Table Of ContentMRI for Radiographers
Springer
Berlin
Heidelberg
New York
Barcelona
Budapest
Hong Kong
London
Milan
Paris
Tokyo
Philip T. English and Christine Moore
MRI for Radiographers
With 175 Figures
Springer
Philip T. English, DCR
Superintendent Radiographer, Neuroradiology, Newcastle General Hospital, Westgate Road,
Newcastle upon Tyne NE4 6BE, UK
Christine Moore, DCR
Senior Radiographer, Neuroradiology, Newcastle General Hospital,
Westgate Road, Newcastle upon Tyne NE4 6BE, UK
ISBN-13: 978-1-4471-3405-3 e-ISBN-13: 978-1-4471-3403-9
DOl: 10.1007/978-1-4471-3403-9
British Library Cataloguing in Publication Data
English, Philip T.
MRI for Radiographers
I. Title II. Moore, Christine
616'()7548
ISBN-13: 978-1-4471-3405-3
Library of Congress Cataloging-in-Publication Data
English, Philip T., 1961-
MRI for radiographersl Philip T. English and Christine Moore.
Includes bibliographical references and index.
ISBN-13: 978-1-4471-3405-3
1. Magnetic resonance imaging. I. Moore, ChrIStine, 1952-
RC78.7.N83E5781994
616.07'548--dc20
DNLM/DLC 94-41660
for Library of Congress CII'
Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the
Copynght, Designs and Patents Act 1988, thiS publication may only be reproduced, stored or transmitted, in any fonn or
by any means, with the prior pemlission in writing of the publishers, or in the case of reprographic reproduction in accor
dance with the terms of licences issued by the Copyright Licensing Agency. Enqlllries concerning reproduction outside
those ternls should be sent to the publishers.
© Springer-Verlag London Limited 1995
Softcover reprint of the hardcover 1st edition 1995
The use of registered names, trademarks, etc. in this publication does not imply, even in the absence of a speCific state
ment, that such names are exempt from the relevant laws and regulations and therefore free for general use.
Product liability: The publisher can give no guarantee for information about drug dosage and application thereof con
tamed in this book. In every individual case the respective lIser must chcck its accuracy by consulting other pharnJaceuti
cal literature.
Typeset by EXPO Holdings
28/3830-543210 Printed on acid-free paper
Preface
One of the most important developments in diagnostic imaging over the last decade has been
magnetic resonance imaging (MRI). Its ability to differentiate between tissues and give patholog
ical information about diseases has led to earlier treatment, thus increasing the likelihood of
recovery. The images produced using this technique give superb anatomical detail in any plane
and are obtained without the use of ionising radiation.
The increased use of MRI has presented radiographers with a number of challenges, and
because we are no longer dealing with ionising radiation understanding the subject can some
times be confusing. We hope that this text will help radiographers and student radiographers to
further their knowledge and unravel the mysteries of MRI.
Philip T. English
Christine Moore
Contents
1 Basic Principles . . . . . . . . . 1
History ..... . 1
Atomic Theory . . 1
Magnetic Theory . 2
Resonance 4
Relaxation. . . . . 5
2 Instrumentation........ 9
The Magnet .. 9
Shim Coils ........... . 12
Gradient Coils ......... . 13
RF Transmitter/Receiver Coils. 14
The Computer . . . . . . . . . 18
3 Pulse Sequences. . . . . . . . . 19
Saturation Recovery (Partial Saturation) 19
Spin Echo (SE) . . . . . . . . . . . . . 20
Multiple Spin Echo. . . . . . . . . . . 22
Fast Spin Echo (FSE) or Turbo Spin Echo (TSE). 23
Inversion Recovery (IR) . . . . . . . . . . . . . 26
Gradient Echo .................. . 28
Magnetisation Transfer Contrast Imaging (MTC) 34
4 Image Production. . . . . . . . 37
Spatial Location of the MR Image 37
Slice Selection Excitation. 37
Slice Position 38
Slice Profile. . . . . . . . 38
5 Image Quality . 45
Signal-to-Noise Ratio (SNR) and Contrast-to-Noise Ratio (CNR). 46
Other Factors Contributing to Image Quality 48
Quality Assurance in MRI . . . . . . . . . . . . . . 48
6 Image Artifacts . . . . . . . . . 51
Magnetic Field Artifacts. . . 51
Hardware-Related Artifacts. 54
Motion-Related Artifacts .. 58
V111 Contents
Pulse Sequence/Data Acquisition Artifacts . 66
Other Artifacts . . . . . . . . . . . . . . . 67
7 Flow ..... 71
Haemodynamics. ....... . 71
Appearance of Flow on MR Images 72
Magnetic Resonance Angiography (MRA). 75
Measurement of Flow. . . . . . . . . . . . 82
8 MR Contrast Agents. . . . . . . . . . . . ....... 83
Basic Principles of a Contrast Agent's Actions. . ....... 83
Paramagnetic Agents . 83
Ferromagnetic Agents. . . . . . . . 87
9 Safety and Patient Management 89
Safety ....... . 89
Patient Management 97
10 Site Planning. 99
Location .... 99
The Magnet .. 99
The Fringe Field · 100
RF Shielding . . · 101
Equipment ... .102
Patient and Staff Requirements .103
Anaesthesia in MRI ..... . · 103
11 Applications and Pathology. . .105
The Brain. . . . . 106
The Spine. . . . . 123
Brachial Plexus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
Head and Neck . 135
Musculoskeletal System . 137
Long Bones. . 150
Abdomen. . . . 156
The Pelvis . 160
Breast Imaging . 164
Chest and Thorax . 166
Cardiovascular System . 166
Paediatric Imaging . . . 173
Suggested Reading .175
Glossary ofMR Terms. .177
Subject Index .185
1 Basic Principles
History electron. Protons and neutrons are found within the
nucleus of the atom and are called collectively "nucle
ons". The electrons orbit the nucleus at various differ
Magnets were first described by the Romans over ent energy levels (or shells). Fig. 1.1 is a diagrammatic
2000 years ago. Over the centuries, as man's under representation of a neon atom showing how the elec
standing of them has increased, so their applications trons orbit the nucleus in energy shells.
have developed. In the late 1940s two independent It can be seen from Table 1.1 that the proton and
groups of scientists led by Felix Bloch (at Stanford) and neutron both have a relative mass of 1 but that the
Edward Purcell (at Harvard) discovered magnetic reso electron has negligible mass. The mass number of an
nance in liquids and solids respectively. These discov element is the total number of protons and neutrons
eries led to them being jointly awarded the Nobel
Prize in 1952.
Magnetic resonance (MR) was initially used for
Table 1.1. RelatIve masses and charges of protons, neutrons and
spectroscopy, and in 1967 MR spectroscopy was used
electrons
to produce MR signals in a live animal. In the early
1970s Paul Lautebur of the State University of New Particle Relative mass Relative charge
York produced two-dimensional proton images of a
Proton 1 +1
water sample. This work soon led to imaging of other
Neutron 1 0
samples, such as lemons and peppers, then live ammals Electron 1/1837 -1
and ultimately humans. Magnetic resonance imaging
(MRI) was born. Initially the technique was referred
to as nucle1r magnetic resonance (NMR), but the
"nuclear" part of the title had "bad" connotations for
the general public and MR.I has become the accepted
term within the medical profession.
Atomic Theory
Before considering the more complex theory of mag
Orbiting
netic resonance we will review a few of the basic prin
Electron
ciples behind the phenomenon.
All matter is composed of atoms, a different atom @=
Nucleus
corresponding to each chemical clement. Under the
correct circumstances clements combine to form com
pounds: for example, sodium and chlorine can com
bine to form sodium chloride (common salt).
Fig. 1.1. Diagrammatic representation of a neon atom showing
Atoms contain three fundamental particles: elec
the relationships between the nucleus (composed of 10 protons and
trons, protons and neutrons. The exception is hydro 10 neutrons) and the electrons. The 10 electrons orbit the nucleus in
gen, which has a single proton and one orbiting two energy levels or shells: the K shell and the L shell.
2 Basic Principles
within its nucleus: magnetic field around themselves. As previously
described all atomic nuclei (except for hydrogen) con
Mass number = no. of protons + no. of neutrons
tain both protons and neutrons. When there is an
or Mass number = no. of nucleons imbalance between the number of protons and the
number of neutrons the nucleus will spin upon its axis,
It can also be seen from Table 1.1 that protons have a
and the spinning of the positively charged protons
positive charge and electrons have a negative charge. It
within the nucleus creates a magnetic field. This field
follows, then, that in order to keep the atom neutral
is too weak to have any effect on bulk matter and
the number of protons must equal the number of elec
requires very sensitive instruments to detect it. This
trons.
instrumentation has been developed in the field of
The atomic number of an element is the number of
magnetic resonance. Table 1.2 lists some of the ele
protons within its nucleus:
ments occurring naturally in biological tissue that pos
Atomic number = no. of protons sess these characteristics.
Seventy-five per cent of the human body consists
The atomic number is also equal to the number of
of water, and therefore it is hydrogen which is the
electrons orbiting the nucleus. The atomic number
most abundant element in the body. In addition to
determines the chemical properties of that element.
being a constituent of water, hydrogen is also found in
lipids. The hydrogen nucleus is particularly susceptible
Magnetic Theory
Table 1.2. Elements found 111 the human body that possess mag
netlc resonance charactenstics
Atomic Magnetism
Element Symbol Nuclear arrangement AtomIC no.
The negatively charged electrons orbiting the nucleus Hydrogen H 1 proton
spin on their axis in one of two directions: clockwise Carbon C 6 protons 7 neutrons 13
Fluorine F 9 protons 10 ncutrons 19
or anticlockwise. In a non-magnetic element there are
Sodium Na 11 protons 12 neutrons 23
the same number of electrons spinning in each direc Phosphorus P 15 protons 16 neutrons 31
tion. In a magnetic element there is an imbalance, Potassium K 19 protons 20 neutrons 39
with more electrons spinning in one direction than the
other. It is this imbalance that creates the potential for
magnetism.
An iron atom, for example, has 26 orbiting elec
trons around its nucleus: 2 electrons in the first (K)
shell, 8 electrons in the second (L) shell, 14 electrons
in the third (M) shell and 2 electrons in the fourth (N)
shell. In the K, Land N shells there are the same
number of electrons spinning clockwise as anticlock
wise. In the M shell, however, there are 9 electrons
spinning in one direction and only 5 spinning in the
other, creating an imbalance which gives iron the
potential for magnetism.
The magnetic potential of an element can be
realised when it is placed in the vicinity of an external
s
magnetic field. The external field encourages the N
atoms in the clement to collect in domains, creating
magnetism.
The SI unit for magnetic field strength is the tesla
(symbol T). Previously field strength was measured in
gauss (1 T = 10 000 gauss).
s
Nuclear Magnetism
Fig. 1.2. The hydrogcn nucleus spinning upon its axis creates a
As well as electrons creating magnetism in bulk matter magnetic moment through its axis which is directly comparoble to a
the nuclei of certain atoms are able to create a small bar Inagnct.
Magnetic Theory 3
Bo
Fig. 1.3a,b The onentation of the mag
netIC moments of the hydrogen protons in
the absence (a) and presence (b) of an
a b
extemally applied magnetIc fIeld (Bo).
to magnetic resonance and its abundance makes it per its axis its magnetic moment precesses around the main
fect for MRI. Although other elements can be imaged magnetic field (Fig. 1.4). This precession is similar to
it is hydrogen imaging that will be considered for the the motion of a spinning top, which spins around its
rest of the text. axis until the speed of the spin slows, allowing gravity
The spinning hydrogen proton can be thought of as to cause it to precess around the direction of the
a tiny bar magnet with two poles (Fig. 1.2). Each pro earth's gravitational field until eventually it falls over.
ton generates a magnetic moment, which is a physical In MR the spinning nucleus precesses in response to
vector (represented as an arrow) parallel to its axis of the main magnetic field (Bo).
rotation that describes the magnetic field strength and The rate of precession of protons in a magnetic field
its direction. is characteristic for that element; it also depends upon
In a neutral environment the billions of hydrogen the strength of the magnetic field the element is placed
protons in tissues have their magnetic moments ran in. This relationship can be expressed as:
domly orientated (Fig. 1.3a). Placed in a strong mag =
Wo -yBo
netic field, such as a magnetic resonance scanner,
however, they align themselves in one of two direc where Wo is the angular frequency (precessional or
tions: "parallel" or "antiparallel" to the field (Fig. Larmor frequency), -y is the magnetogyric ratio (which
1.3b), with a slight majority in the parallel direction. depends on the type of nucleus considered) and Bo is
The protons which are parallel to the main magnetic the magnetic field strength. The equation is known as
field (i.e. aligned towards it) are said to be at a low
energy state, whilst those antiparallel to it are said to
be at a higher energy state (having sufficient energy to Bo
oppose the magnetic field). The actual energy differ
ence between the two states is very small and is
dependent upon the field strength. '~,
/
Thermal energy produced by the various physical / \
and chemical processes within the body causes the two
energy states to be almost equally populated. In fact
for every 1 million atoms antiparallel to the main field
there will be approximately 1 million and six that are
parallel! It is this small population difference which
gives rise to an overall (or net) magnetisation in the
direction of the external main magnetic field (Bo) that
is detected in MRI. This is known as longitudinal
magnetisation.
As shown in Fig. 1.3 the spinning protons do not
Fig. 1.4. Diagrammatic representation of the precession of a
align themselves exactly with the main magnetic field hydrogen nucleus around an extemally applied main magnetic fIeld
but more at an angle to it. As the nucleus spins upon (Bo).
