Independent testing of soft tissue visco-elasticity using indentation and rotary shear deformations

Numerous techniques exist to measure the mechanical properties of soft tissues in vivo, such as mechanical stretching, indentation or shearing, as well as elastographic methods employing ultrasound or other imaging modes. Many groups have reported properties which do not necessarily correspond with each other due to differences in choice of technique, tissue model or other variations. This work deliberately makes use of the two independent modes of indentation and rotary shear, on the same material samples, employing similar modeling approximations, to attempt to determine the common, underlying material properties. This paper introduces the ROSA-2 rotary shear instrument, and presents its mechanical characteristics, as well as presenting validation experiments that were performed to verify non-slip contact with tissue. Measurements made with it are compared with those acquired with the TeMPeST l-D indentation instrument. Initial testing showed reasonably agreement when testing silicone gel samples, over a restricted range of frequencies. When testing bovine liver samples in vitro and porcine liver in vivo, significant discrepancies were found. The potential sources of these differences will be discussed, as will directions for ongoing work.