[Ultrasonic wave propagation characteristics of cancellous bone]

Ultrasonic measurements of bone status or bone mass density are generally performed using ultrasonic parameters consisting of the slope of frequency-dependent attenuation (or broadband ultrasound attenuation: BUA) and the speed of sound (SOS). Many results of in vitro laboratory measurements and in vivo clinical trials have shown the ultrasonic parameters, BUA and SOS correlate significantly with the bone mass density measured by X-ray method. However, there exists some problem inherent in the ultrasonic method on the reproducibility and the uncertainty of measured ultrasonic parameters. The ultrasonic properties of cancellous bone have been experimentally and theoretically studied by author's group to reveal problems inherent in the ultrasonic method. According to experimental and theoretical studies, two longitudinal waves, fast wave and slow wave are clearly observed. The propagation speed of the fast wave increases with the bone density and that of the slow wave decreases very slightly with the bone density. Whereas the attenuation constant of the fast wave is much higher than that of the slow wave and is almost independent of the bone density, but in contrast, the attenuation constant of the slow wave increases considerably with the bone density. Experimental results on transmitted ultrasonic wave through cancellous bone show that the amplitude of the slow wave decreases with the bone density and the amplitude of the fast wave, on the contrary, increases with the bone density. This dependence of the fast wave amplitude on the bone density can not be explained by the attenuation constant. The ultrasonic wave propagation path through cancellous bone is modelized to clarify the propagation phenomenon and to specify the causality between ultrasonic wave parameters and the bone density. The bone density is quantitatively formulated based on the modelization as a function of the amplitude and the propagation speed of the fast wave.