MRI (eBook)

Basic Principles and Applications

Brian M. Dale, Mark A. Brown, Richard C. Semelka (Autoren)

eBook Download: EPUB

2015 | 5. Auflage
John Wiley & Sons (Verlag)
9781119013037 (ISBN)

Infos

This fifth edition of the most accessible introduction to MRI principles and applications from renowned teachers in the field provides an understandable yet comprehensive update.

• Accessible introductory guide from renowned teachers in the field
• Provides a concise yet thorough introduction for MRI focusing on fundamental physics, pulse sequences, and clinical applications without presenting advanced math
• Takes a practical approach, including up-to-date protocols, and supports technical concepts with thorough explanations and illustrations
• Highlights sections that are directly relevant to radiology board exams
• Presents new information on the latest scan techniques and applications including 3 Tesla whole body scanners, safety issues, and the nephrotoxic effects of gadolinium-based contrast media

Brian M. Dale, Ph.D. MBA is Zone Research Manager, MR R&D Collaborations, Siemens Medical Solutions, Inc. Brian is a younger colleague at Siemens of the previous co-author, Dr Mark Brown. Brian has a PhD in biomedical engineering from Case Western Reserve University in Cleveland, OH. His interests are in sequence programming and optimal design.

Mark A. Brown, Ph.D. is Senior Technical Instructor at Siemens Medical Solutions Training and Development Center. He received his Ph.D. in Physical Chemistry from Duke University, in Durham, NC. His research interests include relaxation and exchange phenomena and in vivo nuclear magnetic resonance spectroscopy and imaging.

Richard Semelka, MD, is Director of Magnetic Resonance Services, Professor, and Vice Chairman of Radiology at the University of North Carolina-Chapel Hill Medical School. He received his medical degree and residency training in radiology in his native Canada at the University of Manitoba, and completed a clinical research fellowship in MRI of the body at the University of California at San Francisco. Dr. Semelka has authored over 300 peer-reviewed articles, 12 textbooks including the Wiley Abdominal-Pelvic MRI and Current Clinical Imaging series and is an internationally acclaimed authority in the field.

Inhalt

Preface, ix

ABR study guide topics, xi

1 Production of net magnetization 1

1.1 Magnetic fields 1

1.2 Nuclear spin 2

1.3 Nuclear magnetic moments 4

1.4 Larmor precession 4

1.5 Net magnetization 6

1.6 Susceptibility and magnetic materials 8

2 Concepts of magnetic resonance 10

2.1 Radiofrequency excitation 10

2.2 Radiofrequency signal detection 12

2.3 Chemical shift 14

3 Relaxation 17

3.1 T1 relaxation and saturation 17

3.2 T2 relaxation, T2* relaxation, and spin echoes 21

4 Principles of magnetic resonance imaging - 1 26

4.1 Gradient fields 26

4.2 Slice selection 28

4.3 Readout or frequency encoding 30

4.4 Phase encoding 33

4.5 Sequence looping 35

5 Principles of magnetic resonance imaging - 2 39

5.1 Frequency selective excitation 39

5.2 Composite pulses 44

5.3 Raw data and image data matrices 46

5.4 Signal-to-noise ratio and tradeoffs 47

5.5 Raw data and k-space 48

5.6 Reduced k-space techniques 51

5.7 Reordered k-space filling techniques 54

5.8 Other k-space filling techniques 56

5.9 Phased-array coils 58

5.10 Parallel acquisition methods 60

6 Pulse sequences 65

6.1 Spin echo sequences 67

6.2 Gradient echo sequences 70

6.3 Echo planar imaging sequences 75

6.4 Magnetization-prepared sequences 77

7 Measurement parameters and image contrast 86

7.1 Intrinsic parameters 87

7.2 Extrinsic parameters 89

7.3 Parameter tradeoffs 91

8 Signal suppression techniques 94

8.1 Spatial presaturation 94

8.2 Magnetization transfer suppression 96

8.3 Frequency-selective saturation 99

8.4 Nonsaturation methods 101

9 Artifacts 103

9.1 Motion artifacts 103

9.2 Sequence/Protocol-related artifacts 105

9.3 External artifacts 119

10 Motion artifact reduction techniques 126

10.1 Acquisition parameter modification 126

10.2 Triggering/Gating 127

10.3 Flow compensation 132

10.4 Radial-based motion compensation 134

11 Magnetic resonance angiography 135

11.1 Time-of-flight MRA 137

11.2 Phase contrast MRA 141

11.3 Maximum intensity projection 144

12 Advanced imaging applications 147

12.1 Diffusion 147

12.2 Perfusion 153

12.3 Functional brain imaging 156

12.4 Ultra-high field imaging 158

12.5 Noble gas imaging 159

13 Magnetic resonance spectroscopy 162

13.1 Additional concepts 162

13.2 Localization techniques 167

13.3 Spectral analysis and postprocessing 169

13.4 Ultra-high field spectroscopy 173

14 Instrumentation 177

14.1 Computer systems 177

14.2 Magnet system 180

14.3 Gradient system 182

14.4 Radiofrequency system 184

14.5 Data acquisition system 186

14.6 Summary of system components 187

15 Contrast agents 189

15.1 Intravenous agents 190

15.2 Oral agents 195

16 Safety 196

16.1 Base magnetic field 197

16.2 Cryogens 197

16.3 Gradients 198

16.4 RF power deposition 198

16.5 Contrast media 199

17 Clinical applications 200

17.1 General principles of clinical MR imaging 200

17.2 Examination design considerations 202

17.3 Protocol considerations for anatomical regions 203

17.4 Recommendations for specific sequences and clinical situations 218

References and suggested readings 222

Index 225

Textauszug

Chapter 1
Production of net magnetization

Magnetic resonance (MR) is a measurement technique used to examine atoms and molecules. It is based upon the interaction between an applied magnetic field and a particle that possesses spin and charge. While electrons and other subatomic particles possess spin (or more precisely, spin angular momentum) and can be examined using MR techniques, this book focuses on nuclei and the use of MR techniques for their study, formally known as Nuclear Magnetic Resonance, or NMR. Nuclear spin, or more precisely nuclear spin angular momentum, is one of several intrinsic properties of an atom and its value depends on the precise atomic composition. Every element in the Periodic Table except argon and cerium has at least one naturally occurring isotope that possesses nuclear spin. Thus, in principle, nearly every element can be examined using MR, and the basic ideas of resonance absorption and relaxation are common for all of these elements. The precise details will vary from nucleus to nucleus and from system to system.

1.1 Magnetic fields

Magnetic fields are produced by and surround electric currents, whether these currents are macroscopic currents such as those running through wires or microscopic currents such as those around an atom of iron. The magnetic field can be represented as a vector, meaning that it has both a magnitude and a direction, and is usually denoted by the variable B.1 For example, the B field at the center of a circular loop of current-carrying wire points in the direction of the axis of the loop (perpendicular to the plane of the loop and therefore perpendicular to the current flow) and it has a magnitude that is proportional to the current in the loop. The magnitude of the field is related to the strength of the magnetic force on wires or magnetic materials, and the direction of the field is perpendicular to the direction of the force.

Magnetic fields often vary over time and/or space, and will be coupled to the electric field, producing electromagnetic waves. Magnetic fields, particularly those in electromagnetic waves, are characterized by their frequency (the time between two consecutive “peaks” in the field). In MR, there are magnetic fields, which are constant in time, which vary at acoustic frequencies (a few kilohertz), and which vary at radio frequencies (RF) (several megahertz).

1.2 Nuclear spin

The structure of an atom is an essential component of the MR experiment. Atoms consist of three fundamental particles: protons, which possess a positive charge; neutrons, which have no charge; and electrons, which have a negative charge. The protons and neutrons are located in the nucleus or core of an atom; thus all nuclei are positively charged. The electrons are located in shells or orbitals surrounding the nucleus. The characteristic chemical reactions of elements depend upon the particular number of each of these particles. The properties most commonly used to categorize elements are the atomic number and the atomic weight. The atomic number is the number of protons in the nucleus and is the primary index used to differentiate atoms. All atoms of an element have the same atomic number and undergo the same chemical reactions. The atomic weight is the sum of the number of protons and the number of neutrons. Atoms with the same atomic number but different atomic weights are called isotopes. Isotopes of an element will undergo the same chemical reactions, but at different reaction rates.

A third property of the nucleus is spin or intrinsic spin angular momentum. Classically, nuclei with spin can be considered to be always rotating about an axis at a constant rate. This self-rotation axis is perpendicular to the direction of rotation (Figure 1.1). A limited number of values for the spin are found in nature; that is, the spin, I, is quantized to certain discrete values. These values depend on the atomic number and atomic weight of the particular nucleus. There are three groups of values for I: zero, integral, and half-integral values. A nucleus has no spin (I = 0) if it has an even atomic weight and an even atomic number; for example, 12C (6 protons and 6 neutrons) or 16O (8 protons and 8 neutrons). Such a nucleus does not interact with an external magnetic field and cannot be studied using MR. A nucleus has an integral value for I (e.g., 1, 2, 3) if it has an even atomic weight and an odd atomic number; for example, 2H (1 proton and 1 neutron) or 6Li (3 protons and 3 neutrons). A nucleus has a half-integral value for I (e.g., 1/2, 3/2, 5/2) if it has an odd atomic weight. Table 1.1 lists the spin and isotopic composition for several elements commonly found in biological systems. The 1H nucleus, consisting of a single proton, is a natural choice for probing the body using MR techniques for several reasons. It has a spin of 1/2 and is the most abundant isotope for hydrogen. Its response to an applied magnetic field is one of the largest found in nature. Since the body is composed of tissues that contain primarily water and fat, both of which contain hydrogen, a significant MR signal can be produced naturally by normal tissues.

Figure 1.1 A rotating nucleus (spin) with a positive charge produces a magnetic field known as the magnetic moment oriented parallel to the axis of rotation (a). This arrangement is analogous to a bar magnet in which the magnetic field is considered to be oriented from the south to the north pole (b).

Table 1.1 Constants for Selected Nuclei of Biological Interest

Element

Nuclear composition

Nuclear spin I

Gyromagnetic ratio (MHz T−1)

% Natural abundance

at 1.5 T (MHz)

Protons

Neutrons

1H, protium

1

0

1/2

42.5774

99.985

63.8646

2H, deuterium

1

1

1

6.53896

0.015

9.8036

3He

2

1

1/2

32.436

0.000138

48.6540

6Li

3

3

1

6.26613

7.5

9.39919

7Li

3

4

3/2

16.5483

92.5

24.8224

12C

6

6

0

0

98.90

0

13C

6

7

1/2

10.7084

1.10

16.0621

14N

7

7

1

3.07770

99.634

4.6164

15N

7

8

1/2

4.3173

0.366

6.4759

16O

8

8

0

0

99.762

0

17O

8

9

5/2

5.7743

0.038

8.6614

19F

9

10

1/2

40.0776

100

60.1164

23Na

11

12

3/2

11.2686

100

16.9029

31P

15

16

1/2

17.2514

100

25.8771

129Xe

54

75

1/2

11.8604

26.4

17.7906

Source: Adapted from Ian Mills (ed.), Quantities, Units, and Symbols in Physical Chemistry, IUPAC, Physical Chemistry Division, Blackwell, Oxford, UK, 1989.

While a rigorous mathematical description of a nucleus with spin and its interactions requires the use of quantum mechanical principles, most of MR can be described using the concepts of classical mechanics, particularly in describing the actions of a nucleus with spin. The subsequent discussions of MR phenomena in this book use a classical approach. In addition, while the concepts of resonance absorption and relaxation apply to all nuclei with spin, the descriptions in this book focus on 1H (commonly referred to as a proton) since most imaging experiments visualize the 1H nucleus.

1.3 Nuclear magnetic moments

Recall that the nucleus is the location of the positively charged protons. When this charge rotates due to the nuclear spin, a local magnetic field or magnetic moment is induced about the nucleus. This magnetic moment will be oriented parallel to the axis of rotation. Since the nuclear spin is constant in magnitude, its associated magnetic moment will also be constant in magnitude. This magnetic moment is fundamental to MR. A bar magnet provides a useful analogy. A bar magnet has a north and a south pole, or, more precisely, a magnitude and orientation to the magnetic field can be defined. The axis of rotation for a nucleus with spin can similarly be viewed as a vector with a definite orientation and magnitude (Figure 1.1). This orientation of the nuclear spin and the changes induced in it due to the experimental manipulations that the nucleus undergoes provide the basis for the MR signal.

In general, MR measurements are made on collections of spins rather than on an individual spin....

Produkt-Details

• Erscheint lt. Verlag: 6.8.2015
• Sprache: englisch
• Themenwelt: Medizin / Pharmazie ► Gesundheitsfachberufe
• Themenwelt: Medizinische Fachgebiete ► Radiologie / Bildgebende Verfahren ► Kernspintomographie (MRT)
• Themenwelt: Medizinische Fachgebiete ► Radiologie / Bildgebende Verfahren ► Radiologie
• Themenwelt: Technik
• Schlagworte: 3 Tesla whole body scanners • Artifacts • Brian M. Dale • Clinical applications • clinical scanners • contrast agents • Data Acquisition Techniques • fundamental physics • Image Contrast • Instrumentation • Interventional Radiology • Invasive Radiologie • Magnetic Resonance • Magnetic Resonance Imaging • Magnetresonanztomographie • Mark A. Brown • Measurement Parameters • Medical & Health Physics • Medical Imaging • Medical Science • Medizin • Motion Artifact Reduction Techniques • MR Angiography • MRI • MRI Basic Principles and Applications • MR physics principles • MR spectroscopy • MRT • Net Magnetization • Physics • Physik • Physik in Medizin u. Gesundheitswesen • pulse sequences • Radiofrequency • Radiologie • Radiologie u. Bildgebende Verfahren • radiologists prepare • Radiology & Imaging • radiology boards • Relaxation • Richard Semelka • Safety • Safety Issues • Signal Suppression Techniques • Spin Echoes • T1 Relaxation and Saturation • T2 Relaxation • T2* Relaxation
• ISBN-13: 9781119013037 / 9781119013037