Cover
Magnetic Resonance Technology Hardware and System Component Design
Preface
Contents
Chapter 1 - The Principles of Magnetic Resonance, and Associated Hardware
1.1 Introduction
1.2 The Superconducting Magnet and Nuclear Polarization
1.3 The Transmitter Coil to Generate Radiofrequency Pulses
1.4 Precession
1.4.1 Chemical Shift
1.4.2 Scalar Coupling
1.4.3 Relaxation Processes
1.5 The Receiver Coil for Detecting the MR Signal
1.6 The Receiver: Signal Demodulation, Digitization and Fourier Transformation
1.6.1 Receiver Electronics
1.6.2 Signal Processing
1.7 Shim Coils
1.8 Gradient Coils
1.8.1 Crusher Gradients to Dephase Transverse Magnetization
1.8.2 Gradients for Coherence Selection in High-Resolution NMR
1.8.3 Measurements of Apparent Diffusion Coefficients Using Gradients
1.8.4 Gradient-Based Shimming
1.8.5 Gradients in MRI
1.9 The Deuterium Lock Channel and Field Monitoring
1.10 Magic Angle Spinning Solid-State NMR: Principles and Instrumental Requirements
1.11 Magnetic Resonance Imaging: Principles and Instrumental Requirements
Appendices
Appendix A. Maxwell’s Equations and the Biot–Savart Law
Example A1. Magnetic Field Produced by a Straight Wire
Example A2. Magnetic Field Produced by a Circular Wire Loop
Example A3. Design of a Two-Loop Coil Geometry to Produce a Linear z-Gradient
Example A4. Design of a Two-Loop Coil Geometry to Produce a Homogeneous Magnetic Field
Appendix B. Spherical Harmonic Representation of Magnetic Fields
Visual Description
Mathematical Description of Spherical Harmonics
Example B1. Generation of a Homogeneous Magnetic Field
Example B2. Generating a Linear Magnetic Field Gradient in z
Example B3. Generating a Linear Magnetic Field Gradient in x or y
References
Chapter 2 - Magnets
2.1 Introduction
2.2 Magnet Types
2.2.1 Air-Cored Resistive Magnets
2.2.2 Permanent Magnets
2.2.3 Iron-Cored Resistive Magnets
2.2.4 Iron-Cored Superconducting Magnets
2.2.5 Superconducting Cylindrical Magnets
2.3 Magnetic Field Generation
2.3.1 Basic Physics
2.3.2 Field Homogeneity
2.3.3 Magnetic Shielding
2.3.4 System Shielding from External Interference
2.3.5 Magnetic Field Shimming
2.4 Superconductivity
2.4.1 Superconducting Materials
2.4.2 Energising a Superconducting Magnet
2.4.3 Superconducting Switch
2.4.4 Superconducting Joints
2.4.5 Quenching
2.4.6 Quench Protection
2.4.7 Stress Limits
2.5 Heat Transfer and Cryostat Design
2.5.1 Cryo-Refrigerators
2.5.2 Sub-Atmospheric Operation
2.5.3 Gradient-Induced Heating
2.6 Practical Considerations
2.6.1 Safety
2.6.2 Installation Issues
2.7 Future Developments
2.7.1 High and Ultra-High Field Magnets
2.7.2 Helium-Free Technology
References
Chapter 3 - Radiofrequency Coils
3.1 Introduction
3.2 General Electromagnetic Principles for RF Coil Design
3.2.1 Maxwell’s Equations and the Biot–Savart law
3.2.2 Transmit (B+1) and Receive (B−1) Magnetic Fields
3.2.3 Linear and Circular Polarization
3.2.4 Conservative and Non-Conservative Electric Fields
3.2.5 Electromagnetic Simulations
3.3 Electrical Circuit Analysis
3.3.1 RF Coil Impedance
3.3.2 Resonant Circuits
3.3.3 Capacitive Impedance Matching
3.3.4 Inductive Impedance Matching
3.3.5 Impedance Matching Using Transmission Line Elements
3.3.6 Baluns and Cable Traps
3.3.7 RF Coil Loading—The Effect of the Sample
3.4 RF Coils Producing a Homogeneous Magnetic Field (Volume Coils)
3.4.1 Birdcage Coils
3.4.2 Transverse Electromagnetic Mode (TEM) Resonators
3.4.3 Partial-Volume Coils
3.4.4 Solenoids and Loop Gap Resonators
3.5 Surface Coils
3.5.1 Transmit/Receive Surface Coils
3.5.2 Quadrature Surface Coils
3.6 Detuning Circuits for Transmit-Only Volume Coils and Receive-Only Surface Coils
3.7 Receive Arrays
3.7.1 Array Optimization
3.7.2 Preamplifier Decoupling in Receive Arrays
3.8 Multiple-Frequency Circuits
3.8.1 Multiple-Pole Circuits
3.8.2 Transformer Coupled Circuits
3.8.3 Multiple-Tuned Volume Coils
3.8.4 Multiple-Tuned Surface Coils
3.9 RF coils for NMR Spectroscopy
3.9.1 Probes for High Resolution Liquid-State NMR
3.9.2 Microprobes for High Resolution NMR and Hyphenated Microseparations
3.9.3 Probes for Solid-State NMR
3.9.4 Cryoprobes
3.10 RF Coils for Small Animal Imaging and MR Microscopy
3.10.1 Small Animal Imaging Coils
3.10.2 RF coils for MR Microscopy and Combined MR/Optical Histology
3.11 RF Coils for Clinical Imaging Systems
3.11.1 Single-Channel and Dual-Channel Transmit Coils for Clinical Systems
3.11.2 Receive Arrays for Clinical Systems
3.12 RF Coils for Very High Field Human Imaging
3.12.1 Multi-Channel Transmit Arrays for High Field Imaging
3.13 Dielectric Resonators
3.13.1 HEM11 Mode Resonators
3.13.2 TE01 Mode Resonators
3.14 Antennae for Travelling Wave MRI
Appendix A
Coil Workbench Measurements Using a Network Analyzer
References
Chapter 4 - B0 Shimming Technology
4.1 Introduction
4.2 The Origins of Magnetic Field Inhomogeneity
4.3 Static Spherical Harmonic Shimming
4.3.1 Theory
4.3.2 Magnetic Field Mapping
4.3.2.1 B0 Field Mapping
4.3.2.2 MRI-Based B0 Field Mapping
4.3.3 Calibration of Shim Coil Efficiency
4.3.4 Static Spherical Harmonic Shimming of the Human Brain
4.4 Dynamic Spherical Harmonic Shimming
4.4.1 Principle of Dynamic Shimming
4.4.2 Practical Considerations for Dynamic Shimming
4.4.3 Dynamic Spherical Harmonic Shimming of the Human Brain
4.5 Alternative Shimming Methods
4.5.1 Passive Approaches
4.5.2 Active Approaches
4.5.2.1 Principles and Considerations in Multicoil Shimming
4.5.2.2 Multi-Coil Shimming of the Human Brain
References
Chapter 5 - Magnetic Field Gradients
5.1 Introduction
5.1.1 Linear Magnetic Field Gradients
5.1.2 Spatial Encoding and Geometric Distortion
5.1.3 Classification of Design Methods
5.1.4 Biot–Savart Methods
5.1.4.1 Examples
5.1.5 Current Density Methods
5.1.6 Methods Using Spherical Harmonics
5.1.7 Definition of Gradient Performance Parameters
5.1.7.1 Gradient Efficiency, η, in Units of [mT A−1 m−1]
5.1.7.2 Linearity Radius rLV [m]
5.1.7.3 Slew Rate in Units of [mT m−1 ms−1]
5.1.7.4 Performance Index PI in Units of [mT2 m−2 ms−1]
5.1.8 Developments in “Conventional” Gradient Designs
5.1.9 Integrated Gradient and RF Designs
5.1.10 Increased Bore-Size Systems
5.1.11 Non-Cylindrical Designs
5.2 Gradient System
5.2.1 Overview
5.2.2 Gradient Coil System
5.2.3 Gradient Power Amplifier (GPA) and Connectors
5.2.4 Gradient Cooling System and Temperature Supervision
5.2.5 Gradient Control System and “Safety Watchdog”
5.3 Examples of Specific Gradient Coil Designs
5.3.1 High Strength Gradients for a 7 T Horizontal Bore Animal Magnet
5.3.2 A Whole-Body Modular Gradient Set with Continuously Variable Field Characteristics
5.3.3 A Head Gradient Coil Insert
5.3.4 Ultra-Strong Whole-Body Gradients
References
Chapter 6 - Radiofrequency Amplifiers for NMR/MRI
6.1 Introduction
6.2 Principles of RF Amplification
6.2.1 The RF Power MOSFET
6.2.2 DC Characteristics
6.2.3 RF Characteristics
6.2.4 Amplifier Classes
6.2.5 Switch-Mode Amplifiers
6.2.6 Mechanism of RF Power Amplification
6.3 Matching Networks for Amplifiers
6.3.1 Basics of Matching Networks
6.3.2 Narrowband Matching
6.3.3 Broadband Matching
6.4 Amplifier Performance Considerations
6.4.1 Linearity
6.4.2 Noise Gating
6.4.3 Dynamic Range
6.4.4 Efficiency
6.4.5 Stability
6.4.6 Technical Specifications for Commercial RFPAs for MRI
6.5 Amplifiers for Multi-Channel Transmission
6.5.1 Mutual Coupling in Transmit Arrays
6.6 Current Source Amplifiers
6.6.1 Matching Networks for Current Source Amplifiers
6.6.2 Analysis of the Current Source Network
6.7 Low Output Impedance Amplifiers
6.7.1 Coil Matching Network for an LOI amplifier
6.8 Testing and Comparison of Amplifiers Architectures
6.8.1 Amplifier Bench Measurements
6.8.2 Amplifier Testing Using MRI
6.9 Selection of Amplifier Architecture
References
Chapter 7 - The MR Receiver Chain
7.1 Introduction
7.2 Signal Levels and Dynamic Ranges of MR Data
7.3 Overall Noise Figure of the Receive Chain
7.4 Design of Transmit/Receive Switches
7.5 Low-Noise Preamplifiers
7.6 Data Sampling
7.6.1 Frequency Demodulation
7.6.2 Direct Detection Using Undersampling
7.7 Analogue-to-Digital Converters
7.8 Optical and Wireless Data Transmission
References
Chapter 8 - Electromagnetic Modelling
8.1 Introduction
8.2 Simulating Electromagnetic Fields for Magnetic Resonance
8.2.1 Static Magnetic (B0) Fields
8.2.2 Switched Gradient Fields (Gx, Gy, Gz)
8.2.3 Radiofrequency Magnetic (B1) Fields
8.2.3.1 Analytically Based Methods
8.2.3.2 Finite Difference Time Domain (FDTD) Method
8.2.3.3 Finite Element Method (FEM)
8.2.3.4 Method of Moments
8.2.3.5 Hybrid Simulation Methods
8.2.3.6 Approaches Using Multiple “Ideal” Current or Voltage Sources
8.2.3.7 Circuit Co-Simulation
8.3 The Role of Simulations in Assessing MR Safety and Bioeffects
8.3.1 Static Field Effects
8.3.2 Gradient-Induced Peripheral Nerve Stimulation (PNS)
8.3.3 RF-Induced Heating in the Human Body
8.3.4 Safety of Devices and Implants
8.3.5 MR Safety in Practice
8.4 Calculating the Effects of Electromagnetic Fields on MR Images
8.4.1 Calculation of the Intrinsic Signal-to-Noise Ratio (ISNR)
8.4.2 Simulating MR Images
8.5 Methods for Validating Simulations
Acknowledgements
References
Subject Index