Get Quick Access to Cardiac MRI Techniques
high-quality images!
Williamson
• A review of the most common cardiac MR imaging planes
More than
McGee
This compact guide to cardiac magnetic resonance imaging incorporates the most common techniques with easy-tofollow step-by-step protocols. Physicians and technicians alike get quick access to the information they need at the point of exam. Features include:
200
with step-by-step protocols
imaging protocols and example cases
• A review of the basic physics of cardiac MRI, including pulse sequences and ECG gating, as well as common imaging artifacts and how to prevent them. This easy-to-use reference is the most practical guide for accessing information on all stages of the cardiac MRI exam, from graphical prescription and protocol selection to imaging troubleshooting and interpretation. ABOUT THE AUTHORS KIARAN P. McGEE is Consultant, Department of Radiology, Mayo Clinic, Rochester, Minnesota; Assistant Professor of Biomedical Engineering and Radiologic Physics, College of Medicine, Mayo Clinic. ERIC E. WILLIAMSON is Consultant, Department of Radiology, Mayo Clinic, Rochester, Minnesota; Assistant Professor of Radiology, College of Medicine, Mayo Clinic. PAUL R. JULSRUD is Consultant, Department of Radiology, Mayo Clinic, Rochester, Minnesota; Professor of Radiology, College of Medicine, Mayo Clinic.
Mayo Clinic Guide to Cardiac Magnetic Resonance Imaging
• Descriptions of the most common indications for cardiac MRI, along with typical
Julsrud
• An overview of the various physiologic events that make up the cardiac cycle
Mayo Clinic Guide to Cardiac Magnetic Resonance Imaging
Kiaran P. McGee, PhD Eric E. Williamson, MD Paul R. Julsrud, MD MAYO CLINIC SCIENTIFIC PRESS
Mayo Clinic Guide to Cardiac Magnetic Resonance Imaging
Mayo Clinic Guide to Cardiac Magnetic Resonance Imaging
Editors Kiaran P. McGee, PhD Eric E. Williamson, MD Paul R. Julsrud, MD MAYO CLINIC SCIENTIFIC PRESS AND INFORMA HEALTHCARE USA, INC.
ISBN-13: 978-1-4200-8303-3 Printed in Canada 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. ©2008 Mayo Foundation for Medical Education and Research. All rights reserved. This book is protected by copyright. No part of it may be reproduced, stored in a retrieval system, or transmitted, in any form by any means—electronic, mechanical, photocopying, recording, or otherwise—without the prior written consent of the copyright holder, except for brief quotations embodied in critical articles and reviews. Inquiries should be addressed to Scientific Publications, Plummer 10, Mayo Clinic, 200 First Street SW, Rochester, MN 55905. For order inquiries, contact: Informa Healthcare, Kentucky Distribution Center, 7625 Empire Drive, Florence, KY 41042 USA. E-mail:
[email protected]; Web site: www.informahealthcare.com Library of Congress Cataloging-in-Publication Data Mayo Clinic guide to cardiac magnetic resonance imaging/edited by Kiaran P. McGee, Eric E. Williamson, Paul Julsrud. p. ; cm. Includes bibliographical references and index. ISBN-13: 978-1-4200-8303-3 (pb : alk. paper) ISBN-10: 1-4200-8303-1 (pb : alk. paper) 1. Heart--Magnetic resonance imaging. I. McGee, Kiaran P. II. Williamson, Eric E. III. Julsrud, Paul. IV. Mayo Clinic. V. Title: Guide to cardiac magnetic resonance imaging. [DNLM: 1. Heart Diseases--diagnosis. 2. Magnetic Resonance Imaging--methods. WG 141.5.M2 M473 2008] RC683.5.M35M39 2008 616.1'207548--dc22
2008005368
Nothing in this publication implies that Mayo Foundation endorses any of the products mentioned in this book. 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 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 selections and dosages set forth in this text are in accordance with current recommendations and practices 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 warnings and precautions. This precaution is particularly important when the recommended agent is a new or infrequently used drug. Some drugs and medical devices presented in this publication have U.S. Food and Drug Administration (FDA) clearance for limited use in restricted research settings. It is the responsibility of health care providers to ascertain the FDA status of each drug or device that they plan to use in their clinical practice.
DEDICATION Kiaran P. McGee, PhD To those profound sources of love, joy, and happiness: Nancy, Jeff, Max, Cora, & B.V.M.
Eric E. Williamson, MD To my parents, Byrn and Anita–for teaching me the value of high expectations.
Paul R. Julsrud, MD To my parents for their unconditional love; to my wife and children for their continued support and affection; and to Drs. Richard Van Pragh, Kenneth Fellows, and Ivar Enge for their mentorship and for being extraordinary role models for my career.
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FOREWORD
In 2003, magnetic resonance imaging was formally recognized as one of the most important advances in modern medicine by the awarding of the Nobel Prize in Physiology or Medicine jointly to Paul C. Lauterbur and Sir Peter Mansfield. Although this recognition was a “coming of age” for the field of MRI, in many respects cardiac MRI is still in its infancy, in part because of the unique technical challenges of acquiring an MR image of an organ that changes in size, shape, and location. Despite these difficulties, technological developments in MR scanner hardware and software, as well as postprocessing techniques, have allowed cardiac MRI to become more available and practical in nonacademic institutions such as community hospitals and outpatient imaging centers. What is needed to expand this trend is the ability to reliably and reproducibly perform cardiac MRI examinations in those settings. The Mayo Clinic Guide to Cardiac Magnetic Resonance Imaging is designed to facilitate the dissemination of cardiac MRI from academic centers into the broader MR community. The book’s content is designed to serve multiple groups: technologists, clinicians, and clinical medical physicists. Its organization is such that any institution should be able to rapidly develop their own program without having to seek out a variety of medical and technical texts. Technologists are the target audience of the first chapter, being the ones to acquire the actual cardiac MR data. In chapters 2 and 3, the text focuses on providing clinicians with suggested imaging protocols, along with clinical examples of the actual imaging indication. Finally, chapter 4 provides a more technical review of the underlying physical principles of the various imaging sequences used in cardiac MRI. Although this final section is quite technical, the authors have provided a general overview of the various concepts that encompass the physics of cardiac MRI. I highly recommend this text to experts and novices alike.
Jerome F. Breen, MD Chair, Division of Cardiac Radiology Mayo Clinic
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PREFACE
Over the course of the past decade, cardiac magnetic resonance imaging has developed from a boutique imaging modality to the gold standard for assessment of various functional and morphologic cardiac diseases. Based on the experience within our own institution in training physicians, technologists, and physicists alike, it became apparent that there was an unmet need for a practical “users’ guide” to cardiac MR imaging. We also quickly appreciated that such a text would not only be useful in training within our own walls but also could serve as a reference guide for others interested in establishing a cardiac MR imaging program. This was the motivation for creating a practical handbook for cardiac MR imaging. The handbook is intended to serve three basic purposes: 1) to develop a standard methodology to assist the user in prescribing MR sequences in the commonly used cardiac imaging planes, 2) to provide a set of imaging protocols that address the most common indications for which a cardiac MR exam would be ordered, and 3) to give the user a basic overview of the physical principles of cardiac MR imaging, including electrocardiographic gating, pulse sequence design and types, and typical imaging artifacts and strategies to correct them. In many respects, this work is incomplete in that it represents a broad snapshot of the landscape of cardiac MR in 2008 AD. However, it is our hope that the information contained within these pages will contribute to the growth and dissemination of cardiac MR imaging beyond the boundaries of academic medicine into the broader community.
Kiaran P. McGee, PhD Eric E. Williamson, MD Paul R. Julsrud, MD
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ACKNOWLEDGMENTS The Latin phrase “Quasi nanos, gigantium humeris insidentes, ut possimus plura eis et remotiora videre (We are like dwarfs on the shoulders of giants, so that we can see more than they)” aptly describes those pioneering individuals to whom we owe a debt of gratitude. Without their efforts and contributions this work would not be possible. In particular, we recognize Jerome F. Breen, MD, Joel P. Felmlee, PhD, and Richard L. Ehman, MD, whose diligence and hard work have developed a clinical practice that is the genesis of this text. We also acknowledge all of those colleagues, both internal and external to our institution, who are too numerous to identify individually but whose cumulative efforts have contributed to the growth of cardiac MR.
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PRODUCTION STAFF Mayo Clinic Section of Scientific Publications Alyssa C. Biorn, PhD Roberta Schwartz Kristin M. Nett Ann M. Ihrke
Editor Production editor Editorial assistant Copy editor/proofreader
Mayo Clinic Section of Illustration and Design Karen E. Barrie David T. Smyrk Robert R. Morreale James E. Rownd Deborah A. Veerkamp Kevin M. Youel
Art director Medical animator Medical illustrator Commercial illustrator Presentation designer Presentation designer
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AUTHOR AFFILIATIONS Philip A. Araoz, MD Consultant, Department of Radiology, Mayo Clinic, Rochester, Minnesota; Assistant Professor of Radiology, College of Medicine, Mayo Clinic Matt A. Bernstein, PhD Consultant, Department of Radiology, Mayo Clinic, Rochester, Minnesota; Professor of Radiologic Physics, College of Medicine, Mayo Clinic James F. Glockner, MD, PhD Consultant, Department of Radiology, Mayo Clinic, Rochester, Minnesota; Assistant Professor of Radiology, College of Medicine, Mayo Clinic Paul R. Julsrud, MD Consultant, Department of Radiology, Mayo Clinic, Rochester, Minnesota; Professor of Radiology, College of Medicine, Mayo Clinic Jacobo Kirsch, MD Fellow in Cardiac Imaging, Mayo School of Graduate Medical Education, and an Instructor of Radiology, College of Medicine, Mayo Clinic, Rochester, Minnesota. Present address: Radiology Institute, Cleveland Clinic, Weston, Florida. Kiaran P. McGee, PhD Consultant, Department of Radiology, Mayo Clinic, Rochester, Minnesota; Assistant Professor of Biomedical Engineering and Radiologic Physics, College of Medicine, Mayo Clinic Eric E. Williamson, MD Consultant, Department of Radiology, Mayo Clinic, Rochester, Minnesota; Assistant Professor of Radiology, College of Medicine, Mayo Clinic
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TABLE OF CONTENTS PREFACE
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ix
LIST OF ABBREVIATIONS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xix
CHAPTER 1 Cardiac Anatomy and MR Imaging Planes
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
CHAPTER 2 Cardiac Physiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29 CHAPTER 3 Clinical Indications and Sample Imaging Protocols With Case Examples
. . . . . . . . . . . .37
CHAPTER 4 Pulse Sequence Basics, ECG Gating, and MR Artifacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .107 INDEX
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .181
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ABBREVIATIONS 2D
two-dimensional
3D
three-dimensional
ARVC
arrhythmogenic right ventricular cardiomyopathy
AS
aortic stenosis
bpm
beats per minute
ECG
electrocardiography
EDV
end-diastolic volume
ESV
end-systolic volume
FOV
field of view
Gd-DTPA gadolinium diethylenetriamine penta-acetic acid
R-R
time interval between successive R-wave peaks in ECG waveform
RCA
right coronary artery
RF
radio frequency
RV
right ventricle
SE
spin echo
T1
longitudinal (spin-lattice) relaxation time
T2
transverse (spin-spin) relaxation time
TE
echo time
TI
inversion time
GRE
gradient-recalled echo
TR
pulse repetition rate
HR
heart rate
VCG
vector ECG
IR
inversion recovery
VENC
velocity encoding
LAD
left anterior descending artery
VPS
views per segment
LCX
left circumflex artery
LV
left ventricle
MDE
myocardial delayed enhancement
MR
magnetic resonance
MRI
magnetic resonance imaging
NEX
number of excitations or signal averages
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CARDIAC MR ACRONYMS MANUFACTURER IMAGING COMPONENT & TERM
Siemens Medical Solutions
GE Healthcare
Magnetization Preparation Phase contrast PC PC Chemical fat saturation Fat Sat Fat Sat, Chem Sat Tagging Tagging Tagging Flow compensation Flow comp, GMR Flow comp Inversion recovery IR IR Phase-sensitive inversion PSIR PSIR recovery Echo Formation Gradient echo Spoiled Gradient-recalled echo Spoiled GRE Fast gradient echo Fast gradient echo 3D Volume-interpreted GRE Steady state Balanced steady-state free precession Steady-state free precession–FID Steady-state free precession–echo Spin echo Gradient and spin echo
Philips Medical Systems
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
True FISP
FIESTA
BFFE
FISP
GRASS
FE
PSIF
SSFP
T2-FFE
SE
SE
SE
TurboGSE, TGSE
GRASE
GRASE
THRIVE
*Imaging terms and corresponding acronyms used by different MR scanner manufacturers.
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MANUFACTURER IMAGING COMPONENT Siemens Medical & TERM Solutions Data Acquisition Single-shot spin echo Multishot (echo train) spin echo Number of echoes in spin-echo echo train
Echo planar imaging Rapid Imaging Image based k-space based Imaging Mode Two dimensions Three dimensions Single image Multiframe image
GE Healthcare
Philips Medical Systems
HASTE TSE, RARE
SSFSE FSE
SS TSE TSE
Turbo factor
ETL
Turbo factor
EPI
EPI
EPI
mSENSE GRAPPA
ASSET ARC
SENSE
2D 3D Static Cine
2D 3D Static Cine
2D 3D Static Cine
ARC, autocalibrating reconstruction for Cartesian imaging; ASSET, array sensitivity encoded; 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; LAVA, liver acquisition with volume acceleration; MPRAGE, magnetization prepared rapid acquired gradient echoes; mSENSE, modified sensitivity encoding; PSIF, reversed fast imaging with steady-state free precession; 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; TGSE, turbo gradient spin echo; THRIVE, T1 high-resolution isotropic volume estimation; TFE, turbo field echo; TSE, turbo spin echo; VIBE, volumetric interpolated breath-hold examination.
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CHAPTER 1
CARDIAC ANATOMY AND MR IMAGING PLANES
Chapter 1
CARDIAC ANATOMY AND MR IMAGING PLANES Eric E. Williamson, MD Kiaran P. McGee, PhD Paul R. Julsrud, MD
Introduction The purpose of this chapter is to provide a step-by-step protocol for acquiring common cardiac magnetic resonance (MR) imaging planes. The workflow diagrams presented are not complete in that each represents only one example of a pathway that can be followed to facilitate consistent cardiac examinations. However, they do provide both a workflow that facilitates consistent visualization of the most clinically relevant cardiac anatomy and a method for completing the cardiac examination in a practical time frame for the patient and imaging department alike.
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MAYO CLINIC GUIDE TO CARDIAC MRI
Left Ventricular Imaging MR imaging of the heart most commonly includes visualization of the left ventricle (LV). At least three separate imaging planes are acquired routinely, as shown in Figure 1.1; these include the sagittal localizer, a four-chamber localizer, and a series of so-called “short-axis” views. Three additional imaging planes can also be acquired to further characterize the LV in the long axis. Figure 1.1 shows the order in which these planes are typically acquired (numbers 1 through 6), as well as their temporal and spatial relationships. Each arrow describes the relationship between the image used as the prescription and the resultant imaging plane. For example, the sagittal localizer is used as a prescription image in order to acquire the four-chamber localizer view. Similarly, the four-chamber localizer acts as the prescription image for the short-axis views. Solid arrows represent standard imaging planes and dashed arrows identify optional planes.
Figure 1.1. Imaging planes used to characterize the LV of the heart. The numbers indicate the acquisition order. Solid arrows indicate typical imaging planes; dashed arrows indicate optional planes used to visualize the left heart.
CHAPTER 1
CARDIAC ANATOMY AND MR IMAGING PLANES
5
Sagittal localizer (1)
Four-chamber localizer (2)
Short axis (3)
Left ventricle two chamber (4)
Left ventricle three chamber (5) Left ventricle four chamber (6)
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MAYO CLINIC GUIDE TO CARDIAC MRI
Sagittal Localizer The first step in prescribing the various imaging planes acquired during a cardiac MR examination is to obtain a series of straight sagittal images that include the entire volume of the heart (Figure 1.2). This is an essential first step because all subsequent planes are prescribed from these data. Prescribe enough slices to cover the entire heart. The graphical prescription should start roughly at the sternum and cover more than two-thirds of the left side of the patient’s thorax. The goal is to obtain slices that include the mitral valve plane and cardiac apex.
Figure 1.2. Anatomical reference showing the location of the sagittal localizer with respect to the heart and chest wall, as well as the corresponding MR imaging planes. The localizers start at the position of the sternum and progress laterally toward the apex of the heart.
CHAPTER 1
CARDIAC ANATOMY AND MR IMAGING PLANES
Anatomical reference
Resultant sagittal localizer views
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MAYO CLINIC GUIDE TO CARDIAC MRI
Four-Chamber Localizer Acquisition of the sagittal localizer allows identification of the four chambers and great vessels of the heart. Review of these images shows the oblique orientation of the heart within the thoracic cavity, with the apex lying on top of the diaphragm at the level of the fifth intercostal space and the base of the heart posterior to the apex at the level of the third rib. Scrolling from left to right, the sagittal images traverse the left and then right sides of the heart. These images can be used to acquire a four-chamber localizer view. The resultant images are not true four-chamber views, but they serve as a reference point from which short-axis views are acquired. To obtain a four-chamber localizer image from the sagittal localizer images, perform the following steps: ■
■
■
Scroll through the sagittal images and find an image in which the apex of the LV can be clearly identified. This will not necessarily be the first slice that contains cardiac anatomy. It is most common to identify the slice in which the LV first appears and then choose the next medial slice. Continue scrolling through the images, moving medially from the left to the right side of the heart, until the mitral (bicuspid) valve plane is identified. If the mitral valve is not clearly identified, the root of the aorta can also be used. Prescribe an imaging plane that bisects both the apex and mitral valve planes identified in the two previous images.
Figure 1.3 shows the placement of the imaging plane on each sagittal image (yellow lines). Angulation of the imaging plane in this manner allows the imaging slice to approximately bisect all chambers of the heart. The orientation of the imaging plane in relation to the heart and the resultant image are also shown.
Figure 1.3. Four-chamber localizer. Anatomical reference image, MR prescription imaging planes (yellow lines), and the resultant four-chamber image are shown. The imaging plane bisects the apex of the LV and the mitral (bicuspid) valve, providing a view of all four chambers of the heart.
CHAPTER 1
CARDIAC ANATOMY AND MR IMAGING PLANES
Prescription Anatomical reference
Mitral valve
Apex of LV
Resultant four-chamber localizer
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MAYO CLINIC GUIDE TO CARDIAC MRI
Left Ventricular Short-Axis Views Short-axis images of the heart show a view “down the barrel” of the LV, perpendicular to the long axis or septum of the heart. Figure 1.4 shows the placement of a short-axis imaging plane in relation to the heart and the resultant anatomical cross-section. Multiple short-axis images are typically acquired from the cardiac apex to the base of the heart and are distinguished from one another by the part of the LV imaged (eg, cardiac apex, mid ventricle, or base of the heart). The base image is closest to the great vessels and includes the left and right ventricles, whereas the apex image typically includes only the LV. Acquisition of the four-chamber view provides the anatomical landmarks necessary for prescribing the various left ventricular short-axis views. Depending on the type of study, three or more slices will be acquired. If only three slices are used, these should be through the apex, mid ventricle, and base of the LV. To prescribe short-axis views of the LV, perform the following steps: ■
■
■
From the four-chamber data set acquired previously, identify the image that is approximately at end diastole. This will be the image in which the heart is relaxed and the chamber is maximally dilated. Prescribe an imaging plane that is centered on the LV and is roughly perpendicular to the septum. The plane should be placed at approximately mid ventricle. If multiple slices are prescribed, the planes should track roughly parallel to the septum. A series of images slicing through the base, mid ventricle, and apex of the heart will result.
Figure 1.4 shows the placement of three imaging planes (yellow lines) perpendicular to the septum on the four-chamber view. The orientation of these planes in relation to the anatomical orientation of the heart is also shown.
Figure 1.4. Anatomical reference, MR prescription planes (yellow lines), and resultant MR images of the left ventricular short-axis view are shown. The MR imaging planes should be parallel to the septum of the heart and centered within the middle of the LV.
CHAPTER 1
CARDIAC ANATOMY AND MR IMAGING PLANES
Prescription Anatomical reference
Resultant left ventricular short-axis views
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MAYO CLINIC GUIDE TO CARDIAC MRI
Left Ventricular Long-Axis Views A second set of long-axis views also can be acquired by prescribing a set of imaging planes orthogonal to the short-axis images previously acquired. The three prescriptions that follow (two, three, and four chamber) can all be prescribed off a single short-axis image. If multiple slices are acquired, long-axis views are typically prescribed off the slice that corresponds most closely to the mid ventricle. Left Ventricular Two-Chamber (Vertical Long-Axis) View A two-chamber long-axis (or vertical long-axis) view bisects the LV through its anterior and inferior walls, parallel to the interventricular septum. This orientation is commonly known as a two-chamber view because two chambers (the left atrium and LV) are visualized. To prescribe a two-chamber long-axis view of the LV, perform the following steps: ■ ■
■
■
Select the short-axis slice that is approximately at the location of the mid ventricle. Select the image at this slice location that is approximately at end diastole. This will be the image in which the heart is relaxed and the chamber is maximally dilated. Prescribe an imaging plane that is parallel to the interventricular septum and bisects the LV from the base of the heart to the cardiac apex. The center of the imaging plane should be at the center of the LV.
Figure 1.5 shows the placement of the two-chamber imaging plane (yellow line). The prescribed imaging plane is approximately parallel to the interventricular septum. The orientation of this plane in relation to the anatomical orientation of the heart is also shown. Placement of the imaging plane at mid ventricle produces the resultant image.
Figure 1.5. Anatomical reference, MR prescription plane (yellow line), and resultant MR image of the left ventricular two-chamber vertical long-axis view are shown.
CHAPTER 1
CARDIAC ANATOMY AND MR IMAGING PLANES
Prescription Anatomical reference
Resultant left ventricular two-chamber long-axis view
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MAYO CLINIC GUIDE TO CARDIAC MRI
Left Ventricular Three-Chamber View A three-chamber long-axis view bisects the LV through the aortic root and lateral left ventricular wall at the base of the heart and should extend to the tip of the cardiac apex (to avoid foreshortening of the LV). This orientation allows visualization of the aortic outflow tract, as well as the anterior septum and inferolateral wall of the LV. To prescribe a three-chamber long-axis view of the LV, perform the following steps: ■
■
■
■ ■
Select the short-axis slice that is approximately at the location of the aortic outflow tract. Select the image at this slice location that is approximately at end diastole. This will be the image in which the heart is relaxed and the chamber is maximally dilated. Prescribe an imaging plane through the aortic root, bisecting the LV from the base of the heart to the cardiac apex. The center of the imaging plane should be at the center of the LV. This imaging plane can also be obtained by copying the prescription from the twochamber acquisition and rotating the imaging plane accordingly.
Figure 1.6 shows the placement of the three-chamber imaging plane (yellow line). The orientation of this plane in relation to the anatomical orientation of the heart is also shown. Placement of the imaging plane at mid ventricle produces the resultant image.
Figure 1.6. Anatomical reference, MR prescription plane (yellow line), and resultant MR image of the left ventricular three-chamber long-axis view are shown.
CHAPTER 1
CARDIAC ANATOMY AND MR IMAGING PLANES
Prescription Anatomical reference
Resultant left ventricular three-chamber view
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MAYO CLINIC GUIDE TO CARDIAC MRI
Left Ventricular Four-Chamber (Horizontal Long-Axis) View A final, four-chamber, long-axis view of the left and right ventricles can be obtained by prescribing an imaging plane that is perpendicular to the interventricular septum and centered on the LV. This is a true four-chamber view of the heart because it is perpendicular to the septum and bisects the left and right chambers of the heart along the long axis. To prescribe a true four-chamber long-axis view, perform the following steps: ■
■
■ ■ ■
Select the left ventricular short-axis slice that is approximately at the location of the mid ventricle. Select the image at this slice location that is approximately at end diastole. This will be the image in which the heart is relaxed and the chamber is maximally dilated. Prescribe an imaging plane that bisects the LV and inferior septum. The center of the imaging plane should be at the center of the LV. This imaging plane can also be obtained by copying the prescription from either the two- or three-chamber acquisition and rotating the imaging plane accordingly.
Figure 1.7 shows the placement of the four-chamber imaging plane (yellow line). The orientation of this plane in relation to the anatomical orientation of the heart is also shown. Placement of the imaging plane at mid ventricle produces the resultant image.
Figure 1.7. Anatomical reference, MR prescription plane (yellow line), and resultant MR image of the left ventricular four-chamber long-axis view are shown.
CHAPTER 1
CARDIAC ANATOMY AND MR IMAGING PLANES
Prescription Anatomical reference
Resultant left ventricular four-chamber view
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MAYO CLINIC GUIDE TO CARDIAC MRI
Right Ventricular Imaging Visualization and interpretation of the structure and function of the right ventricle (RV) is an integral part of a routine cardiac MR examination. Under most circumstances, adequate visualization can be achieved from the planes used for imaging the LV. Under certain circumstances, however, additional imaging planes are required. In most of these cases, the addition of a single, conventional, axial imaging plane will suffice. In other specific instances, more complex right ventricular views are required, such as for the diagnosis of arrhythmogenic right ventricular dysplasia or Ebstein anomaly. As with the previous figures illustrating visualization of the LV, solid arrows indicate commonly acquired imaging planes and dashed arrows represent optional planes. The color of the arrow symbolizes the left (red) or right (blue) side of the heart (Figure 1.8). The arrows also identify the relationship between the prescription and resultant image planes.
Figure 1.8. Imaging planes used to characterize the RV of the heart. The numbers indicate the acquisition order. Note that, to image the right side of the heart, it is necessary to first acquire several planes used to visualize the left heart.
CHAPTER 1
CARDIAC ANATOMY AND MR IMAGING PLANES
Sagittal localizer (1)
Four-chamber localizer (2)
Conventional axial (3)
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MAYO CLINIC GUIDE TO CARDIAC MRI
Conventional Axial Right Ventricular View The sagittal localizer image set provides the anatomical landmarks necessary for prescribing the axial right ventricular cine data. Using the axial plane can facilitate tracing the RV for calculating RV volumes and ejection fraction. In general, straight axial data sets provide a more complete and easy-to-identify tricuspid valve plane. To prescribe conventional axial views, perform the following steps: ■
■
From the sagittal localizer image, identify the top of the ascending aorta cranially and the apex of the heart caudally. Prescribe a series of straight axial imaging planes that are centered roughly on the septum of the heart.
Figure 1.9 shows the placement of the axial image planes (yellow lines) on the sagittal localizer covering the ascending aorta and apex of the heart. The orientation of these planes in relation to the anatomical orientation of the heart is also shown.
Figure 1.9. Anatomical reference, MR prescription planes (yellow lines), and resultant MR images of the RV with conventional or true axial views are shown. Note that these planes are prescribed as true axial planes that are orthogonal to the sagittal imaging planes acquired initially.
CHAPTER 1
CARDIAC ANATOMY AND MR IMAGING PLANES
Prescription Anatomical reference
Resultant conventional axial right ventricular views
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MAYO CLINIC GUIDE TO CARDIAC MRI
Additional Right Ventricular Views If additional views of the RV are required, the flow diagram in Figure 1.10 can be followed.
Figure 1.10. Additional imaging planes used to characterize the RV of the heart. The numbers indicate the acquisition order. Solid arrows indicate typical imaging planes and dashed arrows indicate optional planes. Most of these planes are optional.
CHAPTER 1
CARDIAC ANATOMY AND MR IMAGING PLANES
23
Sagittal localizer (1)
Four-chamber localizer (2)
Conventional axial (3)
Right ventricle inflow tract vertical long axis (4)
Right ventricle sagittal outflow tract (5)
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MAYO CLINIC GUIDE TO CARDIAC MRI
Right Ventricular Inflow Tract Long-Axis View Acquisition of the four-chamber localizer view provides the anatomical landmarks necessary for prescribing right ventricular inflow tract views. The right ventricular inflow tract long-axis view should be oriented parallel with the interventricular septum and centered at the midventricular point of the RV. To prescribe right ventricular inflow tract long-axis views, perform the following steps: ■
■
From the four-chamber localizer data, identify the image that is approximately at end diastole. At this point of the cardiac cycle, the heart is relaxed and largest. Prescribe an imaging plane that is centered on the RV and is parallel to the septum. The plane should be centered at approximately mid ventricle.
Figure 1.11 shows the placement of a single imaging plane (yellow line) parallel to the septum on the four-chamber localizer view and centered at the middle of the right ventricular cavity. The orientation of this plane in relation to the anatomical orientation of the heart is also shown. Placement of the imaging plane at mid ventricle produces the resultant image.
Figure 1.11. Anatomical reference, MR prescription plane (yellow line), and resultant MR image of the right ventricular inflow tract in the long-axis view are shown.
CHAPTER 1
CARDIAC ANATOMY AND MR IMAGING PLANES
Prescription Anatomical reference
Resultant right ventricular long-axis view
25
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MAYO CLINIC GUIDE TO CARDIAC MRI
Right Ventricular Sagittal Outflow Tract View Acquisition of straight axial images provides the relevant anatomical landmarks necessary to prescribe a sagittal imaging plane oriented so as to bisect the outflow tract from the RV. To prescribe a right sagittal outflow tract view, perform the following step: ■
On the axial data, prescribe a sagittal imaging plane that is centered on the pulmonary semilunar valve and also bisects the main pulmonary artery. This plane will be slightly oblique to the anterior-posterior axis of the patient and as such is not a true sagittal view.
Figure 1.12 shows the placement of a single imaging plane (yellow line) on an axial image showing the main pulmonary artery. Placement of the imaging plane at mid ventricle produces the resultant image.
Figure 1.12. Anatomical reference, MR prescription plane (yellow line), and resultant MR image of the outflow tract of the RV in the sagittal view are shown.
CHAPTER 1
CARDIAC ANATOMY AND MR IMAGING PLANES
Prescription Anatomical reference
Resultant right ventricular sagittal outflow tract view
27
CHAPTER 2
CARDIAC PHYSIOLOGY
Chapter 2
CARDIAC PHYSIOLOGY Paul R. Julsrud, MD Eric E. Williamson, MD
Introduction In-depth knowledge of the physiology of the heart is essential to diagnosing and distinguishing the multitude of complex cardiac disease processes. The figures in this chapter show the relationships among pressure, electrical activity, and associated images throughout the cardiac cycle. As quantitative assessment of cardiac function becomes more important, so too will the need to understand the interrelationships among these various cardiac events.
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Events of the Cardiac Cycle (Left Heart) Figure 2.1 shows the various 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 electrocardiography [ECG]) are shown. The systolic and diastolic time intervals denote the portions of the cardiac cycle taken up by each phase and are defined by the intervals between the black and red, and red and blue lines, respectively, on the ECG waveform. It is important to note that the percentage values shown for the time intervals of systole (40%) and diastole (60%) are valid only for the given heart rate (HR) of 70 beats per minute. As HR 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 2.1. 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, end-diastolic volume; ESV, end-systolic volume. (Modified from Oh JK, Seward JB, Tajik AJ. The echo manual. 3rd ed. Philadelphia: Lippincott Williams & Wilkins; 2006. Used with permission of Mayo Foundation for Medical Education and Research.)
Electrical potential
CHAPTER 2
Systole 40%
Diastole 60%
QRS
End diastole
P
T
120
Aortic pressure
80
Ventricular pressure
Atrial pressure 10 0
Volume (mL)
HR=70
End systole
P
Pressure (mm Hg)
CARDIAC PHYSIOLOGY
130
EDV
50
ESV
0
Aortic outflow
Mitral inflow
Flow (mL/s)
500
0
Isovolumic contraction
Isovolumic relaxation
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MAYO CLINIC GUIDE TO CARDIAC MRI
MR Imaging Characteristics of the Cardiac Cycle Figure 2.2 shows the relationship between the electrical activity of the heart as a whole and the corresponding magnetic resonance (MR) images acquired as part of a cine imaging series in both four-chamber long-axis and two-chamber short-axis views of the left ventricle (LV). Typical cine acquisitions 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 views of the heart at end systole (red outline) and end diastole (blue outline). At end systole, the cavity of the LV is smallest, with maximal thickness of the myocardium; at end diastole, the LV is most relaxed, with maximal chamber volume and minimal myocardial wall thickness.
Figure 2.2. Four-chamber long-axis (top row) and twochamber 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).
CHAPTER 2
CARDIAC PHYSIOLOGY
R
T
P
S
U
Q
Four-chamber long axis
Two-chamber short axis
35
CHAPTER 3
CLINICAL INDICATIONS AND SAMPLE IMAGING PROTOCOLS WITH CASE EXAMPLES
Chapter 3
CLINICAL INDICATIONS AND SAMPLE IMAGING PROTOCOLS WITH CASE EXAMPLES Jacobo Kirsch, MD James F. Glockner, MD, PhD Philip A. Araoz, MD Paul R. Julsrud, MD Kiaran P. McGee, PhD Eric E. Williamson, MD
Introduction The purpose of this chapter is threefold. First, it provides recommended magnetic resonance (MR) imaging protocols for indications in which cardiac MR has been proved to be clinically useful. Second, it provides example MR images for each indication and the imaging sequences that most clearly illustrate the associated findings. Third, it provides descriptions of each disease process and specific recommendations for image interpretation and analysis. The list of indications is not complete, but it covers a broad spectrum of cardiac diseases, with specific focus on the conditions that are most likely to be encountered in clinical practice. Throughout this chapter, we refer to specific pulse sequences and provide examples to illustrate the appearance of a given disease process or abnormality. It is assumed that the reader is familiar with these cardiac MR imaging (MRI) techniques. For the interested reader, in-depth descriptions of all sequences used in this chapter are provided in Chapter 4. For those who are new to cardiac MRI, we recommend reviewing Chapter 4 before reviewing this chapter.
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Myocardial Perfusion and Viability Assessment of the delivery of blood to the myocardium is of utmost importance in patients with ischemic heart disease or the suspicion of it. Regional assessment of myocardial perfusion is performed by administering an exogenous gadolinium-based contrast agent, followed by pseudo–real-time imaging of the arrival of the contrast into the right and then left chambers of the heart. Assessment of myocardial perfusion is therefore based on the first pass of the contrast through the myocardium. Poorly perfused regions show decreased contrast enhancement and are typically identified as perfusion defects. Myocardial viability is assessed, following a delay after administration of the contrast agent, by using T1-weighted inversion recovery–based MRI sequences. The physiologic basis for this approach relies on the delayed wash-in and wash-out of the contrast agent in poorly perfused or ischemic myocardium. Nulling of normal myocardium is achieved by the appropriate choice of inversion time; regions with increased contrast uptake appear bright because of their contrast-enhanced T1 weighting. Figure 3.1 shows the 17 myocardial segments of the heart and their assigned coronary arteries, as defined by the American Heart Association. The 17th segment located at the apical tip can be supplied by any one of the three coronary arteries and has been identified in this figure as being supplied by the left anterior descending coronary artery.
CLINICAL INDICATIONS AND SAMPLE IMAGING PROTOCOLS
CHAPTER 3
41
Infarct Distributions
Coronary Artery Territories Short axis Apical
Mid
Basal
8
Mid
1
7
13 14
Vertical long axis
12
2
6
16 15
17 9
11
LAD
5
3
10
4
RCA
LCX
Figure 3.1. Assignment of the 17 myocardial segments to the territories of the left anterior descending artery (LAD), the right coronary artery (RCA), and the left circumflex coronary artery (LCX). The image of the heart (top) shows the anatomical location of the three main coronary arteries, as well as the three parallel planes that correspond to the short-axis slices shown at bottom. (From Imaging guidelines for nuclear cardiology procedures: part 2. J Nucl Cardiol. 1999;6[2]:G47-84. Used with permission.)
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Imaging Protocol––Myocardial Perfusion and Viability
Series 1
Plane
Imaging sequence
Sagittal
Bright blood static (balanced or spoiled gradient echo)
Specific parameters
≈ Time/slice (s)
Breath hold Single cardiac phase
20
Localizer scan to identify imaging volume. Prescribe enough slices to include entire heart. Typical left-to-right range, ≈200 mm depending on body habitus (≈120 mm left to 80 mm right of midline). 2
Four chamber long axis
Bright blood cine (balanced gradient echo)
Breath hold 20 cardiac phases
10-16
Single slice through left ventricular apex and mitral valve. Adjust VPS based on heart rate and breath-hold duration; minimize breath-hold time for patient comfort.
3
Short axis
Bright blood cine (balanced gradient echo)
Breath hold 20 cardiac phases
10-16
Center each slice on the center of the LV. Ensure that slices cover the entire ventricle from apex to base. Adjust imaging parameters (VPS, imaging matrix, NEX) based on breathhold duration and patient compliance.
CHAPTER 3
Series 4
Plane
Vertical and horizontal long axis
CLINICAL INDICATIONS AND SAMPLE IMAGING PROTOCOLS
Imaging sequence
Specific parameters
Bright blood cine (balanced gradient echo)
Breath hold 20 cardiac phases
43
≈ Time/slice (s) 10-16
Prescription should be based on midventricular short-axis view with slices bisecting the LV parallel and perpendicular to the septum.
5
Short axis
Perfusion
30-40 temporal phases
50 -70
Patient typically needs more than one breath hold to complete. Coaching is required beforehand to prevent rapid inspiration on second breath hold. Inject 20 mL gadolinium contrast at 4-5 mL/s and start sequence simultaneously. Instruct patient to hold his breath for as long as possible and then to do additional breath holds until the scan is over. After the scan has finished, may inject an additional 20 mL gadolinium contrast at 2 mL/s (to improve contrast on delayed enhancement sequence).
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Series 6
Plane Short axis
Imaging sequence Delayed enhancement
Specific parameters Breath hold TI = 100-300 ms
≈ Time/slice (s) 10-20
TI chosen to null normal myocardium. If imaging sequence does not provide recommended TIs (phase-sensitive inversion recovery), a prior multi-inversion time sequence must be run to determine this value. Imaging performed approximately 10 minutes after administration of first contrast bolus. 7
Long axis
Delayed enhancement
Breath hold TI = 100-300 ms
10-20
TI chosen to null normal myocardium. If imaging sequence does not provide recommended TIs (phase-sensitive inversion recovery), a prior multi-inversion time sequence must be run to determine this value. Imaging performed approximately 10 minutes after administration of first contrast bolus.
LV, left ventricle; NEX, number of excitations; TI, inversion time; VPS, views per segment.
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CLINICAL INDICATIONS AND SAMPLE IMAGING PROTOCOLS
45
Case Examples Subendocardial Myocardial Infarction
Figure 3.2. Short-axis perfusion image (left) acquired shortly after the intravenous administration of gadolinium shows a subendocardial area of low signal in the inferior segments, characteristic of a first-pass myocardial perfusion defect (arrow). A corresponding two-chamber myocardial delayed enhancement (MDE) image (right) shows persistent subendocardial hyperenhancement of the inferior wall of the LV, confirming the presence of an infarction (arrow).
Figure 3.3. Four-chamber MDE image demonstrates persistent subendocardial enhancement along the lateral wall of the LV.
The rationale behind MDE sequences is that cellular disruption occurring in the infarcted region causes an increase in vascular permeability and corresponding expansion of the extracellular space. Injected gadolinium accumulates in the area of infarcted myocardium and is cleared more slowly from this region than from normal, healthy myocardium. These two factors result in the characteristic appearance of persistent hyperenhancement in the region of a myocardial infarction.
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Transmural Myocardial Infarction
Figure 3.4. Four-chamber MDE image demonstrates full-thickness delayed enhancement, characteristic of a large LAD distribution infarction. The “dark core” of the infarction is believed to represent microvascular obstruction, meaning that the arterial occlusion was severe enough to prevent any gadolinium from reaching that area.
Figure 3.5. Short-axis balanced gradient echo (left) and MDE (right) images show thinning of the inferolateral left ventricular wall with corresponding full-thickness enhancement, characteristic of an underlying infarction in the LCX arterial territory.
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47
Figure 3.6. Three-chamber balanced gradient echo image shows focal thinning of the anterior left ventricular wall, consistent with an LAD distribution infarction.
Myocardial infarction spreads over time, like a wave front, from the endocardium to the epicardium. In infarcted myocardium, the subendocardial layer should always be affected. If an area of delayed enhancement spares the subendocardial layer or is not confined to a single vascular territory, nonischemic myocardial diseases should be considered.
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Ischemic Cardiomyopathy
Figure 3.7. Four-chamber balanced gradient echo (left) and MDE sequence (right) images show global dilatation of the left ventricular cavity. Thinning and persistent enhancement of the mid and apical segments of the LV are consistent with an ischemic dilated cardiomyopathy.
The term ischemic cardiomyopathy is commonly used to refer to congestive heart failure due to coronary artery disease and resulting myocardial infarction. After a myocardial infarction, some degree of left ventricular dysfunction is expected; it usually correlates with the extent and location of myocardial injury. The use of gadolinium-enhanced perfusion and MDE images for these patients can be important for distinguishing ischemic from nonischemic causes of dilated cardiomyopathy.
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CLINICAL INDICATIONS AND SAMPLE IMAGING PROTOCOLS
49
Cardiomyopathies The cardiomyopathies make up a heterogeneous group of disorders characterized by dysfunction of the cardiac myocytes. This dysfunction leads to a decrease in the ability of the heart to maintain adequate cardiac output. Depending on the type of cardiomyopathy, the imaging findings can be variable and relatively characteristic. Common forms of cardiomyopathy include dilated, hypertrophic, and restrictive cardiomyopathy. Additional disorders involving dysfunction of cardiac myocytes include arrhythmogenic right ventricular cardiomyopathy (ARVC), apical ballooning syndrome, and myocarditis.
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MAYO CLINIC GUIDE TO CARDIAC MRI
Imaging Protocol––Cardiomyopathies
Series 1
Plane
Imaging sequence
Sagittal
Bright blood static (balanced or spoiled gradient echo)
Specific parameters
≈ Time/slice (s)
Breath hold Single cardiac phase
20
Localizer scan to identify imaging volume. Prescribe enough slices to include entire heart. Typical left-to-right range, ≈200 mm depending on body habitus (≈120 mm left to 80 mm right of midline). 2
Four chamber long axis
Bright blood cine (balanced gradient echo)
Breath hold 20 cardiac phases
10-16
Single slice through left ventricular apex and mitral valve. Adjust VPS based on heart rate and breath-hold duration; minimize breath-hold time for patient comfort.
3
Short axis
Bright blood cine (balanced gradient echo)
Breath hold 20 cardiac phases
10-16
Center each slice on the center of the LV. Ensure that slices cover the entire ventricle from apex to base. Adjust imaging parameters (VPS, imaging matrix, NEX) based on breathhold duration and patient compliance.
CHAPTER 3
Series
Plane
4 Left ventricular outflow tract
CLINICAL INDICATIONS AND SAMPLE IMAGING PROTOCOLS
Imaging sequence
Specific parameters
Bright blood cine (balanced gradient echo)
Breath hold 20 cardiac phases
51
≈ Time/slice (s) 10-16
Disease-specific imaging plane. See specific diseases for actual prescription.
5
Short axis
Perfusion
30-40 temporal phases
50-70 for total study
Patient typically needs more than one breath hold to complete. Coaching is required beforehand to prevent rapid inspiration on second breath hold. Inject 20 mL gadolinium contrast at 4-5 mL/s and start sequence simultaneously. Instruct patient to hold his breath for as long as possible and then to do additional breath holds until the scan is over. After the scan has finished, may inject an additional 20 mL gadolinium contrast at 2 mL/s (to improve contrast on delayed enhancement sequence).
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MAYO CLINIC GUIDE TO CARDIAC MRI
Series 6
Plane Short axis
Imaging sequence Delayed enhancement
Specific parameters Breath hold TI = 100-300 ms
≈ Time/slice (s) 10-20
TI chosen to null normal myocardium. If imaging sequence does not provide recommended TIs (phase-sensitive inversion recovery), a prior multi-inversion time sequence must be run to determine this value. Imaging performed approximately 10 minutes after administration of first contrast bolus. 7
Long axis
Delayed enhancement
Breath hold TI = 100-300 ms
10-20
TI chosen to null normal myocardium. If imaging sequence does not provide recommended TIs (phase-sensitive inversion recovery), a prior multi-inversion time sequence must be run to determine this value. Imaging performed approximately 10 minutes after administration of first contrast bolus.
LV, left ventricle; NEX, number of excitations; TI, inversion time; VPS, views per segment.
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CLINICAL INDICATIONS AND SAMPLE IMAGING PROTOCOLS
53
Case Examples Dilated Cardiomyopathy
Figure 3.8. Four-chamber long-axis (top left) and midventricular short-axis (top right) balanced gradient echo images show marked left ventricular dilatation. A twochamber long-axis MDE image (bottom) shows no evidence of myocardial infarction.
The most common form of cardiomyopathy, dilated (congestive) cardiomyopathy, is characterized by enlargement of the cardiac chambers and decreased contractile function in the absence of ischemic causes. Although most commonly idiopathic, dilated cardiomyopathy can be due to viral infection or exposure to toxic substances (eg, alcohol) or can be associated with pregnancy (peripartum cardiomyopathy).
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Hypertrophic Cardiomyopathy
Figure 3.9. Hypertrophic obstructive cardiomyopathy. Three-chamber long-axis (top) and midventricular short-axis (bottom) balanced gradient echo images demonstrate asymmetric septal hypertrophy, characteristic of hypertrophic obstructive cardiomyopathy. The low signal flow void seen in the left ventricular outflow tract (top) is consistent with obstructive physiology. Cine images in this case showed systolic anterior motion of the anterior leaflet of the mitral valve, which is thought to contribute to the outflow tract obstruction.
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55
Figure 3.10. Hypertrophic cardiomyopathy. Two-chamber long-axis balanced gradient echo (top) and midventricular short-axis MDE (bottom) images. Symmetric left ventricular hypertrophy with mitral regurgitation (top) is most likely due to outflow obstruction. MDE image (bottom) demonstrates characteristic small foci of hyperenhancement at the junction points between the right ventricle (RV) and the interventricular septum.
The second most common form of cardiomyopathy, hypertrophic cardiomyopathy, is characterized by thickening of the LV wall, which can be focal or diffuse and symmetric. In a subtype of the disorder, hypertrophic obstructive cardiomyopathy, the wall thickening can cause obstruction of the normal flow of blood, at either the left ventricular outflow tract (Figure 3.9) or the midventricular level. In the case of obstruction, flow dephasing can be observed using bright blood cine or phase-contrast images. When left ventricular outflow tract obstruction is present, it can be associated with systolic anterior motion of the anterior mitral leaflet (Figure 3.9), which typically results in mitral regurgitation (Figure 3.10).
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Restrictive Cardiomyopathy
Figure 3.11. Cardiac amyloidosis. Fourchamber long-axis balanced gradient echo (top left), two-chamber long-axis MDE (top right), and midventricular short-axis MDE (bottom) images of cardiac amyloidosis. Long-axis images show signs of increased cardiac filling pressures, with atrial dilatation (top left and right) and normal-thickness pericardium (top left). MDE images demonstrate heterogeneous, patchy-appearing myocardial signal with poor “nulling” regardless of the inversion time.
Restrictive cardiomyopathy is a less common form of myocardial dysfunction which can be idiopathic or can occur as a result of systemic diseases (eg, amyloidosis or sarcoidosis). The imaging findings in restrictive cardiomyopathy predominantly result from restricted filling in diastole and include decreased diastolic volume of the ventricles and dilated atria in the presence of preserved systolic function. A few causes of restriction have specific imaging findings on MRI that can be helpful in making the diagnosis (Figures 3.11 and 3.12).
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CLINICAL INDICATIONS AND SAMPLE IMAGING PROTOCOLS
57
Eosinophilic Cardiomyopathy
Figure 3.12. Eosinophilic cardiomyopathy. Four-chamber long-axis balanced gradient echo (top left and right) and two-chamber long-axis MDE (bottom) images of eosinophilic cardiomyopathy. Four-chamber images show signs of increased cardiac filling pressures, with atrial dilatation (top left and right) and normal-thickness pericardium (top right). The MDE image shows diffuse endocardial thickening and hyperenhancement not corresponding to a vascular distribution. Thickening of the mitral valve leaflets and obliteration of the left ventricular cavity also have been described for eosinophilic cardiomyopathy but neither is seen in these images.
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MAYO CLINIC GUIDE TO CARDIAC MRI
Arrhythmogenic Right Ventricular Cardiomyopathy
Figure 3.13. Four-chamber balanced gradient echo images of ARVC at end diastole (left) and end systole (right) demonstrate a small outpouching of the free wall of the RV during systole (paradoxical motion). The RV is mildly enlarged.
ARVC is characterized pathologically by fatty or fibrous fatty tissue replacement of the right ventricular myocardium. The most commonly affected locations include the right ventricular apex, pulmonary infundibulum, and subtricuspid region. The involved myocardium can evoke ventricular arrhythmias originating in the RV that induce syncope and that have been linked to an estimated 5% of sudden deaths in persons younger than 35 years in the United States. MRI has been considered the ideal imaging technique to detect fatty tissue infiltration in the RV among patients with typical ARVC. Other features such as trabecular disarray, wall thinning, regional akinesis/dyskinesis, and, most importantly, an increased right ventricular volume can also be fairly easily detected by MRI. However, care should be taken because many healthy patients can have focal fatty infiltration along the RV free wall or focal akinesis at the attachment site of the moderator band on the free wall of the ventricle.
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CLINICAL INDICATIONS AND SAMPLE IMAGING PROTOCOLS
59
Apical Ballooning Syndrome
Figure 3.14. Systolic two-chamber longaxis balanced gradient echo image (top left), and two-chamber long-axis (top right) and apical short-axis (bottom) MDE images. Long-axis images demonstrate systolic dilatation of the left ventricular apex with no evidence of delayed hyperenhancement (top right and bottom) to suggest myocardial infarction.
Apical ballooning syndrome is a potentially reversible clinical syndrome in which patients typically present with elevated troponin levels and electrocardiographic changes that can be indistinguishable from an acute coronary syndrome. The syndrome occurs most frequently among postmenopausal women and consists of transient hypokinesis, akinesis, or dyskinesis of the LV, typically involving more than one vascular distribution. The wall motion abnormalities seen in apical ballooning syndrome should resolve within days to weeks, and MDE MR images in these patients should not show delayed hyperenhancement in any phase of the disease. Cardiac MRI is therefore an excellent tool for distinguishing between apical ballooning syndrome and myocardial infarction.
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Myocarditis
Figure 3.15. Two-chamber long-axis MDE (top left), midventricular short-axis triple inversion recovery (top right), and midventricular short-axis MDE (bottom) images showing scattered focal areas of subepicardial delayed hyperenhancement (left) and edema (right) consistent with acute myocardial inflammation. Findings are characteristic of acute myocarditis. Follow-up images after treatment can show a decrease in regional edema; however, delayed hyperenhancement usually persists and is thought to represent irreversible myocardial injury.
Acute myocarditis comprises a wide variety of infectious, toxic, and autoimmune causes of myocardial inflammation, which can progress to widespread myocardial damage and even to cardiomyopathy. Unfortunately, clinical symptoms are nonspecific, and the diagnosis of myocarditis can be difficult to establish. Cardiac MRI is a powerful tool that can provide an assessment of regional inflammation and edema, cardiac function, and disease progression. Initial findings of edema and delayed hyperenhancement are believed to represent acute inflammation with myocyte injury, whereas persistence of hyperenhancement after resolution of acute symptoms suggests myocyte necrosis and subsequent fibrosis.
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CLINICAL INDICATIONS AND SAMPLE IMAGING PROTOCOLS
61
Valvular Disease MRI is currently gaining acceptance as a noninvasive method of evaluating the cardiac valves. The high spatial resolution of MRI, along with its inherent ability to distinguish between cardiac structures and the adjacent blood pool without the need for intravenous contrast agents, makes this an excellent means of assessing cardiac valvular anatomy. Additionally, flow-sensitive techniques allow for detection of the turbulent jets typically seen with valvular stenosis and regurgitation. By combining cine images used for the determination of ventricular volumes and phase-contrast images for the quantitation of flow, cardiac MRI can be used as a comprehensive, noninvasive method for assessment of valvular disease.
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MAYO CLINIC GUIDE TO CARDIAC MRI
Imaging Protocol––Valvular Disease
Series 1
Plane
Imaging sequence
Sagittal
Bright blood static (balanced or spoiled gradient echo)
Specific parameters
≈ Time/slice (s)
Breath hold Single cardiac phase
20
Localizer scan to identify imaging volume. Prescribe enough slices to include entire heart. Typical left-to-right range, ≈200 mm depending on body habitus (≈120 mm left to 80 mm right of midline). 2
Four chamber long axis
Bright blood cine (balanced gradient echo)
Breath hold 20 cardiac phases
10-16
Single slice through left ventricular apex and mitral valve. Adjust VPS based on heart rate and breath hold duration; minimize breath-hold time for patient comfort.
3
Short axis
Bright blood cine (balanced gradient echo)
Breath hold 20 cardiac phases
10-16
Center each slice on the center of the LV. Ensure that slices cover the entire ventricle from apex to base. Adjust imaging parameters (VPS, imaging matrix, NEX) based on breathhold duration and patient compliance.
CHAPTER 3
Series 4A
Plane
CLINICAL INDICATIONS AND SAMPLE IMAGING PROTOCOLS
Imaging sequence
Long axis– Bright blood cine left ventricle, (balanced gradient echo) left atrium (mitral regurgitation)
Specific parameters Breath hold 20 cardiac phases
63
≈ Time/slice (s) 10-16
Slices should be prescribed from the short-axis slice containing the base of the heart. Views should be prescribed in 45° increments about the center of the LV. These views should contain the mitral valve and regurgitant jet.
4B
Oblique long Bright blood cine axis– aortic (balanced gradient echo) outflow tract (aortic insufficiency)
Breath hold 20 cardiac phases
10-16
The slice should be prescribed from the short-axis slice containing the base of the heart. This view should show the regurgitant jet.
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MAYO CLINIC GUIDE TO CARDIAC MRI
Series 5
Plane
Imaging sequence
Oblique slice Cine phase contrast through aortic region of interest– ascending, descending
Specific parameters Breath hold 20 cardiac phases VPS = 4-8 VENC = 250-300 Flow sensitization direction = slice
≈ Time/slice (s) 10
For aorta: Slice should be through proximal ascending aorta, valve to coronary ostia, and perpendicular to the aorta for accurate flow measurements. Set VENC to 500-600 if patient has history of or evidence for AS (ie, systolic jet in aorta on three-chamber cine). FOV should be as small as possible to maximize number of pixels within aorta. An additional in-plane slice is useful for measuring peak velocities in AS and should be prescribed oblique to the long axis of the aorta. Most useful flow-encoding direction is along the slice-encoding axis. 6
Valve plane
Bright blood cine (balanced or spoiled gradient echo)
Breath hold 20 cardiac phases
10
Imaging plane bisects the valve plane. Good for aortic outflow tract and aortic valve views; regurgitant jets are often visualized more clearly with spoiled gradient echo sequence, although image quality is poor compared with balanced steady-state acquisition. In general, there also is less turbulent flow artifact. To increase conspicuity of the flow jet, reduce imaging bandwidth. AS, aortic stenosis; FOV, field of view; LV, left ventricle; NEX, number of excitations; TI, inversion time; VENC, velocity encoding; VPS, views per segment.
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CLINICAL INDICATIONS AND SAMPLE IMAGING PROTOCOLS
65
Case Examples Aortic Stenosis, Aortic Insufficiency
Figure 3.16. Aortic insufficiency. Three-chamber balanced gradient echo cine image in diastole (left) shows a dark jet of aortic insufficiency extending from the aortic valve toward the anterior mitral valve leaflet. Transverse diastolic balanced gradient echo cine image through the aortic valve plane (right) shows a small central regurgitant orifice.
Figure 3.17. Aortic stenosis. Coronal oblique balanced gradient echo cine image (left) reveals a dark stenotic jet extending from the aortic valve into the proximal aorta. The corresponding valve plane image (right), also obtained in systole, reveals a bicuspid aortic valve with a very narrow opening.
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Aortic insufficiency can be caused by primary valve disease or aortic root dilatation. The most common cause is idiopathic degeneration of a normal valve, with additional causes including Marfan syndrome, aortic aneurysm, bicuspid aortic valve, rheumatic heart disease, and endocarditis. Combined aortic insufficiency and stenosis is common. Patients are often asymptomatic despite severe left ventricular volume overload and may have irreversible LV dysfunction by the time symptoms appear, which limits the value of valve replacement. For this reason, close monitoring of patients with significant aortic insufficiency is recommended. MRI can be a valuable tool for evaluating patients with aortic insufficiency. It provides accurate measurements of left ventricular size and function, as well as visualization of abnormal valve morphology. Qualitative estimation of the severity of aortic insufficiency by MRI agrees fairly well with that obtained by echocardiography, and quantitative evaluation can be performed using cine phase-contrast flow measurement techniques or by noting the difference in stroke volume between the RV and LV.
QUANTIFYING AORTIC INSUFFICIENCY Regurgitant volume Mild Moderate Moderate/severe Severe
60 mL/beat
Regurgitant fraction Mild Moderate Severe
15%-20% 21%-40% >40%
Aortic stenosis most commonly occurs with idiopathic degeneration of a normal valve but can also be caused by degeneration of a bicuspid valve (typically occurring at a younger age) or by rheumatic heart disease. Supravalvular and subvalvular stenosis usually are congenital lesions, but subvalvular functional stenosis also occurs with hypertrophic obstructive cardiomyopathy. Classic symptoms include dyspnea on exertion, exertional syncope, and angina. After symptoms occur, the clinical course usually deteriorates rapidly unless the valve is replaced. Left ventricular hypertrophy is the primary compensatory mechanism. MRI can directly demonstrate hypertrophy of the LV and provide accurate measurements of ventricular mass and function. Stenosis can be qualitatively estimated by visualizing flow jets; quantitative measurements can be obtained either by direct planimetry of the valve or by applying cine phase-contrast techniques to measure peak velocities across the valve, which can be used to estimate pressure gradients.
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67
QUANTIFYING AORTIC STENOSIS Aortic valve area Normal Mild AS Moderate AS Severe AS
Peak systolic velocity cm2
2.0-4.0 1.1-1.9 cm2 0.75-1.0 cm2 4 m/s
Analysis of aortic insufficiency and stenosis by MRI: Useful cardiac MRI–derived metrics include: ■ Regurgitant volume per beat ■ Regurgitant fraction: regurgitant volume or regurgitant flow divided by positive volume or flow ■ Ejection fraction ■ Cardiac output ■ End-systolic and end-diastolic volumes, as well as short-axis LV diameter at end systole and end diastole at the midventricular level (to duplicate standard echocardiographic measurement) ■ Flow though the pulmonary artery. (Note: in normal patients this should be identical to cardiac output; with aortic insufficiency, the cardiac output is increased because of the regurgitant volume.) In patients with aortic stenosis: Peak pressure gradient = 4(Vmax)2 where Vmax is the maximal velocity of the stenotic jet in the proximal aorta. Mean pressure gradient = (4 ∑[Vmax]2dt)/Δt where dt is the time interval for the phase-contrast cine image in which Vmax is measured and Δt the interval over which these velocities are summed (typically one R-R interval).
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This is the time average of Vmax over systole. For this measurement, it is best to choose a small region of interest that encompasses the highest velocity portion of the flow jet (this is somewhat arbitrary). Aortic valve area = AOT(VOT)/Vmax where AOT is the outflow tract area, VOT the outflow tract maximum velocity, and Vmax the maximum velocity of the stenotic jet in the proximal aorta. This measurement can also be performed directly if good cine images have been made through the stenotic valve. Alternative measurement of regurgitant volume: ■ Phase-contrast based = stroke volume in aorta – stroke volume in main pulmonary artery OR = stroke volume in aorta – mitral valve inflow ■
Short-axis balanced gradient echo based = LV stroke volume – RV stroke volume
CHAPTER 3
CLINICAL INDICATIONS AND SAMPLE IMAGING PROTOCOLS
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Mitral Stenosis, Mitral Regurgitation
Figure 3.18. Mitral stenosis. Short-axis balanced gradient echo cine image at base of heart during diastole (left) and three-chamber cine image (right) show stenotic mitral valve (arrows) with reduced area and thickened leaflets.
Figure 3.19. Mitral valve prolapse and regurgitation. Diastolic (left) and systolic (right) images from three-chamber bright blood cine sequence reveal prolapsing mitral valve leaflets and a small jet of mitral regurgitation (arrow).
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Mitral stenosis is most often a result of rheumatic heart disease. Symptoms typically include shortness of breath and fatigue, which are often exacerbated by exercise. A substantial percentage of patients with moderate or severe mitral stenosis eventually have development of atrial fibrillation, which complicates evaluation with MRI. Elevated left atrial pressure leads to atrial dilatation, pulmonary edema, and signs of pulmonary artery hypertension. MRI can visualize stenotic flow jets across the mitral valve, and mitral valve area can be estimated by planimetry. Peak velocities can be measured with cine phase-contrast sequences to estimate pressure gradients. Left atrial size, as well as right ventricular size and function, can be determined. Mitral regurgitation has many causes, including ischemia and papillary muscle rupture, infective endocarditis, mitral valve prolapse, hypertrophic obstructive cardiomyopathy, rheumatic disease, and idiopathic valvular degeneration. Patients with acute mitral regurgitation may present with pulmonary edema and low cardiac output, whereas those with chronic mitral regurgitation may have symptoms of fatigue and weakness. With significant mitral regurgitation, both the left atrium and LV become dilated. MRI can visualize jets of mitral regurgitation in the left atrium, which can be qualitatively evaluated. Quantitative measurement of mitral regurgitant volumes may be obtained by measuring the difference in left and right ventricular stroke volume, or by the difference in left ventricular stroke volume and forward flow in the aorta (using cine phase-contrast sequences). MRI also provides accurate assessment of left ventricular size and function, as well as left atrial size.
QUANTIFYING MITRAL REGURGITATION Grade Grade I/IV Grade II/IV Grade III/IV Grade IV/IV
Regurgitant volume Mild Moderate Moderately severe Severe
