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Library of Congress Cataloging-in-Publication Data
Westbrook, Catherine.
MRI at a glance / Catherine Westbrook. — Third edition.
pages cm
Includes index.
ISBN 978-1-119-05355-2 (pbk.)
1. Magnetic resonance imaging.Outlines, syllabi, etc. 2. Medical physics.Outlines, syllabi, etc.
I. Title.
RC78.7.N83W4795 2016
616.0-548.dc23
2015022541
A catalogue record for this book is available from the British Library.
Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books.
Cover image: © Getty Images/Yuji Sakai
MRI at a Glance is one of a series of books that presents complex information in an easily accessible format. This series has become famous for its concise text and clear diagrams, which are laid out with text on one page and diagrams relating to the text on the opposite page. In this way all the information on a particular topic is summarized so that the reader has the essential points at their fingertips.
The third edition has been updated with a new companion website that includes some exciting new features. In the book, some chapters have been streamlined and reorganized and there are some updated images and diagrams. Each topic is presented on two pages for easy reference and large subjects have been broken down into smaller sections. In the book and companion website I have included simple explanations and animations, analogies, bulleted lists, simple tables, key points, equations (but only for those who like them), scan tips, ‘Did You Know’ learning points, some questions and answers and plenty of images to aid the understanding of each topic. There are appendices on tradeoffs, acronyms, abbreviations and artefacts. The glossary has also been expanded.
This book is intended to provide a concise overview of essential facts for revision purposes and for those very new to MRI. For more detailed explanations the reader is directed to MRI in Practice and Handbook of MRI Technique. Indeed, the diagrams and images in this book are taken from these other texts and MRI at a Glance is intended to complement them.
Learning MRI physics can be hard work. I hope that this book helps to demystify it!
Once again I thank my friend and colleague John Talbot for his beautiful diagrams and for his support. We make a great team and long may it continue! Thanks again to Philips Medical Systems and GE for supplying the images, and to all my friends and family in Brighton, London, Paris, Witney, Leeds, St Augustine, Atlanta and New York.
CW
Each topic is presented in a double-page spread with clear, easy-to-follow diagrams supported by succlnct explanatory text.
Key Point boxes highlight points to remember.
Your textbook id full of photographs, illustrations and tables.
The website icon indicates that you can find accompanying resources on the book's companion website.
The magnetic susceptibility of a substance is the ability of external magnetic fields to affect the nuclei of a particular atom, and is related to the electron configurations of that atom. The nucleus of an atom, which is surrounded by paired electrons, is more protected from, and unaffected by, the external magnetic field than the nucleus of an atom with unpaired electrons. There are three types of magnetic susceptibility: paramagnetism, diamagnetism and ferromagnetism.
Paramagnetic substances contain unpaired electrons within the atom that induce a small magnetic field about themselves known as the magnetic moment. With no external magnetic field, these magnetic moments occur in a random pattern and cancel each other out. In the presence of an external magnetic field, paramagnetic substances align with the direction of the field and so the magnetic moments add together. Paramagnetic substances affect external magnetic fields in a positive way, resulting in a local increase in the magnetic field (Figure 1.1). An example of a paramagnetic substance is oxygen.
Super-paramagnetic substances have a positive susceptibility that is greater than that exhibited by paramagnetic substances, but less than that of ferromagnetic materials. Examples of a super-paramagnetic substance are iron oxide contrast agents.
With no external magnetic field present, diamagnetic substances show no net magnetic moment, as the electron currents caused by their motions add to zero. When an external magnetic field is applied, diamagnetic substances show a small magnetic moment that opposes the applied field. Substances of this type are therefore slightly repelled by the magnetic field and have negative magnetic susceptibilities (Figure 1.2). Examples of diamagnetic substances include water and inert gasses.
When a ferromagnetic substance comes into contact with a magnetic field, the results are strong attraction and alignment. They retain their magnetization even when the external magnetic field has been removed. Ferromagnetic substances remain magnetic, are permanently magnetized and subsequently become permanent magnets. An example of a ferromagnetic substance is iron.
Magnets are bipolar as they have two poles, north and south. The magnetic field exerted by them produces magnetic field lines or lines of force running from the magnetic south to the north poles of the magnet (Figure 1.3). They are called magnetic lines of flux. The number of lines per unit area is called the magnetic flux density. The strength of the magnetic field, expressed by the notation (B) – or, in the case of more than one field, the primary field (B0) and the secondary field (B1) – is measured in one of three units: gauss (G), kilogauss (kG) and tesla (T). If two magnets are brought close together, there are forces of attraction and repulsion between them depending on the orientation of their poles relative to each other. Like poles repel and opposite poles attract.
A magnetic field is generated by a moving charge (electrical current). The direction of the magnetic field can either be clockwise or counter-clockwise with respect to the direction of flow of the current. Ampere’s law or Fleming’s right-hand rule determines the magnitude and direction of the magnetic field due to a current; if you point your right thumb along the direction of the current, then the magnetic field points along the direction of the curled fingers (Figure 1.4).
Just as moving electrical charge generates magnetic fields, changing magnetic fields generate electric currents. When a magnet is moved in and out of a closed circuit, an oscillating current is produced, which ceases the moment the magnet stops moving. Such a current is called an induced electric current (Figure 1.5).
Faraday’s law of induction explains the phenomenon of an induced current. The change of magnetic flux through a closed circuit induces an electromotive force (emf) in the circuit. The emf is defined as the energy available from a unit of charge travelling once around a loop of wire. The emf drives a current in the circuit and is the result of a changing magnetic field inducing an electric field.
The laws of electromagnetic induction (Faraday) state that the induced emf:
Electromagnetic induction is a basic physical phenomenon of MRI, but is specifically involved in the following:
The key points of this chapter are summarized in Table 1.2.
Atoms make up all matter in the universe and also therefore in the human body. There are approximately 7 octillion (7 × 1027) atoms in the average 70 kg person. Most of the human body (96%) is made up of just four elements. These are hydrogen, oxygen, carbon and nitrogen. Hydrogen is the most common element in the universe and in humans.
The atom consists of the following particles:
Protons
Neutrons
Electrons
The following terms are used to characterize an atom:
Atoms of the same element having a different mass number are called isotopes. In a stable atom the number of negatively charged electrons equals the number of positively charged protons. Atoms with a deficit or excess number of electrons are called ions and the process of removing electrons from the atom is called ionization. Only certain types of atoms are available to us in Magnetic Resonance Imaging (MRI). These are atoms whose charged nuclei move or spin. This is because a moving electrical charge produces a magnetic field (see Chapter 1).
There are three types of motion of particles in the atom:
Each type of motion produces a magnetic field (see Chapter 1). In MRI we are concerned with the motion of particles within the nucleus and the nucleus itself.
Protons and neutrons spin about their own axis within the nucleus. The direction of spin is random, so that some particles spin clockwise and others anticlockwise.
When a nucleus has an even mass number, the spins cancel each other out so the nucleus has no net spin.
When a nucleus has an odd mass number, the spins do not cancel each other out and the nucleus spins.
As protons have charge, a nucleus with an odd mass number has a net charge as well as a net spin. Due to the laws of electromagnetic induction (see Chapter 1), a moving unbalanced charge induces a magnetic field around itself. The direction and size of the magnetic field are denoted by a magnetic moment (Figure 2.2). The total magnetic moment of the nucleus is the vector sum of all the magnetic moments of protons in the nucleus. The length of the arrow represents the magnitude of the magnetic moment. The direction of the arrow denotes the direction of alignment of the magnetic moment.
Nuclei with an odd number of protons are said to be MR active. They act like tiny bar magnets. There are many types of elements that are MR active. They all have an odd mass number. The common MR active nuclei, together with their mass numbers, are:
hydrogen 1 | carbon 13 | nitrogen 15 |
fluorine 19 | sodium 23 | oxygen 17 |
The spin characteristics of the commonest MR active nuclei are shown in Table 2.1.
The isotope of hydrogen called protium is the MR active nucleus used in MRI, as it has a mass and atomic number of 1. The nucleus of this isotope consists of a single proton and has no neutrons. It is used for MR imaging because:
In the rest of this book MR active nuclei, and specifically protium, are referred to as spins.
The key points of this chapter are summarized in Table 2.2.
Access the MCQs relating to this chapter on the book’s companion website at www.ataglanceseries.com/mri