Ultrasonic characterization of tissue hardness in the in vivo human liver

Tissue characterization attempts to provide quantitative information about the state of disease or health of tissue interrogated by ultrasound beams. A feature of diffuse disease processes of the liver is decrease in elasticity or increase in hardness of the tissue, which is observed when the primary cardiac pulsation or the externally forced mechanical vibration at low frequency is applied to the liver. A technique for estimating the local tissue displacement based on the spatio-temporal derivative of echo images is described and a model is presented for the changes in tissue hardness as a change in the strain tensor component of tissue. Measurements are performed using a conventional real-time array transducer with 3.5 MHz center frequency and the sequences of RF echo signal are digitized to estimate the local tissue displacement and then the strain tensor of tissue is computed from the distribution of tissue displacement. In experiments with living soft tissue, such as liver in vivo, the strain is small in less elastic regions. The tissue hardness can be characterized as the strain distribution computed from the local displacement distribution observed in the received RF echo signals