Essentials of MRI Safety. Donald W. McRobbie

      Foreword: essentials

      In recent years MRI safety has received increased media attention amidst growing public concern. From controversies over the use and side‐effects of gadolinium to projectile accidents, a serious adverse MRI event always makes news. It doesn’t have to. It is possible to practice completely without incident, as long as you understand the essence of MRI safety.

      I started my career at the University of Aberdeen investigating the biological effects of magnetic fields because, even back in 1981, the safety of MRI was already on the scientific agenda. Since then there have been innumerable technical and clinical advances, but some of our MRI safety practice has not developed at the same rate. Often departmental practices and individual beliefs (I can’t say “knowledge”) are based more upon historical custom and hearsay than upon science. This book aims to place MRI safety practice firmly on a scientific basis, equipping you to recognise and assess potential risk and also its absence.

      What about all the maths? Surely we don’t need it? We do. It is essential because MRI safety without a solid theoretical and numerical foundation is just guesswork, folklore or worse. If you are allergic to maths, then you may be relieved to learn that all the difficult material is contained in Chapter 2 and the appendices. The other chapters contain the minimum of theory necessary to attain an understanding of each topic, and much of the maths is restricted to illustrative numerical examples. I have avoided complex electromagnetic (EM) modeling and complex algebra. All the examples can be solved approximately using simple “back of an envelope” calculations. Whilst this lacks the rigor or accuracy of EM modeling or solving Maxwell’s equations, it provides insight into how physical parameters relate to safety issues. Examples are purely illustrative, but should be sufficient for readers to identify and make estimates of risk.

      This book is also written in the context of the MRI safety roles: MR Medical Director (MRMD), MR Safety Officer (MRSO), and MR Safety Expert (MRSE). In order to develop the required knowledge for these roles, each chapter contains revision questions for self‐assessment, one hundred in total. Beware: some of the questions have multiple correct answers. For the aspiring MRSE every chapter, including the appendices, is essential. For the other roles and for the general readership of MR practitioners more selective reading is possible. Use of the Further reading and resources at the end of each chapter is highly recommended and, in some instances, essential as they contain authoritative sources, reviews, guidance, and standards.

      Finally, a word of caution and a disclaimer. This book does not provide definitive guidance on the scanning of individual patients with specific implants. For that you must adhere to the most current implant information and observe the MRI conditions, conducting benefit versus risk analyses if required, and with appropriate clinical authorization. Neither does the book endorse any specific practice over and above adhering to MRI implant conditions, pharmaceutical labelling, available professional body guidance, standards, and exposure limits.

      Acknowledgments

      I would like to thank colleagues and friends who have helped me on this MRI safety journey, with the kind provision of materials, or with the production of this book. These include Gregory Brown, Anastasia Papadaki, Frank Shellock, Marc Agzarian, Jeff Hand, Stephen Keevil, Denise Newsom, Adam Waldman, Catriona Todd, Amrish Mehta, Frank de Vocht, Kristel Schaap, Nick Ferris, Emanuel Kanal, Titti Owman, Miles Capstick, Niels Kuster, Kjell Hansson‐Mild, John Powell, Thomas Vaughan, Wellesley Were, Kelly Parker, Daniel Zappia, David Price, Marc Rea, Rebecca Quest, Irving Weinberg, Gary Liney, Anneke Cox, Dawn Phillips‐Jarrett, Julian Byrne, Jessica Driscoll, Celine Duraffourd, Eike Davis, Antoine Daridon, Helen Estall, Colin Robertson, Nicola Baker, Dan Krainak, Simon Grant, Zoltan Nagy, Eugenio Mattei, Kawin Setsompop, Stephen Wastling. My thanks also go to James Watson, Tom Marriott, Baskar Anandraj and Sonali Melwani at Wiley, Mary Malin at Transtype for copy‐editing, and to the team at Wiley for bringing the book to production. Finally I am indebted to my original University of Aberdeen supervisors from whom I learned so much and who set me on this journey: Meg Foster and the late Jim Hutchison in whose memory the book is dedicated. As ever my gratitude to Kathryn, Laura, and Andrew for encouragement and support.

      In memoria James Hutchison 1940–2018

1 Systems and safety: MR hardware and fields

      INTRODUCTION

      Magnetic resonance imaging (MRI) has grown, from its initial development in the late 1970s to early 1980s, to become one of the most utilized diagnostic imaging modalities. In 2015 there were 103 million MR examinations performed in hospitals from a population of 1.1 billion people in 29 developed countries. A total of 33 000 scanners were in use in 36 countries serving a combined population of 1.7 billion [1].

The two greatest advantages of MRI are its superior soft tissue discrimination compared to X‐rays or CT, and a lack of exposure to ionizing radiation. MRI uses a combination of magnetic fields of varying frequencies: radiofrequency in the megahertz (MHz) region; audio or “very low frequencies” (VLF) up to tens of kilohertz (kHz); and a static field (zero hertz). None of these possesses sufficiently localized concentrations of electromagnetic (EM) energy to damage atoms, molecules, or cells (Figure 1.1). The risk of cancer induction from magnetic field exposures encountered in MRI is quite possibly zero – unlike X‐rays, CT, mammography, or the radioactive tracers used in nuclear medicine. This makes MRI very attractive for serial examinations, for scans of children whose tissues are more sensitive to the ionizing radiation used in alternative modalities, or for research studies on groups of healthy volunteers.

Figure 1.1 The electromagnetic spectrum showing frequency and wavelength of radiations, relative scale, and applications.

      So, is MRI safe?

      Obviously not, or there would be no need for this book. Whilst later chapters will show that MRI is relatively benign from a biological point of view, the practice of MRI may involve significant risk to the patient and to others present during the examination. The MRI examination room is potentially the most hazardous environment within the radiology department because of the possibility of catastrophic and fatal accidents where practice is poor or where safety protocols are not fully observed or understood.

Nowhere is this better illustrated than in the tragic case of a six‐year old boy who in 2001 was struck by an oxygen tank which had flown into the scanner, later dying from his injuries. This prompted a root and branches review of MR safety practice within the USA by the American College of Radiology [2] leading to a series of recommendations. It is concerning, that even today, not all these recommendations are routinely followed in every institution. In a 10‐year review of MRI‐related incidents reported to the US Food and Drug Authority

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