The Role of Soft Tissue in Ultrasonic Guided Wave Measurements in Bone

Previously we have reported the possibility of observing the fundamental antisymmetric (A0) Lamb wave propagating in the human tibia and radius. However, A0 velocity (1300-2000 m/s) overlaps that of soft tissue (1500 m/s), and it is therefore possible that signals propagating through soft tissue may be mistaken for an A0 wave in bone. In order to clarify this issue, measurements were performed with water or a silicon rubber layer on top of bone phantoms and specimens, and on human tibia in vivo. It was shown that the intensity of A0 mode attenuated rapidly and its velocity decreased slightly with increasing thickness of soft overlayer. In addition, A0 was detectable better through an overlying layer of water than overlying silicon rubber. Thin soft tissue had only a slight effect on the detectability of A0 mode in vivo and A0 could be measured in 70% of the human subjects.