MRI Physics for Radiologists

Transcription

1 Alfred L. Horowitz MRI Physics for Radiologists A Visual Approach Second Edition With 94 Illustrations Springer-Verlag New York Berlin Heidelberg London Paris Tokyo Hong Kong Barcelona Budapest

2 Alfred L. Horowitz, M.D. Director, Magnetic Resonance Imaging, Resurrection Hospital, Chicago, IL and Clinical Assistant Professor of Radiology, University of Illinois Hospital at Chicago, Chicago, IL USA Library of Congress Cataloging-in-Publication Data Horowitz, Alfred L. MRI physics for radiologists: a visual approach I Alfred L. Horowitz. - 2nd ed. p. cm. Includes bibliographical references and index. ISBN-13: Magnetic resonance imaging. [DNLM: 1. Magnetic Resonance Imaging. 2. Physics. OC 762 H816mj RC78.7.N83H '.36:-dc20 DNLMIDLC for Library of Congress Printed on acid-free paper. 1989, 1992 Springer-Verlag New York, Inc. Originally published under the title MRI Physics for Physicians. All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer-Verlag New York, Inc., 175 Fifth Avenue, New York, NY 10010, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use of general descriptive names, trade names, trademarks, etc., in this publication, even if the former are not especially identified, is not to be taken as a sign that such names, as understood by the Trade Marks and Merchandise Marks Act, may accordingly be used freely by anyone. While the advice and information in this book is believed to be true and accurate at the date of going to press, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein. Production managed by Henry Krell; manufacturing supervised by Jacqui Ashri. Camera ready copy provided by author ISBN-13: e-isbn-13: DOl: /

3 To Paula, Jason and Amy

4 PREFACE When this book was initially published three years ago, it was my goal to delineate the principles of magnetic resonance imaging in a format that could be understood without a sophisticated physics or mathematics background. That is still my goal. However, in the interim, it has become clear to me that many magnetic resonance techniques that we now routinely use are inadequately understood by many of us. Therefore, I have re-structured and expanded the book in the following way. There are now three main sections: sections one and two deal with the contrast and spatial characteristics of the image, as they did in the original text; and an additional section deals with various peripheral but significant magnetic resonance topics. Sections one and two still provide the "meat" of the material through the guise of the spin-echo pulse sequence; but section three goes beyond by explaining other pulse cycles and devices that are commonly used in today's imaging centers. To begin with, since fast scanning has now become a widely used technique, that chapter has been significantly expanded, and now includes a complete but non-mathematical explanation of what a gradient echo is and how fast scan images differ in principle from spin-echo images. Also, the applications of 3DFT and "half-fourier" imaging are graphically covered without mathematical intervention. A large chapter is devoted to motion, including the considerations of motion artifacts and the devices used to control them, as well as the subjects of blood flow, and magnetic resonance angiography. A separate little chapter on aliasing is provided to explain the mystery of the "wrap-around" artifact - both in the phase and frequency encoding directions. The last of the new chapters in section three deals with the interaction of water and fat on the MR image, which includes discussions of chemical shift artifacts as well as chemical shift imaging. The mathematical appendix, which appeared in the first addition, has been replaced by a separate chapter that reveals how a scanner receives and processes a signal to form an image (once again ignoring the "queen of sciences"). This chapter forms the conclusion of the section on the formation of the image, and is placed immediately following the detailed explanations of the frequency and phase encoding processes. vii

5 The change in the title of the book, which now refers to a ''visual approach," was undertaken because of the way in which the book was written: nearly every topic was based on a visual graphic conception. The image came first, and then the text was constructed to fit the picture. Certainly, my basic philosophy for this text remains hinged around my belief that the book can be understood by anybody with a knowledge of basic algebra. In conclusion, I again hope that I have achieved my goal of providing understandable explanations of the principles we use in our daily magnetic resonance imaging activities. I again wish to thank the residents and other physicians in the Department of Radiology at the University of Illinois Hospital in Chicago for their helpful comments and questions, which were especially useful in the preparation of sections one and two in the book. I also wish to acknowledge and thank Pradip M. Pattany, MSc., Director of Research and Development MRI of Colorado, and Norbert J. Pelc, Sc.D., Associate Professor Department of Radiology Stanford University Medical School for their physics expertise, which helped me to understand some of the more difficult concepts.

6 CONTENTS Preface vii Section 1-Image Contrast Overview.... Magnetic Field.... Fields.... Basic 'JYpes of Magnets Permanent Magnet... 5 Superconducting Magnet... 6 Vectors... 8 Paramagnetic, Diamagnetic, Ferromagnetic.... Angular Momentum-Nuclear Spin.... Magnetic Dipole Moment.... Resultant M Vector.... Precession and the Larmor Equation.... Radiofrequency Pulse.... Electromagnetic Waves.... Periodic Functions.... Axis Conventions.... Perturbance of the M Vector.... Rotating Frame of Reference.... Resonance.... M vs the Component MDM Vectors.... The Signal and the Mx Vector.... Controlling the Flip of M.... Motion of M in the X-Y Plane.... Relaxation.... T1 and T2 Components of Relaxation.... Tl Curves.... Pulse Cycles, Pulse Sequences and Tissue Contrast.... TRand TE.... T1 and T2 Weighting.... Balanced (Spin Density) Scans.... T2 and the Spin-echo Pulse Cycle ix

7 Graph of MR Signal-free Induction Decay (FID)..., 41 Envelopes of the Signal T2* Concept of Phase Phase and the MR Signal Dephasing and the MR Signal Rephasing the MR Signal-180 D Refocusing Pulse The Spin-echo Pulse Cycle The ltue T2 Curve T2 Curves for Different Tissues for Long TR'S Tl and T2 Constants T2 Curves for Different Tissues for Short TR'S Section 2-The Image in Space Gradients The Slice Select Gradient Changing Slice Thickness Frequency Gradient The Pixel Grid Sine Functions for Each Pixel Application of Frequency Gradient The Fourier Transform The Spectrum The Fourier Series Fourier Transform of Pixel Grid Rotating Gradients-One Alternative The Phase Encoding Gradient Degrees of Phase Shift Per Row Phase Shift in Sine Functions Simple Summary of Phase Ideas Multiple Repetitions to Form the Image Phase Encoding Repititions and the Pixel Grid Phase Encoding Repetitions and the MDM Inside the "Black Box": From Signal to Image (l)-repetition-time Matrices (2)-Phase-frequency Matrices (3)-Transformation to Image-the 2DFT Wrapping Up Basic Concepts The Gradients in Perspective Imaging in Other Planes x

8 The Multislice Technique Averages, Excitations Exam Time General Wrap-up Section 3-Miscellaneous Topics Some Other Pulse Cycles and Procedures Saturation Recovery and Partial Saturation Inversion Recovery Fast Scans Pulse Cycle Summary Three Dimensional Fourier Imaging Half Fourier Imaging Motion General Considerations Flowing Blood Time-of-flight Phenomena Phase-related Phenomena Thrbulence Even and Odd Echo Effects Flow-related Enhancement Magnetic Resonance Angiography (MRA) Maximum Intensity Projection Algorithm Projection Acquisition Methods to Combat Motion Artifacts Pre-saturation Cardiac Gating Respiratory Ordered Phase Encoding Gradient Moment Nulling Aliasing Aliasing in the Phase Encoding Axis Aliasing in the Frequency Encoding Axis Fat and Water Chemical Shift Artifact Method of Dixon Selective Spectral Excitation Stir Sequences Coils Receiver, Transmitter Coils Gradient Coils Shim Coils xi

9 User Parameter Summary... ' 181 Bibliography Index xii