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Shear waves
Shear waves and compressional waves are the two main modes of propagation of acoustic energy in solids. With shear waves, also called transverse waves, the particles of the medium oscillate at a right angle to the direction of propagation. In compressional waves the oscillations occur in the longitudinal direction or the direction of wave propagation. The wave speeds of these different kinds of waves are governed by two different types of moduli. The compressional wave speed is related to the bulk elasticity modulus of the medium while the shear wave speed is related to the shear elasticity modulus. In soft biological tissues the bulk modulus varies no more than 10% while variation of the shear elasticity modulus may be several orders of magnitude depending on the structure and state of tissue.〔Sarvazyan AP, Skovoroda AR, Emelianov SY, Fowlkes JB, Pipe JG, Adler RS, Buxton RB, Carson PL. Biophysical bases of elasticity imaging. In: Acoustical Imaging. Ed. Jones JP, Plenum Press, New York and London, 1995; 21: 223-240.〕〔Sarvazyan AP, Urban MW, Greenleaf JF. Acoustic waves in medical imaging and diagnostics. Ultrasound Med Biol. 2013 Jul;39(7):1133-46. . Epub 2013 Apr 30. Review. PMID 23643056〕
==Shear Waves in Elastography==
The bulk modulus is defined by short range molecular interaction forces and depends mainly on molecular composition of tissue which is typically 75% water with little variation. In contrast to that, the shear modulus is defined by long range interactions and is highly sensitive to structural changes. The wide range of variability makes the shear elasticity modulus and, respectively, the shear wave speed, highly sensitive to physiological and pathological structural changes of tissue. For this reason, the use of shear waves in new diagnostic methods and devices has been extensively investigated over the last two decades. Numerous new methods were developed most notable of which are Shear Wave Elasticity Imaging (SWEI), Magnetic Resonance Elastography (MRE), Supersonic Shear Imaging (SSI), Shearwave Dispersion Ultrasound Vibrometry (SDUV), Harmonic Motion Imaging (HMI), Comb-push Ultrasound Shear Elastography (CUSE), and Spatially Modulated Ultrasound Radiation Force (SMURF).〔Sarvazyan AP, Rudenko OV, Swanson SD, Fowlkes JB, and Emelianov SY, Shear wave elasticity imaging: a new ultrasonic technology of medical diagnostics. Ultrasound Med. Biol. 1998; 24: 1419-35.〕〔Muthupillai R, Lomas DJ, Rossman PJ, Greenleaf JF, Manduca A, and Ehman RL, Magnetic resonance elastography by direct visualization of propagating acoustic strain waves. Science 1995; 269: 1854-7.〕〔Bercoff J, Tanter M, and Fink M, Supersonic shear imaging: a new technique for soft tissue elasticity mapping. IEEE Trans. Ultrason. Ferroelectr. Freq. Control 2004; 51: 396-409.〕〔Chen S, Urban MW, Pislaru C, Kinnick R, Zheng Y, Yao A, and Greenleaf JF, Shearwave dispersion ultrasound vibrometry (SDUV) for measuring tissue elasticity and viscosity. IEEE Trans. Ultrason. Ferroelectr. Freq. Control 2009; 56: 55-6.〕〔Vappou J, Maleke C, and Konofagou EE, Quantitative viscoelastic parameters measured by harmonic motion imaging. Phys. Med. Biol. 2009; 54: 3579-3594.〕〔Song P, Zhao H, Manduca A, Urban M W, Greenleaf J F, and Chen S, "Comb-push ultrasound shear elastography (CUSE): a novel method for two-dimensional shear elasticity imaging of soft tissues," IEEE Trans. Med. Imaging, vol. 31, pp. 1821-1832, 2012.〕〔9.
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