Ultrasound Elastography: Principles and Techniques

Diagnostic and Interventional Imaging (2013) 94, 487—495 . CONTINUING EDUCATION PROGRAM: FOCUS Ultrasound elastography: Principles and techniques ∗ J.-L. Gennisson , T. Deffieux, M. Fink, M. Tanter Institut Langevin, ondes et images [Waves and Images], ESPCI ParisTech, CNRS UMR 7587, Inserm ERL U979, université Paris VII, 1, rue Jussieu, 75238 Paris cedex 05, France KEYWORDS Abstract Ultrasonography has been widely used for diagnosis since it was first introduced Ultrasound in clinical practice in the 1970’s. Since then, new ultrasound modalities have been developed, elastography; such as Doppler imaging, which provides new information for diagnosis. Elastography was devel- Quasi-static method; oped in the 1990’s to map tissue stiffness, and reproduces/replaces the palpation performed Dynamic method; by clinicians. In this paper, we introduce the principles of elastography and give a technical Impulse summary for the main elastography techniques: from quasi-static methods that require a static elastography; compression of the tissue to dynamic methods that uses the propagation of mechanical waves Shear wave in the body. Several dynamic methods are discussed: vibro-acoustography, Acoustic Radiation elastography Force Impulsion (ARFI), transient elastography, shear wave imaging, etc. This paper aims to help the reader at understanding the differences between the different methods of this promising imaging modality that may become a significant tool in medical imaging. © 2013 Éditions françaises de radiologie. Published by Elsevier Masson SAS. All rights reserved. Ultrasonography is a widely used medical imaging technique with many clinical applications. Used in clinical practice for more than 40 years, it is highly regarded for its ease of use, real-time capability, portability and low cost. Based on the propagation of mechanical waves and more particularly on high frequency compressional waves aka ultrasound, it allows the construction of morphological images of organs, but lacks a fundamental and quantitative information on tissue elastic properties; indeed the bulk modulus that governs the propagation of ultrasound is almost homogeneous in the different biological tissues and does not depend on tissue elasticity [1]. Elastography, whose development started about 20 years ago, aims at imaging tissue stiffness, which provides an additional and clinically relevant information. Mapping the stiffness can either be estimated from the analysis of the strain in the tissue under a stress (quasi-static methods), or by the imaging of shear waves, mechanical waves, whose propagation is governed by the tissue stiffness rather than by its bulk modulus. ∗ Corresponding author. E-mail address: [email protected] (J.-L. Gennisson). 2211-5684/$ — see front matter © 2013 Éditions françaises de radiologie. Published by Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.diii.2013.01.022 488 J.-L. Gennisson et al. From a physics point of view, elastography aims to diagnostic-imaging devices, as a simple yet indirect infor- quantitatively image the Young’s E modulus, the physi- mation on the tissue stiffness. cal parameter corresponding to the stiffness. This has two important advantages: Dynamic methods • the Young’s modulus, noted E, exhibits important variations between different biological tissues, which makes it In dynamic methods, a time-varying force is applied to the ideal for the characterization of different tissues with an tissue, it can either be a short transient mechanical force or excellent contrast [1]; an oscillatory force with a fixed frequency. A time-varying • the Young’s modulus characterizes the stiffness of a tis- mechanical perturbation will propagate as mechanical sue, which is exactly the quantitative reproduction of a waves which in a solid body can be compressional waves clinician’s palpation and has relevant diagnostic value. or shear waves (Fig. 1). The compressional waves propagate very quickly in the human body (∼1500 m/s), and at high This simple and intuitive relationship between palpa- frequencies, these waves, also known as ultrasounds, can be tion and elastography calls for many applications of this used to image the body. Shear waves, which are only gener- ‘‘palpation imaging’’ such as breast tumor characterization ated at low frequencies (10 Hz to 2000 Hz) due to absorption and hepatic fibrosis staging where it was successfully been at higher frequencies, propagate more slowly, and their validated. Wherever palpation has been shown to have a ∼ speed ( 1—50 m/s) is directly related to the medium shear clinical value, elastography can be seen as a relevant tool ␮ modulus ( = ␳ VS 2), where ␳ is the density of the area for diagnosis. ∼ 3 ( 1000 kg/m ). Moreover, although palpation requires a direct contact In biological tissues, which are almost incompressible, and can only be applied to superficial organs, many elastog- the Young’s modulus can be approximated as three times the raphy techniques can also be applied to deep organs opening shear modulus (E = 3 ␮). The shear wave propagation speed new possibilities of diagnosis. can thus be used to map the Young’s modulus quantitatively. To assess the Young’s modulus of the tissue, all elastog- Dynamic elastography techniques, which rely on shear raphy techniques rely on the same basis: an external force waves propagation, can produce quantitative and higher is applied to the studied tissue and the resulting movements resolution Young’s modulus map compared to quasi-static are then followed. The external force can be classified methods. The use of shear waves, however, requires a more according to two means of excitation: the static methods complex system, able to generate the shear wave (mechan- (or the quasi-static method) and the dynamic methods. ical vibrator or ultrasound radiation pressure) and to image the small displacements induced by the shear wave (ultra- fast or stroboscopic ultrasound). The different methods Comparison of the two approaches Quasi-static methods Both static and dynamic methods use ultrasound to track In the case of quasi-static elastography, a constant stress is the displacements in the tissue either due to a static stress applied to the tissue. The displacement and the generated or to the propagation of the shear wave. Both approaches ␧ strain are then estimated using two-dimensional correla- are similar in that they apply an external stress and then tion of ultrasound images. The Young’s modulus is then given monitor the induced strain, although the frequency range is ␴ ␧ via the Hooke’s law ( = E. ), which links stress and strain very different: 0 Hz in static elastography and 50 to 500 Hz in a purely elastic medium. In practice, since the applied in dynamic elastography. stress is unknown, only the strain is displayed, this strain Quasi-static elastography cannot give a quantitative map is sometimes called the elastogram. This technique has value for the Young’s modulus since only the strain can be the advantage of being easy to implement but the unknown estimated and the applied stress is unknown. It is thus impos- stress distribution prevents any quantitative estimation of sible to recover the Young’s modulus using Hooke’s law. the local Young modulus in kilo-Pascal. Nevertheless, this By using the wave equation of shear waves, dynamic method has been deployed on many commercial ultrasound elastography do not require to know the stress distribution Figure 1. a: the longitudinal wave (P) spreads by successive volume variations of the medium. The displacement of the medium u is parallel to its propagation direction with a speed of VL. The ultrasounds used in ultrasonography are longitudinal waves. Sound is also a longitudinal wave in the range of audible frequencies; b: the shear wave (S) spreads by successive movements that are perpendicular to the direction of propagation with a speed of VS. Ultrasound elastography: Principles and techniques 489 to estimate the local Young’s modulus. However, this tech- then calculated by spatial derivation following one or pos- nique suffers from the presence of both compression and sibly two directions for the most evolved approaches. This shear waves in the studied medium. Thus the overlapping was the first elastography technique, developed by the Ophir waves require the use of tools that reduce the quality of the Group [2] at the beginning of the 1990s. Today, the tech- estimations. In addition, this technique is very sensitive to nique is being tested in breast lesion characterization with boundary conditions (the waves rebound at the interfaces interesting results [3]. and are mixed together), which makes it very difficult to Several commercial implementations of this process were distinguish between compression and shear waves. developed at the same time by different constructors as for Faced with these limitations, transient elastography example: • was developed at the same time and provides several Hitachi with its ‘‘Real-time Tissue Elastography’’. Based technological improvements. The main advantage of tran- on a quasi-static elastography method, it allows to quali- sient excitation is to naturally separate shear waves from tatively show the stiffness of tissue in a color image super compression waves, since shear waves are three times imposed on the standard ultrasound B-mode image; • slower than compression waves. Thus measurements of the Siemens with its ‘‘eSie Touch Elastography Imaging’’ propagation speed become relatively more straightforward. method. The

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