Totally Accessible MRI (eBook)

Dedication Page 5
Foreword 6
Why This Book? 8
A User's Guide 10
Acknowledgments 13
Table of Contents 15
Part I In the Beginning: Generating, Detecting, and Manipulating the MR (NMR) Signal 20
1 Laying the Foundation: Nuclear Magnetism, Spin, and the NMR Phenomenon 21
The Overall Aim 21
Spatial Resolution 21
Contrast Resolution 22
Where Does the MRI Signal Come From? 22
Nuclear Magnetic Resonance 22
Spin Semantics 23
Prerequisites to NMR: Nuclear Magnetism 24
Prerequisites to NMR: Nuclear Spin Angular Momentum 25
What Is Spin? 27
Interaction of Protons with a Static Magnetic Field (B) 28
Describing a Real-Life Sample: Dealing with Many Spins 30
The Energy Configuration Approach: A Painless (Really!) Bit of Quantum Mechanics 30
Overview 30
Probability and Certainty 31
Energy Levels 32
Minimizing the Energy Configuration 33
The Uncertainty Principle 34
Summary 34
Making the Quantum Mechanical Approach Specific to MRI 34
One More Thing .. . What Exactly Is the MRI Signal That We Measure? 36
2 Rocking the Boat: Resonance, Excitation, and Relaxation 37
Introduction: How Can We Find a Signal to Measure? 37
At Rest, Signal Is Not Detectable 37
Net Transverse Magnetization Is Detectable 37
Generating Net Transverse Magnetization 38
Resonance 38
What Is a Rotating Magnetic Field (Radiofrequency Field)? 39
What Happens When the Radiofrequency (BI ) Is Turned On? 41
Generating MR Signal 43
Points of View: The Laboratory and Rotating Frames of Reference 43
3 Relaxation: What Happens Next? 45
What Happens When the Radiofrequency Is Shut Off? 45
Separate, but Equal (Sort of): Two Components of Relaxation 45
Recovery of Longitudinal Magnetization 46
Loss of Transverse Magnetization 46
Relaxation Mechanisms 47
Spin-Lattice (Longitudinal) Relaxation 48
The Effects of Variation in B0: T2´ 49
The Spin Echo 50
A Different Case of the Spin Echo 53
The Stimulated Echo 54
Measuring the MR Signal 55
4 Image Contrast: Tl, T2, T2*, and Proton Density 56
T2/T2* Contrast 56
T1 Contrast 58
Multiple Repetitions Required 58
Change the Starting Point 58
Contrast Agents and Their Effect on Tl 60
Proton Density Contrast 62
Putting Things Together to Control Image Contrast 63
5 Hardware, Especially Gradient Magnetic Fields 65
Why This Chapter? 65
The Bo Magnetic Field 65
Requirements 65
Magnet Designs for Generating Bo 66
Permanent Magnets and Vertical Fields 67
Superconducting Magnets and Horizontal Fields 69
Keeping Your Cool: The Cryogen System 70
Losing Your Cool: Magnet Quench 71
Shimming 72
Radiofrequency Transmission 73
Radiofrequency Calibration 73
The Gradient Magnetic Field 73
The RF Coils 78
Tuning 79
Impedance Matching 79
Surface RF Coils and Coil Sensitivity 80
Quadrature RF Coils 80
Phased Array Coils 82
The Receiver (A2D) 83
The Computer 84
Shielding 86
B0 Fringe Field 86
Eddy Currents 87
RF Shielding 88
The Prescan Process 89
Center Frequency Determination 89
Receiver Gain Adjustment 90
Part II User Friendly: Localizing and Optimizing the MRI Signal for Imaging 91
6 Spatial Localization: Creating an Image 92
What Is an Image? 92
Image Geometry 93
Understanding and Exploiting B0 Homogeneity 94
The Gradient Magnetic Field: A Review 94
Gradient Magnetic Fields Are Linear 94
Location Is Frequency 95
A First Look at Signal Localization 95
Slice Selection Using the Gradient Magnetic Field 95
Slice Thickness 96
Slice Location 98
Plane of Section 99
Localizing Signal Within the Plane of the Slice: Background for Frequency and Phase Encoding 99
Dividing a Single Signal 99
Setting Up for Spatial Localization 100
Localization Within the Slice 100
Frequency Encoding: The Next Stage 101
Separating Frequencies with the Fourier Transform (Don't Be Afraid!) 102
Fourier Analysis Applied to the Frequency-Encoded Signal 103
How Many Samples Do We Need? 104
Frequency Encoding: The Bottom Line 105
The One-dimensional Image 106
Phase Encoding and the Two-dimensional Fourier Transform 107
How We Don't Do It 107
Why Phase Encode? 108
How Can Phase Be Detected? 109
How "Rapidly" Does Phase Change? 110
Filling in the Data 111
Some Comments Regarding k-Space 114
What Location in the Image Does a Point ink-Space Represent? 114
Topology of k-Space 115
7 Defining Image Size and Spatial Resolution 118
How Much Area Will Be Included in the Image? 118
Specifying the Field of View 119
How Do We Detect Specific Frequencies? 120
Aliasing and Its Fixes 120
Sampling Rate, the Nyquist Rule, and Undersampling 120
A voiding Aliasing: Oversampling 122
How About the Phase-Encoding Direction? 123
How Does My MRI Vendor "Stop" Aliasing? 123
Refining the Field of View 124
Gradient Strength Determines Field of View 124
Sampling Rate Determines Field of View 124
Receiver Bandwidth 125
Receiver Bandwidth and Field of View 125
How Do We Specify Field of View in Real Life? 125
How Small Can the Field of View Be? 125
A Footnote Regarding Receiver Bandwidth 126
8 Putting It All Together: An Introduction to Pulse Sequences 127
Putting It All Together 127
What Exactly Is a Pulse Sequence? 127
The Pulse Sequence Diagram 128
It's Just a Timeline 128
Notation 128
Building the Pulse Sequence 130
The Spin Echo Pulse Sequence: A First Example 130
The Spin Echo Pulse Sequence in a Nutshell 132
What Happens After TE: Multiple Echoes andMultiple Slices 133
Multislice Imaging 133
Multiecho Imaging 135
The Gradient Echo Pulse Sequence: Gradient Recalled Echo, Fast Field Echo, Fast Low Angle Shot, and So Forth 136
What Is a Gradient Echo? 137
The Gradient Echo Pulse Sequence in a Nutshell 139
What Makes Gradient Recalled Echo Different? 140
Partial (AKA Modified) Flip Angles 141
Introducing Saturation 141
The Ernst Angle 142
Controlling T1 Contrast with the Flip Angle 143
Contrast Modification in SE and GRE Imaging 144
9 Understanding, Assessing, and Maximizing Image Quality 145
What Is the Measure of a Good Image? 145
What Is Noise? 146
Signal-to-Noise Ratio: Measuring Image Quality 146
What Affects Signal to Noise? 148
Contrast-to-Noise Ratio: Measuring Diagnostic Utility 151
Quality Assurance 152
The Quality Assurance Measurements 152
The Quality Assurance Phantom 153
Signal-to-Noise Ratio 153
Uniformity 153
Geometry 154
Contrast 155
General Checks 155
10 Artifacts: When Things Go Wrong, It's Not Necessarily All Bad 156
Things Do Go Wrong ... but It's Not All Bad News 156
Motion 156
Undersampling (Wraparound Artifact) 158
Susceptibility Effects: Signal Loss and Geometric Distortion 161
Signal Loss 161
Geometric Distortion 161
Truncation (Gibbs Artifact) 163
Radiofrequency Leak (Zipper Artifact) 166
k-Space Corruption: (Corduroy, Herringbone, andSpike Artifacts) 167
Chemical Shift Artifact 168
Slice Profile Interactions (Cross-Talk Artifact) 170
11 Safety: First, Do No Harm 171
Who Cares? 171
The Safety of MRI Versus Iatrogenic Injury 171
Types of MRI Risk 172
Projectiles 172
Bioimplant Malfunction 173
Risk to the Patient 173
Risk to the Device 174
Burns 174
Beware Conductive Materials and Loops 174
Nerve Stimulation 175
Hearing 175
Psychological Distress 175
Pregnancy 176
Keeping It Safe: S4 176
Safety 176
Security 177
Screening 177
Surveillance 177
Part III To the Limit: Advanced MRI Applications 180
12 Preparatory Modules: Saturation Techniques 181
Inversion-Recovery Imaging 181
Spectral Saturation Techniques 185
Chemical Shift 185
Exploiting Chemical Shift to Saturate Tissue 186
Hybrid Techniques 186
Selective Excitation 186
Spatial Saturation 187
Magnetization Transfer Contrast 188
13 Readout Modules: Fast Imaging 190
Gradient Echo Approaches: Turbo-Fast Low Angle Shot, Fast Gradient Recalled Echo, Turbo Field Echo 190
Residual Transverse Magnetization and Spoiling 191
Steady-State Free Precession: Balanced Turbo Field Echo, True-Fast Imaging with Steady-State Precession, Fast Imaging Employing Steady-State Acquisition 193
Manipulating k-Space: Rapid Acquisition with Relaxation Enhancement, Turbo Spin Echo, Fast Spin Echo 193
Fast Spin Echo Contrast 195
Extreme Speed: Single-Shot Turbo Spin Echo, Single-Shot Fast Spin Echo, Half Fourier Acquisition Single-Shot Turbo Spin Echo 196
T1 Contrast in Single-Shot FSE 197
Contrast Inversion: MR Hydrography 197
Gradient and Spin Echo 198
Hyperspace: Echoplanar Imaging 198
Further Exploits in k-Space 201
14 Volumetric Imaging: The Three-dimensional Fourier Transform 203
Multislice Versus Volumetric Imaging: Three-dimensional Versus Two-dimensional 203
Two-dimensional Imaging: How Do We Do It? 203
Three-dimensional Imaging: How Do We Do It? 204
Slab Selection 204
A Variation on Phase Encoding 204
The Price: Imaging Time 207
The Payoff Spatial Resolution and Signal to Noise 208
15 Parallel Imaging: Acceleration with SENSE and SMASH 210
Why Another Imaging Technique? 210
So What's New? 210
Basics of Parallel Techniques 211
Parallel Radiofrequency Systems 211
Coil Sensitivity 211
Acquisition Schemes 212
Sensitivity Encoding: SENSE, Integrated Parallel Acquisition Techniques-Modified SENSE (iPAT-mSENSE), Array Spatial Sensitivity Encoding Technique (ASSET) 213
Simultaneous Acquisition of Spatial Harmonics: SMASH, Integrated Parallel Acquisition Techniques-Generalized Autocalibrating Partially Parallel Acquisition (iPAT-GRAPPA) 214
What Do We Actually Gain and at What Cost? 214
Speed 214
Susceptibility 215
Signal to Noise 215
16 Flow and Angiography: Artifacts and Imaging of Coherent Motion 216
What Is Magnetic Resonance Angiography Anyway? 216
Basic Principles of Flow for Students of MRI 217
Laminar Flow 218
Turbulent Flow 218
Impact of Flow on the MR Signal 219
High- Velocity Signal Loss 219
Dephasing Due to Movement Along a Gradient Magnetic Field 221
Phase Misregistration 222
Odd Echo Dephasing and Even Echo Rephasing 223
Gradient-Moment Nulling (AKA Flow Compensation) 224
Dephasing Due to Turbulence 226
Time-of-Flight Effect 226
Entry Slice Phenomenon 227
Time-af-Flight MRA 228
Maximizing Signal from Flow 228
Suppressing Background Signal 229
What Type of Contrast Do MRA Images Have Anyway? 230
Arteries or Veins 231
Two-dimensional or Three-dimensional 232
What Do We Mean by 2D TOF-MRA? 232
There Is Another Way: 3D TOF-MRA 232
Limitations of 3D MRA and Their Solutions 234
Which Way Do You Go? Two-dimensional Versus Three-dimensional 236
It Really Is Better if You Go Both Ways: Multiple Overlapping Thin-Slab Acquisition 237
Something Different: Contrast-Enhanced MRA 237
Don't Forget This Pitfall! 241
Phase-Contrast MRA 242
The Concept 242
Direction Sensitivity in PC-MRA? 243
Velocity Encoding and Another Type of Aliasing 244
Making the PC-MRA Image 245
Direction and Quantification of Flow Derived from PC-MRA Images 246
Where Do We Go from Here? 248
17 Diffusion: Detection of Microscopic Motion 249
Introduction 249
What Is Diffusion? 249
Effect of Diffusion on the MR Signal 250
Making the MR Image Sensitive to Diffusion 250
What Do Diffusion-Sensitized Images Look Like? 258
Quantitative Diffusion Imaging: The ADC 260
The ADC "Map" 260
Accuracy in Measurement of the ADC 261
Directional Information: DTI 261
Real Life: Anisotropy 262
Encoding the Direction of Diffusion 262
Describing Diffusion Direction: The Diffusion Tensor 262
Measuring Anisotropy 263
Tractography 265
18 Understanding and Exploiting Magnetic Susceptibility 267
What Is Magnetic Susceptibility (X) Anyway? 267
Effects on Magnetic Field Strength (Bnet) 267
Where It Really Counts: Effects on Bo HomogeneityWhat if 268
Principle Magnetic Susceptibility-Related Effects: Signal Loss and Distortion 269
Proton-Electron Dipole Interactions: The Other Face of Paramagnetism 270
Susceptibility-Related Effects I: Artifacts 270
Susceptibility-Related Effects II: Hemorrhage 271
Oxyhemoglobin: The "Hyperacute" Phase 273
Deoxyhemoglobin: The "Acute" Phase 273
Methemoglobin: The "Subacute" Phase 274
Location, Location, Location! 274
Hemosiderin and Ferritin: The "Chronic" Phase 275
Susceptibility-Related Effects III: Contrast Agents 276
Exploiting the PEDI Effect 276
Creating Spatial Variability in the Magnetic Field 276
Susceptibility-Related Effects IV: Perfusion Imaging 277
Time Series 277
Hemodynamic Measures 278
Hemodynamic Parameter Images 280
Susceptibility-Related Effects V: Functional MRI 281
Physiology offMRI 281
The fMRI Acquisition 283
fMRI Data Processing 283
19 Spectroscopy and Spectroscopic Imaging: In Vivo Chemical Assays by Exploiting the Chemical Shift 285
Introduction 285
The Chemical Basis of MRS 285
What Then Is Spectroscopy (MRS) and How Is It Different from MRI? 286
Abundance, Resolution, and Detection 287
The Importance of Field Homogeneity 288
Localization: Single-Voxel Methods 289
Stimulated Echo Acquisition Mode 289
Point- Resolved Spectroscopy 290
Localization: Chemical Shift Imaging 292
Brain Chemistry: Brief Overview of the Proton Spectrum 294
Neuronal Marker: N-Acetylaspartate 294
Membrane Marker: Choline 294
The Reference Standard: Creatine 295
Necrosis: Lactate 296
Tissue Loss: Myo-Inositol 296
Neurotransmitters: Glutamate 296
Appendices 297
Appendix 1 Understanding and Manipulating Vectors 298
What Are They? 298
What Do We Do with Them? 298
Appendix 2 Glossary of Terms 300
Appendix 3 Glossary of Common MRI Acronyms, Abbreviations, and Notations 312
Appendix 4 Resources for Reference and Further Study 318
Index 320