Ultrasonic slow waves in air-saturated cancellous bone

This paper describes preliminary observations of ultrasonic wave propagation in air-saturated defatted cancellous bone from the human vertebra. Using a broadband pulse transmission system, attenuation and phase velocity were measured over a wide frequency range (100 kHz-1 MHz). The observed behaviour was consistent with that expected for the decoupled slow wave predicted by Biot's theory. Velocity was lower than that of free air, and there was marked frequency-dependent attenuation and velocity dispersion. The tortuosity (alpha) of the trabecular microstructure was estimated from the high frequency limit of the dispersion curve, with a mean value of alpha = 1.040 +/- 0.004 obtained in five specimens. Ultrasonic measurements in air represent a valuable new approach, capable of yielding parameters that directly characterise bone structure. Furthermore, they may give useful insights into wave propagation in bone in vivo, where the trabecular framework is saturated with marrow fat rather than air.