Practical Small Animal MRI E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 462

Veröffentlichungsjahr: 2011

Beschreibung

Practical Small Animal MRI is the seminal reference for clinicians using Magnetic Resonance Imaging in the diagnosis and treatment of veterinary patients. Although MRI is used most frequently in the diagnosis of neurologic disorders, it also has significant application to other body systems. This book covers normal anatomy and specific clinical conditions of the nervous system, musculoskeletal system, abdomen, thorax, and head and neck. It also contains several chapters on disease of the brain and spine, including inflammatory, infectious, neoplastic, and vascular diseases, alongside congenital and degenerative disorders.

Leseprobe

Contents

Preface

CHAPTER ONE PHYSICS

Diagnostic Radiology

Nuclear Medicine or Gamma Scintigraphy

Computed Tomography

Ultrasonography

Magnetic Resonance Imaging

T2 Sequence

T1 Sequence

STIR Sequence

FLAIR Sequence

Gradient Echo Sequence

Hardware Artifacts

Phase and Frequency Artifacts

Chemical Shift Artifact

Fold-Over or Aliasing Artifact

Truncation Artifact

Magnetic Susceptibility Artifact

Volume Averaging Artifact

Magic Angle Artifact

Cross-Talk Artifact

Safety

CHAPTER TWO VETERINARY CLINICAL MAGNETIC RESONANCE IMAGING

Neuroanatomical Considerations

Supratentorial Nervous System

Infratentorial Nervous System

Individual Cranial Nerves

Non-Neural Intracranial Elements

Important and Definable Landmarks

Diagnosis of Pathophysiological Conditions of the Intracranial Nervous System

Cerebral Edema

Intracranial Vascular Disease

Seizures

Ventricular Obstruction (Also See Section “Hydrocephalus” In This Chapter)

Increases in Intracranial Pressure

Terminal Effects of Compartmentalized ICP Increases—Brain Herniation

Diagnosis of Specific Intracranial Diseases

Intracranial imaging artifacts

Bibliography

Anatomy

General Types of Spinal Cord Pathophysiology

Diagnosis of Spinal Cord Disease Processes

MR Bibliography

Anatomical Considerations

Specific Diseases

MR Bibliography

CHAPTER THREE ORTHOPEDIC

Imaging Technique

Brachial Plexus Disease

Pelvic Region

Nerve Sheath Tumors

Stifle Joint

Tarsus and Carpus Regions

Bibliography

CHAPTER FOUR MAGNETIC RESONANCE IMAGING OF ABDOMINAL DISEASE

Normal Abdominal Anatomy

Abnormalities of the Liver and associated structures

Abnormalities of the kidneys and adrenal glands

Other Associated Organs

Portosystemic Shunts

Other vascular abnormalities

Bibliography

CHAPTER FIVE THORAX

Anatomy

Imaging Procedure

Cardiac

Mediastinal

Pulmonary

Bibliography

CHAPTER SIX HEAD—NON-CNS

Anatomy

Imaging Procedure

Nasal Cavity

Oral Cavity

External, Middle, and Inner Ear

Orbit Conditions

Other

Conclusion

Bibliography

CHAPTER SEVEN CANCER IMAGING

Introduction

Technical Considerations for Cancer MR

Whole Body MR for Cancer Staging

MR Imaging Techniques for Radiation Therapy Planning

Imaging Residual Tumor and Tumor Recurrence

Physiological and Molecular Imaging With MR

Specific Areas

Conclusion

Acknowledgments

Bibliography

Color plates

Index

Edition first published 2009© 2009 Wiley-Blackwell

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Library of Congress Cataloging-in-Publication Data

Gavin, Patrick R.Practical small animal MRI/Patrick R. Gavin, Rodney S. Bagley.p.; cm.Includes bibliographical references and index.ISBN-13: 978-0-8138-0607-5 (alk. paper)ISBN-10: 0-8138-0607-0 (alk. paper)1. Veterinary radiography. 2. Magnetic resonance imaging. I. Bagley, Rodney S. II. Title.[DNLM: 1. Magnetic Resonance Imaging–veterinary. SF 757.8 G283p 2009]SF757.8G38 2009636.089’607548–dc222008040608

Disclaimer

The contents of this work are intended to further general scientific research, understanding, and discussion only and are not intended and should not be relied upon as recommending or promoting a specific method, diagnosis, or treatment by practitioners for any particular patient. The publisher and the author make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of fitness for a particular purpose. In view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of medicines, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each medicine, equipment, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. Readers should consult with a specialist where appropriate. The fact that an organization or Website is referred to in this work as a citation and/or a potential source of further information does not mean that the author or the publisher endorses the information the organization orWebsite may provide or recommendations it may make. Further, readers should be aware that Internet Websites listed in this work may have changed or disappeared between when this work was written and when it is read. No warranty may be created or extended by any promotional statements for this work. Neither the publisher nor the author shall be liable for any damages arising herefrom.

DEDICATION

This work is dedicated to our former and current radiology and neurology residents, the animal patients, and their owners.

Rod Bagley

Pat Gavin

PREFACE

Dr. Rod Bagley and I discussed writing a textbook on Veterinary MRI for several years. Rod had already published a textbook, and knew the tremendous amount of work involved. We tackled this text with enthusiasm, but also with a degree of trepidation due to the daunting task.

Our goal in writing this textbook was not to have an exhaustive referenced rehash of material that is already present in text. Our goal was to provide a useful, clinical Veterinary MRI text. We were fortunate in starting magnetic resonance imaging of the brain in 1986. Magnetic resonance imaging for spontaneous brain tumors in dogs was a foundation for our research project. MRI continued to be a cornerstone of our research project to monitor the response of the brain tumor treatment and to evaluate normal tissue toxicities.

Washington State University had the oldest teaching hospital in North America in the 1990s. In 1996, we moved into a new facility that included a 1.0 Tesla superconducting magnetic resonance unit. We were the first veterinary medical college in an U.S. university with a state-of-the-art in-house superconducting MR unit. We were enthusiastic to expand beyond the imaging of brain tumors, and our initial efforts were in spinal disease. Our initial studies were not very good, and we had to methodically evaluate numerous pulse sequences and image planes to arrive at a clinically useful study. Magnetic resonance imaging was evolving rapidly in all fields, and some of the anatomical differences with our small patients necessitated a change from previous human protocols. We became comfortable in the spine, and then advanced into imaging for other conditions of the head, orthopedic disease, thorax, and abdomen. We also embarked on the imaging of the limbs of live adult horses. Our equine studies will not be covered in this textbook and will need to be treated in a separate volume.

The superior visualization of soft tissues of the body has allowed for the imaging of virtually all disease processes. The improved conspicuity allows clinicians of multiple disciplines to have clear visualization of the disease process. This text is to provide examples of our experiences that have been gained over the past 21 years. In addition to our experiences, some of the examples come from my active collaboration with other sites. These include the IAMS Pet Imaging Centers in Vienna, Virginia, Raleigh, North Carolina, and Redwood City, California. Examples have also been used from Dr. Michael Broome’s Advanced Veterinary Medical Imaging Center in Tustin, California. These centers, coupled with the Washington State University cases make up the bulk of the material for the figures in the text, but other images come from Dr. Kelley Collins, Veterinary Imaging Center in Ambler, Pennsylvania, Oakridge Veterinary Imaging in Edmond, Oklahoma, Tacoma Veterinary MRI in Tacoma, Washington, and Veterinary Neurological Centers in Phoenix, Arizona.

The images chosen are realistic examples of common abnormalities. We have endeavored to provide good quality images, but not ones that cannot be readily obtained virtually all superconducting magnets. There are no permanent magnet images due to lack of availability of such images in our files. The images are shown with the patients right on the viewers left, unless otherwise indicated. The images are generally shown with the dorsal anatomic area to the top of the image, even when acquired differently. If the dependence of the image is important, that will be given in the figure legend.

There is intentional overlap of some diseases in the various chapters. For example, neoplasia of a peripheral nerve may be covered in Chapter 2 (Section 3) on the peripheral nervous system, Chapter 3 on orthopedic disease, and Chapter 7 on MR imaging for cancer. The studies may have been requested for different reasons, that is loss of function, lameness, or for radiation therapy planning, and this redundancy should help the reader find the material for the clinical problem as presented.

We considered an exhaustive library of normal images, but discarded that notion. All studies have variability due to species, individual variation, technique, and volume averaging. Therefore, it is impossible to show an example that would fit all needs. We have given limited normal information, and have endeavored to illustrate common misunderstandings.

The text has limited information on physics, sequence selection, and artifacts. There are many superb texts that delve into these topics in great detail. We have only provided a skeleton of that material to facilitate the discussion of the case material presented in the various chapters. Magnetic resonance imaging is just now becoming an accepted modality throughout the veterinary profession. We have been fortunate to be among the early adaptors of this exciting technology. We hope this text will aid you in the continued exploration and discovery of new information.

We would like to thank Dr. Susan Kraft, DVM, PhD DACVR and Dr. Shannon Holmes for their superb contributions. Finally, we would like to thank the many students, interns, residents, and colleagues that helped us learn from our mistakes.

CHAPTER ONE

PHYSICS

SECTION 1

Comparative Imaging

Patrick R. Gavin

Diagnostic imaging has always been a mainstay of the armamentarium for the veterinarian. Veterinarians have limited resources available as regards history and routine screening procedures. Therefore, diagnostic imaging has a major role in the workup of numerous veterinary patients. An overreliance on diagnostic imaging has been observed by numerous clinicians; however, the move toward less invasive diagnostic procedures with a high precision of diagnosis has continued to drive this phenomenon. This chapter deals with the advances in diagnostic imaging through the last 60 years.

DIAGNOSTIC RADIOLOGY

Diagnostic radiology was invented in the late 1800s. The use of diagnostic radiology was rewarding primarily in the study of skeletal structures. However, due to the cost of the equipment, lack of education, and potential risks, the modality did not penetrate veterinary medicine until approximately the 1950s. Initially, these were the colleges of veterinary medicine in North America that possessed the equipment to perform diagnostic radiographic examinations. There were no trained radiologists at that time and in some places the studies were often performed and interpreted by non-veterinarians. Clinicians did not know what to expect as they had no prior knowledge of the diagnostic modality. Clinicians were often asked if they wanted a V/D or lateral and would merely say “yes” at the answer and accept the outcome. Much was to be learned.

Diagnostic radiology advanced rapidly in veterinary medicine, and the first examinations for veterinary radiologists were performed by charter diplomates for the American College of Veterinary Radiology in 1965. Following this beginning veterinary radiology advanced rapidly. Diagnostic radiology was utilized in multiple species throughout colleges of veterinary medicine and in selected practices. By the early to mid-1970s, advanced radiographic procedures including fluoroscopy and angiography were available, though primarily at colleges of veterinary medicine. The use of diagnostic radiology expanded with improved knowledge, especially with better understanding of its diagnosis of various pathologic conditions. The use of diagnostic radiology abated somewhat with the advance of diagnostic ultrasonography; however, it has remained the stalwart of diagnostic imaging in the veterinary profession. At the current time, there is a major push to move from conventional analog film screen technology to computed and/or digital radiography. It is presumed that veterinary radiology will continue to follow the progression realized in human radiology.

NUCLEAR MEDICINE OR GAMMA SCINTIGRAPHY

The previously used term, nuclear medicine, fell out of favor with the antinuclear movement of the 1970s. Medical personnel were quick to adopt the softer terminology of gamma scintigraphy that facilitated its continued development as an imaging modality. While gamma scintigraphy has the advantage of visualizing physiologic and temporal pathologic changes, for the most part its greatest use in veterinary medicine has been static studies for the diagnosis of skeletal disease. The use of the modality for the diagnosis of skeletal disease is well documented. The challenges of using nuclear isotopes, radiation safety concerns, and time delays are well documented. Some studies have become rather routine in veterinary medicine. These include studies of the thyroid gland that have been published and have led to a better understanding of thyroid disease.

While this modality has been present since the turn of the century, it became rather commonplace in veterinary medicine in the 1980s. Its involvement as a diagnostic modality has undergone little evolution in the last two decades.

COMPUTED TOMOGRAPHY

Computed tomography (CT) was first utilized in the mid-1970s in veterinary medicine, primarily for the diagnosis of intracranial disease. The modality was modified for the study of large animal species shortly thereafter. CT has had a large expansion in the veterinary medical field. Virtually all colleges of veterinary medicine provide this diagnostic modality. In the last 10 years, extension into private veterinary practices has significantly expanded its availability. There are now numerous large specialties, and even general practices, with CT on site. Many units were purchased as used equipment, but many include state-of-the-art helical units.

CT uses the same basic physical principles as diagnostic x-ray, except it depicts the shades of gray in cross-section. It is also possible to better visualize different tissues and the pathologic change within them, if present. Therefore while the modality is similar to diagnostic x-ray, CT is superior in diagnosis because the axial images are far superior to the two-dimensional radiographic projections. CT has led a renaissance in the understanding of three-dimensional anatomy and physiologic principles.

ULTRASONOGRAPHY

Ultrasonography became a clinical imaging modality in veterinary medicine in the late 1970s. It languished in veterinary colleges through much of the 1980s as the technology advanced. The initial technology of static B-mode machines was replaced by real-time machines that allowed an approximate 80% reduction in scanning time. The resolution and utility of the studies improved at the same time. However, diagnostic ultrasonography did not hit its stride and become mainstream in the United States until approximately the 1990s. Now, most large veterinary practices (and certainly referral practices) have diagnostic ultrasonography. This modality is also available in many smaller private practices. There have been numerous technologic advancements that have improved the quality of this modality. Increased availability of traveling diagnostic radiologists and/or interpretation via teleradiology have improved diagnostic outcomes.

Other specialists utilizing diagnostic ultrasonography, including cardiologists and internists, have further fueled the expansion of this modality in veterinary medical practice. Currently, most ultrasonographic examinations are performed by licensed veterinarians. It is this author’s opinion that in the future, many of these procedures will be performed by trained ultrasonographers and interpreted by radiologists, just as occurs in the human field. In the human field, there is a greater medical liability issue, and if physician radiologists can make it work, certainly veterinary radiologists can work in this format to further advance this modality’s utility in the diagnosis of our veterinary patients.

MAGNETIC RESONANCE IMAGING

Magnetic resonance imaging (MRI) came into clinical utility in the mid-1980s. It was utilized in veterinary medicine primarily as a research tool in the 1980s and early 1990s. In the mid-1990s, some areas began to use MR as a routine clinical modality. The procedure was applied to large animal imaging a few years later. However, the attitude of “not invented here” plagued the inclusion of MRI for the diagnosis of veterinary patients at many sites in the early years. Many veterinary sites had antiquated equipment or equipment with poor reliability, which gave it the aura of an unreliable diagnostic modality. However, as more sites gained modern diagnostic equipment, the utility of the modality became apparent.

Following the change of the millennium, MR became the modality of choice for the veterinary neurologist for the examination of disease processes involving the brain and spinal cord. Efforts to expand the use of the modality included corporate sponsorship of diagnostic facilities. At the time of this writing, this author is aware of more than 40 sites dedicated to MR imaging of animals using what would be considered modern state-of-the-art equipment. One limitation has been the non-availability of appropriately trained veterinary radiologists with expertise in this modality capable of providing accurate diagnoses of clinical conditions. Currently, the American College of Veterinary Radiology does not require training time minimums in MRI for their core curriculum, as not all training sites have this modality available. Therefore many veterinary radiologists, and others, must essentially undergo “on the job training” in the use of this modality.

There is a broad spectrum of equipment options. These options span from the currently available best, including machines capable of functional MRI, commonly utilized super conducting magnets, cost-effective mid-field units, to even less expensive but less capable low field permanent magnets. It is this author’s opinion that equipment generally costs what it is worth. Therefore, equipment that is more expensive is of more diagnostic worth, and conversely, equipment that costs less has less diagnostic capability. The equipment purchase balance will be finding equipment that provides the utility required for the financial reality of the practice. There has been a rapid development of equipment in the last few years.

SECTION 2

Basic Physics

Patrick R. Gavin

It is beyond the scope of this text to do an extensive treatise of the physics of MRI. There are several excellent texts, as well as numerous study guides, and even impressive volumes of free information on the Internet that can be consulted for more in-depth information on patient MR physics. This chapter outlines the salient features of the physics of MRI to allow a better understanding of image, and artifact, production, and visualization.

Current clinical applications for MRI rely on visualization of the hydrogen atom’s nucleus. This physical property was previously known as nuclear magnetic resonance, that is, the hydrogen atom nuclei resonate. The word nuclear does not refer to radioactivity, but merely refers to the nucleus of the atom. For more politically correct names it has become known as MRI. The basic physical principle is that a moving electrical charge produces a magnetic field. The size of the magnetic field is dependent on the speed of movement (magnetic movement) and the size of charge. While the hydrogen nucleus has a small electric charge it spins very fast. These physical attributes in concert with the abundance of the hydrogen nucleus within the body produce a detectable magnetic field.

Magnetic field strengths are measured in units of gauss (G) and tesla (T). One tesla is equal to 10,000 gauss. The earth’s magnetic field is approximately 0.5 G. The strength of MRI is similar in strength to the electromagnets used to pick up large heavy scrap metal. Materials can be ferromagnetic, paramagnetic, supraparamagnetic, or diamagnetic. Ferromagnetic materials generally contain iron, nickel, or cobalt. These materials can become magnetized when subjected to an external magnetic field. In MR images, these materials cause large artifacts characterized by the properties of signal and distortion of the image. These artifacts can be seen in MR images even when the ferromagnetic substances are too small to be seen on conventional radiography. Commonly seen sources of these artifacts are microchips, ameroid constrictors, certain bone plates, gold-plated beads, and colonic contents.

Paramagnetic materials include ions of various metals such as iron (Fe), manganese (Mg), and gadolinium (Gd). These substances can also have magnetic susceptibility, but only about 1/1,000 that of ferromagnetic materials. These substances increase the T1 and T2 relaxation rates. Because of this property, chelates of these elements make ideal components of MR contrast agents. Gadolinium chelates are the most common agents and generally cause an increase in T1-weighted signal. This is seen as increased hyperintensity (brightness) in T1-weighted images. At very high gadolinium concentrations, as seen in the urinary bladder, loss of signal can be seen as a result of T2 relaxation effects dominating.

Supraparamagnetic elements are materials that have ferromagnetic properties. The most commonly used is super paramagnetic iron oxide (SPIO), which is an iron (Fe)based contrast agent for liver imaging. These have been used minimally in veterinary MR. Diamagnetic materials have no intrinsic magnetic moment, but can weakly repel the field. These materials include water, copper, nitrogen, and barium sulfate. They will cause a loss of signal and have been seen as a loss of MR signal in images made after the administration of barium sulfate suspensions.

Since hydrogen is the common element used to make an MR clinical image, we will discuss the process of image formation. When hydrogen is placed within a large external magnetic field, the randomly spinning protons (hydrogen nucleus) will come into alignment with the external field. Some of the protons align with the field and some align against the field, largely canceling each other out. A few more align with the field than against it. The net number aligning with the magnetic field is very small. Approximately, three protons align with the field for every one million protons as 1.0 T. This number is proportional to the external magnetic field strength. While this number appears very small, the abundance of hydrogen allows for high-quality images. For example, in a typical volume imaging element termed a voxel, the number of protons aligned with the field would be roughly 6 × 1015.

Basic physics dictate that the energy is proportional to the nuclei’s unique resonant frequency in MR; this is called the Larmor frequency. The frequency of the spinning of the hydrogen nuclei is relatively low. The resonance frequency is proportional to the external magnetic field, which for hydrogen is equal to 42.56MHz/T. MRI is able to make high-quality images, not because of the energy of the spinning protons, but due to the abundance of hydrogen protons present in the body. The spinning or “resonating” of nuclei occurs because of unpaired electrons in the orbital shell. Each nucleus with this characteristic will resonate at a unique frequency. The spinning protons act like toy tops that wobble as they spin. The rate of wobbling is termed precession. These precess at the resonance or Larmor frequency for hydrogen.

If a radiofrequency (RF) pulse is applied at the resonance frequency, the protons can absorb that energy. The absorption of energy causes the protons to jump into a higher energy state. This causes the net magnetization to spiral away from the main magnetic field, designated B0. The net magnetization vector, therefore, moves from its initial longitudinal position a distance proportional to pulse, which is determined by its temporal length and strength. After a certain length of time, the net magnetization vector would rotate 90° and lie in a transverse plane. It is at this position that no net magnetization can be detected. When the RF pulse is turned off, three things start to happen simultaneously:

1. The absorbed energy is retransmitted at the resonance frequency.

2. The spins begin to return to their original longitudinal orientation, termed the T1 relaxation.

3. While the precessions were initially in-phase, they begin to de-phase, termed T2 relaxation.

The return of the excited nuclei from the high energy state to their ground state is termed T1 relaxation (or spin–lattice relaxation). The T1 relaxation time is the time required for the magnetization to return to 63% of its original longitudinal length. The T1 relaxation rate is the reciprocal of the T1 time (1/T1). T1 relaxation is dependent on the magnetic field strength that dictates the Larmor frequency. Higher magnetic fields are associated with longer T1 times.

T2 relaxation occurs when spins in high and low energy states exchange energy but do not lose energy to the surrounding lattice as occurs in T1 relation. It is, therefore, sometimes referred to as spin–spin relaxation. This results in loss of transverse magnetization. In biological materials, T2 time is longer than T1 time. T2 relaxation occurs exponentially like T1 and is described as the time required for 63% of the transverse magnetization to be lost. In general, T2 values are unrelated to field strength. In patients, the magnetic signal decays faster than T2 would predict. There are many factors creating imperfections in the homogeneity of the magnetic field, including the magnet and patient inhomogeneities including surface contours, air–tissue interfaces, and any metal the patients may have within them, including dental work, staples, and orthopedic appliances. The sum effect of all of these inhomogeneities pronounces an effect called T2*. The T2 relaxation comes from random interactions, while T2* comes from a combination of random and fixed causes including magnet and patient inhomogeneity.

To attempt to negate the fixed causes, a 180° refocusing pulse is used. Consider the following analogy, three cars in a race going at different speeds. At the start, all the cars are obviously together, and can be thought of as being in-phase. At some time after the start of the race, there is a noticeable difference between them due to different speeds; they are in essence out-of-phase. At that time, everybody will turn around and go back toward the starting line. If it is assumed that everyone is still going at the same rate as before, then they will all arrive at the starting line together and in-phase. The time required for the atoms to come back in-phase is equal to the time it took for them to lose phase. This total time is called the “TE” or echo time. The 180° pulse is used to reverse the T2* de-phasing process. As soon as the spins come back into phase, they will immediately start to go out-of-phase again. The two variables of interest in spin echo (SE) sequences are (1) the repetition time (TR) and (2) the echo time (TE). All SE sequences include a slice-selective 90° pulse followed by one or more 180° refocusing pulses. This refocusing pulse can be applied multiple times. The use of multiple refocusing pulses is the basis for fast or turbo spin–echo imaging, FSE, or TSE respectively.

Images of T1 and T2 relaxation are produced by sampling the signal at various times. Both effects are always present; however, we will often accentuate one effect over the other such that the sequences are often properly termed T1-weighted or T2-weighted images. To produce the cross-sectional images, gradient coils are needed, which produce deliberate variation in the main magnetic field. There is one gradient coil in each Cartesian plane direction (X, Y, and Z planes). These slight variations in the magnetic field will allow for slice selection and phase and frequency encoding. The slice selection gradient will be the Z, X, and Y gradients for a patient in supine position for the transverse, sagittal, and dorsal plane sequences, respectively.