THE DEPENDENCE OF ULTRASONIC PROPERTIES ON ORIENTATION IN HUMAN VERTEBRAL BONE

Speed of sound (SOS) and broad-band ultrasound attenuation (BUA) were measured in cubes of human trabecular bone from lumbar vertebrae, in t he three major anatomical axes. There were significant differences in SOS and in BUA when measured in the different axes, indicating a struc tural component to the ultrasonic measurement. Qualitatively different behaviour was observed in the cranio-caudal (CC) axis compared to the transverse directions: SOS was approximately 500 m s-1 greater than i n either the lateral (LT) or antero-posterior (AP) axes, and BUA was a pproximately 23 dB MHz-1 cm-1 greater. Small, but significant, differe nces existed between the AP and LT axes for both SOS and BUA. In the A P and LT directions, strong linear correlations existed between SOS an d apparent density (r = 0.90), and between BUA and apparent density (r = 0.96). In the CC axis, correlations with density were poorer. The a nomalous behaviour in the CC axis was due to a transient travelling ah ead of the main wavefront, and it is suggested that this represents pr opagation of ultrasound directly through the trabecular framework as a bar wave. This can only occur in the CC axis where the majority of tr abeculae are orientated parallel to the direction of propagation. Meas urements on cubes in air, as opposed to water, supported this hypothes is. Modifications to the experimental technique necessary to consisten tly detect this phenomenon are described.