berlin, Germany
Corrected 2nd printing 2008
ISBN 978-3-540-30067-0 e-ISBN 978-3-540-37845-7
DOI 10.1007 / 978-3-540-37845-7
Library of Congress Control Number: 2006924129
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T itel V
Preface
It is with great pleasure that we present this completely revised English edi-
tion of our book How Does MRI Work? An Introduction to the Physics and
Function of Magnetic Resonance Imaging only two years after publication of
the first English edition. We are particularly pleased that our introductory
textbook met with great approval in the English-speaking world and not just
in the German-speaking countries. This success has been an enormous in-
centive for us to further improve and update the text. For this reason, we are
now presenting a second edition. All chapters have been thoroughly revised
and updated to include the latest developments in the ever-changing field of
MRI technology. In particular, the chapter on cardiovascular imaging has
been improved and expanded. We gratefully acknowledge the contribution
of Daniel Nanz, PhD, the author of this chapter. Moreover, two completely
new chapters have been added: “Fat Suppression Techniques” and “High-
Field Clinical MR Imaging”.
Notwithstanding these additions, the intended readership of our book
remains the same: it is not a book for MR specialists or MR physicists but
for our students, residents, and technologists, in short, all those who are
interested in MRI and are looking for an easy-to-understand introduction
to the technical basis of this imaging modality at the beginning of their MRI
training.
The second English edition presented here corresponds to and appears
together with the completely revised fifth German edition.
The authors gratefully acknowledge the support of numerous persons
without whose contributions the new German and English editions of our
book would not have been possible. First of all, we thank our readers, in
particular those who bought and read the preceding versions and provided
oral and written comments with valuable suggestions for improvement.
We should furthermore like to thank Klaas P. Pruessmann, PhD, and Jo-
hannes M. Froehlich, PhD, for their excellent introductions to parallel im-
aging and MR contrast agents.
Special thanks are due to our translator, Bettina Herwig, who very know-
ledgeably and with great care translated the entire text and provided valu-
able advice in preparing the new edition.
Finally, we would like to thank Springer-Verlag, in particular Dr. U. Heil-
mann, W. McHugh, and Dr. L. Ruettinger, for their cooperation.
For the authors:
Dominik Weishaupt, MD January 2006
Titel VI I
Contents
1 Spin and the Nuclear Magnetic Resonance Phenomenon ...... 1
2 Relaxation .................................................................. 7
2.1 T1: Longitudinal Relaxation .............................................. 7
2.2 T2/T2*: Transverse Relaxation ........................................... 8
3 Image Contrast ............................................................ 11
3.1 Repetition Time (TR) and T1 Weighting ............................... 12
3.2 Echo Time (TE) and T2 Weighting ...................................... 13
3.3 Saturation at Short Repetition Times ................................... 15
3.4 Flip Angle (Tip Angle) .................................................... 17
3.5 Presaturation ................................................................ 18
3.6 Magnetization Transfer .................................................... 19
4 Slice Selection and Spatial Encoding ................................. 21
4.1 Three-Dimensional Spatial Encoding ................................... 26
4.2 K-Space ...................................................................... 27
5 Factors Affecting the Signal-to-Noise Ratio ........................ 29
5.1 Pixel, Voxel, Matrix ........................................................ 30
5.2 Slice Thickness and Receiver Bandwidth ............................... 30
5.3 Field of View and Matrix .................................................. 32
5.4 Number of Excitations ..................................................... 36
5.5 Imaging Parameters ........................................................ 36
5.6 Magnetic Field Strength ................................................... 36
5.7 Coils .......................................................................... 37
5.7.1 Volume Coils ................................................................ 37
5.7.2 Surface Coils ................................................................ 37
5.7.3 Intracavity Coils ............................................................ 37
5.7.4 Phased-Array Coils ........................................................ 38
6 The MR Scanner ........................................................... 41
6.1 The Magnet .................................................................. 42
6.2 The Gradient System ....................................................... 44
6.3 The Radiofrequency System .............................................. 45
6.4 The Computer System ..................................................... 45
VIII
7 Basic Pulse Sequences ................................................... 47
7.1 Spin Echo (SE) Sequences ................................................ 48
7.2 Black Blood Effect .......................................................... 49
7.3 Multislice Imaging ......................................................... 50
7.4 Inversion Recovery (IR) Sequences ...................................... 50
7.5 STIR Sequences ............................................................. 52
7.6 FLAIR Sequences .......................................................... 52
7.7 Gradient Echo (GRE) Sequences ........................................ 52
7.8 Multiecho Sequences ...................................................... 55
8 Fast Pulse Sequences .................................................... 57
8.1 Fast or Turbo Spin Echo Sequences ..................................... 57
8.2 Single-Shot Fast Spin Echo (SSFSE) Sequences ........................ 58
8.3 Fast or Turbo Inversion Recovery (Fast STIR) Sequences ............ 59
8.4 Fast Gradient Echo (GRE) Sequences ................................... 59
8.5 Echo Planar Imaging (EPI) Sequence ................................... 60
8.6 Hybrid Sequences .......................................................... 60
8.7 Gradient and Spin Echo (GRASE) Sequence ........................... 61
8.8 Spiral Sequences ............................................................ 61
8.9 Echo Time and T2 Contrast in Fast Sequences ........................ 62
9 Fat Suppression Techniques ............................................ 63
9.1 Chemical Shift Imaging ................................................... 63
9.2 Frequency-Selective Fat Saturation ...................................... 65
9.3 Short TI Inversion Recovery (STIR) ..................................... 66
9.4 Spectral Presaturation with Inversion Recovery (SPIR) .............. 66
10 Parallel Imaging ........................................................... 69
10.1 Background ................................................................. 69
10.2 Principles of Parallel Imaging ............................................ 69
10.3 Special Requirements ...................................................... 71
10.4 Applications ................................................................. 71
11 Cardiovascular Imaging ................................................. 73
11.1 Angiography ................................................................ 74
11.1.1 Bright Blood Imaging ...................................................... 74
11.1.2 Black Blood Imaging ....................................................... 86
11.1.3 Time-Resolved MR Angiography ........................................ 88
11.2 Perfusion-Weighted Imaging ............................................. 89
11.3 Diffusion-Weighted Imaging ............................................. 91
Contents IX
11.4 The BOLD Effect in Functional Cerebral Imaging .................. 95
11.5 Cardiac Imaging .......................................................... 97
11.6 Cardiac Imaging with SSFP Sequences ............................... 99
11.7 Myocardial Perfusion Imaging ......................................... 100
11.8 Late-Enhancement Imaging ............................................ 101
11.9 Detection of Increased Myocardial Iron Concentrations .......... 102
12 MR Contrast Agents .................................................... 103
12.1 Chemical Structure ...................................................... 107
12.2 Relaxivity .................................................................. 109
12.3 Pharmacologic Properties ............................................... 112
12.3.1 Extracellular Contrast Agents .......................................... 112
12.3.2 Intravascular or Blood Pool Contrast Agents ........................ 115
12.3.3 Liver-Specific Contrast Agents ......................................... 116
12.3.4 RES Contrast Agents ..................................................... 118
12.3.5 Lymph Node-Specific Contrast Agents ............................... 119
12.3.6 Tumor-Targeted Contrast Agents ...................................... 119
12.3.7 Other Emerging Tissue-Specific Contrast Agents ................... 120
12.3.8 Hyperpolarized Gases ................................................... 120
12.3.9 Oral MR Contrast Agents ............................................... 121
12.4 Outlook .................................................................... 122
13 MR Artifacts .............................................................. 129
13.1 Motion and Flow Artifacts (Ghosting) ................................ 129
13.2 Phase Wrapping .......................................................... 131
13.3 Chemical Shift ............................................................ 133
13.4 Magnetic Susceptibility .................................................. 135
13.5 Truncation Artifacts ..................................................... 135
13.6 Magic Angle ............................................................... 136
13.7 Eddy Currents ............................................................ 136
13.8 Partial Volume Artifacts ................................................. 136
13.9 Inhomogeneous Fat Suppression ...................................... 136
13.10 Zipper Artifacts .......................................................... 137
13.11 Crisscross or Herringbone Artifacts ................................... 137
14 High-Field Clinical MR Imaging ...................................... 139
14.1 Tissue Contrast ........................................................... 140
14.2 Magnetic Susceptibility .................................................. 140
14.3 Chemical Shift ............................................................ 140
14.4 Radiofrequency (RF) Absorption ...................................... 141
X
15 Bioeffects and Safety .................................................. 143
Glossary ........................................................................... 147
Subject Index ..................................................................... 165
T itel XI
Abbreviations
FID Free induction decay
FSE Fast spin echo
GRE Gradient echo
IR Inversion recovery
MHz Megahertz
MR Magnetic resonance
MRA Magnetic resonance angiography
MRI Magnetic resonance imaging
msec Milliseconds
NMR Net magnetization vector
PC MRA Phase-contrast MR angiography
PD Proton density
ppm Parts per million
RF Radiofrequency
SAR Specific absorption rate
SE Spin echo
SNR Signal-to-noise ratio
T Tesla
TE Echo time
TOF Time of flight
TR Repetition time