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Viscoelastic imaging methods using acoustic radiation force Frew, Samuel Mark


Elastography is a method of imaging the viscoelastic properties of soft tissue and similar media. It can be used for identifying various forms of cancer and fibrosis in soft tissue. Elastography can thus aid in diagnosis of such diseases, and in treatment procedures, such a biopsy needle guidance. Acoustic radiation force (ARF) imaging is one form of elastography whereby the ARF of focused ultrasound displaces the medium at an internal, localised position. The same ultrasound system can be used to monitor the displacement, from which the viscoelastic properties can be recovered. This work presents a new ARF imaging method, called axial relaxation imaging, that uses transfer function techniques. The relaxation response at a point in the medium is monitored after a period of ARF application. By assuming the response corresponds to a negative step response of a linear system, the relative force-displacement transfer function is computed. This is then related to a Voigt model over a range of frequencies to obtain a relative elastic parameter and a frequency-dependent viscous parameter governed by a power law. The method was applied to homogeneous phantoms made with different gelatine concentrations. Relative elastic parameters of 1, 2.3 and 4.4 and relative viscous parameters of 1, 1.8 and 3.0 were obtained for 2, 3 and 4 wt% gelatine concentrations, respectively, with consistent results between phantoms of the same type and agreement with values from other estimation techniques. The viscous power law frequency dependencies were governed by flow index values of –0.10, –0.14 and –0.18, respectively. The good separation between parameters in the results shows the method holds potential for application to tissue characterisation.

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