Garra BS, & Garra, B. S. (2007). Imaging and estimation of tissue elasticity by ultrasound. Ultrasound Quarterly, 23(4), 255–268. https://doi.org/10.1097/ruq.0b013e31815b7ed6
Ultrasound (US) elasticity imaging is an extension of the ancient art of palpation and of earlier US methods for viewing tissue stiffness such as echopalpation. Elasticity images consist of either an image of strain in response to force or an image of estimated elastic modulus. There are 3 main types of US elasticity imaging: elastography that tracks tissue movement during compression to obtain an estimate of strain, sonoelastography that uses color Doppler to generate an image of tissue movement in response to external vibrations, and tracking of shear wave propagation through tissue to obtain the elastic modulus. Other modalities may be used for elasticity imaging, the most powerful being magnetic resonance elastography. With 4 commercial US scanners already offering elastography and more to follow, US-based methods may be the most widely used for the near future.
Gennisson, J.-L., Deffieux, T., Fink, M., & Tanter, M. (2013). Ultrasound elastography: Principles and techniques. Diagnostic and Interventional Imaging, 94(5), 487–495. https://doi.org/10.1016/j.diii.2013.01.022
Ultrasonography has been widely used for diagnosis since it was first introduced in clinical practice in the 1970's. Since then, new ultrasound modalities have been developed, such as Doppler imaging, which provides new information for diagnosis. Elastography was developed in the 1990's to map tissue stiffness, and reproduces/replaces the palpation performed by clinicians. In this paper, we introduce the principles of elastography and give a technical summary for the main elastography techniques: from quasi-static methods that require a static compression of the tissue to dynamic methods that uses the propagation of mechanical waves in the body. Several dynamic methods are discussed: vibro-acoustography, Acoustic Radiation 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.
Eagle, M. (2006). Doppler ultrasound—Basics revisited. British Journal of Nursing, 15(Sup2), S24–S30. https://doi.org/10.12968/bjon.2006.15.Sup2.21238
Palpation of pedal pulses alone is known to be an unreliable indicator for the presence of arterial disease. Using portable Doppler ultrasound to measure the resting ankle brachial pressure index is superior to palpation of peripheral pulses as an assessment of the adequacy of the arterial supply in the lower limb. Revisiting basics, this article aims to aid the clinician to understand and perform hand-held Doppler ultrasound effectively while involving the client or patient in the process. The author describes the basics of Doppler ultrasound, how to select correct equipment for the process, and interpretation of results to further enhance clinicians' knowledge.
Chen K, Yao A, Zheng EE, Lin J, & Zheng Y. (2012). Shear wave dispersion ultrasound vibrometry based on a different mechanical model for soft tissue characterization. Journal of Ultrasound in Medicine, 31(12), 2001–2011. https://onlinelibrary.wiley.com/doi/abs/10.7863/jum.2012.31.12.2001
Ultrasound vibrometry can measure the propagation velocity of shear waves in soft tissue noninvasively, and the shear moduli of tissue can be estimated inversely from the velocities at multiple frequencies. It is possible to choose the appropriate model for tissue characterization from mathematical methods and analysis of model behaviors. The three classic models, Voigt, Maxwell, and Zener, were applied to fit the velocity measurements and estimate shear moduli of porcine livers with different thermal damage levels and different storage times. The Zener model always provided the best estimation of the moduli with the minimum errors in our experiments. Unlike the Voigt and Maxwell models, the moduli of the Zener model cannot be used to indicate damage levels in porcine livers directly, but the creep and relaxation behaviors of the Zener model are effective.
Song, P., & Chen, S. (2018). Shear Wave Dispersion Ultrasound Vibrometry. In Ultrasound Elastography for Biomedical Applications and Medicine (pp. 284–294). John Wiley & Sons, Ltd. https://doi.org/10.1002/9781119021520.ch19
This chapter introduces an ultrasound imaging method called shearwave dispersion ultrasound vibrometry (SDUV) that can quantify both tissue elasticity and viscosity noninvasively. It also discusses a few practical challenges of this method. SDUV generates multi-frequency wide-band harmonic shearwaves using both acoustic radiation force (ARF) and external mechanical vibration to obtain the shear wave speed dispersion curve, from which tissue viscoelasticity can be robustly estimated. Different configurations of SDUV are developed using ARF and external mechanical vibration. Then, the chapter extends the concept of SDUV to using a transient shear wave signal and dispersion analysis to quantify tissue viscoelasticity. Consequently, this method is performed in both ex vivo and in vivo tissues, including liver, heart, artery, kidney, prostate, and muscle. In addition, the chapter reviews several applications of SDUV and other similar techniques that provide viscoelasticity measurements.
Dietrich, C. F., Barr, R. G., Farrokh, A., Dighe, M., Hocke, M., Jenssen, C., Dong, Y., Saftoiu, A., & Havre, R. F. (2017). Strain Elastography—How To Do It? Ultrasound International Open, 03(04), E137–E149. https://doi.org/10.1055/s-0043-119412
Tissue stiffness assessed by palpation for diagnosing pathology has been used for thousands of years. Ultrasound elastography has been developed more recently to display similar information on tissue stiffness as an image. There are two main types of ultrasound elastography, strain and shear wave. Strain elastography is a qualitative technique and provides information on the relative stiffness between one tissue and another. Shear wave elastography is a quantitative method and provides an estimated value of the tissue stiffness that can be expressed in either the shear wave speed through the tissues in meters/second, or converted to the Young’s modulus making some assumptions and expressed in kPa. Each technique has its advantages and disadvantages and they are often complimentary to each other in clinical practice. This article reviews the principles, technique, and interpretation of strain elastography in various organs. It describes how to optimize technique, while pitfalls and artifacts are also discussed.
The assessment of left ventricular (LV) function is a fundamental requirement in clinical situations. Numerous techniques are used and parameters can be employed, and ejection fraction (EF) is the most established. Echocardiography is the most widely used technique for the assessment of LV function, but echocardiographic assessment of EF is subject to limitations. The development of strain as a clinical tool over the last 30 years is accurate and reliable. The year 2020 represents a culmination of this work to produce an additional assessment of LV function. This chapter provides the technical background regarding this modality, summarizes potential applications, and prepares the reader about the main clinical applications of myocardial strain. The adoption of a new tool can be difficult. Fundamental to finding the motivation for this effort is recognition of the limitations of assessment of both global and regional function using current techniques. There are two aspects of a desirable measurement: validity and reliability. Validity pertains to the comparison of the measurement against an external standard. The accuracy of strain has been validated experimentally against in vivo measurement of tissue excursion with sonomicrometry and clinically against magnetic resonance tagging techniques. While validity is important to clinicians, in most circumstances we are less worried about consistent bias (e.g., when the measurement is within the reference standard) but more concerned about random variation of the measurement that would preclude our ability to compare individuals or compare measurements at different points of time. This is a reflection of reliability. In sequential testing, the tool has to be sufficiently sensitive to pick up small differences, and reliability can be measured as the ratio of true variance and true+error variance. What is important is that both validity and reliability of existing techniques are inadequate and especially of limited reproducibility.
Since its inception, strain imaging in echocardiography has demonstrated significant improvement in technology and has established itself as a sensitive biomarker for assessing cardiac function. Recently, many studies began using strain parameters as reproducible and robust endpoints. Emerging technologies and fields, including three-dimensional strain, principal strain analysis, fusion imaging, and machine learning, are further reinforcing its usefulness. Additionally, in future research, such as trials in silico and tissue engineering, strain imaging will play an important role in therapeutic evaluation. In this chapter, we discuss the newly evolving applications of strain imaging and its potential impact in clinical cardiology.