Mayo Clinic Guide to Cardiac Magnetic Resonance Imaging [2 ed.] 9780199941186, 2014030936

Table of contents :
Cover
Series
Mayo Clinic Guide to Cardiac Magnetic Resonance Imaging
Copyright
Dedication
Contents
Contributors
Section I Basic Principles of Cardiac MRI
1 ECG Gating and Associated Artifacts
2 Cardiac Anatomy
3 Standard Imaging Planes in Cardiac MR Imaging
4 Pulse Sequence Basics
5 Modular Cardiac MR Imaging Protocols
Section II Clinical Applications and Case Studies
6 Myocardial Ischemia and Infarction
7 Nonischemic Myocardial Disease
8 Pericardial Disease
9 Cardiac Masses
10 Congenital Disease
11 Valvular Heart Disease
Section III Troubleshooting—MRI Artifacts and Safety
12 Common MR Imaging Artifacts
13 Cardiac MRI Safety
Section IV Review Questions
14 Board-Type Questions and Answers
Index

Citation preview

M AYO C L I N I C G U I D E T O C A R D I AC M AG N E T I C R E S O N A N C E I M AG I N G SECOND EDIT ION

MAYO CLINIC SCIENTIFIC PRESS Mayo Clinic Atlas of Regional Anesthesia and Ultrasound-Guided Nerve Blockade Edited by James R. Hebl, MD, and Robert L. Lennon, DO Mayo Clinic Preventive Medicine and Public Health Board Review Edited by Prathibha Varkey, MBBS, MPH, MHPE Mayo Clinic Challenging Images for Pulmonary Board Review Edited by Edward C. Rosenow III, MD Mayo Clinic Infectious Diseases Board Review Edited by Zelalem Temesgen, MD Mayo Clinic Antimicrobial Handbook: Quick Guide, Second Edition Edited by John W. Wilson, MD, and Lynn L. Estes, PharmD Just Enough Physiology By James R. Munis, MD, PhD Mayo Clinic Cardiology: Concise Textbook, Fourth Edition Edited by Joseph G. Murphy, MD, and Margaret A. Lloyd, MD Mayo Clinic Internal Medicine Board Review, Tenth Edition Edited by Robert D. Ficalora, MD Mayo Clinic Internal Medicine Board Review: Questions and Answers Edited by Robert D. Ficalora, MD Mayo Clinic Electrophysiology Manual Edited by Samuel J. Asirvatham, MD Mayo Clinic Gastrointestinal Imaging Review, Second Edition By C. Daniel Johnson, MD Arrhythmias in Women: Diagnosis and Management Edited by Yong-Mei Cha, MD, Margaret A. Lloyd, MD, and Ulrika M. Birgersdotter-Green, MD Mayo Clinic Body MRI Case Review By Christine U. Lee, MD, PhD, and James F. Glockner, MD, PhD Mayo Clinic Gastroenterology and Hepatology Board Review, Fifth Edition Edited by Stephen C. Hauser, MD

M AYO CLI NIC GUIDE TO CA R DI AC M AGNETIC R ESONA NCE I M AGI NG SECOND EDITION

Editors Kiaran P. McGee, PhD

Eric E. Williamson, MD

C O N S U LTA N T, D E PA RT M E N T O F R A D I O L O G Y

CH A I R , DI V I S ION OF C A R DIOVA S C U L A R R A DIOL O G Y

M AY O C L I N I C , R O C H E S T E R , M I N N E S O TA

M AY O C L I N I C , R O C H E S T E R , M I N N E S O TA

A S S O C I AT E P R O F E S S O R O F M E D I C A L P H Y S I C S A N D

ASSOCIATE PROFESSOR OF R ADIOLOGY

A S S I S TA N T P R O F E S S O R O F B I O M E D I C A L E N G I N E E R I N G

M AY O C L I N I C C O L L E G E O F M E D I C I N E

M AY O C L I N I C C O L L E G E O F M E D I C I N E

Matthew W. Martinez, MD D I R E C T O R , S P O RT S C A R D I O L O G Y & H Y P E RT R O P H I C C A R D I O M Y O PAT H Y P R O G R A M S A N D C A R D I O VA S C U L A R I M A G I N G , L E H I G H VA L L E Y H E A LT H N E T W O R K , A L L E N T O W N , P E N N S Y LVA N I A ; A S S O C I AT E P R O F E S S O R O F M E D I C I N E , M O R S A N I C O L L E G E O F M E D I C I N E , U N I V E R S I T Y O F S O U T H F L O R I D A , TA M PA , F L O R I D A F O R M E R LY, A S S I S TA N T P R O F E S S O R O F C A R D I O VA S C U L A R D I S E A S E S M AY O C L I N I C C O L L E G E O F M E D I C I N E

MAYO CLINIC SCIENTIFIC PRESS

OXFORD UNIVERSITY PRESS

The triple-shield Mayo logo and the words MAYO, MAYO CLINIC, and MAYO CLINIC SCIENTIFIC PRESS are marks of Mayo Foundation for Medical Education and Research.

3 Oxford University Press is a department of the University of Oxford. It furthers the University’s objective of excellence in research, scholarship, and education by publishing worldwide. Oxford  New York Auckland  Cape Town  Dar es Salaam  Hong Kong  Karachi  Kuala Lumpur Madrid Melbourne Mexico City Nairobi  New Delhi  Shanghai  Taipei  Toronto  With offices in Argentina Austria Brazil  Chile Czech Republic France Greece  Guatemala Hungary Italy Japan Poland Portugal Singapore  South Korea Switzerland Thailand Turkey Ukraine Vietnam Oxford is a registered trademark of Oxford University Press in the UK and certain other countries. Published in the United States of America by Oxford University Press 198 Madison Avenue, New York, NY 10016

© 2008, 2015 by Mayo Foundation for Medical Education and Research. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior permission of Mayo Foundation for Medical Education and Research. Inquiries should be addressed to Scientific Publications, Plummer 10, Mayo Clinic, 200 First St SW, Rochester, MN 55905. You must not circulate this work in any other form and you must impose this same condition on any acquirer. Library of Congress Cataloging-in-Publication Data Mayo Clinic guide to cardiac magnetic resonance imaging / [edited by] Kiaran P. McGee, Eric E. Williamson, Matthew W. Martinez.—Second edition.   p. ; cm. Guide to cardiac magnetic resonance imaging Includes bibliographical references and index. ISBN 978–0–19–994118–6 (alk. paper) I.  McGee, Kiaran P., editor.  II.  Williamson, Eric E., editor.  III.  Martinez, Matthew W., editor.  IV.  Mayo Clinic, issuing body.  V.  Title: Guide to cardiac magnetic resonance imaging. [DNLM:  1.  Heart Diseases—diagnosis.  2.  Magnetic Resonance Imaging—methods.  WG 141.5.M2] RC683.5.E5 616.1′207547—dc23 2014030936 Mayo Foundation does not endorse any particular products or services, and the reference to any products or services in this book is for informational purposes only and should not be taken as an endorsement by the authors or Mayo Foundation. Care has been taken to confirm the accuracy of the information presented and to describe generally accepted practices. However, the authors, editors, and publisher are not responsible for errors or omissions or for any consequences from application of the information in this book and make no warranty, express or implied, with respect to the contents of the publication. This book should not be relied on apart from the advice of a qualified health care provider. The authors, editors, and publisher have exerted efforts to ensure that drug selection and dosage set forth in this text are in accordance with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, readers are urged to check the package insert for each drug for any change in indications and dosage and for added wordings and precautions. This is particularly important when the recommended agent is a new or infrequently employed drug. Some drugs and medical devices presented in this publication have US Food and Drug Administration (FDA) clearance for limited use in restricted research settings. It is the responsibility of the health care providers to ascertain the FDA status of each drug or device planned for use in their clinical practice. 9 8 7 6 5 4 3 2 1 Printed in the United States of America on acid-free paper

Kiaran P. McGee, PhD To Cora, whose faith, courage, perseverance, and strength of character inspire me, motivate me, and fill me with admiration every day. Keep the faith, fight the good fight, and never give up.—Dad Eric E. Williamson, MD To my parents, Bryn and Anita, for teaching me the value of high expectations. Matthew W. Martinez, MD To my wife, Anna, whose strength and continued support in all aspects of our life together are unmatched. Nothing matters until I share it with you.

FOR EWOR D

This second edition of the Mayo Clinic Guide to Cardiac Magnetic Resonance Imaging is written as a practical guide, designed to provide truly useful information to radiologists, along with other physicians, fellows, residents, technologists, and medical physicists. The book will be at home beside the magnetic resonance imaging (MRI) scanner console, in the reading room, and on the desk of anyone training in the art of cardiac MRI. Since the publication of the first edition, this book has undergone major revision and updating, including an expansion from 4 to 14 chapters. New chapters covering anatomy, imaging protocols, and MRI safety have been included. Another new feature of the second edition is the inclusion of 120 board-style multiple-choice questions and answers. More than 60 clinical case studies are included and illustrated with hundreds of clinical images. Topics of the case studies include myocardial ischemia and infarction, nonischemic myocardial disease, pericardial disease, cardiac masses, and valvular disease. The second edition clearly reflects the growth of this remarkable modality. Within Mayo Clinic, clinical cardiac MRI patient volumes have grown in excess of 10% each year during the past decade. Growth has been driven by new capabilities stemming from engineering advances such as

high-speed gradients, multiple-channel receive coil technology, adaptation of gadolinium-based contrast agents, parallel imaging techniques, and advanced 2-dimensional and 3-dimensional imaging sequences that provide anatomic and functional information unique to MRI. At Mayo Clinic, the steady growth of this modality is also a testament to visionary radiologists such as Dr. Paul Julsrud and Dr. Jerome Breen, who saw the potential of this nascent technology long before it attained its present capabilities. Finally, this book reflects the remarkable and synergistic power resulting from the collaboration between clinical imaging physicians, medical physicists, and engineers. It can truly be said that cardiac MRI, unlike other medical imaging modalities, is an imaging technology that was invented by its users. The steady pace of invention is well illustrated in this second edition. I highly recommend this book to all.

Richard L. Ehman, MD Consultant, Department of Radiology, Mayo Clinic; Professor of Radiology Mayo Clinic College of Medicine

vii

PR EFACE

The Mayo Clinic Guide to Cardiac Magnetic Resonance Imag­ ing, Second Edition, represents a major expansion of the first edition of this text. Not only has the format of the text changed, but new sections and chapters have been added to keep this work current and topical. In keeping with the highly clinical focus of this text, additional case studies and examples have been provided to expand the clinical section of the text. This section includes a medical history for each case, the indication for performing the cardiac magnetic resonance imaging (MRI) examination, and a discussion of the imaging findings from each study. In addition, 120 questions, answers, and corresponding explanations are provided in the final section of the text. The objective of this section is to assist in learning of the sometimes complex and challenging field of cardiac MRI. The overall focus of the text remains unchanged, which is to provide a practical handbook for cardiac MRI that will be useful for physicians at all phases of their professional careers, technologists both new and experienced, and medical physicists involved in clinical cardiac MRI. We would be remiss if we did not recognize our colleagues who contributed both directly and indirectly to the content of this text. We would like to identify, among the professional staff of Mayo Clinic, Ron Kuzo, MD, Nandan Anavekar, MD, Thomas Foley, MD, Philip Araoz, MD, Terri Vrtiska, MD, James Glockner, MD, PhD, Phillip Young, MD, Robert Watson, MD, PhD, Nila Akhtar, MD, and Ethany Cullen, MD, for all of their input and assistance. We would also like to recognize our gifted and talented team of MRI technologists who help to “keep us honest” on a daily basis. Finally, we thank the members of our 3-dimensional imaging laboratory and our colleague and friend David Larson for all of their hard work in ensuring that we provide the best imaging services to our patients and colleagues. As cardiac MRI continues to grow, we hope that this text will contribute to the dissemination of knowledge necessary to perform complex, and at times challenging, MRI examinations of the heart, increase the knowledge base of all practitioners, and facilitate the growth and dissemination of cardiac MRI beyond the boundaries of academic medicine into the broader community. Finally, we would like to recognize the landmark contributions to the development of cardiovascular MRI, not only within Mayo Clinic but worldwide, made by our good friends and colleagues Paul Julsrud, MD, and Jerome Breen, MD.

Paul and Jerry, this book is testimony to your leadership and hard work, and we thank you. I N T RODUC T ION TO T H E S E C O N D E DI T ION OF T H E M AYO C L I N I C G U I DE TO C A R DI AC M AG N ET I C R E S O N A NC E I M AGI NG This book is designed to meet the needs of 3 distinct yet integrated groups, namely MR technologists, clinicians, and MRI scientists and physicists. To achieve this goal, this book is divided into 4 separate sections. Section I  provides information for MRI technologists who wish to acquire consistent high-quality cardiac MR images. This includes an overview of electrocardiographic gating, standard anatomy of the heart, methods for acquiring standard imaging planes of the heart, pulse sequences used for cardiac MRI, and modular imaging protocols for the most common indications for a cardiac MRI examination. Section II provides case studies covering 6 broad disease categories representing the most common indications for cardiac MRI. Within each disease category, numerous clinical cases are presented, beginning with a description of the initial indication for cardiac MRI, followed by the recommended imaging protocol, identification of imaging findings, and a summary of these findings within the context of the identified disease. The target audience of this section includes clinicians in training (residents and fellows), as well as those involved in academic or routine clinical practice. Section III provides technical information relevant to MRI scientists and physicists involved in cardiac MRI. Emphasis is placed on common imaging artifacts and methods to either reduce or eliminate them, as well as a description of basic MRI safety as it pertains to cardiac MRI. Finally, Section IV provides “board-type” questions and answers designed to test the knowledge of clinicians involved in cardiac MRI. Examination questions cover a range of topics including MRI physics, anatomy and physiology, and identification of key imaging findings of diseases described throughout the text. Kiaran P. McGee, PhD Eric E. Williamson, MD Matthew W. Martinez, MD

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CONT ENTS

Contributors xiii

9. Cardiac Masses Ethany L. Cullen, MD, and Philip A. Araoz, MD 10. Congenital Disease Nandan S. Anavekar, MB, BCh, and Paul R. Julsrud, MD 11. Valvular Heart Disease James F. Glockner, MD, PhD

S E C T ION I B A S IC PR I N C I PL E S OF C A R D I AC M R I 1. ECG Gating and Associated Artifacts Kiaran P. McGee, PhD, and Matthew W. Martinez, MD 2. Cardiac Anatomy Matthew W. Martinez, MD 3. Standard Imaging Planes in Cardiac MR Imaging Matthew W. Martinez, MD, Eric E. Williamson, MD, and Kiaran P. McGee, PhD 4. Pulse Sequence Basics Kiaran P. McGee, PhD, and Matthew A. Bernstein, PhD 5. Modular Cardiac MR Imaging Protocols Phillip M. Young, MD, Eric E. Williamson, MD, and James F. Glockner, MD

3

183 203

16 S E C T ION I I I T ROU B L E S H O O T I N G —M R I A RT I FAC T S A N D SA FET Y

27 45

12. Common MR Imaging Artifacts 231 Kiaran P. McGee, PhD, and Matthew A. Bernstein, PhD 13. Cardiac MRI Safety 246 Kiaran P. McGee, PhD, Robert E. Watson Jr, MD, PhD, and Eric E. Williamson, MD

74

S E C T ION I I C L I N IC A L A PPL IC AT IO N S A N D CA SE ST U DI E S 6. Myocardial Ischemia and Infarction Thomas A. Foley, MD 7. Nonischemic Myocardial Disease Nila J. Akhtar, MD, Matthew W. Martinez, MD, and Eric E. Williamson, MD 8. Pericardial Disease Phillip M. Young, MD

163

S E C T ION I V R E V I E W QU E S T IO N S 83 105 143

xi

14. Board-Type Questions and Answers Kiaran P. McGee, PhD, Matthew W. Martinez, MD, and Eric E. Williamson, MD

259

Index 

313

CONT R I BUTOR S

Nila J. Akhtar, MD Senior Associate Consultant, Department of Radiology, Mayo Clinic, Rochester, Minnesota; Assistant Professor, Mayo Clinic College of Medicine

Paul R. Julsrud, MD Supplemental Consultant, Department of Radiology, Mayo Clinic, Rochester, Minnesota; Professor of Radiology, Mayo Clinic College of Medicine

Nandan S. Anavekar, MB, BCh Senior Associate Consultant, Division of Cardiovascular Diseases, Mayo Clinic, Rochester, Minnesota; Assistant Professor of Medicine, Mayo Clinic College of Medicine

Matthew W. Martinez, MD Director, Sports Cardiology & Hypertrophic Cardiomyopathy Programs and Cardiovascular Imaging, Lehigh Valley Health Network, Allentown, Pennsylvania; Associate Professor of Medicine, Morsani College of Medicine, University of South Florida, Tampa, Florida Formerly, Assistant Professor of Cardiovascular Diseases, Mayo Clinic College of Medicine

Philip A. Araoz, MD Consultant, Department of Radiology, Mayo Clinic, Rochester, Minnesota; Professor of Radiology, Mayo Clinic College of Medicine

Kiaran P. McGee, PhD Consultant, Department of Radiology, Mayo Clinic, Rochester, Minnesota; Associate Professor of Medical Physics and Assistant Professor of Biomedical Engineering, Mayo Clinic College of Medicine

Matthew A. Bernstein, PhD Consultant, Department of Radiology, Mayo Clinic, Rochester, Minnesota; Professor of Medical Physics, Mayo Clinic College of Medicine Ethany L. Cullen, MD Physician, Mayo Clinic Health System—Austin, Austin, Minnesota

Robert E. Watson Jr, MD, PhD Chair, Division of Neuroradiology, Mayo Clinic, Rochester, Minnesota; Assistant Professor of Radiology, Mayo Clinic College of Medicine

Thomas A. Foley, MD Consultant, Division of Diagnostic Radiology, Mayo Clinic, Rochester, Minnesota; Assistant Professor of Radiology, Mayo Clinic College of Medicine

Eric E. Williamson, MD Chair, Division of Cardiovascular Radiology, Mayo Clinic, Rochester, Minnesota; Associate Professor of Radiology, Mayo Clinic College of Medicine

James F. Glockner, MD, PhD Consultant, Division of Diagnostic Radiology, Mayo Clinic, Rochester, Minnesota; Assistant Professor of Radiology, Mayo Clinic College of Medicine

Phillip M. Young, MD Chair, Division of Body Magnetic Resonance Imaging, Mayo Clinic, Rochester, Minnesota; Associate Professor of Radiology, Mayo Clinic College of Medicine

xiii

Cardiac MR Acronyms a M ANUFACTUR ER IM AGING COMPONENT & TER M

SIEMENS MEDICAL SOLUTIONS

GE HEALTHCAR E

PHILIPS MEDICAL SYSTEMS

Magnetization Preparation Phase contrast Chemical fat saturation Tagging Flow compensation Inversion recovery Phase-sensitive inversion recovery T2 signal preparation T1 signal preparation

PC Fat Sat Tagging Flow comp, GMR IR, TIR PSIR T2-Prep MOLLI, T1-Scout

PC Fat Sat, Chem Sat Tagging Flow comp IR, MPIR, FastIR PSIR

PC SPIR, SPAIR Tagging Flow comp IR-TSE PSIR

GRE FLASH TurboFLASH MPRAGE, 3D FLASH VIBE

GRE SPGR FGRE, FSPGR 3D FGRE, 3D FSPGR FAME, LAVA

FFE T1-FFE TFE 3D TFE THRIVE

True FISP FISP PSIF SE TurboGSE, TGSE

FIESTA GRASS SSFP SE GRASE

BFFE FE T2-FFE SE GRASE

HASTE TSE, RARE Turbo factor

SSFSE FSE ETL

SS TSE TSE Turbo factor

DIXON EPI TWIST NATIVE-trueFISP

IDEAL EPI TRICKS Inhance Inflow IR

mDIXON EPI TRACS B-TRANCE

mSENSE GRAPPA

ASSET ARC

SENSE

2D 3D Static Cine

2D 3D Static Cine

2D 3D Static Cine

CineIR

Echo Formation Gradient echo Spoiled Gradient-recalled echo Spoiled GRE Fast gradient echo 3D Fast gradient echo Volume-interpreted GRE Steady state Balanced SSFP SSFP–FID SSFP–echo Spin echo Gradient and spin echo Data Acquisition Single-shot spin echo Multishot (echo train) spin echo Number of echoes in spin-echo echo train Iterative Dixon fat-water separation Echo planar imaging Time-resolved MR angiography Balanced SSFP–based noncontrast MR angiography Rapid Imaging Image based k-Space based Imaging Mode Two dimensions Three dimensions Single image Multiframe image

Abbreviations: ARC, autocalibrating reconstruction for Cartesian imaging; ASSET, array spatial sensitivity encoding technique; B-TRANCE, balanced-SSFP–triggered angiography, non–contrast enhanced; ETL, echo train length; FAME, fast acquisition with multiple excitation; FE, field echo; FFE, fast-field echo; FID, free induction decay; FIESTA, fast imaging employing steady-state acquisition; FISP, fast imaging with steady precession; FLASH, fast low angle shot; FSE, fast spin echo; GMR, gradient moment recalled; GRAPPA, generalized autocalibrating partially parallel acquisition; GRASE, gradient and spin echo; GRASS, gradient-recalled acquisition in the steady state; HASTE, half Fourier-acquired single-shot turbo spin echo; IDEAL, iterative decomposition of water and fat with echo asymmetry and least-squares estimation; IR, inversion recovery; LAVA, liver acquisition with volume acceleration; MOLLI, modified Look-Locker imaging; MPIR, multiplanar inversion recovery; MPRAGE, magnetization prepared rapid acquired gradient echoes; MR, magnetic resonance; mSENSE, modified sensitivity encoding; NATIVE, noncontrast MR angiography of arteries and veins; PSIF, reversed fast imaging with SSFP; RARE, rapid acquisition with relaxation enhancement; SENSE, sensitivity encoding; SPAIR, special attenuation with inversion recovery; SPGR, spoiled gradient-recalled echo; SPIR, spectral attenuation with inversion recovery; SSFP, steady-state free precession; TFE, turbo field echo; TGSE, turbo gradient spin echo; THRIVE, T1 high-resolution isotropic volume estimation; TIR, turbo inversion recovery; TRACS, time-resolved angiography using CENTRA (contrast-enhanced timing robust angiography) and SENSE; TRICKS, time-resolved imaging of contrast kinetics; TSE, turbo spin echo; TWIST, time-resolved angiography with interleaved stochastic trajectories; VIBE, volumetric interpolated breath-hold examination. a

Imaging terms and corresponding acronyms used by different MR scanner manufacturers.

SEC T ION I BA SIC PR I NCI PL E S OF C A R DI AC M R I

1. ECG GATING A ND ASSOCI ATED ARTIFACTS Kiaran P. McGee, PhD, and Matthew W. Martinez, MD

I

n routine clinical cardiac magnetic resonance (MR) imaging, most morphologic (ie, static) and functional (ie, cine) imaging involves the use of segmented data acquisition methods that are synchronized with the patient’s electrocardiographic (ECG) waveform. The process of synchronizing data acquisition with the ECG waveform of the patient is known as ECG gating. The purpose of this chapter is to provide an overview of the importance of ECG gating, describe the relationship between a normal ECG waveform and the appearance of the heart on MR imaging (MRI) throughout the cardiac cycle, and provide an overview of methods for ensuring robust ECG gating in the MR environment. Finally, different types of artifacts introduced into the ECG waveform by the MR environment and methods to reduce them will be discussed.

collected will reflect a given phase of the cardiac cycle, with data acquired shortly after the R wave, reflecting systole, and data acquired toward the end of the R-R interval, reflecting diastole. Irregular heart rates caused by atrial fibrillation or frequent premature ventricular contractions will result in data inconsistencies, with some data collected and arranged into the wrong phase of the cardiac cycle. Collection of data in this manner often results in degradation of image quality and production of artifacts, which in turn can decrease the reproducibility of measurements derived from these images. The most common reason for failure of a cardiac MR examination is the inability to synchronize data acquisition with the correct phase of the cardiac cycle. Because the ECG waveform is the method of synchronization, ensuring that a robust and reliable waveform is achieved for each patient is essential to success (Box 1.1).

S E G M E N T E D DATA AC QU I S I T ION A N D E C G G AT I NG

E V E N T S OF T H E C A R DI AC C YC L E (L E F T H E A RT )

In most cases, cardiac MR data are not acquired in a continuous fashion. Rather, only a fraction or segment of the total image data is acquired at one time (ie, at a given phase of the cardiac cycle). The R wave of the ECG waveform is usually detected first (the trigger point), because it is typically the most prominent and reproducible feature of the ECG tracing, followed by the acquisition of a given segment of data after a predetermined delay. To complete the imaging process, data are acquired over multiple cardiac cycles (ie, R-R intervals); this is most commonly referred to as segmented data acquisition. Segmented data can be acquired both prospectively and retrospectively. If data acquisition is prospective, the MR scanner will begin collecting image data after a predetermined delay, known as the trigger delay. If retrospective data collection is performed, data are acquired continuously throughout the cardiac cycle, and the trigger point is used to determine in which portion of the cardiac cycle the data were acquired. After enough data are collected to reconstruct 1 or more images, as in the case of a cine series, data are retrospectively sorted according to their temporal location within the cardiac cycle, as determined from the time between the ECG trigger and data acquisition. If the heart rate is regular, all the data

The heart consists of 2 separate but interrelated systems that control the distribution throughout and return of blood from the body. The left side of the heart (“left heart”) is the functional unit that regulates the systemic circulation of blood throughout the body, while venous return of blood is governed by the pulmonary circulation and therefore controlled by the right side of the heart (“right heart”). Accurate and reproducible assessment of cardiac function is important; therefore, a thorough understanding of the interrelationship between

Box 1.1  TAKE-HOME POINTS—WHY USE ECG GATING IN MR STUDIES? •

To enable accurate temporal sampling of data throughout the cardiac cycle, which is necessary for cine acquisitions • To allow acquisition of static images at a given phase of the cardiac cycle • To allow correct sorting of static and dynamic data obtained over multiple R-R intervals using segmented acquisition schemes

3

4  •   S ection I :   B asic P rinciples of C ardiac M R I

the various cardiac events is also important. Figure 1.1 shows the physiologic events of the left heart throughout the cardiac cycle. The relationships of pressure, volume, and flow to the electrical potential of the heart as a whole (as measured by ECG) are shown. Note that the percentage values shown for the time intervals of systole (40%) and diastole (60%) are

valid only for the given heart rate of 70 beats per minute. As heart rate increases, the systolic time interval remains relatively unchanged, while the diastolic interval decreases. This results in a percentage increase in systole and a percentage decrease in diastole. Consequently, end diastole is the most variable phase of the cardiac cycle.

Figure 1.1  Important cardiac physiologic waveforms during the cardiac cycle. The bottom 3 traces show the pressure, volume, and flow curves

within the left-sided cardiac chambers throughout the cardiac cycle, correlated with the ECG waveform at the top. EDV indicates end-diastolic volume; ESV, end-systolic volume; HR, heart rate. (Modified from Oh JK, Seward JB, Tajik AJ. The echo manual. 3rd ed. Philadelphia: Lippincott Williams & Wilkins; c2006. Used with permission of Mayo Foundation for Medical Education and Research.)

1.  E C G G ating and A ssociated A rtifacts  •   5

M R I C H A R AC T E R I S T IC S OF T H E C A R DI AC C YC L E Figure  1.2 shows the relationship between the electrical activity of the heart as a whole and the corresponding MR images acquired as part of a cine imaging series in both the 4-chamber long-axis and 2-chamber short-axis views of the left ventricle. Typical cine sequences acquire 20 images corresponding to fixed time points or phases of the cardiac cycle. In this figure, only 10 images for both views are reproduced, representing every even or odd phase of the cardiac cycle. Viewed as a dynamic display, these cine loops simulate real-time imaging and are used to interpret the contractility of the heart. The figure also shows enlarged 4- and 2-chamber views of the heart at end systole (red outline) and end diastole (blue outline). At end systole, the cavity of the left ventricle is smallest, with maximal thickness of the myocardium; at end diastole,

the ventricle is most relaxed, with maximal chamber volume and minimal myocardial wall thickness. M E A S U R I N G T H E H E A RT ’ S E L E C T R IC A L AC T I V I T Y:   T H E E I N T HOV E N L E A D A R R A NGEM EN T The heart’s electrical activity was first assessed in 1889 by Augustus Desiré Waller, who measured the electrical potential difference (voltage) across electrode pairs placed at 5 separate anatomical locations (2 on the arms, 2 on the legs, and 1 at the mouth). Waller’s method of measuring the time-varying voltage signals across various electrode pairs (lead combinations) is commonly referred to as the electrocardiogram, or ECG. The original configuration suggested by Waller was modified by

Figure 1.2  Four-chamber long-axis (top row) and 2-chamber short-axis (bottom row) cine series corresponding to the electrical potential trace of a

cardiac cycle. Enlarged MR images at bottom are those acquired at end systole (red outline) and end diastole (blue outline).

6  •   S ection I :   B asic P rinciples of C ardiac M R I

Willem Einthoven in 1908. The so-called Einthoven configuration or Einthoven’s triangle required placement of electrodes at 3 locations; the left and right arms and the left leg (Figure 1.3). The differences in electrical potential between electrodes are known as limb leads I  (potential difference between left and right arms), II (potential difference between right arm and left leg), and III (potential difference between left arm and left leg). The Einthoven diagram also shows the electrical model of the heart—a dipole (positive and negative charge separated in space) whose magnitude and orientation vary throughout the cardiac cycle. This model is also known as the vector ECG (VCG). Measurement of the time-varying voltage of leads I, II, or III is the mathematical equivalent of the projection of the VCG onto the respective lead. This projection produces the characteristic ECG waveform (Figure 1.4). This complex waveform can be decomposed into separate waveforms that describe the various phases of the cardiac cycle. Key components of the ECG waveform include the P wave, which

signifies the onset of atrial depolarization, the QRS complex, which represents ventricular depolarization, and finally the T wave, which indicates ventricular repolarization. PAT I E N T PR E PA R AT ION F OR G AT I N G I N M R I ECG gating in MRI involves identification of the appropriate anatomical locations for placement of the ECG electrodes, followed by adequate preparation of the patient’s skin surface. Identification of the appropriate lead locations is described in more detail in the next section, but 3 rules should be observed: 1) Lead locations should be placed as far as possible from the sternum, particularly if the patient has had prior cardiothoracic surgery and sternal wires are present.

Figure 1.3  Three-lead ECG configuration and Einthoven’s triangle. (From Malmivuo J, Plonsey R. Bioelectromagnetism: principles and

applications of bioelectric and biomagnetic fields. New York: Oxford University Press; c1995. Used with permission.)

1.  E C G G ating and A ssociated A rtifacts  •   7

Figure 1.4  ECG and components of the cardiac cycle. (From Bernstein MA, et al. Handbook of MRI pulse sequences. Amsterdam: Elsevier

Academic Press; c2004. Used with permission.)

2) For women with large, pendulous breasts, an alternative location along the chest wall and inferior to the breast should be identified. Large amounts of breast tissue can attenuate the ECG signal. 3) Leads should be placed on the anterior chest surface as opposed to the posterior. The heart is generally located closer to the anterior chest wall, and the ECG voltage is generally greater there. After identification of the appropriate lead locations, the patient’s skin surface should be prepared. For men, this typically requires shaving the chest around the region of the electrodes. Caution should be exhibited if this procedure is performed in the MR scan room because most disposable razors use steel blades that can represent a projectile hazard if brought too close to the MR scanner. Commercial abrasive gels are available to remove keratinized skin from the epidermis; this reduces the electrical impedance of the skin and thereby creates a better electrical contact between the skin and electrode. Fine-grit sandpaper also can be used but is generally not recommended. The use of a gel has the added advantage of cleaning the skin’s surface to remove oil and dirt. When the appropriate electrode locations are identified and the skin has been prepared, the electrodes can be applied. MR-compatible electrodes should be used at all times. The vendor of the MR scanner provides the appropriate type of electrode either directly or through a third-party supplier.

E C G L E A D PL AC E M E N T A major contributor to a prolonged cardiac examination is the amount of time spent attempting to optimize lead placement either immediately before or during the cardiac MR examination. The Division of Cardiac Radiology at Mayo Clinic has developed several lead-placement configurations (“the Mayo configuration”) that hold the most promise for gating success. A second lead-placement set is also recommended when the MR scanner is equipped with VCG gating. M AYO C ON F IGU R AT ION

Figure 1.5 describes the 4 lead placement configurations recommended by Mayo Clinic. The Mayo lead placements are not optimized on the basis of electrophysiologic models of the ideal patient; rather, they represent the most successful configurations derived from extensive experience with various placement iterations. M R-I N DUC E D E C G A RT I FAC T S Figure 1.6 shows an ECG waveform from lead II for a healthy volunteer. Unfortunately, the MR environment is particularly hostile for ECG measurement. Table  1.1 lists some of the more common sources of interference that exist in a typical MR environment, along with their amplitude and frequency ranges.

8  •   S ection I :   B asic P rinciples of C ardiac M R I

Figure 1.5  The Mayo Configuration. The 4 recommended configurations for ECG lead placement for male and female patients. Electrode locations

are variations of the precordial locations used in a 12-lead ECG. Lead color coding: black, left arm; green, right leg; red, left leg; white, right arm.

M AG N E TOH Y DRODY N A M IC E F F E C T

Blood is composed of formed elements (45% by volume) and plasma. Plasma is composed of approximately 90% solvent and 10% solute. The solvent is water and the solute comprises salts (sodium, potassium, calcium, magnesium, chloride, and bicarbonate), plasma proteins (albumin, fibrinogen, and globulins), and other substances (nutrients, waste products, respiratory gases, and hormones). When dissolved, the salts in the plasma disassociate into ions and, left alone, do not contribute to the ECG signal. However, when in motion

and in the presence of a magnetic field, they will experience an external Lorentz force. Charge separation occurs, inducing a time-varying electrical dipole signal that is a function of the velocity and direction of flow within the field of the MR scanner. This dipole is detected as a time-varying electrical signal superimposed onto the ECG waveform. This second electrical signal precedes ventricular contraction and consequently is detected after the QRS complex of the ECG waveform. Ventricular repolarization, as described by the ST segment, is detected at approximately the same time

1.  E C G G ating and A ssociated A rtifacts  •   9

Figure 1.6  ECG waveform from lead II from a normal (healthy) volunteer.

as the flow of blood through the aorta and, hence, is often superimposed onto the T wave of the ECG. This is most commonly known as T-wave swelling (or T-wave elevation) and can cause the T wave to be of even greater amplitude than the R wave. T-wave swelling often results in unreliable triggering and poor image quality. Figure 1.7 shows a normal ECG waveform of a volunteer in the absence of an external magnetic field (ie, non-MR environment) and in the presence of a large external magnetic field (ie, inside an MR scanner).

Effect on ECG Waveform Increase in amplitude of the ST segment of the QRST complex, also known as T-wave swelling.

Effect on MR Data Acquisition If ECG triggering is based on the peak voltage of the waveform, the scanner could trigger off the T wave or switch between the R-wave and T-wave peaks. Detection-peak

Table 1.1  SOURCES OF ECG NOISE IN MR IMAGING INDUCED ELECTR ICAL VOLTAGE (TYPICAL R ANGE)

SOURCE

FR EQUENCY SPECTRUM (TYPICAL R ANGE)

ECG reference signal

0.2–3 mV

0.05–100 Hz

Magnetohydrodynamic effect

Several mV; can be greater than the ECG waveform