Some aspects of the atypical acoustic behavior of cancellous bone

Abstract

Cancellous bone, consisting of a porous framework of solid trabeculae saturated in marrow fluid, is a challenging material in terms of ultrasonic characterization. This study considers some unusual aspects of the ultrasonic behavior of cancellous bone. These anomalies highlight the boundaries of our knowledge and, arguably, point us towards promising directions for future progress. Profoundly different wave propagation regimes can exist along different anatomical axes. Along the main axis of trabecular orientation, two longitudinal waves are observed, but only one wave is observed for propagation transversely across the trabeculae. In the latter case, a positive attenuation slope and negative velocity dispersion are seen, in apparent violation of the Kramers–Kronig relations. The properties of the saturating fluid have major effects on acoustic properties, and temperature studies also point towards the importance of the fluid. In some clinical studies, velocity and attenuation in the heel show opposite changes, which is difficult to explain without considering the possibility of changes in the marrow as well as the bone. Shear wave propagation in fluid-filled cancellous bone has received little attention. Measurements in whale bone suggest that shear waves propagate in dry bone, but are rapidly attenuated in the presence of saturating fluid.