Foreword ix
Guy FRIJA
Chapter 1. Ultrasound Medical Imaging 1
Didier VRAY, Elisabeth BRUSSEAU, Valérie DETTI,François VARRAY, Adrian BASARAB, Olivier BEUF, Olivier BASSET,Christian CACHARD, Hervé LIEBGOTT, PhilippeDELACHARTRE
1.1. Introduction 1
1.2. Physical principles of echography 3
1.2.1. Ultrasound waves 3
1.2.2. Wavefronts 4
1.2.3. Stress/Strain relation 5
1.2.4. Propagation equation 6
1.2.5. Acoustic impedance 7
1.2.6. Acoustic intensity 7
1.2.7. Mechanical Index 9
1.2.8. Generation, emission 9
1.2.9. Resolution 10
1.2.10. Propagation of a plane wave in a finite isotropic medium11
1.2.11. Propagation of a plane wave in a non-homogeneous medium13
1.2.12. Speckle 15
1.2.13. Nonlinear waves 16
1.2.14. Contrast agents 17
1.3. Medical ultrasound systems 18
1.3.1. Principle 18
1.3.2. The different stages in image formation 19
1.3.3. Ultrasound imaging probe 21
1.3.4. Modes of imaging, B-mode and M-mode, and harmonic imagingmodes 24
1.3.5. Doppler imaging 27
1.4. The US image 34
1.4.1. Properties of speckle, echostructure and statisticallaws 34
1.4.2. Segmentation of US images 38
1.4.3. Simulation of US images 41
1.5. Recent advances in ultrasound imaging 44
1.5.1. Generation/emission of ultrasounds 44
1.5.2. Signal- and image processing 49
1.5.3. Multimodal imaging 60
1.6. A bright future for ultrasound imaging 65
1.7. Bibliography 65
Chapter 2. Magnetic Resonance Imaging 73
Dominique SAPPEY-MARINIER and André BRIGUET
2.1. Introduction 73
2.2. Fundamental elements for MRI 76
2.2.1. Introduction 76
2.2.2. Vectorial description of nuclear magnetic resonance (NMR)78
2.2.3. RF pulses and their effect on magnetizations 88
2.2.4. Elementary pulse sequences using the refocusingtechnique 97
2.2.5. Spatial discrimination of signals using gradients:fundamental principle of MRI 106
2.2.6. Multi-parameter aspect of MRI 110
2.3. Instrumentation 115
2.3.1. Introduction 115
2.3.2. Recording the signal 117
2.3.3. Magnetic systems 129
2.3.4. A typical MRI installation in a clinical environment136
2.3.5. Operation and safety 139
2.4. Image properties 144
2.4.1. Introduction 144
2.4.2. Field of view 144
2.4.3. Spatial resolution 148
2.4.4. Contrast and signal 155
2.4.5. Contrast elements in MRI practice 162
2.5. Imaging sequences and modes of reconstruction 168
2.5.1. Introduction 168
2.5.2. Overall view of acquisition sequences 168
2.5.3. Modes of reconstruction 195
2.6. Application of MRI: uses and evolution in the biomedicalfield 208
2.6.1. Introduction 208
2.6.2. Spectroscopy and imaging: technical and clinicalcomplementarity 210
2.6.3. Diffusion MRI: a morphological and functionalapproach 217
2.6.4. Functional MRI (fMRI) of cerebral activation 236
2.6.5. Bi-modal approach to MRI: the example ofMR/PET 239
2.7. Bibliography 244
List of Authors 263
Index 265
Chapter 1
Ultrasound Medical Imaging
1.1. Introduction
Ultrasound imaging accompanies each of us, from several months before we are born, throughout our lives. To monitor our development, 17 million ultrasound examinations are performed each year in France in the private sector, and around twice that if we add those done in the public sector. Thus, ultrasound imaging is the most widely used type of imaging for diagnostics after radiography. The world market for ultrasonography is still growing, and is worth an estimated 4.9 billion dollars (data from 2009). Ultrasonography plays a central role, both in hospitals and in doctors’ surgeries. The reasons for its ever-growing success are mainly based on its portability, its reasonable cost in comparison with other methods, its performances in terms of yielding results in real time and the fact that it uses non-ionizing waves. Historically, ultrasonography is associated with specialties in obstetrics for pregnancy-monitoring and cardiology. Today, ultrasound medical imaging covers a far wider range of specialties like no other type of imaging. Imaging of the digestive system, breasts, liver with elastography, thyroid or prostate, are examples of the most commonly performed operations [HTT 09].
In the future, significant progress is expected which will enable a doctor to reach a more certain diagnosis in a shorter period of time. For this to happen, technological innovations will be accompanied by methodological developments in signal- and image processing. 3D or 4D imaging techniques, particularly in the area of cardiovascular care, are likely to emerge in the near future, with the development of new probes and new modes of acquisition, e.g. using sparse sampling techniques. At the same time, progress in modeling, simulation and image processing will be at the heart of new quantitative analysis software built into ultrasound machines. The diagnosis of myocardial infarction, the replacement of the heart valves or the detection of atherosclerosis are examples of medical exams for which these innovations will be essential (Figure 1.1).
Figure 1.1. Program for measuring the volume of the left ventricle of the heart, running on a Vivid 6 clinical echogram machine (GE HealthCare). For a color version of this figure, see www.iste.co.uk/fanet/medimagnet.zip
Other means of imaging based on estimation of the physical properties of healthy and diseased tissues will also supplement conventional imaging tools. Different modes of quasi-static, dynamic or transient elastography should be able to quantify the elasticity of tissues for diagnosing liver disorders or quantify the development of cancer, for instance.
In this chapter, we are going to present both the physical basics of ultrasound (US) imaging and the main advances expected of the echography of tomorrow. The chapter begins with a presentation of the physical principles upon which ultrasound imaging is based. Then, we shall detail the different modes of imaging in ultrasound systems: B-mode, M-mode, Doppler modes, contrast and harmonic imaging. The hardware aspects will also be touched upon, with a discussion of the different types of linear or sectorial probes used in clinical practice, depending on the compromise between resolution and penetration. We shall then return to statistical analysis of the US image using the properties of the ultrasound speckle. This knowledge will help to simulate realistic images and sequences which are useful to validate the image formation models and processing methods. The final part of this chapter will be given over to the advances in ultrasound image acquisition and processing which will serve as a diagnostic aid to doctors. We shall present the most recent probe technologies which are based on new materials to transmit and receive ultrasounds in 2D and 3D with sensor matrices. These probes can be based on innovative methods of image formation using the methods of synthetic aperture, “tagging” or sparse sampling. We shall also illustrate the contribution made by new techniques such as elastography, nonlinear imaging or parametric imaging, and the performances of real-time tracking methods or motion estimation more generally. The end of the chapter will be devoted to multimodality imaging. Using the example of bi-modal US/Optical imaging, we shall show that the combination of the anatomical information provided by the US image with the functional or metabolic information provided by the other modes of imaging facilitates a more effective aid to diagnosis and monitoring of the evolution of diseases.
1.2. Physical principles of echography
1.2.1. Ultrasound waves
US waves are pressure waves whose frequency is greater than the maximum audible frequency of 20 kHz. These US waves are mechanical vibrations. They need a source to give rise to them, and a support medium (a solid, liquid or gas) in order to propagate. If the vibration generated by the source is oscillating, the particles of the medium initially at rest will oscillate around their equilibrium position when the US wave passes through. There are two modes of oscillation: a longitudinal mode where the particles oscillate along the direction of the wave’s propagation, forming a longitudinal or compression wave, and a transversal mode, where the particles oscillate in the direction transversal to the direction of the wave’s propagation, forming a transversal or shear wave [SZA 04].
In most diagnostic echogram exams, it is soft tissues which are explored, and the range of frequencies of oscillation of the wave is between 2 and 20 MHz for the most common external applications and 30 to 50 MHz for internal explorations, such as intra-vascular imaging. At the frequencies used for diagnostic imaging, the shear waves are greatly attenuated and can therefore be neglected. In addition, as soft tissues are composed primarily of water, the propagation of the US wave in these tissues is very similar to its propagation in liquids. In a liquid, the particles oscillate along the direction of propagation of the wave, forming a longitudinal wave. In water, for the applications in question here, the movement of the particles is roughly a few tens of nanometers and their velocity is a few cm/s, whereas the phase velocity c at which the wave propagates in the medium is around 1500 m/s.
When a sinusoidal disturbance propagates in a liquid medium, regions of compression and dilatation form in the medium when the wave passes through. This phenomenon of compression and dilatation is periodic and is observable in two different ways: at a given time or at a given position. It is due to the displacement of the particles. Particle displacement is greater in the dilatation zone than in the compression zone. The periodicity of the displacement is called the wavelength ? if we observe that displacement as a function of the position at a given time, and period T if we observe it as a function of time at a given position. These two values are linked by the phase velocity ? = cT or indeed c = ?f when f = 1/T, representing the frequency of the oscillations emitted by the source.
In order to study the propagation of a US wave, we need to distinguish two types of media: isotropic and anisotropic media. An isotropic medium is one which exhibits no single prevailing direction of propagation. This means that it is the source generating the initial disturbance which imposes the direction of propagation, rather than the medium. Conversely, with an anisotropic medium it is the medium which imposes the direction of propagation. Such is the case, for instance, in most tissues with an oriented fibrous structure.
1.2.2. Wavefronts
As we have just seen, there are two types of waves which are distinguished by their mode of propagation. These waves are also differentiated by the shape of what is called their wavefront, i.e. the set of points of a medium which simultaneously experience the same change in pressure as the wave passes. Thus, waves can be classified into three wave shapes:
– spherical waves, which are generated over 4
? steradians by a source whose dimension is very small in relation to the wavelength emitted; – cylindrical waves, which are generated by a cylindrical source of infinite length, vibrating in a radial direction; – plane waves, which are generated by a flat (or “plane”) surface such as a piston of infinite dimensions.
In reality, any given wave is a combination of the three forms described above. For reasons of simplicity, we shall only examine the interaction of a plane wave with the medium. By way of example, consider the configuration represented in Figure 1.2. A point source generates a spherical wave. Target 1, which is “near” to the source, is then subjected to the influence of that spherical wave. That is, if the dimension of that target is larger than the wavelength, not all the points on the anterior face are subject to the same pressure at the same time. On the other hand, as regards Target 2, which is a long way from the source, the wavefront which reaches this target can be considered to be plane.
Figure 1.2. The source emits a spherical wave. Target 1 receives a spherical wave whereas Target 2, which is further away, receives a plane wave
1.2.3. Stress/Strain relation
We consider a perfectly elastic medium, i.e. a medium which, when subjected to a stress, deforms with no internal friction (with no losses) and regains its original form exactly when the stress is removed. As the wave propagates in this elastic and isotropic...
