Breast MRI: Fundamentals and Technical Aspects

257 Pages·2008·4.448 MB·English

by  R. Edward Hendrick PhD, FACR (auth.)

Breast MRI
R. Edward Hendrick, PhD, FACR
Breast MRI
Fundamentals and Technical
Aspects
R. Edward Hendrick, PhD, FACR
Research Professor and Director, Breast Imaging Research (retired)
Lynn Sage Comprehensive Breast Center
Northwestern University’s Feinberg School of Medicine
Northwestern Memorial Hospital
Chicago, IL
USA
Library of Congress Control Number: 2007932969
ISBN: 978-0-387-73506-1 e-ISBN: 978-0-387-73507-8
© 2008 Springer Science+Business Media, LLC
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This book is dedicated to my mother,
Enid Winifred Kimes Hendrick Thurston,
who has taught me so much about life,
and
to my wife,
Jean Rachelle Paquelet,
who makes every day worth living.
Foreword
Edward Hendrick has a unique position among physicists. Not only does he under-
stand the physics of breast imaging, but he understands the fundamentals of breast
cancer screening, detection, and diagnosis. I am fairly certain that most who read
this text will be unaware that Dr. Hendrick played a major role in our understanding
of breast cancer screening, particularly the benefit of screening women in their for-
ties. He performed meta-analyses of the data from the randomized, controlled trials
of screening and demonstrated a statistically significant benefit for screening women
ages 40–49 when others were arguing incorrectly that there was no demonstrable
benefit. His work permitted the rest of us to clarify the errors made by others in the
interpretation of the data and it is in large measure due to Dr. Hendrick’s efforts that
life saving mammography screening is available for women in their forties.
Dr. Hendrick played a critical role in the development of quality standards for
mammography, and guided the American College of Radiology Mammography
Accreditation program and the effort to establish quality standards for mammogra-
phy screening. He was a major force in trying to guide the FDA to understand the
burden that unnecessary requirements placed on radiologists.
Dr. Hendrick is the Breast Imager’s physicist. Recognizing the benefit of screen-
ing mammography, he has now turned his attention to the next major advance in
breast evaluation – Magnetic Resonance Imaging (MRI). It seems somewhat
strange to think of MRI as being a new “advance.” Several of us were trying to
apply MRI to the breast twenty years ago when it first became possible to use the
technology for breast evaluation. We soon learned that without a contrast agent, the
relaxation values of normal breast tissue and cancer were not sufficiently different
and MRI lost favor until gadolinium became available. It is not clear why it has
taken so long for breast MRI to come back, but my own belief is that MRI physi-
cists found it easier to work with a more rigid structure like the brain and neurologi-
cal system, and breast MRI had to wait for these applications to become robust
before attention returned to the breast.
There is huge potential for MRI of the breast as we move into an era of
renewed interest. For radiologists it is apparent that it is easier to “detect” a bright
focus of enhancement against a black background (MRI) than it is to try to appre-
ciate a lighter gray structure against a gray background (mammography). The
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huge expense of MRI is daunting when we think of applying it to screening, but
costs can be reduced. Requiring the injection of a substance intravenously is also
a limitation, but, as we have seen with the decision by the American Cancer
Society to endorse MRI screening for high risk women, the opportunity to further
reduce the death rate from the most feared of cancers among women will push the
barriers back and more and more women will be having MR examinations.
Having personally participated in the often heated debates about the merits of
mammography screening, I am well aware of, and a strong supporter of, the
importance of rigorous scientific validation for breast cancer screening tests.
Unlike diagnostic evaluation where the anecdotal experience of the treating physi-
cians may be the only way that decisions can be made and lives saved, screening
involves “healthy” individuals who may be made “ill” by a false positive screen-
ing test. There are breast cancers that if left undetected would never be lethal, and
there are breast cancers that have metastasized even before a new screening test
can find them. If these are the cancers that a new screening study such as MRI
finds, then there will be no benefit, and only “harm”. It is for these reasons that I
would strongly urge large randomized, controlled trials of MRI screening to prove
a benefit. It has been argued that these would be prohibitively expensive. However,
the gradual drift into MR screening of more and more women will be far more
expensive, and the benefit will not be clear.
Regardless of how the use of breast MRI continues to evolve, it will continue
to evolve and expand, and Breast Imaging radiologists need to understand breast
MRI so that the information it provides can be integrated into the care of the
patient along with the results from mammography and ultrasound and other tests
as they are brought to bear on breast problems. I am personally grateful, and we
should all be grateful that having helped us to understand x-ray physics and mam-
mography, Dr. Hendrick has taken on MRI of the breast with equal energy, atten-
tion, and expertise. As he points out, breast MRI provides a major opportunity to
increase our ability to detect early breast cancers, and this will likely translate into
further reductions in the death rate from the most common cancer among
women.
Thank you, Ed, for all of your important contributions.
Dan Kopans, MD, FACR
Professor of Radiology, Harvard Medical School
Founder and Senior Radiologist, Breast Imaging Division,
Massachusetts General Hospital
Boston, Massachusetts
Preface
From years of being both student and teacher, I’ve found that the best way to learn
a new field is one-on-one, with an interested student, a willing teacher, and a pad
of paper between them. As a result, the tone of this book is not that of a didactic
classroom lecture, but an informal exchange between two colleagues. When you
read each chapter, imagine that we are sitting side-by-side and that I am doing my
best to explain the basics of magnetic resonance imaging (MRI) and the essentials
of breast MRI to you. The figures and the occasional formula illustrate the essential
concepts on the pad of paper between us.
An informal, one-on-one approach affords the opportunity to include a little his-
tory and provide a personal emphasis that is often difficult to communicate in the
classroom. Therefore, do not be surprised by the occasional historical aside about
developments in basic physics related to MRI, to the development of MRI, and
breast MRI.
When I told Dr. Dan Kopans that I was writing a book to covey the basics of
MRI to breast imagers, he had three words of advice: “keep it simple”. I have tried
to do that. Of course, Dan never keeps advice to just three words. He went on to say
that the MRI physics explanations had to be simple enough that even he could
understand them. At that point, I enlisted Dan, and a number of other practicing
radiologists, to serve as reviewers of this book, to make sure the level and content
are appropriate. In addition to Dr. Dan Kopans, I want to thank Dr. Jean R. Paquelet,
Dr. Lora Barke, Dr. Eric Berns, and Dr. Richard Pacini for reading and comment-
ing on each of the chapters in this book. Their efforts have made the task easier for
you. Special thanks go to Dr. Frank Shellock for reviewing Chapter 12 on MRI
safety and for providing the patient and personnel screening forms that are reprinted
in the Appendix.
The one thing I ask of you as you start this book is that you give the story a
chance to unfold. MRI is not a one-act play. It requires learning a number of
fundamentals and then putting them all together before you can experience the
breakthrough of understanding how MRI really works. I will try to minimize
the unnecessary, extraneous facts, and will try to focus on the essentials that
prepare you for that breakthrough.
One of the essentials of learning MRI is to equip yourself with models that con-
vey a better intuitive feeling for how MRI actually works. This is certainly done in
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other texts, but this book places special emphasis on giving you pictorial models for
the essential aspects of MRI. Hence, there will be more pictures and graphs that you
probably want to take the time to look at, but each picture will be helpful in putting
it all together to understand how MRI and breast MRI work.
My goal for this book is that when you finish the fundamentals, Chapters 1–7,
you will understand how MRI works. At that point, you should have a clear under-
standing of why T2-weighted pulse sequences make cystic lesions bright without
contrast and why T1-weighted imaging makes lesions bright with contrast.
Moreover, you should be beginning to arm yourself with the tools you need to
understand the intricacies of MR pulse sequences. If you already know all this, then
skip to Chapter 8 for the start of breast MRI. My goal with the second part of the
book is that by the time you get to Chapter 11, you will have a good idea of how
breast MRI works and how you can best perform breast MRI in your own practice,
including the selection of breast protocols. By the end of Chapter 11, you should
know the differences between good and bad breast MRI and how you can maximize
image quality with your current equipment.
Chapter 12 is on MR safety, with an emphasis on breast imaging. Chapter 13
focuses on newer techniques that may help improve the sensitivity and specificity
of breast MRI. This section describes some of the new techniques being developed
that might make breast MRI (and possibly MRS) that rare examination that has
both high sensitivity and high specificity for breast cancer.
Finally, I would appreciate your feedback on this book. I’m sure that even after
careful review and editing, it won’t be free of errors or perfectly clear to everyone
who reads it. If you see ways that I can correct or improve the book, please let
me know by e-mail at: [email protected]. If you like certain aspects
of the book, I would appreciate hearing about that, too. Both forms of feedback
complete the one-on-one student-teacher relationship, where neither person is
entirely student or entirely teacher, and both benefit from the experience. I sincerely
hope that you do.
Chicago, Illinois R. Edward Hendrick, PhD, FACR
Contents
Foreword by Dan Kopans. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Preface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix
1 Fundamentals of Magnetic Resonance Imaging. . . . . . . . . . . . . . . . . . . 1
Subatomic Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
The Atom. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Magnetic Dipole Moments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Nuclear Magnetic Moments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Tissue Magnetization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Magnetic Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Precession and Magnetic Resonance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Tissue Excitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Measuring the Magnetic Resonance Signal. . . . . . . . . . . . . . . . . . . . . . . . . 15
The Basic Nuclear Magnetic Resonance Experiment . . . . . . . . . . . . . . . . . 16
Chapter Take-home Points. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2 Tissue Relaxation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Nuclear Magnetic Resonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
T1 Relaxation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
T2 Relaxation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Distinguishing T2 and T2* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
The Physical Basis of Relaxation Times . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Chapter Take-home Points. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3 Spatial Resolution in Magnetic Resonance Imaging. . . . . . . . . . . . . . . . 31
Basic Components of a Magnetic Resonance Imaging System. . . . . . . . . . 31
Magnets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Radiofrequency Transmitter Coils. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
xi