Preface

This book is intended for the NMR or MRI operator who sits down at the console for the first, second or even one hundredth time and wonders “why is this component designed this way, what does this button REALLY do, why didn’t they do it this way?” Historically, many of the pioneers of magnetic resonance (Bloch, Gutowsky, Schlichter, Lauterbur and Mansfield, to name a few) also constructed their own equipment. The seed of a new experiment required a deep knowledge of the quantum mechanical behaviour of the spin system, but to actually perform an experiment it was necessary to build the equipment oneself, and not to get on the cell phone to complain to the relevant vendor. While acknowledging that the days of being able to fix a system with a sheath of circuit diagrams and a well-aimed soldering iron are regrettably over, we believe that understanding how a system works expands the opportunities for new science and engineering to flourish.

This book was inspired by the seminal books of authors such as Eiichi Fukushima and David Hoult: volumes that provide a deep level of understanding coupled with unbridled enthusiasm and excitement for the subject matter. In a world of ever-increasing specialization, it is perhaps worth noting that these eminent scientists are also renowned in the mountain climbing and operatic worlds, respectively. To this end, a mixture of academic and industrial scientists were invited to contribute chapters, the latter to provide insights that too commonly are not amply represented in the academic literature. Regrettably, the list is male dominated: as one who has personally and professionally been inspired by the contributions of female engineers in many related fields, this is a sign of a too-slowly changing society, at all levels of education. Should further editions evolve, we hope that the author contributions become naturally more balanced.

Chapter 1 provides a summary of the phenomenon of magnetic resonance, linked to the relevant hardware components, described more fully in the later chapters of this book. The appendices provide an outline of mathematical constructions and approaches, including the Biot–Savart law and spherical harmonics, which are widely used in the design of many of the different hardware components covered in the specific chapters.

Chapter 2 covers the principles of designing superconducting magnets for magnetic resonance. In high resolution NMR, field strengths have broken through the 23.5 tesla ceiling required for 1 GHz operation, and new hybrid magnets with low temperature superconductors supplemented by high temperature superconducting inserts will soon result in field strengths above 30 tesla. Similarly, human-sized MRI magnets of 11.7 tesla are now available, coupled with new designs for making high field magnets of similar size and footprint to conventional lower field systems.

Chapter 3 describes the wide variety of radiofrequency coils that have been developed for high resolution NMR, and human and animal MRI. Basic electromagnetic principles behind the geometries used, methods of impedance matching, multiple-frequency tuning, active detuning and other concepts are all explained from a basic level. The ubiquitous use of multi-element receive arrays and increasing use of transmit arrays are also covered, with a final section devoted to new types of RF coil used for very high field human MRI.

Chapter 4 is concerned with the design of shim coils, which are used to maximize the static magnetic field homogeneity within the volume of interest for the particular magnetic resonance experiment. Shim coil design based on spherical harmonics is described: these coils are used for high resolution MR systems as well as human MRI systems. The chapter also considers alternative designs that are particularly applicable to human MRI at high field.

Chapter 5 describes the design of magnetic field gradients that enable spatial information to be encoded for MRI, and also form the basis of coherence selection in high resolution NMR, and molecular diffusion measurements in both solution state NMR and human MRI. Specific examples of gradient coil design are outlined including strong small diameter gradients for animal imaging, as well as the strongest yet designed for human use, the so-called connectome gradients.

Chapter 6 concentrates on the basis of designing radiofrequency power amplifiers, which are used to provide power to the RF coils. The basic operation of a MOSFET amplifier is used to provide detailed analysis of amplifier behaviour and design. Different types of amplifier are considered, including new developments in current source and low output impedance amplifiers, and the pros and cons of each design discussed.

Chapter 7 provides an outline of the receive chain of the magnetic resonance system. The system is analyzed in terms of minimizing the noise figure of the chain. Specific designs of preamplifiers and quadrature hybrids are outlined. Different forms of data sampling, including the use of undersampling, are discussed as well as the future use of optical and wireless techniques for massively parallel receive systems.

Chapter 8 describes methods and applications of electromagnetic simulations for magnetic resonance. Interactions of the human body with the main magnetic field, magnetic field gradients and electric fields produced by the RF coil are discussed in detail. The combination of electromagnetic simulations with the Bloch equations provides a platform for simulating reconstructed images produced by different imaging sequences.

Andrew Webb