Computer and experimental simulation of a cortical end-plate phase cancellation artefact in the measurement of BUA at the calcaneus

It has been experimentally demonstrated that for the measurement of broadba nd ultrasound attenuation (BUA) at the human calcaneus the cortical end-pla te creates a measurement artefact of the order of 7 dB MHz(-1). It has been suggested that the origin of this artefact may be a phase cancellation of the ultrasound pulse resulting from inconsistencies in propagation time acr oss the ultrasound beam. Experimental and computer simulations were performed on samples of varying degrees of curvature and hence varying propagation times across the ultraso und beam. The experimental simulation incorporated Perspex samples of 35, 5 0 and 75 mm radius. The computer simulation was implemented using Matlab an d Simulink, with the propagation time represented by a transport delay. The wavelet-based simulation incorporated a digitized transmitted ultrasound p ulse derived from the experimental simulation. The experimental and computer-derived frequency spectra for the varying rad ii samples were comparable, demonstrating, firstly, that there is a signifi cant dependence of measured BUA upon radius of curvature and, secondly, tha t the response in measured BUA to radius of curvature is similar in magnitu de and trend for both experimental and computer simulations. The current st udy suggests that the BUA artefact observed in vitro corresponds to a radiu s of approximately 58 mm. Although the radius of curvature was not recorded in the original in vitro study, this value appears to be reasonable. This study indicates that the assumptions within the computer simulation we re manifested within the experimental validation, and, hence, the observed BUA measurement artefact is related to the presence of the calcaneal cortic al end-plate and is due to phase cancellation of the propagating ultrasound pulse.