A visualization of nonlinear elasticity property of tissues by ultrasound

Imaging of tissue elasticity is promising as a technique for visualizing tissue variations such as tumors from the point of view of hardness. Evaluation of the elasticity has hitherto been carried out mainly in the range in which the stress and strain generated within the tissue under infinitesimal deformation are related linearly. Under a finite deformation, such a linear relationship does not hold but a nonlinear phenomenon (strain hardening) takes place, in which Young's modulus increases with the strain. Since such nonlinearity is a property characteristic of the tissue, this paper deals with this nonlinearity and discusses a method of visualizing it and obtaining new information for tissue diagnosis. As a basic study, an experiment is carried out using a phantom containing the gizzard and liver of a fowl. The rate of increase of Young's modulus due to substantial deformation is visualized as a nonlinear parameter. It is found that gelatin and liver, with similar values of Young's modulus, cannot be distinguished by the conventional Young's modulus image but can be distinguished by visualizing the incrementation rate of each Young's modulus. Hence, the nonlinear elastic parameter can be effective for tissue discrimination.