MRI Physics - Stephen J. Powers

. .

Résumé :

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

3D FSE: CUBE/SPACE/VISTA 97

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