Advances in Acoustic Microscopy and High Resolution Imaging E-Book

• Herausgeber: Wiley-VCH
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 640

Veröffentlichungsjahr: 2013

Beschreibung

Novel physical solutions, including new results in the field of adaptive methods and inventive approaches to inverse problems, original concepts based on high harmonic imaging algorithms, intriguing vibro-acoustic imaging and vibro-modulation technique, etc. were successfully introduced and verified in numerous studies of industrial materials and biomaterials in the last few years. Together with the above mentioned traditional academic and practical avenues in ultrasonic imaging research, intriguing scientific discussions have recently surfaced and will hopefully continue to bear fruits in the future. The goal of this book is to provide an overview of the recent advances in high-resolution ultrasonic imaging techniques and their applications to biomaterials evaluation and industrial materials. The result is a unique collection of papers presenting novel results and techniques that were developed by leading research groups worldwide.
This book offers a number of new results from well-known authors who are engaged in aspects of the development of novel physical principles, new methods, or implementation of modern technological solutions into current imaging devices and new applications of high-resolution imaging systems. The ultimate purpose of this book is to encourage more research and development in the field to realize the great potential of high resolution acoustic imaging and its various industrial and biomedical applications.

Leseprobe

Table of Contents

Related Titles

Title page

Copyright page

List of Contributors

Introduction

Author Biographies

Part One: Fundamentals

1: From Multiwave Imaging to Elasticity Imaging

1.1 Introduction

1.2 Regimes of Spatial Resolution

1.3 The Multiwave Approach

1.4 Wave to Wave Generation

1.5 Wave to Wave Tagging

1.6 Wave to Wave Imaging: Mapping Elasticity

1.7 Super-resolution in Supersonic Shear Wave Imaging

1.8 Clinical Applications

1.9 Conclusion

2: Imaging via Speckle Interferometry and Nonlinear Methods

2.1 General Introduction

2.2 Part I: Speckle Interferometry

2.3 Part II: Nonlinear Imaging

2.4 Summary and Closing

Part Two: Novel Developments in Advanced Imaging Techniques and Methods

3: Fundamentals and Applications of a Quantitative Ultrasonic Microscope for Soft Biological Tissues

3.1 General Introduction: Basic Idea of an Ultrasonic Microscope for Biological Tissues

3.2 Sound Speed Profile

3.3 Acoustic Impedance Profile

3.4 Summary

4: Portable Ultrasonic Imaging Devices

5: High-Frequency Ultrasonic Systems for High-Resolution Ranging and Imaging

5.1 General Introduction

5.2 High-Frequency Ultrasonic System Components

5.3 Engineering Concepts for High-Frequency Ultrasonic Imaging

5.4 High-Frequency Ultrasound Imaging in Biomedical Applications

5.5 Summary

6: Quantitative Acoustic Microscopy Based on the Array Approach

6.1 General Introduction

6.2 Measurement of Velocity and Attenuation of Leaky Waves

6.3 Measurement of Bulk Wave Velocities and Thickness of Specimen

6.4 Conclusions

Part Three: Advanced Biomedical Applications

7: Study of the Contrast Mechanism in an Acoustic Image for Thickly Sectioned Melanoma Skin Tissues with Acoustic Microscopy

7.1 Introduction

7.2 Physical and Mathematical Modeling for Five Layer Wave Propagation in an Acoustic Microscope

7.3 Sample Preparation

7.4 Digital Imaging – Optical and Ultrasonic

7.5 High Frequency Acoustic Microscopy

7.6 Conclusions

Acknowledgment

8: New Concept of Pathology – Mechanical Properties Provided by Acoustic Microscopy

8.1 Introduction

8.2 Principle of Acoustic Microscopy

8.3 Application to Cellular Imaging

8.4 Application to Hard Tissues

8.5 Application to Soft Tissues

8.6 Ultrasound Speed Microscopy (USM) [39]

8.7 Articular Tissues

8.8 Summary

9: Quantitative Scanning Acoustic Microscopy of Bone

9.1 Introduction

9.2 Quantitative SAM-Based Impedance of Bone

9.3 Tissue Mineralization, Acoustic Impedance, and Stiffness

9.4 Elastic Anisotropy at the Nanoscale (Lamellar) Level

9.5 Elastic Anisotropy at the Microscale (Tissue) Level

9.6 Applications in Musculoskeletal Research

9.7 Conclusions

Part Four: Advanced Materials Applications

10: Array Imaging and Defect Characterization Using Post-processing Approaches

10.1 Introduction

10.2 Modeling Array Data

10.3 Imaging with 1D Arrays

10.4 Imaging with 2D Arrays

10.5 Scattering Matrices and Their Experimental Extraction

10.6 Defect Characterization and Sizing

10.7 Conclusions

11: Ultrasonic Force and Related Microscopies

11.1 Introduction

11.2 Mechanical Diode Detection

11.3 Experimental UFM Implementation

11.4 UFM Contrast Theory

11.5 Quantitative Measurements of Contact Stiffness

11.6 UFM Picture Gallery

11.7 Image Interpretation – Effects of Adhesion and Topography

11.8 Superlubricity

11.9 Defects Below the Surface

11.10 Time-Resolved Nanoscale Phenomena

Acknowledgments

12: Ultrasonic Atomic Force Microscopy

12.1 Introduction

12.2 Principle

12.3 Theory

12.4 Instrumentation

12.5 Experiments

12.6 Observation of Defects in Layered Materials

12.7 Conclusion

13: Acoustical Near-Field Imaging

13.1 Principle of Near-Field Imaging

13.2 Near-Field Acoustical Imaging and Atomic Force Microscopy

Acknowledgment

Index

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The Editor

Prof. Roman Gr. Maev

NSERC Indust. Research Chair

University of Windsor

401, Sunset Avenue

Windsor ON N9B 3P4

Canada

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© 2013 Wiley-VCH Verlag & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany

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List of Contributors

Walter Arnold

Saarland University

Department of Material Science and Technology

Campus D 2.2

66123 Saarbrücken, Germany

and

Göttingen University

1. Phys. Institut

Friedrich-Hund Platz 1

37077 Göttingen, Germany

Andrew Briggs

Oxford University

Department of Materials

16 Parks Road

OX1 3PH Oxford, UK

Bruce W. Drinkwater

University of Bristol

Faculty of Engineering

University Walk

Bristol BS8 1TR, UK

Helmut Ermert

Ruhr-Universität Bochum

Department of Electrical Engineering and Information Technology

High Frequency Engineering Research Group

Building ID 03/343

44780 Bochum, Germany

Mathias Fink

Ecole Supérieure de Physique et de Chimie Industrielles de la Ville de Paris

CNRS

INSERM

Institut Langevin

10 rue Vauquelin

75005 Paris, France

Mathilde Granke

Université Pierre et Marie Curie

CNRS UMR 7623

Laboratoire d’Imagerie Paramétrique

15 rue de l’ecole de médecine

75006 Paris, France

Naohiro Hozumi

Toyohashi University of Technology

1-1 Hibarigaoka, Tempaku-cho

Toyohashi 441-8580, Japan

Kazuto Kobayashi

Honda Electronic Co., Ltd.

20 Oyamazuka, Oiwa-cho

Toyohashi 980-857, Japan

Oleg V. Kolosov

Lancaster University

Department of Physics

Room A30, Physics Building

Bailrigg, LA1 4YW Lancaster, UK

Pascal Laugier

Université Pierre et Marie Curie

CNRS UMR 7623

Laboratoire d’Imagerie Paramétrique

15 rue de l’ecole de médecine

75006 Paris, France

Roman Gr. Maev

University of Windsor

Institute for Diagnostic Imaging Research

401 Sunset Avenue

Windsor, ON N9B3P4, Canada

Chiaki Miyasaka

Pennsylvania State University

Department of Engineering Science and Mechanics

212 Earth-Engineering Sciences Building

University Park, PA 16802, USA

Kay Raum

Universitätsmedizin Berlin

Julius-Wolff Institut & Berlin-Brandenburg School for Regenerative Therapies, Charité, Augustenburger Platz 1

13353 Berlin, Germany

Jeffrey Sadler

University of Windsor

Institute for Diagnostic Imaging Research

401 Sunset Avenue

Windsor, ON N9B 3P4, Canada

Amena Saïed

Université Pierre et Marie Curie

CNRS UMR 7623

Laboratoire d’Imagerie Paramétrique

15 rue de l’ecole de médecine,

75006 Paris, France

Yoshifumi Saijo

Tohoku University

Biomedical Imaging Laboratory

Graduate School of Biomedical Engineering

4-1 Seiryomachi

Aoba-ku, Sendai 980-8575, Japan

Fedar M. Severin

University of Windsor

Institute for Diagnostic Imaging Research

401 Sunset Avenue

Windsor, ON N9B3P4, Canada

David Shum

Hotel Dieu Grace Hospital

Windsor Regional Hospital

Leamington District Memorial Hospital

1995 Lens Avenue

Windsor, ON N8W 1L9, Canada

Mickael Tanter

Ecole Supérieure de Physique et de Chimie Industrielles de la Ville de Paris

CNRS

INSERM

Institut Langevin

10 rue Vauquelin

75005 Paris, France

Sergey A. Titov

Russian Academy of Sciences

N.M. Emanuel Institute of Biochemical Physics

4 Kosygin st.

Moscow 119344, Russia

Sergey Titov

Russian Academy of Sciences

N.M. Emanuel Institute of Biochemical Physics

4 Kosygin st.

Moscow 119344, Russia

Bernhard R. Tittmann

Pennsylvania State University

Department of Engineering Science and Mechanics

212 Earth-Engineering Sciences Building

University Park, PA 16802, USA

Toshihiro Tsuji

Tohoku University

Department of Materials Processing

Aoba 6-6-02, Aoba-ku

Miyagi, Sendai 980-8579, Japan

Alexander Velichko

University of Bristol

Faculty of Engineering

Room 1.2, Queen’s Building, University Walk

Clifton, Bristol BS8 1TR, UK

Michael Vogt

Ruhr-Universität Bochum

Department of Electrical Engineering and Information Technology

High Frequency Engineering Research Group

Building ID 03/340

44780 Bochum, Germany

Paul D. Wilcox

University of Bristol

Faculty of Engineering

Room 2.57, Queen’s Building, University Walk

Clifton, Bristol BS8 1TR, UK

Kazushi Yamanaka

Tohoku University

Department of Materials Processing

Aoba 6-6-02, Aoba-ku

Miyagi, Sendai 980-8579, Japan

Introduction

While a picture is worth a thousand words, in science a single image is often problematic. Imaging technology is largely based on manipulating optical waves, but since optics does not provide all of the information we need, in the twentieth century we turned to other technologies. Acoustic imaging is now an integral and important part of our continuing effort to extend our ability to “see.” Although ultrasonic images do not provide the fine details found in magnetic resonance images (MRI) or X-ray methods, the acoustic imaging system provides significant information at one-tenth the cost of MRI, with the added advantage of being completely safe for the patient’s health. This form of imaging is particularly useful for obtaining data from inside the human body, for delineating the interfaces between solid and spaces in muscles and soft tissues. Ultrasound renders live images where the operator can dynamically select the most important sections for documenting the changes in structure without long-term side effects in the patient. The introduction of high-resolution acoustic imaging systems in the early 1960s facilitated the examination of the internal microstructure of nontransparent solids and the monitoring of internal stress. In addition to measuring elastic properties, this technique is also used to examine adhesion in multilayered structures and has many other applications. Acoustic microscopy has become not only a new imaging method extensively used in many areas of physics, biology, and technology but also a new efficient tool of quantitative characterization of the microstructure of various species and materials.

The role of high-resolution ultrasonic imaging in academic studies of condensed matter and various applications for microstructural material characterization in physics, biology, and technology is rapidly increasing. The whole spectrum of original physical and methodological approaches to ultrasonic imaging results in a significant improvement in the quality of developed technology. New generations of ultrasonic imaging system devices continue to decrease in size and will soon enter the realm of pocket-sized dimensions. New transducer materials, includ­ing advanced composites and recent MEMS applications to novel array solutions, also contribute to substantial changes in the design of ultrasonic imaging systems.

The goal of this book is to provide an overview of recent advances in high-resolution ultrasonic imaging techniques and their applications to biomaterials evaluation and industrial materials. In this book we were lucky to bring together a unique collection of papers presenting novel results and techniques that were developed by leading research groups worldwide.

Novel physical solutions, including new results in the field of adaptive methods and inventive approaches to inverse problems, original concepts based on high harmonic imaging algorithms, and intriguing vibro-acoustic imaging and vibro-modulation technique, have been successfully introduced and verified in numerous studies of industrial materials and biomaterials in the last few years. Together with the above-mentioned traditional academic and practical avenues in ultrasonic imaging research, intriguing scientific discussions have recently surfaced in various fields and will hopefully continue to bear fruit in the future.

This book offers several new results from well-known authors who are engaged in aspects of the development of novel physical principles, new methods, or implementation of modern technological solutions into current imaging devices and new applications of high-resolution imaging systems. I believe that this book will help encourage more research and development in the field to realize the great potential of high-resolution acoustic imaging and its various industrial and biomedical applications. We have also included a biography of every contributor to this book, through which you may be able to trace the progression and future direction of this field. I sincerely hope that you will enjoy reading about these exciting research results.

In closing, I am grateful to all my colleagues, the distinguished contributors to this book, and the co-authors who shared their results and insights, thus lending a unique perspective and voice to this book. I would also like to thank Sabina Baroniciu for her invaluable assistance in the preparation of this book, especially with the arduous task of putting together the collection of all the manuscripts from each respected author. But she did it and did it amazingly well!

Undoubtedly, I would not have been able to work on this book without the support of my family, without the understanding and patience of my wife, Elena Maeva, and my children, Anna and Grigori, who forgave my inattention to them and my preoccupation with the work on this book.

Many thanks to you all

Roman Gr. Maev

Windsor, Ontario, Canada

2 July 2012

Author Biographies

Walter Arnold received a diploma in physics in 1970 and a PhD in solid state physics in 1974, both from the Technical University Munich, Germany. He then held various positions as a postdoctoral researcher and scientific staff member at the CNRS, Grenoble, France and the Max-Planck-Institute for Solid State Physics, Stuttgart, Germany, the IBM T.J. Watson Research Center, Yorktown Heights, NY, and the Brown Boveri Research Centre, Baden, Switzerland, working on low temperature physics, solid state physics, and applied physics. From 1980 until his retirement in December 2007 he was employed at the Fraunhofer-Institute for Non-Destructive Test­ing, Saarbrücken, as head of the research department. Parallel to this position, Dr. Arnold was appointed professor of materials technology at the University of Saarbrücken. Since his retirement, he has continued research work with colleagues at the Saarland University and as a guest professor at the 1. Physikalische Institut, Universität Göttingen, Germany.

Dr. Arnold has authored and co-authored about 300 papers including 170 peer-reviewed papers. He has supervised 31 PhD theses and approximately 150 master and diploma students.

Andrew Briggs received his PhD from the Cavendish Laboratory, Cambridge, in 1976. He came to Oxford University as a Research Fellow in 1980 and was appointed a University Lecturer in 1984. He wrote the definitive monograph Acoustic Microscopy, which was published by Oxford University Press (OUP) in 1992. The second edition, with a new chapter on acoustically excited probe microscopy written with Oleg Kolosov, was published by OUP in 2010. For his pioneering work in applications of acoustic and scanned probe microscopy he was elected Honorary Fellow of the Royal Microscopical Society in 1999. In 2002 he was appointed professor of nanomaterials at Oxford. From 2002 to 2009 he was Director of the Quantum Information Processing Interdisciplinary Research Collaboration. His current research interests focus on carbon nanomaterials for quantum technologies.

Bruce Drinkwater (PhD, CEng, FIMechE, FInstNDT, DIC) was born in Hexham, England in 1970. He received B.Eng and PhD degrees in mechanical engineering from Imperial College, London, England in 1991 and 1995, respectively. From 1996 to the present he worked as an academic in the Mechanical Engineering Department at the University of Bristol, England. During this time he has published over 80 journal articles on a range of topics connected with ultrasonics and non destructive evaluation.

Between 2000 and 2005 he was an EPSRC Advanced Research Fellow researching ultrasonic wheel probes and the ultrasonic measurement of adhesive joints, thin layers, and interfaces. During this period both his work on array-wheel probes and on bearing condition monitoring was com­mercialized. He was promoted to professor of ultrasonics in 2007. He currently leads a large collaborative research program that aims to develop ultrasonic array devices for the manipulation of biological particles for applications such as tissue engineering.

Helmut Ermert received a Dipl.-Ing. degree in electrical engineering and a Dr.-Ing. degree from the Technical University (RWTH) Aachen, Germany in 1965 and 1970, respectively. In 1975 he received a Dr.-Ing. habil. degree (Habilitation) from the Engineering Faculty at the University of Erlangen-Nuremberg, Germany.

From 1966 to 1970 he worked on millimeter wave and microwave engineering at the Technical University (RWTH) Aachen. From 1970 to 1975 he was involved in teaching and research in microwave integrated circuits, microwave ferrites, and microwave measurement techniques at the University Erlangen-Nuremberg. From 1978 to 1987 he was a professor of electrical engineering in Erlangen working on microwave and acoustic imaging using various fields and waves (ultrasound, microwaves, thermal waves, and eddy current fields) for diagnostic purposes in medicine and engineering. Since 1987 he has been a professor of electrical engineering and Director of the Institute of High Frequency Engineering at the Ruhr-University in Bochum, Germany.

At present, he is continuing research on measurement techniques, diagnostic imaging, and sensors in the RF and microwave area as well as in the ultrasonic area for applications in medicine, nondestructive testing, and industry.

Mathias Fink received an MS degree in mathematics from Paris University, France, in 1967, and a PhD degree in solid state physics in 1970. He then moved to medical imaging and received the Doctorates-Sciences degree in 1978 from Paris University. His Doctorates-Sciences research was in the area of ultrasonic focusing with transducer arrays for real-time medical imaging.

Dr. Fink is a professor of physics at the Ecole Superieure de Physique et de Chimie Industrielles de la Ville de Paris (ESPCI ParisTech), Paris, France. In 1990 he founded the Laboratory Ondeset Acoustique at ESPCI, which became the Langevin Institute in 2009. In 2002, he was elected to the French Academy of Engineering, in 2003 to the French Academy of Science, and in 2008 at the College de France to the Chair of Technological Innovation.

Dr. Fink’s area of research is concerned with the propagation of waves in complex media and the development of numerous instruments based on this basic research. His current research interests include time-reversal in physics, super-resolution, metamaterials, medical ultrasonic imaging, ultrasonic therapy, multiwave imaging, acoustic smart objects, acoustic tactile screens, underwater acoustics, geophysics, and telecommunications. He has developed different techniques in medical imaging (ultrafast ultrasonic imaging, transient elastography, supersonic shear imaging), wave control, and focusing in complex media with time-reversal mirrors. He holds more than 55 patents and has published more than 350 peer-reviewed papers and book chapters.

Mathilde Granke obtained her Engineer Diploma in computational structural mechanics from the Ecole Centrale Nantes, France in 2007; she then spent six months as a visiting scholar in the Department of Mechanical and Industrial Engineering at the University of Illinois in Chicago. She received a Master’s degree in biomechanics from the Ecole des Arts et Métiers Paris in 2008. Since 2008, she has joined the Laboratoire d’Imagerie Paramétrique at Univer­sity Pierre et Marie Curie, Paris VI, France as a PhD student.

For the past four years, her area of interest has been focused on bone mechanics, with particular applications in biomaterials, bone numerical modeling, and bone pathologies. Her current research is focused on the relationship between the material properties and mechanical characteristics of bone tissue evaluated by scanning acoustic microscopy.

Naohiro Hozumi was born in Kyoto, Japan on April 2, 1957. He received his BS, MS, and PhD degrees in 1981, 1983, and 1990, respectively, from Waseda University. He was employed by the Central Research Institute of Electric Power Industry (CRIEPI) from 1983 to 1999. He was an associate professor of Toyohashi University of Technology from 1999 to 2006, and a professor at the Aichi Institute of Technology from 2006 to 2011. Since 2011, he has been a professor at Toyohashi University of Technology.

He has been engaged in the research of insulating materials and diagnosis for high voltage equipment, as well as the use of acoustic measurement for biological and medical applications.

Kazuto Kobayashi was born in Aichi, Japan on June 8, 1952. He received a BS degree in electrical engineering from Shibaura Institute of Technology, Tokyo, Japan in 1976. He is currently a director of the Department of Research and Development at Honda Electronics Co. Ltd. in Toyohashi, Japan. His research activities and interests include medical ultrasound imaging, signal processing, and high frequency ultrasound transducers.

Oleg Kolosov received his PhD from the Moscow Institute of Physics and Technology in 1989, and conducted research at the Russian Academy of Sciences in the group of Roman Maev, authoring his first patents in the field of acoustic microscopy. In 1991 he became a Fellow of the Science and Technology Agency of Japan, working at the Mechanical Engineering Laboratory and Joint Research Centre of Atom Technology, where he filed first patents on ultrasonic force mi­­croscopy jointly with Kazushi Yamanaka. He joined the Materials Department at Oxford University as a Re­search Fellow in 1994, and from 1996 as an Advanced Fellow of Engineering and Physical Sciences Research Council. At Oxford he continued his exploration of ultrasound in scanning probe microscopy, which included the development of time resolved and nanoscale subsurface imaging using ultrasonic scanning force microscopy. In 2000 he was appointed a Group Leader and in 2002 a Director of Innovation of Symyx Technologies – a world pioneer in combinatorial materials discovery, and authored more than 45 patent applications worldwide in this field. In 2006 he was appointed a Reader in Experimental Condensed Matter Physics at Lancaster University.

His research interests span the characterization of microscale and nanoscale properties of materials, combinatorial methods for materials discovery, and nanomechanical quantum sensors.

Pascal Laugier holds a PhD in physical acoustics from the University Denis Diderot in Paris, France. He is currently a Research Director at the French National Scientific Research Center (CNRS) and is head of the Laboratory of Parametric Imaging at University Pierre and Marie Curie, Paris, France. Laugier has 15 years experience in osteoporosis research and more than 25 years experience in ultrasonic biomedical imaging science, developing high frequency imaging and applying tissue characterization techniques to various fields of medicine such as skin, cartilage, eye, and bone.

He was involved in several European research projects and has been an investigator with the European Space Agency on a Microgravity Research Program. He serves as a reviewer of more than 30 scientific journals. He also serves as an Expert Reviewer for major national and international institutions.

He has co-authored over 150 articles in peer-reviewed journals, 160 Conference proceedings papers, and over 750 conference abstracts. He holds 12 patents, all of them in the field of ultrasound imaging for medical applications.

Roman Gr. Maev was born in Moscow, Russia. He received his Master of Science degree in theoretical nuclear physics from the Moscow Physical Engineering Institute followed by a PhD in the “Theory of Semiconductors” from the Physical P.N. Lebedev Institute of the USSR Academy of Sciences. In 1990 he received a Fellowship from Gore-Chernomirdin and as a result successfully attended a course project for the Scientific Business Management Fund for one semester of study at the Harvard Business School (Boston, USA). In 2001 he received a DSc degree from the Russian Academy of Sciences, and in 2005 he received a Full Professor diploma in Physics from the Government of the Russian Federation.

Dr. Roman Gr. Maev is the founding director-general of The Institute for Diagnostic Imaging Research – a multi-disciplinary, collaborative research and innovation consortium with one of its directions in nanotechnology. Dr. Maev is also a full faculty professor in the Department of Physics at the University of Windsor, Canada, and in 2007 was granted the title of University Professor Distinguished.

The diverse range of disciplines encompassed by Dr. Maev includes theoretical fundamentals of physical acoustics, experimental research in ultrasonic and nonlinear acoustical imaging, nanostructural properties of advanced materials and its analysis. He is the author of four monographs, editor and co-editor of nine books, has published over 350 articles in leading international journals, and holds 23 international patents.

Elena Maeva received bachelor’s and master’s degrees in 1978 and 1980, respectively, from the Moscow D. Mendeleev Chemical-Technological University. Subsequently, in 1997 she received her PhD in physics and chemistry at the Institute of Chemical Physics within the Russian Academy of Sciences.

Dr. Maeva is an associate professor in physics (cross-appointed with chemistry and bio-chemistry) at the University of Windsor, Windsor, Canada. Her research is mostly related to the area of applied physics and chemistry. She is deeply involved in the application of advanced non-destructive methods of evaluation for the investigation of the structure of different materials and tissues. Her research is focused in three main areas: adhesive and nanocomposite structures, the study of the properties and degradation process of bio and ecology clean material based composites, and investigation of hard and soft biological tissue in biomedical projects.

During her academic career Dr. Maeva has published 26 articles in peer-reviewed journals, 66 articles in peer-reviewed conference proceedings, and has presented invited talks at major national and international conference and symposiums. She also holds seven patents.

Chiaki Miyasaka is a Dr. of Engineering (received from Tokyo Institute of Technology in1996) and adjunct professor in the Department of Engineering Science and Mechanics at Pennsylvania State University.

He is interested in developing sensors (e.g., acoustic lens operating at an ultra high frequency) and applications (e.g., mathematical modeling and novel experimental method) in scanned image microscopy, that is, acoustic microscopy, atomic force microscopy, laser scanning microscopy, scanning electron microscopy, and the like, in the field of biomedical physics. His recent research activities in the biomedical field are focused on medical ultrasonic imaging relating to skin and breast cancers.

Kay Raum graduated from the Martin-Luther-University of Halle-Wittenberg with Diploma and PhD degrees in physics in 1997 and 2002, respectively. From 1995 to 1996 he was with the Bioacoustics Research Laboratory at the University of Illinois at Urbana-Champaign as a Visiting Scholar. From 1997 until 2003 he was a research assistant at the Medical Faculty of the Martin Luther University. In 2004 he received a post-doctoral fellowship from the French National Center of Scientific Research (CNRS) and joined the Laboratoire d’Imagerie Paramétrique at University Pierre et Marie Curie, Paris, France. In 2006 he became the Research Head of the Interdisciplinary Center for Musculoskeletal Diseases and in 2008 he received his Habilitation in “Experimental Orthopedics” at the Medical Faculty of the Martin Luther University. Since 2008 he has been a professor of engineering at the Berlin-Brandenburg Graduate School for Regenerative Therapies, and Head of the Ultrasound Biomicroscopy group of the Julius-Wolff-Institute at Charité-Universitätsmedizin Berlin.

He has been working with high frequency ultrasound for more than 15 years, and he has contributed specifically to the establishment and validation of quantitative acoustic microscopy in bone research. His current research is focused on the development of innovative parametric imaging techniques and their application in musculoskeletal research.

Jeffrey Sadler received his Hon. BSc degree in physics from the University of Guelph, and MSc and PhD degrees in physics from the University of Windsor. Recently, he joined the Institute for Diagnostic Imag­ing Research as a Post Doctoral Fellow. Past research has involved various situations in the area of acoustics, including computer simulations of acoustic waves in various plate structures, calculating the acoustical properties of composite materials, and acoustical imaging though complicated structures.

Amena Saïed received a PhD degree in physics from the University of Science and Technology, Montpellier, France in 1985. For several years she has been involved in the development of VHF acoustic micro­scopy and specific transducers for nondestructive testing of materials. In 1990, she joined the research department of Schlumberger Industrie (France) where she was in charge of the development of new gas flow-metering techniques using ultrasound. She joined the CNRS (Centre National de la Recherche Scientifique) in 1992 as a research scientist working in the Laboratoire d’Imagerie Paramétrique UMR 7623 (L.I.P) at the University Pierre et Marie Curie in Paris. She was the head of the high frequency ultrasound group. Her research program included technological innovations in the fields of biomedical imaging and the development of new methods of ultrasound signal and image processing for high resolution and quantitative evaluation of tissue composition and pathologies. In particular, she was involved in the development of three-dimensional, high frequency quantitative ultrasonography of eye and articular cartilage.

Dr. Saïed is currently continuing research at L.I.P. on topics including scanning acoustic microscopy of bone tissue and microbubble-mediated sonoporation. Her general research interests include biomedical imaging, tissue characterization, high-frequency transducers, acoustic microscopy, and microbubble-mediated sonoporation for intracellular gene delivery.

Yoshifumi Saijo was born in Yokohama, Japan on July 21, 1962. He received M.D. and PhD degrees in 1988 and 1993, respectively, from Tohoku University. He is currently a professor of the Biomedical Imaging Laboratory at the Graduate School of Biomedical Engineering of Tohoku University. He is concurrently engaged with the Graduate School of Medical Sciences, School of Engineering, Institute of Development, Aging and Cancer of Tohoku University and the Department of Cardiovascular Surgery of Tohoku University Hospital.

His main research interests are high frequency ultrasonic imaging of biological tissues and cells, parametric imaging of intravascular ultrasound, blood flow dynamics imaging by echocardiography and MRI, photoacoustic imaging of biological tissues, and mobile ultrasonic imaging by developing portable ultrasound devices.

In 1997 he received an award for his outstanding research paper in Ultrasound in Medicine and Biology. He is a member of The Japan Society of Ultrasonics in Medicine, Japanese Society of Echocardiography, and Japan Circulation Society.

Fedar Seviaryn was born in 1963 in Belarus. He received combined BSc and MSc degrees in physics from the Chair of Acoustics at Moscow State University. After defending his PhD thesis, “Nonlinear acoustical phenomena in layered structures” in 1989, he held the position of researcher at the B. I. Stepanov’s Institute of Physics of National Academy of Science of Belarus. Since 1998 Dr. Seviaryn has worked as a research associate in the Department of Physics at the University of Windsor, Windsor, Canada.

As a member of the Centre for Imaging Research and Advanced Materials Characterization he participates in numerous research projects in physical acoustics and the development of ultrasonic nondestructive evaluation applications.

David Shum graduated from the University of Hong Kong, Faculty of Medicine and completed his anatomical pathology residency training at the University of Western Ontario in 1980, becoming a Fellow of the Royal College of Physicians and Surgeons, and a Diplomat of the American Board of Pathology that year. He joined the Department of Pathology at Victoria Hospital and the Faculty of Medicine at the University of Western Ontario and was an associate professor in the Department of Pathology and the Division of Dermatology until 2000. He served as the Head of Surgical Pathology in the London Health Sciences Centre from 1995 to 2000 before becoming the senior dermatopathologist in the Department of Pathology in Vancouver General Hospital. In 2004, he became the Medical Director and Chief Pathologist in the Integrated Hospital Laboratories Service that includes Hotel Dieu Grace Hospital, Leamington District Memorial Hospital, and Windsor Regional Hospital. He is now an adjunct professor in pathology with the Schulich School of Medicine and Dentistry and he is also the Cancer Care Ontario Pathology Lead in Region 1 of the Local Health Integration Network (LHIN).

His medical practice is focused on diagnostic surgical pathology and dermatopathology. His publications include peer-reviewed articles, research papers, book chapters, and an Atlas of Histopathology of Skin Diseases. His current research interest is on 3D reconstruction of microscopic images and the use of ultrasound microscope in surgical pathology.

Mickael Tanter is a research professor at the French National Institute for Health and Medical Research (INSERM). For five years, he has headed the team Inserm ERL U979 “Wave Physics for Medicine” at Langevin Institute, ESPCI ParisTech, France. In 1999, he obtained his PhD degree from Paris VII University in physics.

His main activities are centered on the develop­ment of new approaches in wave physics for medical imaging and therapy. His current research interests cover a wide range of topics: elastography using shear wave imaging, high intensity focused ultrasound, ultrasonic imaging using ultrafast ultrasound scanners, adaptive beam forming, and the combination of ultrasound with optics and MRI.

Dr. Tanter holds 17patents in the field of ultrasound imaging and is the author of more than 80 technical peer-reviewed papers and book chapters.

Sergey A. Titov was born in Saratov, Russia in 1957. He received a combined BS and MS degree in physics from the Moscow State University in 1980, and a PhD degree in radio physics from the Moscow Institute of Radio Engineering, Electronics and Automation in 1991. Dr. Titov was an assistant professor with the Moscow Institute of Radio Engineering, Electronics and Automation from 1982 to 1992. In 1992, he was appointed associate professor at the same institute. Currently, he is a research associate at the Institute for Diagnostic Imaging Research, University of Windsor, Windsor, Ontario, Canada. In addition, Dr. Titov is a Senior Research Fellow of the Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Moscow, Russia.

He has published over 100 research papers and tutorials, and holds 11 patents. His research interests include quantitative acoustic microscopy, nondestructive testing, material characterization, electronics, and digital signal processing.

Bernhard R. Tittmann received his PhD from the University of California at Los Angeles in 1965. From 1966 to 1978 he held a post as Member of the Technical Staff at the North American Science Center and from 1979 to 1989 as Manager of the Materials Characterization Department at the Rockwell International Science Center. Then he joined the Engineering Science and Mechanics Department at the Pennsylvania State University at University Park, Pennsylvania as chaired full professor where in 1995 he established a Centre for Engineering Nano Characterization.

Professor Tittmann’s research interests focus on the fundamentals of condensed matter, physical acoustics, ultrasonic imaging, and acoustic microscopy. He has contributed to several books, published more than 400 scientific papers, and holds six patents.

Toshihiro Tsuji was born in Shiga, Japan. He was awarded BE, ME, and PhD degrees by the Department of Materials Processing in Tohoku University in 1998, 2000, and 2003, respectively. In 2003, he worked as a JSPS postdoctoral fellow in the Mechanical Engineering Laboratory, Advanced Industrial Science, and Technology, Japan on materials characterization by ultrasonic atomic force microscopy (UAFM). Since 2004, he has worked as a research associate and, since 2006, has been an assistant professor in the Department of Materials Processing, Tohoku University.

His research interests are in materials evaluation, nondestructive testing and sensors, and scanning probe microscopy. One of his present activities is the development and application of UAFM and ball surface acoustic wave (SAW) sensors.

Alexander Velichko was born in Krasnodar, Russia, in 1975. He received a MSc degree in applied mathematics from the Kuban State University, Krasnodar, Russia, in 1998 and a PhD degree from the Rostov State University, Rostov-on-Don, Russia, in 2002. Dr. Velichko has been a researcher in the Department of Mechanical Engineering at the University of Bristol, England since 2003 and was recently appointed to a lectureship.

His current research interests include mathematical modeling of propagation and scattering of elastic waves, ultrasonic imaging using arrays, and guided waves and signal processing.

Michael Vogt was born in Hagen, Germany in 1969. He received a Dipl.-Ing. degree in electrical engineering and a Dr.-Ing. degree from the Ruhr-University Bochum, Germany, in 1995 and 2000, respectively. In 2008, he qualified as a university lecturer at the Ruhr-University Bochum (Habilitation). Since 1995, he has been working on ultrasound imaging and measurement techniques, signal and image processing, and high frequency electronics at the High Frequency Engineering Research Group of the Ruhr-University Bochum. From 2001 to 2006, Dr. Vogt led the inter­disciplinary research project “High-Frequency Ultrasound” within the Ruhr Center of Excellence for Medical Engineering (KMR) Bochum, Germany. In 2007, he joined Krohne Messtechnik GmbH, Duisburg, Germany, as an R&D scientist working on ultrasonic flowmeters and electromagnetic level measurement systems.

His research interests include medical imaging systems, high frequency metrology, radar systems, and electromagnetic field simulations. Dr. Vogt is a Senior Member of the Institute of Electrical and Electronics Engineers (IEEE).

Paul D. Wilcox was born in Nottingham, England in 1971. He received an M.Eng. degree in engineering science from the University of Oxford in 1994 and a PhD from Imperial College, London, England in 1998.

From 1998 to 2002 he was a research associate in the Non-Destructive Testing (NDT) research group at Imperial College where he worked on the development of guided wave array transducers for large area inspection. From 2000 to 2002 he also acted as a consultant to Guided Ultrasonics Ltd., a manufacturer of guided wave test equipment. Since 2002 Dr. Wilcox has been at the University of Bristol (Bristol, England) where he is a professor in dynamics and an EPSRC Advanced Research Fellow.

His current research interests include long-range guided wave inspection, structural health monitoring, array transducers, elastodynamic scattering, and signal processing.

Kazushi Yamanaka was born in Tokyo, Japan. He was awarded BS and MS degrees by the Department of Applied Physics at the University of Tokyo in 1975 and 1977, respectively.

Since 1978, he has worked in the Mechanical Engineering Laboratory, Ministry of International Trade and Industry, Japan on materials characterization by acoustic microscopy. He obtained his PhD degree from Tohoku University in 1987. From 1987 to 1988, he was a Summit Postdoctoral Researcher at the Industrial Materials Research Institute, Canada. Since 1997, he has been a professor in the Department of Materials Processing, Tohoku University.

His research interests are materials evaluation, nondestructive testing, and sensors, using acoustic microscopy, laser ultrasound and scanning probe microscopy. Some present activities include the development and application of ultrasonic atomic force microscopy (UAFM), subharmonic phased array for crack evaluation (SPACE), and ball surface acoustic wave (SAW) sensors.

Part One

Fundamentals

1

From Multiwave Imaging to Elasticity Imaging

Mathias Fink and Mickael Tanter

1.1 Introduction

Different kinds of waves can be used to provide images of the human body. They propagate in tissues with very different wavelengths ranging from a fraction of micrometer for light, to some tenths of a millimeter for ultrasound, some centimeters for sonic shear waves, to some kilometers for low frequency electromagnetic waves. Each of these waves can provide an image whose contrast and spatial resolution depend on the way the wave interacts with tissues. For example, density and compressibility are the contrasts revealed by ultrasonic waves, while shear waves carry information on the viscoelasticity of tissues (shear modulus and viscosity). For electromagnetic waves, low frequency waves are sensitive to electrical conductivity while optical waves are sensitive to optical absorption coefficient and dielectric permittivity.

1.2 Regimes of Spatial Resolution

The spatial resolution of an image, contrary to common thinking, is not always controlled by the interrogating beam wavelength, as in conventional optical microscopy. There are indeed three different potential regimes describing wave propagation through tissues: coherent, diffusive, and near-field regimes. It is only in the coherent regime that wavelength determines resolution.

Ultrasonic waves, for example, can propagate tens of centimeters without losing their coherence. Since that distance is several orders of magnitude greater than the typical wavelength, spatial resolution in ultrasound depends on the wavelength.

In contrast, light at optical frequencies rapidly loses its coherence when propagating through opaque tissues and scattering off individual heterogeneities. The light transport mean free path l* – about 1 mm – characterizes the distance after which light loses any memory of its initial direction. For applications like diffuse optical tomography [1] in which the propagation distances are much longer than the mean free path, the spatial resolution is on the order of the observation depth.

Most low frequency electromagnetic imaging methods correspond to the so-called near-field regime, which is characterized by an observation depth much smaller than the wavelength. An example is electrical impedance tomography. With that technique, one generates low frequency alternating currents at multiple electrodes placed on the skin and infers tissue conductivity from potential measurement at the electrodes. In this regime, detectors are able to sense the exponentially decaying evanescent waves radiated by the medium. The spatial resolution is also on the order of the observation distance independent of wavelength.

A simple classification between these three various regimes can be made by comparing the three spatial scales that control any imaging experiment, that is, observation depth z, wavelength λ, and transport mean free path l*.

The first situation, λ < z < l*, corresponds to the coherent regime with the possibility of wave focusing and is encountered in the field of ultrasonic imaging and optical computed tomography. The second situation, λ < l* < z, corresponds to the diffusive regime, where the wave losses its coherence through tissue interactions. The third situation, z < λ < l*, corresponds to near-field imaging and is encountered in the field of near-field optics, EMG or EEG imaging, and electrical impedance tomography. It is only in the first situation that spatial resolution depends on the wavelength while in the other two regimes the spatial resolution is on the order of the observation distance z.

In all these imaging situations, physicists have striven to reach the optimal limits of the spatial resolution associated with each kind of wave. Today, after having pushed for decades the technological limits of these modalities, physicists are facing the inherent physical limits of the contrast/resolution couple in each modality. For medical imaging and diagnosis, physicians understood rapidly that one way to overcome these limits was to combine different imaging modalities, such as PET/CT, PET/MRI or ultrasound/X-ray mammography. The basic idea of multimodality imaging, such as, for example, the combination PET/CT, is to associate the high-resolution morphological image of a first modality (CT) with an image of the second modality (PET) that is poorly resolved but provides a clinically interesting contrast (i.e., metabolic activity). However, such multimodality imaging remains extremely costly and is limited by the inherent physical limits of each separate modality.

1.3 The Multiwave Approach

A very exciting solution to avoid the use of multimodality imaging is the multiwave imaging concept. It was proposed independently by different groups in the physicist community. It consists of productively combining two very different waves – one to provide contrast, another to provide spatial resolution – to build a new kind of image. Contrary to multimodality imaging that remains the superposition of two images limited by their respective contrast/resolution couples, multiwave imaging [2] overcomes this limitation by providing a unique image of the most interesting contrast with the most interesting resolution.

Multiwave imaging can benefit from three different potential interactions between waves:

In a first case, the interaction of the first wave with tissues during its propagation can generate a second kind of wave. This is the case in thermoacoustic and photoacoustic imaging [3–5] where any kind of electromagnetic wave is absorbed in some region causing a transient change in temperature that radiates an ultrasonic wave through thermal expansion.In a second case, a first wave that carries the information about the desired contrast but either completely losses its coherence during propagation through tissues or has a large wavelength can be tagged locally by a second kind of wave that remains coherent and well focused. The tagging focal spot can then be steered at different locations to build a complete image. This is the case of acousto-optical imaging (or acousto-optical tomography) [6, 7] where tissue displacements induced by a focused ultrasound beam modulates the optical speckle pattern of photons traveling through tissues. An image of the optical absorption is built with the sub-millimetric resolution of the ultrasonic wave.In a third case, a first wave traveling much faster than the second one can be used to produce a movie of the slow wave propagation. This is the case of transient elastography, where ultrafast ultrasonic scanners can track the motion of tissues scatterers induced by the propagation of low speed shear waves. This last case is relatively unique as it allows us to observe remotely the full movie of the near field of the shear wave around each obstacle (even if these obstacles are located in the far field of the two waves). A local inversion algorithm performed on this near-field movie produces a shear elasticity image relying on a sub-millimetric resolution while the shear wavelength is centimetric.

The concept of multiwave imaging is particularly interesting for the estimation of three physical parameters that remained difficult to map up until recently with a good spatial resolution: shear modulus and shear viscosity that give access to mechanical parameters that doctors feel during palpation, optical absorption, which gives access to tissues color, and finally electrical conductivity that depends on ion concentration and mobility in tissue and on the amount of intra- and extracellular fluids. In this chapter we will focus mainly on the third approach, which allows mapping tissue elasticity and viscosity with high precision.

1.4 Wave to Wave Generation

All techniques based on wave/wave generation are related to some dissipative processes that transform one part of a pulsed electromagnetic energy in some transient tissue motion that radiates coherent ultrasonic waves. From the recording of the ultrasonic field on an array of piezoelectric transducers, one can deduce an image of the ultrasonic sources. The fact that the ultrasonic speed is practically uniform in all tissue and has a well-known value greatly simplifies the reconstruction process. An image of the sources is then built with the sub-millimetric resolution of the ultrasonic wave.

In the thermoacoustic approach both microwaves [3, 4] and optical waves can be used. An image of the optical absorption or of the tissue conductivity is built with the sub-millimetric resolution of the radiated ultrasound. Microwave penetration allows deeper exploration and the first conductivity images of breast have been obtained with this modality, while vascular images on small animals have been obtained with the photo-acoustic approach. Figure 1.1 illustrates a spectacular application using as a heating source a laser with 532 nm wavelength and a wideband ultrasonic transducer with a 2.25 MHz central frequency to receive the photo-acoustic wave. Figure 1.1b shows that blood vessels in the cortical surface of small animals can be imaged with the skin and the skull intact. The imaging depth is limited to 1 cm, which is enough to image the entire brain of a small animal.

Figure 1.1 (a) Non-invasive photo-acoustic image of a superficial lesion (1 mm × 4 mm) in the right cortex on rat’s cerebra acquired through skull. RH is the right cerebral hemisphere, LH the left cerebral hemisphere, and L the lesion. The blood vessels are clearly imaged. (b) Open skull photograph of the rat surface acquired after the photo-acoustic experiment. See Reference [4].

Others modalities has been proposed to improve electrical impedance tomography, known as magnetoacoustic tomography (MAT), where tissue is displaced by an electric or a magnetic stimulation [5, 6] to produce ultrasound. In the most interesting technique with magnetic induction (MAT-MI) tissues are put both in a strong static magnetic field and in a time-varying magnetic field (MHz range). The time-varying magnetic field induces eddy currents that interact with the static magnetic field to produce a Lorentz force that induces ultrasonic waves, which can also be recorded by ultrasonic transducers. In this approach the acoustic wave amplitude is proportional to the electrical conductivity in the MHz range.

Different approaches can be conducted to map the source terms and the inverse problem image reconstruction is typically based on the fact that in a medium with constant speed the data recorded on the transducers array are spherical integrals of pressure source. A back-propagation algorithm allows recovery of function from integrals over spheres (spherical Radon transform). This step can also be accomplished by time-reversing and back-propagating the acoustic data in a computer model of constant speed.

1.5 Wave to Wave Tagging

Acousto-optic imaging combines, thanks to acousto-optic effects, ultrasound and light in a different way to photoacoustic imaging that is directly related to a dissipative process. A focused ultrasonic beam induces locally an ultrasonic modulation of a light beam traversing a scattering medium. Light transmitted through an organ contains thus different frequency components: the main component (the carrier) is centered at the incident coherent optical beam frequency. It is related to the scattered photons that do not interact with ultrasound. The sideband components are shifted by the ultrasound frequency. The sideband photons that result from the interaction between light and ultrasound are called the “tagged photons.” The weight of these tagged photons components depends on the optical absorption in the region of interest. Acousto-optic imaging detects selectively the tagged photons. An image related both to the optical absorption and to optical diffusion is then built up in scanning the focused ultrasonic beam over the whole organ. Marks investigated this tagging technique for the first time in the early 1990s [7]. Since then, many different groups have contributed to this field [8–12]. Two main mechanisms participate in the ultrasonic modulation of light in a scattering media. One is based on the variation of the optical phase in response to ultrasound-induced displacements scatterers. The displacement of scatterers modulates the physical path lengths of light traversing the ultrasonic field. Multiply scattered light accumulates modulated path lengths. Therefore, the intensity of the speckle associated with multiply scattered light fluctuates with the ultrasonic frequency. A second mechanism is based on the variation of the optical phase in response to ultrasonic modulation. As the result of ultrasonic modulation of the index of refraction, the optical phase between scattering events is modulated and the modulated phase causes also the speckle intensity to vary with ultrasound.

Many coherent detection techniques have been proposed to detect the tagged photons. One of the most interesting is the parallel detection scheme that uses a source synchronized lock-in technique in which a CCD camera works as a detector array [10, 11]. An interesting improvement was proposed to increase the axial resolution of the acousto-optic images, which was not as good as the lateral resolution with monochromatic ultrasound. Wang and Ku replaced the ultrasound monochromatic excitation by a frequency-swept (chirped signal) that modulated also the gain of the optical detectors [11].

However, the main difficulty of this technique in living tissues results from both the motion of the scatterers due to the Brownian motion of the scatterers and to the tissue inner motions (blood flow). This speckle decorrelation broadens the carrier and sideband lines. Typically, with 4 cm breast thickness, the speckle decorrelation time is in the ms range and yields a 3 kHz broadening. Detector bandwidth in this range is needed. With a mono-detector (photodiode) there is of course no problem but to achieve a good signal-to-noise one needs a large optical etendue in detector plane (product of the detector area and the detector acceptance angle) to fit the etendue of the tagged photons source, that is, to maximize the scattered light collection. This is why multi-detectors, such as a CCD camera, have been investigated, but they suffer from a low image frequency rate (typically 100 Hz) that is not enough to avoid the broadening effect in living tissues. Faster cameras are not sensitive enough. More recently different groups [12] have proposed very promising tagged-photons detection techniques based on photorefractive crystal based interferometry, which can give both a large etendue and a detection bandwidth in the kHz range (response time of GaAs photorefractive material is of the order of 1 ms).

Interestingly, the tagging concept could be used not only in acousto-optics but in many other fields of medical imaging such as, for example, electric impedance tomography tagged by ultrasonic remote vibrations.

Here we have discussed the concept of wave to wave tagging using two kinds of totally different waves. However, although it is not a multiwave technique, MR imaging can also be interpreted as a tagging technique. It uses only one kind of wave combined with a static field: a radio-frequency electromagnetic wave that causes protons to absorb some of its energy and to release it later at a resonance radio-frequency. The spatial tagging is achieved here through the addition to a static magnetic field of non-uniform magnetic fields whose spatial gradient modifies the local Larmor frequency, allowing during the reception mode a spatial resolution much better than the RF wavelength through a frequency analysis of the received signal.

1.6 Wave to Wave Imaging: Mapping Elasticity

The third approach to multiwave imaging is perhaps the most fascinating. Indeed, the wave interaction is here produced such that the near field of the slow wave around each obstacle can be filmed by the faster wave. In this approach, the playground consists of sonic shear waves and ultrasonic waves. These waves interact to produce a quantitative and highly resolved image of the stiffness of deep organs.

Stiffness is characterized by the Young’s modulus E (in kPa) and is an important parameter in medicine. Stiffness changes are often linked to pathology [13] and the significant dependence of E on structural changes in the tissue is the basis for the palpatory diagnosis of various diseases, such as detection of cancer nodules in the breast or prostate. Although it is strongly subjective, manual palpation is not only useful for screening and diagnosis but also during interventions to effectively guide the surgeon towards the pathological area. The concept of stiffness imaging was introduced in the early 1990s by J. Ophir et al. and named elastography [14]. Their technique is based on the ultrasonic imaging of tissue deformations induced by a quasi-static compression of organs applied by the operator at the surface of the body. Tissue deformations are obtained by acquiring two pre-compression and post-compression ultrasonic images of the organ using a conventional ultrasound scanner. Consequently, static elastography is inherently a single wave approach (based on the single use of ultrasonic waves), which implies some important drawbacks. Comparison of the two images enables only the mapping of local tissue strain. This strain image, called an elastogram, is linked to stiffness as soft regions tend to exhibit a higher strain than stiffer areas. However, even for a simple one-dimensional model, the underlying link between local strain ξ and stiffness E (Young’s modulus in kPa) is strongly dependent on the local and unknown stress τ via the well-known relation E = τ/ξ. Unfortunately, applying a quasi-static compression at the surface of the body can create a very complex spatial distribution of stress that both prevents the assessment of local stiffness and induces image artifacts.

To understand how to map tissue elasticity without the artifacts observed with static elastography, we will focus on a different approach that uses low frequency shear waves to obtain more precise tissue elasticity information.

To understand the connection between the Young modulus and the shear wave velocity, one can in a first approximation consider soft tissue as an isotropic elastic medium. The mechanical behavior of such a soft solid is characterized by two parameters, K (inverse of the compressibility κ = 1/K) and μ, which are, respectively the bulk and shear modulus. The relationship between stiffness and these parameters is described by:

(1.1)

The property K >> μ is a kind of definition of “soft solids” and human soft tissues belong to this category. It implies straightforwardly a direct link between stiffness E and the shear modulus, E  3μ. Therefore, an elegant way of accessing stiff­­ness properties consists in using shear waves whose speed cs depends simply on the shear modulus, . Another important consequence of the big discrepancies between K and μ in a soft solid is that the compressional wave speed is much larger than that of the shear waves (from 1540 m s–1 for cp to some m s–1 for cs). This is a unique case where two mechanical waves exhibit totally different wave speeds.

In conventional “single wave” imaging, only the compressional wave (and consequently the contrast of bulk modulus) is used. This is the successful field of medical ultrasound imaging. Now, could we use shear waves to image the shear modulus contrast of tissues? As we have just seen, the MHz frequency range is forbidden due to shear tissue viscosity and shear waves can only propagate on centimetric distances at low sonic frequencies. The typical shear wave frequency ranges between 10 Hz and 1 kHz. For example, to propagate on a 5 cm distance, we are limited to frequencies lower than 100 Hz, corresponding to typical wavelengths of several centimeters. The use of shear waves in a “single wave” imaging approach can only lead to poor results as it will rely on very bad spatial resolution. However, the contrast sensed by shear waves remains very relevant information for the diagnosis.

How can we solve this problem using a “multiwave” approach? We can benefit from the huge discrepancy between shear and compressional wave speeds. The idea is to use the compressional waves at ultrasonic frequencies to observe the tissue motion induced by the propagation of low speed sonic shear waves. During their propagation, shear waves induce local tissue displacements of the order of some tens of microns around their equilibrium position. One approach proposed by Sato and the group of K. Parker [15, 16] consists in observing the shear wave effect with ultrasound Doppler techniques. They use a sinusoidal shear wave excitation to produce a stationary vibrating pattern containing a set of nodes and antinodes. This approach was called sono-elastography. The distance between antinodes is used to deduce the shear wave wavelength and allows an estimate of the shear modulus with a spatial resolution of the order of the shear wavelength (centimeters).

Another approach that allows us to obtain shear modulus estimations with millimetric resolution (instead of centimetric resolution) is to use a short transient shear excitation instead of a sinusoidal shear excitation. This is the field of transient elastography [17, 18]. In this case, one needs to produce ultrafast images of tissues to image the propagation of the transient sonic shear waves. Here, the goal is to obtain a movie of the transient shear wave propagating inside organs with millimetric resolution. As the typical shear wave speed varies between 1 and 10 m s–1, one needs at least to reach 10 000 frames per second to follow the shear wave-front millimeter by millimeter. Using such an ultrafast scanner, it could be possible to estimate these local displacements between successive images.

Our group developed such an ultrafast scanner [18]. This is the first ultrasonic device able to reach more than 10 000 frames per second of deep-seated organs. In this device, whose architecture emerged from the concept of time reversal mirrors [19, 20], one transmits several thousand times per second an ultrasonic beam widely spread in the whole area of interest. This imaging sequence is very different from the one used in conventional ultrasound scanners that insonicate the medium using only a very thin ultrasonic focused beam that needs to be translated step by step to sequentially map the imaged area. Such a conventional echographic image results in more than 128 successive insonications. Taking into account the time of flight of backscattered ultrasound (a 20 cm back and forth propagation requires some 130 μs), a typical frame rate of about 50 images per second can be reached for sequential imaging. Contrarily, in an ultrafast scanner, for each transmit beam, the backscattered echoes coming from a very large region of interest are recorded by an array of some hundreds of piezoelectric transducers and stored in large memories. Then, a fast algorithm transforms several thousand times per second the backscattered echoes into an echographic image. Because the ultrasonic wave speed is known and constant, this operation can be obtained through a numerical time reversal refocusing. To track the local displacements induced by shear wave propagation, successive ultrasonic images are compared. This is possible because the ultrasonic images are dominated by the so-called “speckle” noise that originates from the random distribution of weak scatterers (Rayleigh scatterers much smaller than the wavelength) that exist everywhere in tissues. Note that in soft tissue, contrary to optics, ultrasonic backscattering is dominated by a single scattering process, thus insuring an unambiguous correspondence between the arrival time of the speckle noise and the spatial location of the scatterers distribution. By cross-correlating in the time domain the speckle noise observed from one frame to the other, a motion speckle tracking algorithm enables the reconstruction of a complete movie of the tissue displacements field along the ultrasonic beam direction (Figure 1.2). From this movie, one can locally deduce the shear wave speed and thus the shear modulus μ.

Figure 1.2