A COMPARISON OF TIME-DOMAIN AND FREQUENCY-DOMAIN APPROACHES TO ULTRASONIC VELOCITY-MEASUREMENT IN TRABECULAR BONE

Different methods for ultrasonic velocity determination using broad-ba nd pulse transmission have been investigated in 70 human calcanae in v itro. The work took place within the context of the EC BIOMED1 concert ed action Assessment of Quality of Bone in Osteoporosis. Ultrasonic ve locities were determined using three different transit time definition s: first arrival (TTV1), thresholding (TTV2), and first zero crossing (TTV3). Phase velocity (PV) was determined over a range of frequencies from 200 to 800 kHz using a new phase spectral analysis technique. Th e different velocity measurements were compared in terms of their magn itudes and their inter-correlations. There were significant difference s of up to 260 m s(-1) between different transit time velocities (p < 0.0001), indicating the sensitivity of the measurement to the arrival criteria used. Phase velocities were lower than all of the transit tim e velocities (p < 0.0001) and decreased with increasing frequency (p < 0.005). A strong correlation (r(2) = 0.968) was observed between PV a t 400 kHz (PV400) and TTV3, with much weaker correlations between PV a nd the other transit time velocities. Reproducibility for transit time velocity measurement was optimal for TTV3 (coefficient of variation, cv = 0.41%), and for PV it was optimal at 600 kHz (cv = 0.34%). These data indicate that transit time measurements may be subject to errors due to the modification of the pulse shape during propagation through bone by attenuation and dispersion. Velocity measurement by phase spec tral analysis appears to offer advantages over the transit time approa ch, and should be the method of choice for velocity measurement in tra becular bone. Where transit time velocity measurements are made, the f irst-zero-crossing criterion appears to be have some advantages over o ther arrival criteria. We also note that PV measurements provide new i nformation on dispersion which could prove to be relevant to the struc tural and mechanical characterization of trabecular bone.