AN INVESTIGATION OF PULSE-TIMING TECHNIQUES FOR BROAD-BAND ULTRASONICVELOCITY DETERMINATION IN CANCELLOUS BONE - A SIMULATION STUDY

Berlage wavelets are used to simulate ultrasonic pulses in an unbounde d, homogeneous, isotropic and absorptive medium. Intrinsic absorption of the medium is properly described by Kolsky's attenuation, which con siders velocity dispersion to meet the causality condition. Several cu rrent time-domain velocity measurement techniques have been investigat ed using numerically simulated pulses for three normalized BUA values: 20, 40 and 60 dB MHz(-1) cm(-1) which mimic experimentally determined values for cancellous bone. The velocities, calculated using first mo tion transit times, are used as references supported by the Fermat pri nciple of least time. The simulated results for fixed sample thickness indicate that pulse-broadening increases with the transit time of the reference point and the intrinsic absorption of the medium. Compariso n shows that the first zero-crossing method yields 3-6% errors in velo city results, better than the cross-correlation method. However, the z ero-crossing method gives inconsistent velocity measurement for a medi um of 40 dB MHz(-1) cm(-1) absorption and three different thicknesses: 0.2, 0.4 and 0.6 cm. A novel technique for velocity measurement is pr esented using the peak of the envelope of a signal as a reference poin t to measure transit time difference. The envelope of a signal represe nts the instantaneous amplitude of the associated analytic signal. The velocities derived using this method differ from the true velocities by only 1.2-2.4%, more accurate than those obtained by the first zero- crossing method. The envelope peak has the additional merits of easy d etection and robustness. Most importantly, the envelope technique may be used to yield accurate velocity measurement in cases where an accur ate determination of the first motion transit time is sometimes prohib ited due to the presence of noise.