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MRI Physics: Tech to Tech Explanations

ISBN: 9781119615026
ISBN: 9781119615026
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Technologists must have a solid understanding of the physics behind Magnetic Resonance Imaging (MRI), including safety, the hows and whys of the quantum physics of the MR phenomenon, and how to competently operate MRI scanners. Generating the highest quality images of the human body involves thorough knowledge of scanner hardware, pulse sequences, image contrast, geometric parameters, and tissue suppression techniques.

MRI Physics: Tech to Tech Explanations is designed to help student MRI technologists and radiotherapists preparing for Advanced MRI certification examinations to better understand difficult concepts and topics in a quick and easy manner.

Written by a highly experienced technologist, this useful guide provides clear and reader-friendly coverage of what every MR Technologist needs to know. Topics include safety considerations associated with the magnetic field and RF, pulse sequences, artifacts, MRI math, the much-feared gradients, and I.V. contrast.

Provides basic guidance on safety considerations, protocols options, critical thinking, and image contrast optimization
Simplifies the challenging topic of MRI physics using straightforward language and clear explanations
Covers content for American Registry of Radiologic Technologists (ARRT) and Continuing Qualifications Requirements (CQR) exams
Features numerous illustrations and photographs of various MRI concepts, pulse sequence design, artifacts, and the application of concepts in clinical settings

MRI Physics: Tech to Tech Explanations is a must-have resource for the experienced and training MRI technologist, medical students, and radiology residency rotations.

Stephen J. Powers, BSRT (R), (CT), (MR) is currently an MR Technologist for South Coast Hospital Systems in the USA, and former MRI Clinical Applications Specialist for General Electric Health Care. Stephen has over twenty years of classroom, lecture and clinical instructor experience.


About the Author xv

Preface xvii

Acknowledgements xix

Introduction 1

1 Hardware: Magnet Types and Coils 15

Magnets 15

Coils 17

2 The Basics 23

Why the Hydrogen Molecule? 24

The Net Magnetization Vector 26

MRI is a Sequence of Events 27

Free Induction Decay (FID) 32

Relaxation 33

Proton Density 38

Image Contrast 38

The IQ Triangle: Contrast, SNR, Resolution 39

B0 and B1 43

Free and Bound Protons 44

3 Image Weighting 47

Where Does Image Weighting Come From? 48

Time of Repetition (TR) 50

Time of Echo (TE) 52

TE and TR 54

Why Different TR Ranges for Different Field Strengths? 54

How Does TR Control T1? 55

What Does TR Affect? 56

Interpreting the T1 Relaxation Curve 57

Time of Repetition: Effects of the TR 57

TE: The T1 and T2 of it 58

Interpreting the T2 Relaxation Curve 60

Effects of TE on Image Contrast 62

What Do the Lines on the Curves Really Mean Anyway? 62

One Last Weighting Triangle 65

T1 and T2 Contrast Review 66

4 Introduction to the Basic Pulse Sequences 69

What is a Pulse Sequence? 69

Spin Echo (SE) 70

Gradient Echo/Gradient Recalled Echo (GRE) 73

Line Diagram Anatomy 74

The Ernst Angle 77

5 Multi Echo Spin Echo Sequence 81

Introduction to k-Space 82

k-Space: Phase Encoding 85

With FSE, Watch the Speed Limit! 86

k-Space, ETL, and Image Contrast 87

Filling k-Space 89

Pros and Cons of FSE 89

Another Way to View T2* and 180°s 91

Where Do Relaxation and Decay Curves Come From? 92

A T2* Curve Compared to the T2 Curve 93

Metal Artifact Reduction (MARS) 94

Driven Equilibrium: A “Forced T1” 95


Single Shot FSE/HASTE 98

6 Tissue Suppression 105

Tissue Saturation versus Suppression 107

Inversion Recovery – Part One: STIR 108

Inversion Recovery: STIR with Vectors 109

Inversion Recovery Part Two: T2 FLAIR 113

IR Sequences: T1 and T2 FLAIR 116

IR Weightings: STIR, T1 and T2 FLAIR 117

Inversion Recovery – Part Two 119

The Rupture View 120

Tissue Saturation: Chemical Shift 121

Chemical Saturation at Low Fields 123

Tissue Saturation: SPAIR and SPIR 124

The Dixon Technique 126

Water Excitation 126

Saturation Pulses or Bands 129

Subtractions 131

Magnetization Transfer 135

IR Prepped Sequences 137

How is an RF Pulse Selective or Non-Selective? 140

Water Excitation Sequences 142

7 The Gradient Echo Sequence 145

GRE Sequence Structure 147

Phase Dispersion and Gradient Reversal 148

Analog to Digital Converter (ADC) 149

GRE Sequence Image Weighting 149

Two Different Kinds of T2 Relaxation 152

The GRE Weighting Triangle 153

GRE and SE Differences 156

Different Gradient Echo Types 157

In and Out of Phase TEs 161

In Phase/Out of Phase at 1.5 T 163

8 Gradient Echo Magnetic Resonance Angiography 167

Time of Flight MRA 168

TOF Angiography: Two Golden Rules 171

Types of MRA Sequences 171

TOF Concept in MRA versus MRV 172

2D versus 3D 172

2D TOF MRAs 175

3D TOF MRAs 176

In-Plane Saturation 178

In-Plane Saturation Avoidance 179

Magnetization Transfer (MT) 181

Options for Better MRAs 183

Phase Contrast MRA 185

9 k-Space 191

What Is Fourier Transform? 192

k-Space Filling 192

10 Echo Planar Sequences 203

Diffusion Weighted Imaging 205

Diffusion Tensor Imaging or White Matter Tractography 215

Susceptibility Weighted Imaging 216

Brain Perfusion 218

Arterial Spin Labeling 222

Spectroscopy 225

11 Geometric Parameters: Trade-offs and Effects on Image Quality 231

Field of View (FOV) Is Your Film Size 232

Nex, ACQ, NSA, and SNR 235

Scan Matrix 237

Frequency Matrix 237

Echo Train Length 238

Echo Spacing 239

Echo Train Balancing 240

Slice Thickness and Slice Gap 242

Fractional Echo 243

Bandwidth 244

Rectangular (Rec.) FOV 249

No Phase Wrap/Phase Oversampling/Fold-Over Suppression 251

Concatenations or Acquisitions 254

Sequential Order Acquisition 255

12 Image Artifacts 257

Motion 258

Flow Artifact/Phase Mis-registration 262

RF Artifacts 265

Wrap/Aliasing/Fold-over Artifact 265

Gibbs Artifact (Ringing/Truncation) 268

Chemical Shift Artifact 271

Cross-talk 276

Cross-excitation 278

Gradient Warp or Distortion 281

Metal Artifacts 281

Corduroy Artifact 283

Annifact 284

Moiré Fringe Artifact or Zebra Artifact 285

Magnetic Susceptibility Artifact 286

Dielectric Effect or Standing Wave 288

Magic Angle Artifact 290

13 Gradients 295

Physical Gradients 296

Logical Gradients 302

14 MRI Math 313

The Larmor Equation: W0 = γB0 314

Acquisitions or Nex or NSA 314

Scan Time Equations 315

Pixel Size and Voxel Volume 317

How to Convert Hz per Pixel to MHz 318

In and Out of Phase TEs 319

Dixon Method or Technique 320

SNR and the 3D Sequence 321

15 Parallel Imaging 325

Parallel Imaging: What Is It? 325

When and Where to Use the Speed 326

Parallel Imaging: How Does It Work? 327

Parallel Imaging: Pros and Cons 330

16 IV Gadolinium 335

Why We Use Gad 336

How Does Gad Shorten the T1 of Tissues? 337

The Blood–Brain Barrier 341

Post Contrast T2 FLAIR Imaging 342

Imaging Gadolinium 345

Eovist® 347

Glossary 351

Suggested Reading 388

Index 389