MRI in Clinical Practice (eBook)

Contents 7
Introduction 9
Acknowledgments 10
Part I 11
Chapter 1 The Basics 12
1.1 SOME MR PHYSICS 12
1.2 IMAGE FORMATION 20
1.3 PULSE SEQUENCES 24
1.4 A WORD ON HARDWARE 31
Chapter 2 Safety 35
2.1 INTRODUCTION 35
2.2 STATIC FIELD 36
2.3 TIME-VARYING MAGNETIC FIELD 37
2.4 RADIO-FREQUENCY FIELD 39
2.5 OTHER ISSUES 39
2.6 PATIENT SCREENING 41
Chapter 3 Scan Parameters and Image Artifacts 43
3.1 SCAN PARAMETERS 43
3.2 IMAGE ARTIFACTS 48
Chapter 4 Quality Assurance 62
4.1 THE BASICS 62
4.2 MEASUREMENTS AND ANALYSIS 64
Part II 70
Chapter 5 Brain and Spine 72
5.1 BRAIN—INTRODUCTION 72
5.2 CONVENTIONAL TECHNIQUES 73
5.3 ADVANCED APPLICATIONS 77
5.5 CONVENTIONAL TECHNIQUES 83
5.6 ADVANCED APPLICATIONS 85
Chapter 6 Breast 86
6.1 INTRODUCTION 86
6.2 CONVENTIONAL TECHNIQUES 87
6.3 ADVANCED APPLICATIONS 89
Chapter 7 Abdomen and Pelvis 93
7.1 LIVER—INTRODUCTION 93
7.2 CONVENTIONAL TECHNIQUES 93
7.3 ADVANCED APPLICATIONS 95
7.4 MR CHOLANGIOPANCREATOGRAPHY (MRCP)—INTRODUCTION 96
7.5 CONVENTIONAL TECHNIQUES 97
7.6 ADVANCED APPLICATIONS 99
7.7 PROSTATE—INTRODUCTION 99
7.8 CONVENTIONAL TECHNIQUES 101
7.9 ADVANCED APPLICATIONS 102
7.10 GYNECOLOGY—INTRODUCTION 104
7.11 CONVENTIONAL TECHNIQUES 105
7.12 ADVANCED APPLICATIONS 108
Chapter 8 Cardiac MRI 109
8.1 INTRODUCTION 109
8.2 CONVENTIONAL TECHNIQUES 109
8.3 ADVANCED APPLICATIONS 114
Chapter 9 MR Angiography 116
9.1 INTRODUCTION 116
9.2 CONVENTIONAL TECHNIQUES 117
9.3 ADVANCED APPLICATIONS 121
Chapter 10 Musculoskeletal MRI 125
10.1 KNEE—INTRODUCTION 125
10.2 CONVENTIONAL TECHNIQUES 125
10.3 ADVANCED APPLICATIONS 127
10.4 SHOULDER—INTRODUCTION 128
10.5 CONVENTIONAL TECHNIQUES 129
10.6 HAND AND WRIST—INTRODUCTION 130
10.7 CONVENTIONAL TECHNIQUES 130
10.8 ADVANCED APPLICATIONS 131
10.9 FOOT AND ANKLE—INTRODUCTION 131
10.10 CONVENTIONAL TECHNIQUES 132
10.11 HIPS—INTRODUCTION 132
Appendix I Pulse Sequence Acronyms 134
Appendix II Miscellaneous 144
A2.1 LIST OF CONTRAST AGENTS 144
A2.2 THE TRANSITION TO HIGH FIELD 145
Appendix III Screening Form 146
Index 148

Chapter 2 Safety (p. 26)

2.1 INTRODUCTION

A major advantage MRI has over other imaging modalities is its lack of ionizing radiation and inherent safety. Since its introduction into clinical practice in the early 1980s, its use has grown to such an extent that it is estimated that over 20 million scans are carried out worldwide each year. The vast majority of these scans are performed without incident. From my own personal experience, I have volunteered for countless scans, at several different field strengths with no problems (as far as I know!).

Although MRI is an extremely safe form of medical imaging there are three speci.c interactions between the scanner and the patient that need addressing when quantifying any potential safety hazards. These correspond to the three separate magnetic fields used in the imaging process:

• The static magnetic field produced by the scanner (B0)

• The time-varying magnetic field produced by the gradient coils (dB/dt)

• The radio-frequency field produced by the imaging coils (B1) The following discusses each of these effects in turn together with their appropriate safety limits. In the United Kingdom the Medical and Healthcare Products Regulatory Agency (MHRA) has published guidelines (2002) that do not set exposure limits but intend to draw the attention of users to the limits published by the National Radiological Protection Board (NRPB), and the International Electrotechnical Commission (IEC).

The document de.nes three distinct modes of scanner operation, namely: normal, controlled, and research. Much clearer advice has been proposed by the U.S. Food and Drug Administration (FDA) in 1997 and more recently updated in 2003, which I have used here, and compared alongside the other recommendations.

2.2 STATIC FIELD

Biological Effects

There are no known long-term or irreversible biological effects from exposure to static fields of less than 10 Tesla. This is even more reassuring when you consider that all clinical systems currently operate at no higher than 3.0 Tesla.

There are three forces to consider when examining biological interactions between the human body and the static field of the scanner, these are: translational forces on ferromagnetic objects (the projectile effect), torque or turning forces exerted on implants and tissues, and the force induced on moving charged particles.

Potentially hazardous effects on human biology involving minute ferromagnetic particles in tissues and moving electrons and ions in cells have been studied and it is well established that these simply could not occur at the field strengths in use. Some transiently occurring phenomena have been observed, however, at high field.

For example, magnetophosphenes (flashing lights in the eyes) have been anecdotally reported at 4.0 Tesla. Other mild sensory effects such as vertigo, nausea, and a metallic taste have also been reported. These have usually been associated with rapid head movements near the entrance of the scanner bore.

In addition there is some alteration in the T-wave demonstrated on an electrocardiograph (ECG) trace, which increases at higher field. This is due to the induced electrical potential in moving blood but is of negligible consequence.