In vivo imaging of viscoelasticity and hardness of biological soft tissue using ultrasound.

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Changes in tissue elasticity are related to diffuse disease processes of living soft tissue. The goal of this study is to characterize tissue elasticity based on ultrasonic measurement of displacement and strain when the primary cardiac pulsation or the external mechanical force is applied to the soft tissue. A model is presented for the changes in tissue elasticity as a change of internal strain tensor, which results in variation in the Young's or shear modulus of elasticity. Mesurements are performed using real-time ultrasound and the sequences of RF echo signal returned from tissue are digitized to estimate local tissue displacement. In this paper the 2-dimensional distribution of internal displacement is estimated using the spatio-temporal derivative of RF echo envelope signals, from which components of internal strain tensor are computed. Experiments on tissue mimicking phantoms indicate that 2-dimensional fields of components of strain tensor can be estimated with the spatial resolution of 8mm*8mm. The distribution of the relative shear modulus based on strain images is reconstructed. In experiments with living soft tissue, such as liver in vivo, the relative shear modulus is large in less elastic regions. The elasticity of soft tissue can be characterized as the relative shear modulus computed from the local strain tenpor observed in the received RF echo signals.