Totally Accessible MRI: A User\'s Guide to Principles, Technology, and Applications (Paperback) > Radiology

This is a practical guide that offers a lucid introduction to the principles of MRI physics. The author, recognized in the imaging community for his exceptional teaching methods and lectures, has written an easy to understand text. Each chapter explains the "why" and "how" behind MRI physics. Readers will understand how altering MRI parameters will have many different consequences for image quality and the speed in which images are generated. Practical topics, selected for their value to clinical practice, include progressive changes in key MRI parameters, imaging time, and signal to noise ratio. A wealth of high quality illustrations, complemented by concise text, enables readers to gain a thorough understanding of the subject without requiring prior in-depth knowledge.

Foreword by Emanuel Kanal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Why This Book? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix
A User’s Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv
Part I In the Beginning: Generating, Detecting, and Manipulating the MR (NMR) Signal
1 Laying the Foundation: Nuclear Magnetism, Spin,
and the NMR Phenomenon . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
The Overall Aim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Where Does the MRI Signal Come From? . . . . . . . . . . . . . . . . . 4
Interaction of Protons with a Static Magnetic Field (B) . . . . . . 10
The Energy Confi guration Approach: A Painless (Really!)
Bit of Quantum Mechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
One More Thing . . . What Exactly Is the MRI Signal
That We Measure? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2 Rocking the Boat: Resonance, Excitation,
and Relaxation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Introduction: How Can We Find a Signal to Measure? . . . . . . . 19
Generating Net Transverse Magnetization . . . . . . . . . . . . . . . . . . 20
3 Relaxation: What Happens Next? . . . . . . . . . . . . . . . . . . . . . . . 27
What Happens When the Radiofrequency Is Shut Off? . . . . . . 27
Separate, but Equal (Sort of): Two Components
of Relaxation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
The Spin Echo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4 Image Contrast: T1, T2, T2*, and Proton Density . . . . . . . . . . 38
T2/T2* Contrast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
T1 Contrast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Proton Density Contrast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Putting Things Together to Control Image Contrast . . . . . . . . . 45
5 Hardware, Especially Gradient Magnetic Fields . . . . . . . . . . . . 47
Why This Chapter? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
The B0 Magnetic Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
Radiofrequency Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
The Gradient Magnetic Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
The RF Coils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
The Receiver (A2D) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
The Computer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
Shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
The Prescan Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
Part II User Friendly: Localizing and Optimizing the MRI Signal for Imaging
6 Spatial Localization: Creating an Image . . . . . . . . . . . . . . . . . . . 75
What Is an Image? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Understanding and Exploiting B0 Homogeneity . . . . . . . . . . . . . 77
Slice Selection Using the Gradient
Magnetic Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Localizing Signal Within the Plane of the Slice: Background for Frequency
and Phase Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
Frequency Encoding: The Next Stage . . . . . . . . . . . . . . . . . . . . . 84
Phase Encoding and the Two-dimensional
Fourier Transform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
Some Comments Regarding k-Space . . . . . . . . . . . . . . . . . . . . . . 97
7 Defi ning Image Size and Spatial Resolution . . . . . . . . . . . . . . . 101
How Much Area Will Be Included in the Image? . . . . . . . . . . . 101
Specifying the Field of View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
Aliasing and Its Fixes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
Refi ning the Field of View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
A Footnote Regarding Receiver Bandwidth . . . . . . . . . . . . . . . . 109
8 Putting It All Together: An Introduction to
Pulse Sequences. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
Putting It All Together . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
What Exactly Is a Pulse Sequence? . . . . . . . . . . . . . . . . . . . . . . . 110
The Pulse Sequence Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
Building the Pulse Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
The Spin Echo Pulse Sequence: A First Example . . . . . . . . . . . 113
What Happens After TE: Multiple Echoes
and Multiple Slices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
The Gradient Echo Pulse Sequence . . . . . . . . . . . . . . . . . . . . . . . 119
Contrast Modifi cation in SE and GRE Imaging . . . . . . . . . . . . . 127
9 Understanding, Assessing, and Maximizing
Image Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
What Is the Measure of a Good Image? . . . . . . . . . . . . . . . . . . . 128
What Is Noise? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
Signal-to-Noise Ratio: Measuring Image Quality . . . . . . . . . . . . 129
What Affects Signal to Noise? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
Contrast-to-Noise Ratio: Measuring
Diagnostic Utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
Quality Assurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
10 Artifacts: When Things Go Wrong, It’s Not Necessarily
All Bad . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
Things Do Go Wrong . . . but It’s Not All Bad News . . . . . . . . 139
Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
Undersampling (Wraparound Artifact) . . . . . . . . . . . . . . . . . . . . 141
Susceptibility Effects: Signal Loss and
Geometric Distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
Truncation (Gibbs Artifact) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
Radiofrequency Leak (Zipper Artifact) . . . . . . . . . . . . . . . . . . . 149
k-Space Corruption: (Corduroy, Herringbone, and
Spike Artifacts) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
Chemical Shift Artifact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
Slice Profi le Interactions (Cross-Talk Artifact) . . . . . . . . . . . . . 153
11 Safety: First, Do No Harm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
Who Cares? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
The Safety of MRI Versus Iatrogenic Injury . . . . . . . . . . . . . . . . 154
Types of MRI Risk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
Keeping It Safe: S4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
Part III To the Limit: Advanced MRI Applications
12 Preparatory Modules: Saturation Techniques. . . . . . . . . . . . . . . 165
Inversion-Recovery Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
Spectral Saturation Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
Hybrid Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
Selective Excitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
Spatial Saturation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
Magnetization Transfer Contrast . . . . . . . . . . . . . . . . . . . . . . . . . 172
13 Readout Modules: Fast Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . 174
Gradient Echo Approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
Steady-State Free Precession . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
Manipulating k-Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
Hyperspace: Echoplanar Imaging . . . . . . . . . . . . . . . . . . . . . . . . . 182
Further Exploits in k-Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
14 Volumetric Imaging: The Three-dimensional
Fourier Transform. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
Multislice Versus Volumetric Imaging:
Three-dimensional Versus Two-dimensional . . . . . . . . . . . . . 187
Two-dimensional Imaging: How Do We Do It? . . . . . . . . . . . . 187
Three-dimensional Imaging: How Do We Do It? . . . . . . . . . . . 188
15 Parallel Imaging: Acceleration with SENSE
and SMASH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
Why Another Imaging Technique? . . . . . . . . . . . . . . . . . . . . . . . . 194
So What’s New? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
Basics of Parallel Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
Sensitivity Encoding: SENSE . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
Simultaneous Acquisition of Spatial Harmonics:
SMASH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
What Do We Actually Gain and at What Cost? . . . . . . . . . . . . . 198
16 Flow and Angiography: Artifacts and Imaging of
Coherent Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
What Is Magnetic Resonance Angiography
Anyway? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
Basic Principles of Flow for Students of MRI . . . . . . . . . . . . . . 201
Impact of Flow on the MR Signal . . . . . . . . . . . . . . . . . . . . . . . . 203
Time-of-Flight MRA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
Something Different: Contrast-Enhanced MRA . . . . . . . . . . . . . 221
Don’t Forget This Pitfall! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
Phase-Contrast MRA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226
Where Do We Go from Here? . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
17 Diffusion: Detection of Microscopic Motion . . . . . . . . . . . . . . 233
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233
What Is Diffusion? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233
Effect of Diffusion on the MR Signal . . . . . . . . . . . . . . . . . . . . . 234
Making the MR Image Sensitive to Diffusion . . . . . . . . . . . . . . . 234
What Do Diffusion-Sensitized Images Look Like? . . . . . . . . . . 236
Quantitative Diffusion Imaging: The ADC . . . . . . . . . . . . . . . . . 238
Directional Information: DTI . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
18 Understanding and Exploiting Magnetic
Susceptibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
What Is Magnetic Susceptibility (c) Anyway? . . . . . . . . . . . . . . . 245
Proton-Electron Dipole Interactions: The Other Face of
Paramagnetism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
Susceptibility-Related Effects I: Artifacts . . . . . . . . . . . . . . . . . . . 248
Susceptibility-Related Effects II: Hemorrhage . . . . . . . . . . . . . . 249
Susceptibility-Related Effects III: Contrast Agents . . . . . . . . . . . 254
Susceptibility-Related Effects IV: Perfusion Imaging . . . . . . . . . 255
Susceptibility-Related Effects V: Functional MRI . . . . . . . . . . . . 259
19 Spectroscopy and Spectroscopic Imaging: In Vivo
Chemical Assays by Exploiting the Chemical Shift . . . . . . . . 263
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263
The Chemical Basis of MRS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263
What Then Is Spectroscopy (MRS) and How Is It
Different from MRI? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264
Abundance, Resolution, and Detection . . . . . . . . . . . . . . . . . . . . 265
The Importance of Field Homogeneity . . . . . . . . . . . . . . . . . . . . 266
Localization: Single-Voxel Methods . . . . . . . . . . . . . . . . . . . . . . . 267
Localization: Chemical Shift Imaging . . . . . . . . . . . . . . . . . . . . . 270
Brain Chemistry: Brief Overview of
the Proton Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272
Appendices
Appendix 1 Understanding and Manipulating Vectors . . . . . . . . . 277
Appendix 2 Glossary of Terms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279
Appendix 3 Glossary of Common MRI Acronyms,
Abbreviations, and Notations. . . . . . . . . . . . . . . . . . . . 291
Appendix 4 Resources for Reference and Further Study . . . . . . . 297
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299

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