Study about the propagation of airborne ultrasonic wave through a heel for bone-density estimation

Bone density and elasticity can be measured from propagation parameters of acoustic waves. In typical bone-density estimation, a propagation path of the ultrasonic wave has to be filled with mediums, whose acoustic properties are similar to tissues, to reduce attenuation by boundary reflection. Therefore, transducers are brought into contact with examined regions. In this study, non-contact bone-density estimation for a calcaneus using airborne ultrasonic waves has been proposed. When airborne ultrasonic waves pass through a heel, the received signals are attenuated more than -70 dB, which is even estimated from ideal flat boundaries of air, soft tissue, solid bone and cancellous bone. Therefore, pulse compression using M-sequence is employed to improve the S/N of received signals. When the order of M-sequence is n-th order, S/N improvement of received signals by cross correlation of the received signal and the reference signal, which is corresponding to the transmitted signal is √(2ⁿ - 1) times. The pass-through waves in a heel can be detected by amplification of 60 dB and S/N improvement of more than 45 dB in the experiments. However, the TOF of pass-through waves can be estimated because first-arrival pass-through waves are not clarified. For measurement of the propagation velocity of pass-through waves, wideband transducers or S/N improvement is required to clarify the first-arrival pass-through waves.