Ultrasonic imaging based on measurements of viscoelastic hysteresis in soft tissue

Ultrasonic tissue elasticity imaging, which displays stiffness distributions such as strain and Young's modulus based on the detection of echo shifts under quasi-static deformation applied to the body surface using an ultrasonic probe, is useful for the detection of lesions, such as cancers, that affect the stiffness of tissue. While the viscoelasticity of tissue makes diagnosis by elasticity imaging difficult, because the image quality depends on the rate of deformation, the differences in viscoelasticity reflect the tissue type and state of degeneration. In this paper, with the aim of achieving tissue discrimination based on viscoelasticity, an ultrasonic imaging technique using the hysteresis parameter (HP), which characterizes the local viscoelastic hysteresis, is investigated. In order to obtain the HP, quasi-static and cyclic loading and unloading to the body surface are applied. As result of the direct mechanical testing of porcine thigh muscle before and after formalin immersion, the post-immersed HP value decreased to about 46% of the pre-immersed HP value. Moreover, the ability to distinguish tissue components has also been confirmed. Therefore, the sensitivity of the HP in discriminating tissue degeneration and its components is found to be adequate. In the HP imaging experiment involving human breast diagnosed as having fibroadenoma, while the conventional B-mode and strain images depict the fibroadenoma as circular and homogeneous regions, the HP image depicts these as non-circular and inhomogeneous areas. From these results, HP imaging is proposed as method for providing different information from that provided by conventional imaging such as B-mode and strain images.