Getting Started in Clinical Radiology E-Book

George W. Eastman

• Herausgeber: Thieme
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 709

Veröffentlichungsjahr: 2005

Beschreibung

"I read the book for enjoyment and pleasure, as well as enlightenment. It was a delightful learning experience." --Thomas Lee Bucky, MD This book teaches radiology in a way that mimics a lively setting on the wards. To have fun in learning the theoretical basis of imaging and the interpretation of radiographs and other modalities, in the context of clinical examination and findings, this is the book for you. The basics of imaging are described using analogies from daily life to make them as understandable and memorable as possible. The material of radiology is described using actual cases; the most common differential diagnoses are presented. A great amount of image material supports the learning process. A storyline runs through the book: four students in their final year of medical school are involved in active discussion of the cases, so that the reader also feels a part of the diagnostic process.

Leseprobe

Getting Started in Clinical Radiology

From Image to Diagnosis

George W. Eastman, M.D.Professor of RadiologyVirchow Campus of the CharitéHumboldt University and Free University of BerlinBerlin, Germany

Christoph Wald, M.D., Ph.D.Assistant Professor of RadiologyTufts University School of MedicineBoston, USADepartment of RadiologyLahey ClinicBurlington, MA, USA

Jane Crossin, M.D.Senior Lecturer Medical ImagingDepartment of Medical ImagingRoyal Brisbane HospitalBrisbane, Australia

1035 illustrations

ThiemeStuttgart · New York

This book is dedicated to Gustav Bucky—radiologist, inventor, teacher.And with love to Mary and Jerry Crockett.

Library of Congress Cataloging-in-Publication Data

Eastman, George W.

Getting started in clinical radiology : from image to diagnosis / George W. Eastman, Christoph Wald, Jane Crossin.

   p. ; cm.

Includes index.

ISBN 3-13-140361-6 (GTV : alk. paper) – ISBN 1-58890-356-7 (TNY : alk. paper)

   1. Radiology, Medical–Outlines, syllabi, etc.

   2. Diagnostic imaging–Outlines, syllabi, etc.

[DNLM: 1. Diagnostic Imaging–Problems and Exercises.

2. Radiology–methods–Problems and Exercises.

WN 18.2 E13g 2005] I. Wald, Christoph. II. Crossin, Jane.

III. Title.

RC78.17.E37 2005

616.07’57–dc22

2005016549

Illustrator: Andrea Schnitzler, Innsbruck, Austria

© 2006 Georg Thieme Verlag,

Rüdigerstrasse 14, 70469 Stuttgart, Germany

http://www.thieme.de

Thieme New York, 333 Seventh Avenue,

New York, NY 10001 USA

http://www.thieme.com

Typesetting by Mitterweger & Partner, Plankstadt

Printed in Germany by Grammlich, Pliezhausen

ISBN 3-13-140361-6 (GTV)

ISBN 1-58890-356-7 (TNY)

Important note: Medicine is an ever-changing science undergoing continual development. Research and clinical experience are continually expanding our knowledge, in particular our knowledge of proper treatment and drug therapy. Insofar as this book mentions any dosage or application, readers may rest assured that the authors, editors, and publishers have made every effort to ensure that such references are in accordance with the state of knowledge at the time of production of the book.

Nevertheless, this does not involve, imply, or express any guarantee or responsibility on the part of the publishers in respect to any dosage instructions and forms of applications stated in the book. Every user is requested to examine carefully the manufacturers’ leaflets accompanying each drug and to check, if necessary in consultation with a physician or specialist, whether the dosage schedules mentioned therein or the contraindications stated by the manufacturers differ from the statements made in the present book. Such examination is particularly important with drugs that are either rarely used or have been newly released on the market. Every dosage schedule or every form of application used is entirely at the user’s own risk and responsibility. The authors and publishers request every user to report to the publishers any discrepancies or inaccuracies noticed. If errors in this work are found after publication, errata will be posted at www.thieme.com on the product description page.

Some of the product names, patents, and registered designs referred to in this book are in fact registered trademarks or proprietary names even though specific reference to this fact is not always made in the text. Therefore, the appearance of a name without designation as proprietary is not to be construed as a representation by the publisher that it is in the public domain.

This book, including all parts thereof, is legally protected by copyright. Any use, exploitation, or commercialization outside the narrow limits set by copyright legislation, without the publisher’s consent, is illegal and liable to prosecution. This applies in particular to photostat reproduction, copying, mimeographing, preparation of microfilms, and electronic data processing and storage.

Foreword

The opening sentence says it all: “Radiology can be a lot of fun!” It summarizes what is unique about this book.

Radiology books designed for medical students have as their main purpose an introduction to the science and art of medical imaging. Behind this obvious purpose is an implicit intent also to fascinate students, and thereby to inspire some of the most susceptible and capable to choose a career in radiology. An early attempt to inspire students grew out of a classroom medical student teaching program, in which the radiologist Lucy Frank Squires was assisted by students and radiology trainees like myself. That course was wildly successful and attracted many students to a lifetime interest in radiology. What made this program unique was its light-hearted approach and the use of everyday household objects to explain radiological principles to the students, and to make them feel comfortable in the process.

This text by George W. Eastman, Chris Wald, and Jane Crossin is, in many ways, an extension of that successful humanistic formula for medical student teaching. The authors have captured our attention by introducing the subject through the eyes of fictional medical students to whom they have given form, substance, and personalities with emotions and fears. Although fictional, the characters are realistic in their foibles. What is new and different in this book is its clever use of these students to make us inquisitive about them as well as the real subject matter. This process relieves some of the inherent dryness of the topic by involving our hearts in the sharing of the uncertainties and concerns of the characters, and it captures our attention.

The thread of human connection to our fictional students weaves its way through the book. In the introduction we learn of the diverse backgrounds of the students, something of their private lives, and gain an inkling of their interactions with each other. In the chapter on chest radiology, we sympathetically experience the challenge of the subject material through their eyes.

The complexity of modern radiology is reflected in the organization and content of the book. The students’ introduction to radiology starts with technical aspects of basic image acquisition and extends to the fundamentals of psychophysics in image perception, an important topic often overlooked in radiology texts. What follows includes principles of disease detection, disease diagnosis, and appropriate examination selection. As one who was a radiology trainee in the 1960s, I never cease to be amazed at how simple life was at that time. One chose between either film radiography or fluoroscopy; there was nothing else but nuclear medicine, which was then still in its infancy. Now, the wide range of imaging modalities makes it essential to learn how to choose between them to make the best use of imaging.

For this voyage of the medical student into the world of radiology, the authors have set sail toward a unique polar star that encompasses humanism as well as comprehensive imaging science. The text promises to introduce and guide a new generation of students into the fascinating world of radiological imaging.

Reginald Greene

A Word of Thanks

We would like to thank all who have so generously contributed to the development of the overall concept and final realization of this book. First of all there are the many students and residents we persuaded to act as “didactic guinea pigs” for us. Their remarks were helpful and encouraging, sometimes keenly observed: “Awkward style!” Their contributions were substantial. The same holds true for a number of residents and fellows as well as staff radiologists in our respective departments poring over parts of the book and sharing their views on particular features. To ensure that the cases provided were not only radiologically correct but also reflected the referring physicians’ point of view, we asked quite a number of colleagues from other specialties to review the respective chapters. In particular we would like to thank Professor Hartmann (Ophthalmology), Dr. Schlunz (Facial and Plastic Surgery), Dr. Matthias (Ear Nose and Throat Surgery), Dr. Kandziora (Trauma Surgery), all from the Charité Hospital in Berlin; Professor Wagner (Radiology) from Marburg University; and Professor von Kummer (Neuroradiology) from Dresden University. All analogies used in Chapter 3, “Tools in Radiology”, were double-checked for correctness from an engineering point of view by Dr. Anton of Siemens Medical Systems. We would also like to acknowledge the support of Thavaganeshan Vasuthevan of GE Medical Systems.

We owe special thanks to Professor Wermke of the Charité for the permission to use his ultrasound images for Chapter 9, “Gastrointestinal Radiology.”

We are grateful to a long list of colleagues (see opposite) who have supported this book by supplying us with some of their best case material or in other ways.

None of this would have happened had it not been for the support of the publishers, Thieme. Special thanks go to Cliff Bergman, Juergen Luethje, and Antje Voss. They readily adopted the concept and enhanced or smoothed over parts of it where this was felt to be necessary. They accompanied the book—with patience and motivation—through the production phase.

Each one of us has—at different times in our professional lives—benefited from working with inspired radiologists who had the ability to plant the enthusiasm for practicing and teaching radiology in our heads and hearts. On G.W.E.’s side these were Drs. Jürgen Freyschmidt, Hans-Stefan Stender, Klaus Langenbruch, Reginald Greene, Dan Kopans, Ad van Voorthuizen, and Jan Vielvoye. Among others, Drs. Robert E. Wise, Frank Scholz, Alain Pollak, and Roger Jenkins from the Lahey Clinic in Boston have been an invaluable inspiration for C.W. to remain in an academic career and look beyond the obvious. J.C. thanks Drs. Gord Weisbrod, Steve Herman, and Naeem Merchant in Toronto for sharing both their enthusiasm for radiology and their encyclopedic radiological knowledge. All of us loved to learn with books by Benjamin Felson, Clyde Helms, and Lucy Frank Squire.

Our families have, of course, felt the ups and downs of this project the most. The ease and the many different ways in which our children learn about this world we live in were a great source of ideas. The critical minds of our spouses put an end to many initial little afterthoughts, that, on reflection, it would have been unwise to include in this book. Many thanks for their patience.

Finally, this book—like all of radiology—is a dynamic affair. Any comments, criticisms, and suggestions for improvement are most welcome and will be considered in its further development. All those involved in teaching who would like to contribute first-rate didactic material are also invited to do so. All contacts can be made via [email protected].

George W. EastmanChris WaldJane Crossin

Colleagues and Co-workers Who HaveContributed Images to this Book

Chapter 6 Paul Bode of LeidenFig. 6.7Ulrike Engert of BerlinFig. 6.25Hans-Holger Jend of BremenFig. 6.31Matthias Jürgens and Michaela Fahrenkrug of BerlinFig. 6.33Udo Kaisers of BerlinFig. 6.43cAjay Chavan of OldenburgFig. 6.56 Chapter 7 Hans-Frank Böttcher of BerlinFig. 7.2 b, cJörg Hendrik Seemann of BerlinFigs 7.2d–f; 7.16c, d; 7.18a–dMatthias Gutberlet of BerlinFig. 7.2g, h, jPeter Ewert of BerlinFig. 7.2iJens Ricke of BerlinFig. 7.3a–cUlf Karl-Martin Teichgräber of BerlinFig. 7.14Hans-Joachim Wagner of MarburgFig. 7.12a, bPetr Podrabsky of BerlinFig. 7.13a, b Chapter 8Walter T. Kating of BerlinFig. 8.11Thomas Schnalke and Christa ScholzFigs. 8.19b; 8.21bJohannes Hierholzer of PotsdamFig. 8.24cHelga Bertram of BerlinFig. 8.26Gerwin Lingg and Corinna Schorn of Bad KreuznachFigs. 8.48–8.51; 8.72–8.75Special thanks to the Rugby Club of BerlinFig. 8.29c Chapter 9 Rainer Roettgen of BerlinFigs. 9.1a, b; 9.25bUlrike Engert of BerlinFig. 9.2cDieter Gläser of BerlinFigs. 9.7b; 9.70lJoachim Werner Kaufmann of BerlinFigs. 9.20a; 9.25aJohannes Hierholzer of PotsdamFig. 9.22a, bThomas Riebel of BerlinFig. 9.26a–dPetr Podrabsky of BerlinFig. 9.37a–cHelga Bertram of BerlinFig. 9.37d, eWolfram Wermke of BerlinFigs. 9.41a; 9.42a; 9.43a (left), b; 9.45a; 9.47a; 9.48a; 9.49a; 9.50a; 9.56a, 9.59Michael Westphal of BerlinFig. 9.70kMatthias Grothoff of BerlinFig. 9.70n Chapter 10 Ricarda Rühl of BerlinFig. 10.1a, b Chapter 11 Thomas Liebig of HanoverFigs. 11.4; 11.30Stefan Niehus and Michael Werk of BerlinFig. 11.7f–iKarl-Titus Hoffmann of BerlinFigs. 11.15; 11.28Harald Bruhn of BerlinFig. 11.32 Chapter 11 Uta Zaspel of BerlinFig. 12.5Regina Bartezko of BerlinFig. 12.17aHanno Stobbe of BerlinFig. 12.17a, bMagdalena Bostanioglo of BerlinFigs. 12.11–12.14 Chapter 13 Arne Lemke of BerlinFig. 13.18Rüdiger von Kummer of DresdenFig. 13.19b–d Chapter 14 Walter T. Kating of BerlinFig. 14.9a, b

Contents

1   Why Another Textbook of Radiology?

Can You Imagine Radiology to be Fun?

What Is So Special about Learning (and Teaching) Radiology?

What Makes This Textbook Different to Others?

How Is This Book Structured?

Who Will Accompany You through This Book?

What Is There to Say about the Style of the Book?

2   Radiology’s Role in Medicine

What Is So Different in Radiology as Opposed to Other Clinical Disciplines?

Which Other Special Aspects Are There to Consider?

What Else Could Improve Your Compassion for the Radiologists?

A Short Run through Radiological Basics

3   Tools in Radiology

3.1 Projection Radiography

Generation of X-Rays

Attenuation of X-Rays

Detection of X-Rays

Techniques of Exposure

Contrast Media Examinations

Image Processing

3.2 Computed Tomography

Working Principle

Contrast Media

3.3 Ultrasonography

Working Principle

3.4 Magnetic Resonance Tomography

Generation of the MR Signal

What Is So Special about the “External” and “Internal“ Magnetic Fields?

How Do We Generate an MR Signal in a Salami?

Spatial Allocation of the MR signal

Analysis of the MR Signal

3.5 Our Perception

4   Phenomena in Imaging and Perception

4.1 What Do I Need to Know for Image Analysis?

Is the Quality of the Study Technically Adequate?

How Do I Analyze an Image?

Tissue Characteristics on Radiographic Images

What Is a Normal, What Is a Pathological Finding?

Where is the Pathology?

What Can Go Wrong in Perception?

4.2 Can We Reach a Diagnosis that Approaches Histological Certainty?

Are There Any Volume Changes?

What Happens to the Surrounding Anatomy?

What Is the Internal Structure Like?

What Pathology Commonly Occurs in a Particular Anatomical Region?

5   Risks, Risk Minimization, and Prophylactic Measures

5.1 The Nonindicated Study

5.2 The Ill-Prepared Study

5.3 Studies with Contrast Media

Contrast Media in Radiography and CT

Contrast Media in Magnetic Resonance Tomography

Contrast Media in Ultrasonography

5.4 The False Finding

5.5 Risks of Radiological Procedures

Risks of Projection Radiography and Computed Tomography

Risks of Ultrasound

Risks of Magnetic Resonance Tomography

5.6 Risks of Intervention

From Detection to Diagnosis and Beyond

6   Chest

6.1 How Do I Analyze a Radiograph of the Chest?

First Determine the Image Quality

Now Go Ahead and Analyze the Thorax

Now Get Additional Information from the Lateral Chest Radiograph

I See an Abnormality—What Do I Do Now?

6.2 Opacities in the Lung

Solitary, Circumscribed Opacity of the Lung

Multiple Lesions in the Lung

Diffuse, Homogeneous Opacity of the Lung

6.3 Acute Pulmonary Changes

Acute Diffuse Linear, Reticular, Reticulonodular (Interstitial) Pattern

Acute Diffuse Acinar, Confluent (Alveolar) Pattern

6.4 Chronic Lung Disease

Chronic Linear, Reticular, Micronodular (Interstitial) Pattern

6.5 Pulmonary Symptoms without Correlating Findings in the Chest Radiograph

6.6 Lesions in the Mediastinum

Widening of the Upper Mediastinum

Abnormal Findings of the Lower Mediastinum

6.7 Enlargement of the Hila

6.8 The Ultimate Exam

7   Cardiovascular and Interventional Radiology

7.1 Interventions in Vascular Occlusive Disease

Arterial Occlusion

Venous Obstruction

7.2 Tissue Biopsies

7.3 Insertion of a Drain

7.4 Implantation of a Transjugular Intrahepatic Portosystemic Stent-Shunt (TIPSS)

7.5 Implantation of a Vena Cava Filter

7.6 Implantation of a Port

7.7 Embolization

7.8 Neural Blockades

7.9 Gregory’s Test

8   Bone and Soft Tissues

8.1 How Do You Analyze a Bone Image?

Bone

Joints

Soft Tissues

I See an Abnormality—What Do I Do Now?

8.2 Diseases of the Bone

Focal Bone Lesions

Generalized Bone Diseases

8.3 Diseases of the Spine

8.4 Diseases of the Joints

Joints of the Upper Extremity

Joints of the Lower Extremity

8.5 Fracture and Dislocation

8.6 Soft Tissue Tumors

8.7 Gregory’s Test

9   Gastrointestinal Radiology

9.1 How Do We Analyze an Abdominal Radiograph?

What Can You Evaluate on an Abdominal Radiograph?

Why Are You Interested in the Standard Chest Radiograph in a Patient with Abdominal Pain?

I See an Abnormality—What Do I Do Now?

9.2 Patient with Acute Abdominal Pain

9.3 Diseases of the Esophagus

9.4 Diseases of the Small Bowel

9.5 Diseases of the Large Bowel

9.6 Problems with Defecation

9.7 Diseases of the Liver and the Intrahepatic Biliary System

Focal Liver Lesion

Diffuse Liver Disease

9.8 Diseases of the Extrahepatic Biliary System

9.9 Diseases of the Pancreas

9.10 Diseases of the Peritoneum and Retroperitoneum

9.11 Gregory’s Test

10  Genitourinary Tract

10.1 How Do You Assess a Renal Ultrasound?

I See an Abnormality—What Do I Do Now?

10.2 Renal Masses

10.3 Renal Volume Loss/Renal Atrophy

10.4 Increase in Renal Volume

10.5 Renal Calculi

10.6 Adrenal Tumors

10.7 Where Is Greg?

11  Central Nervous System

11.1 How Do You Analyze a Sectional Study of the Head?

Key Points of Thorough Image Analysis

I See an Abnormality—What Do I Do Now?

Are You Ready for Your First Case?

11.2 Perfusion Disturbances of the Brain

Brain Hemorrhage

Cerebral Infarction

11.3 Brain Tumors

Perisellar Brain Tumors

Tumors of the Cerebellopontine Angle

11.4 Neurodegenerative Diseases

11.5 Congenital Disorders of the Brain

11.6 Spinal Cord Tumors

11.7 Gregory’s Vernissage

12  Breast

12.1 How Do You Analyze a Mammogram?

How Do You Evaluate the Image Quality?

What Do You Have to Pay Attention to in Image Analysis?

Ready for Your First Case? Let's Go!

12.2 Tumorlike Lesions and Tumors of the Breast

12.3 Breast Implant

12.4 Tumors of the Male Breast

12.5 Dr. Skywang’s Test

13  Face and Neck Imaging

13.1 Diseases of the Nose and Sinuses

13.2 Disease of the Ears

13.3 Diseases of the Temporomandibular Joint

13.4 Injuries and Diseases of the Orbit

13.5 Diseases of the Neck

13.6 The Teeth You Need

14  Trauma

14.1 Polytrauma

14.2 Luxations and Fractures

14.3 Hannah’s Test

Solutions to the Test Cases

Post Scriptum

Index

1  Why Another Textbook of Radiology?

Can You Imagine Radiology to be Fun?

Radiology can be a lot of fun! It is this very personal experience of the authors that will accompany you throughout this book and hopefully throughout the rest of your medical life. It is also the main reason why we considered this book to be necessary. Can diagnostic imaging and the therapy of patients in need be a pleasant task? The answer is a resounding “yes.” Successful management in medicine relies on keeping a certain distance from the events. Empathy and respect are essential for a trustful relationship with the patient. The optimal path to the right diagnosis and subsequent adequate therapy, however, requires primarily clear thinking. Clear thinking, in turn, greatly profits from motivation, optimism, and enjoyment of what one is doing. The enthusiasm for a “great case,” which temporarily seems to ignore the often tragic personal fate of the patient, must not be taken away from the radiologist. The same is true for learning about radiology—as a student, as a young doctor: One has to enthuse the neophytes for the fascinating field of radiology!

What Is So Special about Learning (and Teaching) Radiology?

Radiology is a gigantic, continually growing specialty that gets ever more complex by the month. It is, for several reasons, not to be learned by heart. The tools of image acquisition and image analysis have to be mastered, i.e., their principles have to be understood. Understanding the principles of imaging—just like the understanding of any individual image—is primarily an intellectual challenge. It is on this foundation that specific knowledge can be accumulated, of course through reading the literature but most of all through very personal transfer of experience: “There is no substitute for a seasoned radiology teacher.” In few medical fields can the exchange of knowledge between the teacher and the trainee be as intense, interactive, and multifaceted as in radiology. Radiology for that reason is a didactic specialty “par excellence.” Using exemplary image material, most relevant diagnostic techniques can be taught and learned. That is the great opportunity of academic radiology—we just have to seize it.

What Makes This Textbook Different to Others?

Well … a lot of things. But one of the main ideas we try to convey in this book is the overriding importance of a sound indication for every radiological examination or therapy. The number of nonindicated examinations is unfortunately high; the driving forces are manifold: litigation, examinations that are “en vogue,” overworked referring doctors who would rather get the scan and then examine the patient, and the practice of self-referral by nonradiologists who have a financial interest in imaging the patient in their own private practice or institution. All lead to many unnecessary diagnostic examinations with unintended consequences for our patients. Overutilization also poses a threat for the future—i.e., your professional life and our healthcare systems—as it is not economically sustainable in any of today’s societies. We would like to infuse you with the right attitude and give a proper orientation of what is indicated when. The indication guidelines of the British Royal College of Radiologists under the title “Making the best use of a Department of Clinical Radiology” have thus been inserted into and adapted to this book.

How Is This Book Structured?

The first part of this book, entitled “A Short Run Through Radiological Basics,” will describe and hopefully allow you to understand the essentials of imaging. For starters, you are going to be fed the technical principles of image acquisition. To keep this part digestible, “normal life” analogies have been recruited wherever complex technologies made this necessary and where it was felt to be didactically appropriate. Subsequently we’ll take you through the phenomena and procedures that help you tackle image analysis in diagnostic imaging. We take special care to alert you to the importance of psychophysical perception: in a world filled with fantastically expensive imaging equipment it is still your visual and central nervous system that detects and categorizes disease. This fundamental truth is frequently underrepresented in other texts. Last but not least, you are going to learn about the obvious and not so obvious risks of imaging and image-guided therapy.

The second, the clinical, part of this book is entitled “From Detection to Diagnosis and Beyond.” You will get to know not only the specific examination modalities for each organ system but also the most efficient diagnostic work-up in emergency radiology—under circumstances you will encounter in your not too distant future professional medical life when you are most likely to make crucial decisions yourself. You will be confronted with cases to solve just as if you were already engulfed in clinical routine. Every individual problem is approached by a combination of image analyses, taking into account relevant available history, and whatever clinical symptoms you might be able to verify yourself. The path to the right diagnosis is then laid out—you just have to stay on it. The differential diagnoses are described in the approximate order of likelihood, if that does not interfere with the didactic point to be made. The traditional pathologically oriented approach thus takes a step back to leave center stage to radiological morphology: it is just you and the image you have to evaluate.

Who Will Accompany You through This Book?

Five medical students will see you through this book: Giufeng, Hannah, Joey, Paul, and Ajay. All of them are bright, highly motivated kids, well prepared by their teachers and eager to solve cases on their own. It goes without saying that they eventually present their findings to “their” radiologist in charge—to get the final blessing and to learn even more. Their first few weeks in radiology have made them inspired diagnosticians, running down interesting cases and not giving up before they find a convincing diagnosis. They are also a truly international bunch, having been attracted to this academic hospital in “down under” Sydney for a variety of reasons. (Hannah, Giufeng, Joey, Paul, and Ajay are, of course, fictitious persons. All stories relating to them are also pure fiction. We would like to thank our young colleagues and collaborators Juliane Stoll, Il-Kang Na, Ralph Patrick Chukwuedo, Ansgar Leidinger and Tino Bejach for the permission to use their pictures. Working together with them was a lot of fun. A great thanks goes to our pleasant young colleague Gero Wieners who posed as Gregory. The patients’ names are also fictitious. Similarities to real persons are not intended and are pure coincidence. The cases are didactically optimized and compressed to fit the objective of this book.)

Giufeng (Chinese for “the gentle one”) (Fig. 1.1) is a native of Sydney, to where her parents moved in the eighties straight from Singapore. As you can undoubtedly tell from the picture, she has developed a special interest in neuroradiology. She knows everything about the cranial nerves, their tracts and nuclei. The sensory organs are another one of her specialties. For that and other reasons, Gregory, the senior resident assigned to neuroradiology, frequently visits with her.

Hannah (Fig. 1.2) has come from Berlin for her final year in medical school. Her love of the sun, the beach, and classical music got her to the “emerald city.” If she had to pick a favorite field in radiology, she would probably choose musculoskeletal radiology. She has already made up her mind to try her luck in radiology, but if that doesn’t work out she will try to become an orthopod. She never loses control, however mixed-up things may be. Wiseguys get finished off by her with just a few carefully chosen words. Her private passion is—you guessed it already—surfing on Sydney’s Bondi beach.

Paul (Fig. 1.3) says he sucked radiology in with his mother’s milk. His father is a medical physicist, his mother a successful painter of abstract art, his brother a Melbourne investment broker almost unscathed by any bear attacks. Paul loves to dive into complex cases much like others get submerged in the latest thriller by Michael Crichton. In any case: He finds radiology a very attractive field—almost as attractive as … well, as far as Paul is concerned, he is getting sick and tired of this neuro guy and his interventions.

What Is There to Say about the Style of the Book?

Radiology is a thriving field with fashions, moods, fascinating personalities, and a lot of history to go around. Radiologists love to assign names to phenomena, signs, and techniques. Most of these are globally understood—radiology was a truly global thing from the very beginning. So there are a lot of Latin, German, and French terms—add a Greek cracker now and then. If they help us understand, we should use them. Some remind us of great physicians who were inventors, researchers, teachers. It does not hurt to acknowledge their accomplishments, and we support that by giving a little worthwhile or possibly useless information about them now and then in this book.

Ajay (Fig. 1.4) is originally from Johannesburg, South Africa, where his grandfather used to work with a certain Mahatma Ghandi. The family is rumored to be obscenely rich—car manufacturing, real estate, you name it. He is already married at the age of 25, much to the sorrow of the women around him. His wife is dashingly beautiful and three handsome kids are coming right after their father. Ajay has an untamable urge to tell delicate jokes to everyone, in one of four languages. He is interested in radiology because he loves to handle expensive hardware.

Joey (Fig. 1.5) has just managed to make the right histological diagnosis off just one radiograph—and seems to enjoy the experience. He will hopefully make this a habit. Joey just loves intervention. Every time he watches a difficult angiographic or drainage procedure, his fingers grab for imaginary catheters, guide wires, and needles. The interventional folks have recognized his passion for their trade and let him work with them whenever it is possible. As for his social life, he comes across as the “big loner.” Apart from that he is a cheerful guy from New York who has left that city for the first time in his life to do his radiology “down under.”

And then there is Gregory (Fig. 1.6), of course. As already mentioned, he is the young and enterprising senior resident with a special interest in neuroradiology. He has made it a habit to take care of the medical students—with very definite preferences and in more than one way. He is hoping for an academic career. His hormonal status is acknowledged with benevolent interest by many in the department. A nice guy at heart, he can turn into a son of a … at times. When you come right down to it, he is just one of us normal guys in academia.

2  Radiology’s Role in Medicine

What Is So Different in Radiology as Opposed to Other Clinical Disciplines?

A radiologist primarily approaches the patients by looking at images, in a procedure quite similar to the one pathologists normally follow but quite unlike what any other clinical specialist would do. The unbiased analysis of the image is the first, and undoubtedly an abstract, intellectual step. This certainly implies that radiologists must be pretty brainy, or else they can lay down their arms right there. Thus, there should be a little Sherlock Holmes in every one of them, although county sheriffs have also been reported to survive. It is in a secondary step that we study the clinical symptoms in order to verify, improve, or—yes—dump our diagnosis and go back to square one. This procedure has many advantages, but it makes radiologists vulnerable when information is withheld or cannot be correctly evaluated.

Which Other Special Aspects Are There to Consider?

The radiology department is basically a consultative service unit for the hospital. Few other disciplines can do without it. For that reason, communication with colleagues from other fields is tremendously important and not always without glitches. At the same time it is rather transparent to the referring doctors what the radiologists do and do not do; few colleagues talk about and document their work as well as radiologists do. Patient management and the administration of reporting as well as image distribution are further cornerstones for swift and effective diagnoses and interventions.

What Else Could Improve Your Compassion for the Radiologists?

A few, admittedly cocky, statements might get you on the road. A radiologist is:

The heroic person who presents—swiftly and accurately—hundreds of images to a bunch of hotheaded trauma surgeons in their morning round, some of whom have studied those very images with much more time and with the patient and her or his symptoms at hand. Any surgeon will tell you: There is nothing like chewing up a radiologist for breakfast before a great day in the operating room. You need a big heart and a lot of sympathy for all these colleagues whose psychological pressure at times surpasses that suffered by the radiological profession. The person who—on a single day—pronounces hundreds of patients to be healthy in heart and lung just on the basis of a single chest film. He or she then dares to put this down in writing, for all colleagues to see and question from then on to eternity. The person who—on the basis of rudimentary clinical data, if any—presents available image material at the noontime general medical radiology meeting, with listing of delicately weighted differential diagnoses for every patient, while at the same time out of the dark of the back of the room miraculously appears the hitherto unknown information that renders two-thirds of these differential diagnoses ridiculous. The person who has to reconsider all diagnostic and interventional procedures every half year because rapid technological and scientific developments in radiology make this absolutely necessary. To call it an end, the person who starts to shiver, groan, and giggle foolishly when finally coming across the splendid example of a pigmented villonodular synovitis that has been the missing link in the personal teaching file.

This has to be sufficient as justification for this book and as a peek into the soul and life of radiology.

A Short Run through Radiological Basics

3  Tools in Radiology

3.1 Projection Radiography

Good old projection radiography remains one of the staples of radiology, although a little over 100 years old. And it is by no means obsolete even in times of multimillion-dollar high-tech imaging equipment. The bulk of all diagnostic imaging studies is still done with this technology. Mammography, a prominent representative of this group, is the only imaging study that has been proven to lower patient mortality significantly—if performed correctly and, of course, only in women. The basic technical principle of projection radiography is simple. However, the complete chain of events from generating the x-ray beam to viewing the developed image can be full of surprises to keep even the “pro” busy making sure everything is done properly and the radiograph at hand is a quality product. With insufficient knowledge or lack of experience and care, things can easily derail—there are enough catastrophic studies to prove that point.

Generation of X-Rays

A high-voltage current is built up between a cathode and an anode, all of this inside a vacuum tube (Fig. 3.1). The cathode is heated to about 2000°C by a specific heating filament. Electrons are emitted by the cathode, accelerated by the electric field between cathode and anode, and hit the anode with considerable energy, where they induce electromagnetic radiation of the type called x-rays. These rays are richer in energy the higher the applied voltage. The area where the electrons hit the anode is called the focus. As a lot of heat is generated in the process, the anode consists of a heat-resistant disk covered with tungsten in most cases. The disk rotates quickly to disperse the heat along its circumference, thus forming a focal track. The vacuum tube is surrounded by oil inside a lead-lined housing that features only one small opening for the radiation to escape.

The generated radiation has a spectrum, or spread of energies, only a part of which can be used for imaging. Some of the so-called “soft” or very low-energy rays would be completely absorbed by the body’s soft tissues and thus only increase the dose to the patient without contributing anything to the image. For that reason, they are filtered out, typically by an aluminum or copper sheet. In addition the radiation exiting the tube housing is also constrained by lead collimators that keep the beam strictly limited to the body area of interest.

Attenuation of X-Rays

X-rays are attenuated as they pass through the patient’s body. Two processes play a role: absorption and scatter. With lower-energy radiation (corresponding to lower exposure voltage) absorption dominates. It correlates well with the atomic number of the irradiated matter. Mammography makes proper use of this characteristic and employs low-energy radiation to detect minute spots of calcium in the breast that may indicate cancer.

With high-energy radiation (corresponding to high exposure voltage) scatter is mainly responsible for attenuation. In this process the radiation beam loses energy and is diverted in all directions (scattered). The scattered radiation increases with irradiated body volume. It is hazardous for patients and their immediate vicinity, i.e., the angiographer standing alongside the patient to work with his or her catheters. When scatter reaches the detector, it causes an unstructured shade of gray that diminishes the contrast of the image. A scatter grid (Fig. 3.1) positioned in front of the detector reduces this “diverted” radiation.

The Guy Who Took Care of the ScatterGustav Bucky’s name is known to radiologists all over the world for his invention of the scatter grid in 1912. After the initial presentation at a medical convention, some colleagues suggested that the images were so good it must be a hoax. Having been forced into emigration by the Nazis, he left Berlin for New York, where he continued his innovative work. With his invention of the grid that is in use in every x-ray machine to this very day he eventually earned the lump sum of $25—ingenuity is definitely not a monetary unit.

Detection of X-Rays

A variety of detectors can make x-rays visible. The simplest is photographic film; because of the high spatial resolution one can achieve, it is used in nondestructive testing of industrial materials such as alloy car wheels or gas pipelines. To expose film alone an incredible dose of x-rays is necessary, but that does not matter in this instance. Film is much less sensitive to x-rays than to light—any airport security x-ray scan will show you the inside of your camera without significantly damaging your valuable vacation photos, which proves the point. As light exposes film much better, in diagnostic radiology a combination is used of film and intensifying screens that are made of rare earth materials (gadolinium, barium, lanthanum, yttrium). These screens fluoresce when irradiated (just like the foil of “Bariumplatincyanür” that Wilhelm Conrad Roentgen used in his initial experiments) and thus expose the film. Usually the film is sandwiched between two intensifying screens inside a light-tight cassette.

Generation of x-Rays

Fig. 3.1a The figure shows the generation of x-rays, their attenuation due to scatter, and their detection. b This is a modern digital projection radiography unit used primarily for skeletal work (by Philips Medical Systems).

Film–screen combinations vary greatly in their x-ray sensitivity and spatial resolution and thus have to be selected according to the specific imaging problem to be solved. If the depiction of fine detail is important, the required dose is generally higher. If the dose must be kept as low as possible, such as in children, fine detail must often be sacrificed.

Some intensifying screens emit the main fraction of their light only after stimulation by a laser beam. These screens are called storage phosphors. After their exposure they are scanned in a read-out system and their information content is immediately digitized. These screens can register a larger bandwidth of radiation intensity, which is why “over- or underexposure” is widely tolerated by the digital system. The information content of the image and the dose to the patient, however, may be inadequate although the image looks normal at first glance.

Another digital detector that is currently becoming popular consists of a layer of cesium iodide crystals on top of an amorphous silicon photodiode panel. The crystals light up when hit by x-rays and their light is then converted into an electronic charge by the photodiode. This is immediately read out by special electronics.

Digital Subtraction Angiography (DSA)

Fig. 3.2a The arterial vasculature of the brain is very complex. The bony skull is not simple either.b If a precontrast image is subtracted from the image after contrast administration, the bony structures, especially at the skull base, disappear and the visualization of the vascular tree improves considerably.

For fluoroscopy (e.g., in small-bowel follow-through or in vascular intervention) image intensifier systems are used. A luminescent layer that covers a large-area cathode absorbs the x-rays. The emitted light liberates electrons in the cathode material. These electrons are focused by electronic lenses and hit a small screen that serves as anode. All this happens inside an evacuated large tube. The resulting very bright image is registered by an external television camera and shown on a viewing monitor.

Other digital detectors are used in computed tomography (see p. 9) or are being tried out for projection radiography. The resulting signal is always a digital one, permitting post-processing of images and archiving and image communication with an ease unheard of in analog systems.

Techniques of Exposure

Projection radiography: The usual radiograph is a summation image of the exposed body part. A nodule seen over the lung fields, for example, cannot generally be assigned to the lung, the anterior or posterior chest wall, or even the skin surface, because all these structures are superimposed on each other. Clinical inspection, a little brainwork, a lateral projection, a fluoroscopy, or a conventional or computed tomography might help.

In projection radiography, a decrease in transparency or a “shadow” (e.g., a tumor) is bright; an increase in transparency (e.g., air in the bowel) is dark.

Conventional tomography: In conventional tomography, only a single slice of the body (e.g., in the hip joint) is depicted while all others are blurred by motion. During the exposure the x-ray tube and the detector move in opposite directions parallel to the imaging plane. A steel beam connects the two and swivels around a movable axis. The position of the axis marks the body layer that is imaged motion-free—the tomographic plane. By moving the beam axis ventrally or dorsally, other planes can be selected. Conventional tomography is a beautiful but dying art—well-equipped departments continue to use it for special, mostly skeletal, studies.

Fluoroscopy: In a considerable number of diagnostic and interventional examinations, the function and morphology of, for example, hollow organs are first evaluated in real time under fluoroscopy with image intensifier systems. Exposures of specific regions, projections, and findings are then performed separately but often with these same systems. The exposures can be viewed immediately on a monitor.

Contrast Media Examinations

To take a closer look at the gastrointestinal tract, it is filled with iodinated contrast solution or a barium suspension. Iodine and barium have high atomic numbers; they therefore absorb x-rays splendidly and are very visible on the radiograph. Barium suspensions can also be prepared and instilled to beautifully coat the interior wall of the air-filed or fluid-filled bowel (for example, in double contrast barium enemas).

To look at the vascular system, for example, in interventional procedures such as balloon dilations of the arteries, iodinated contrast solution is injected into the vessel. In angiography, subtraction is used to improve the depiction of vessels: the images before contrast are subtracted from the images after contrast administration. The resulting radiographs show only the vascular tree without the anatomical background. This is especially helpful in the abdomen and the skull base (Fig. 3.2).

Image Processing

Rest assured that the chemistry of traditional film processing or the post-processing of digital radiographs is all but trivial. The effects on image quality and patient dose can be tremendous. It is a regular and exciting pastime of experienced radiologists to detect and correct any mistakes that the numerous systems may come up with.

3D Reconstruction

Fig. 3.3 This complete 3D reconstruction of a child’s head was performed as a special service to the plastic surgeons: They wanted a precise documentation before surgically approaching a congenital skeletal abnormality. The left part of the image shows the head with surrounding soft tissue and also the finding that worried the patient’s parents. What do you make of it? There is an accessory median suture of the frontal bone.

3.2 Computed Tomography

Computed tomography (CT) is currently the workhorse of radiology. Recent technical developments permit extremely fast volume scans that may serve to generate two-dimensional slices in all possible orientations as well as sophisticated three-dimensional reconstructions (Fig. 3.3). The radiation dose, however, remains high and continues to require a very strict indication for every intended CT.

Working Principle

In computed tomography the x-ray tube continuously rotates around the cranio-caudal axis of the patient. A beam of radiation passes through the body and hits a ring or a moving ring segment of detectors. The incoming radiation is continuously registered, the signal is digitized and fed into a data matrix taking into account the varying beam angulations (Fig. 3.4). The data matrix can then be transformed into an output image. In today’s modern CT machines the tube rotation continues as the patient is fed through the ringlike CT gantry, thus generating not single slice scans but spiral volume scans of larger body segments. For each picture element (pixel) the attenuation of the radiation is calculated and expressed as Hounsfield units (HU) (Table 3.1). Water has, by definition, a Hounsfield unit value of 0.

Table 3.1 Attenuation of different body components

Body componentHounsfield units (HU) Bone1000 to 2000Thrombus60 to 100Liver50 to 70Spleen40 to 50Kidney25 to 45White brain matter20 to 35Gray brain matter35 to 45Water–5 to 5Fat–100 to –25Lung–1000 to –400

Working Principle of Computed Tomography

Fig. 3.4a The x-ray tube rotates continuously around the longitudinal axis of the patient. A rotating curved detector field opposite to the tube registers the attenuated fan beam after it has passed through the patient. Taking into account the tube position at each time point of measurement, the resulting attenuation values are fed into a data matrix and further computed to create an image. b This is a modern volume CT scanner (by GE Medical Systems).

Contrast Media

Contrast media are used in CT to visualize vessels and the vascularization of different organ systems. They attenuate radiation because of their high atomic number (e.g., iodine and barium). Contrast media containing gadolinium (which also has a high atomic number) normally intended for use in magnetic resonance tomography could theoretically also be used in CT if the administration of iodine is contraindicated. They are, however, incredibly expensive and not registered for this use yet. To better appreciate the inside of hollow viscera, iodine or barium contrast media are also given orally or instilled into the rectum.

Fat and air are always black in CT; bone cortex and high-atomic-number contrast media are always white.

3.3 Ultrasonography

Ultrasonography (“ultrasound”) is the cheapest and most “harmless” technology in radiology. For these reasons many physicians outside radiology also use the modality. Wherever ultrasound provides sufficient information and wherever radiation dose must be minimized at any cost (pediatrics and obstetrics), it is the primary imaging modality of choice. For the examination of vessels and blood flow, color-coded Doppler ultrasound may be used.

Working Principle

Ultrasound technology is simple—any bat knows how to do it. In medical ultrasonography the sound waves are generated artificially by means of piezoelectric crystals. These crystals are magic gadgets: when connected to an alternating current of a certain frequency, they will vibrate and thus emit a sound wave of the same frequency. If, on the other hand, they are exposed to sound waves of a certain frequency, they will produce an alternating current of that frequency.

For medical purposes sound waves of 1–15 MHz frequency are used—inaudible ultrasound waves.

If, by way of ultrasound gel, the crystal is brought into direct contact with the body, the emitted ultrasound waves spread through the tissue. The tissue absorbs, scatters, or reflects them.

Absorption and spatial resolution increase with higher frequencies. For that reason the maximum penetration of ultrasound waves and the depiction of fine image details correlate with frequency: in breast imaging high-resolution 7.5–10 MHz systems may be used, while in abdominal imaging 3.5–5 MHz systems are adequate to view also the deeper regions. Bone and calcifications absorb sound totally, which is why we see an acoustic “shadow” behind them (Fig. 3.5). Very little sound is absorbed in fluid-filled viscera, leading to the opposite effect: the echo-signal behind the fluid is stronger that in the tissue around it.

Only the reflection of sound back to the piezoelectric crystal will result in a signal as the basis for an image. Large and minute tissue interfaces reflect the sound. If it is an interface between soft tissue and air/gas, reflection is total—structures behind it cannot be imaged, also resulting in an acoustic shadow (Fig. 3.5). The ultrasound scanner calculates a two-dimensional image—howon earth does it do that? From the time passing between seeing a lightning discharge and hearing its resulting thunder we can estimate our distance to the thunderstorm. The ultrasound system measures, for each crystal separately, the time between each emitted sound pulse and the received echo pulses reflected by the tissue. The elapsed time defines the pixel matrix row that the signal is assigned to. The intensity of the echo pulse defines the respective gray value of the pixel. Hundreds of piezoelectric crystal elements are arranged in a row, and their combined data are fused into one two-dimensional ultrasound image.

In ultrasound, cystic structures are dark and show signal increase behind them. Bone and air are bright and cause an acoustic shadow.

Color-coded Doppler ultrasound: By listening to the sound of a passing motorcycle we can find out whether it is coming or going and estimate how fast it is. If ultrasound waves are reflected by moving interfaces (such as erythrocytes in flowing blood) at an angle of 10–60°, the same effect (the Doppler effect) comes into play: the echo undergoes a frequency shift dependent on the speed and direction of the blood flow. This information can be color coded into a normal ultrasound image. In color-coded Doppler ultrasound, color type and intensity tell us the direction and speed of the blood flow. As a convention, venous, centripetal flow is coded as blue; arterial, centrifugal flow as red. But take note: You accidentally rotate the scanner probe by 180° and the colors switch! And as your probe approaches a 90° angle relative to the vessel, your Doppler signal vanishes altogether. Special ultrasound contrast media further increase the Doppler effect.

3.4 Magnetic Resonance Tomography

Magnetic resonance tomography is the technically most complex imaging modality in radiology but it also holds the largest diagnostic potential. Many are terrified by the prospect of having to understand the basic principles of magnetic resonance (MR). All of this is completely unnecessary, of course: the thing is in essence nothing but a bicycle dynamo. But let’s start at the beginning.

Working Principle of Ultrasonography

Fig. 3.5a If an alternating electric current is sent through a piezoelectric crystal, it vibrates with the frequency of the current, producing sound waves of that frequency. In medical ultrasound, typical frequencies vary between 1 and 15 MHz. Ultrasound gel acoustically couples the ultrasound transducer to the body, where the ultrasound waves can then spread. Inside the body the sound is absorbed, scattered, or reflected. Fluid filled (cystic) structures appear dark and show acoustic enhancement behind them. Bone and air appear bright because they absorb and reflect the sound, showing an “acoustic shadow” behind them.b This is a modern US scanner (by Toshiba Medical Systems).c These are transducers for different purposes.

Generation of the MR Signal

Do You Know about the Larmor Frequency?

Anyone who has sat on a swing moving legs and trunk in slow rhythm to swing ever higher, or who was the “swing pusher on duty” for a little sister or brother, daughter, or son, realizes that objects have a certain inherent frequency at which they swing (or resonate): their resonance frequency. If you do not know or feel this frequency or are not able to move your body accordingly (like a small child), you will never be able to swing on your own. If you are, however, able to apply the frequency appropriately, you will go a long way with very little force. The same holds true for atoms and molecules, of course.

The nuclei of atoms spin about their axes with high frequency and some nuclei (such the hydrogen nucleus—the proton) have resultant magnetic moments. We are actually looking at small rapidly spinning “magnets.” As the atoms move randomly, these “magnets” tumble about chaotically and thus neutralize each other’s magnetic fields. A call to order is necessary before anything good can come out of this.

Magnetic Resonance Tomography

You probably remember this physics experiment from back in school: iron dust arranges itself along the lines of a magnetic field. In MR a constant external magnetic field (called B0 by the MR physicists) calls the little nuclear “magnets” to order. The protons align themselves along the axis of the magnetic field and, in addition to their spin, begin to rotate around the axis of the B0 magnetic field much like gyroscopes wobble in the Earth’s gravitational field.

This rotational frequency is identical to the resonance frequency, which is also named the Larmor frequency. This frequency varies with the strength of the magnetic field.

The Irishman Whose Frequency We Cannot Do WithoutSir Joseph Larmor was an Irish physicist who taught in Cambridge, England, around the beginning of the last century. One of his special fields was the mathematical theory of electromagnetism. The Larmor frequency is just one of several physical phenomena that carry his name. He was a conservative man, at times opposing most of Einstein’s ideas and the introduction of baths in his college in Cambridge: “We have done without them for 400 years, why begin now?” As it turns out, he became an avid bather right after the public baths were installed.

Fig. 3.6a The coordinate system with three axes Z, X, and Y inside the MR machine is shown. Inside of the gantry you can see the salami and the antenna alongside it in which the MR signal is induced. b If a gradient is superimposed on the static field along the Z-axis (Z-gradient), every slice of the salami gets its own “Larmor frequency address.” An excitation pulse B1 of a frequency E will now only excite slice E. c Right after the B1 excitation of slice E, a temporary gradient is superimposed along the Yaxis (Y-gradient). As protons within the slice now rotate with different Larmor frequencies, the signals dephase except in the rod that keeps the original frequency. The phase shift persists until read-out. d During read-out, a third gradient is superimposed along the X-axis (X-gradient). Each cube in the rod now has its own “Larmor frequency address.” The measured signal of that specific frequency can now be assigned to a specific voxel in the image. e This is a modern whole-body MRI scanner (by Siemens Medical Systems).

What Is So Special about the “External” and “Internal” Magnetic Fields?

The magnets for the external applied magnetic field (B0) are large and incredibly strong (0.5, 1.0, or 1.5 tesla, the last corresponding to 30 000 times the force of the natural terrestrial magnetic field). Why do we need such a strong field? Our protons do align themselves along the field axis and wait patiently for coming sensations—they may, however, choose a parallel and an antiparallel orientation. This is where the simple magnet story comes to an end. The parallel orientation is the least energy-consuming, which is why more than half of the protons choose it. The other protons assume the antiparallel orientation. As the external magnetic field increases in power, the antiparallel orientation requires ever more energy and thus becomes less and less popular. The dominance of the parallel protons increases and thereby the magnetization of the examined body. This “internal” magnetic field initially has the same orientation as the external field B0. Its axis corresponds to the longitudinal axis of the MR gantry, also called the Z-axis (Fig. 3.6a). Now the stage is set: Enter a biological sample to examine—how about a nice salami?

How Do We Generate an MR Signal in a Salami?

It so happens that protons (i.e., nuclei of hydrogen atoms)—which can be beautifully studied by MR—are abundant in salamis and other organic material: in excess of 90% of organic material consists of hydrogen. After having been moved into the B0 external magnetic field of the MR system the majority of protons inside the sausage have aligned themselves parallel to B0 and have generated an “internal” magnetic field. If we now want them to tell all, we’d better get them excited. This is done by a radiofrequency pulse (RF pulse), a temporary outer RF magnetic field that oscillates with the Larmor frequency of hydrogen (also called B1 by MR physicists). Remember: Hydrogen protons could not care less about RF pulses of higher or lower frequencies. The longer the B1 RF pulse is active and the stronger it is, the more the axis of the protons is tilted away from the Z-axis into the X–Y-plane. For simplicity’s sake, let us consider a pulse that has the power and duration to tilt the proton axis by 90°. As this happens not only to one proton but synchronously to many protons in the salami, the “internal” magnetic field also tilts 90° and rotates with the Larmor frequency of hydrogen, much like a propeller—or the magnet inside the bicycle dynamo (in the X–Y plane; Fig. 3.6a). If you now position a wire coil along the sausage (corresponding to the receive coil or antenna of the MR machine), a measurable alternating current is induced—much like in the coils of a bicycle dynamo.

This current is the MR signal we can start our work with. Remember for later that the field signal is strongest if all protons are in phase (“listen to the same beat”) which is always the case right after the B1 pulse.

After the RF pulse B1 and the resulting 90° tilt of the “internal” magnetic field, the current measured by the antenna—our signal—decreases again. The reasons are twofold: For one thing, the axis of the “internal” magnetic field moves back to the Z-axis—remember that the “external” magnetic field B0 is always present and is very strong. For another thing, the protons lose the phase synchronization they have been forced into by the RF pulse B1. As they dephase, the “internal” magnetic field power also shrinks. You will learn more about these processes later.

We now have proof that there are protons inside that salami; of course we had a hunch there would be. To look at slices of the sausage we have to assign the signals to locations in a three-dimensional coordinate system.

Spatial Allocation of the MR signal

The frequency with which I swing or push my swinging child depends, besides other things, on the terrestrial gravity. The Larmor frequency with which I can excite a proton depends on the strength of the magnetic field surrounding it. Magnetic fields can be built asymmetrically so that their strength increases along an axis. These types of fields are called gradients.

Z-gradient: If such a gradient is positioned along the longitudinal or Z-axis of the system (Z-gradient) (Fig. 3.6a) the magnetic field increases along the length of the salami, giving every slice of the sausage a different Larmor frequency address. If we now give the B1 pulse, it excites not the whole salami but only one slice—the one with the Larmor frequency of the B1 pulse (Fig. 3.6b). The bandwidth and form of the B1 pulse determine the thickness of the selected slice.

Y-gradient: After the excitation B1 pulse is over, a second gradient is positioned along the Y-axis of the system (Y-gradient). During the duration of this gradient the protons thus have different Larmor frequencies depending on their position along the Y-axis; that is, they rotate with different speeds. The subsequent phase shift persists after the Y-gradient is turned off again. The sausage slice now consists of rods of different phase (Fig. 3.6c). Here is the analogy to illustrate the phenomenon: If three different cars drive on a three-lane highway and adhere to a speed limit, they stay side by side. Once the speed limit is lifted, they drive with different speeds and the gap between them grows. As the speed limit is enforced again, they drive at the same speed (same Larmor frequency for the protons) and the gap between them (the phase shift) persists. This applies to law-abiding drivers only, of course. The gradient can be designed to leave the Larmor frequency unchanged in one rod that subsequently does not undergo the phase shift. Frequency and phase are thus identical to the original B1 pulse. We will now dice this rod into volume elements (voxels).

X-gradient: The last gradient is switched on during the read-out phase and is positioned along the X-axis (X-gradient). It divides the rod into cubes, assigning a Larmor frequency address to each (Fig. 3.6d).

Now we have the single cubes (or voxels) that we need for a two-dimensional image: a selectively excited slice of a defined thickness, and a rod in correct phase that is subdivided into cubes of different Larmor frequencies assignable to locations in a coordinate system. To calculate the image, a separate measurement must be performed for every rod (voxel or pixel row) of the image matrix; that is, for a matrix of 256 × 256 voxels, we need to repeat the process 256 times. The rest is complex electrical engineering.

Analysis of the MR Signal

Which Phenomena Do We Need to Know?

As has been described above, the MR signal measurable directly after the RF pulse decays quickly. This is due to two phenomena that can be quantified separately:

Longitudinal relaxation: This is the process of the “internal” magnetic field returning to the original orientation (Z-axis) along the “external “ magnetic field B0. This is a pretty fast process. The corresponding parameter is the T1 value. Transversal relaxation: This is the process of signal loss due to dephasing of the protons. Starting with the same rotational frequency and phase right after the B1 pulse, different protons in different locations are influenced by the magnetic forces of their neighboring atoms and the general inhomogeneity of the field and thus lose their synchronization (they “lose their common beat”). Another analogy to illustrate the phenomenon: Imagine a string orchestra fiddling away at a musical score. It is a special orchestra—the musicians hear only their own music and see nobody but the conductor. It is the conductor who gives the sign to start (B1 pulse). If he were to leave right after the beginning, the individual musicians could continue to play their score but the orchestra’s music would quickly turn disharmonic—or dephase. As the protons dephase, the power of the rotating “internal” magnetic field decreases. This process takes time; it is called transversal relaxation and is described by the T2 value.