Experimental and computer simulation validation of ultrasound phase interference created by lateral inhomogeneity of transit time in replica bone: marrow composite models.

The measurement of broadband ultrasound attenuation in cancellous bone for the assessment of osteoporosis follows a parabolic-type dependence with bone volume fraction, having minima values corresponding to both entire bone and entire marrow. Langton has recently proposed that the primary attenuation mechanism is phase interference due to variations in propagation transit time through the test sample as detected over the phase-sensitive surface of the receive ultrasound transducer. This fundamentally simple concept assumes that the propagation may be considered as an array of parallel 'sonic rays'. The transit time of each ray is defined by the proportion of bone and marrow propagated, being a minimum (t(min)) solely through bone and a maximum (t(max)) solely through marrow, from which a transit time spectrum, may be defined describing the proportion of sonic rays having a particular transit time. The aim of this study was to test the hypothesis that there is a dependence of phase interference upon the lateral inhomogeneity of transit time by comparing experimental measurements and computer simulation predictions of ultrasound propagation through a range of relatively simplistic solid:liquid models. From qualitative and quantitative comparison of the experimental and computer simulation results, there is an extremely high degree of agreement of 94.2%-99.0% between the two approaches. This combined experimental and computer simulation study has successfully demonstrated that lateral inhomogeneity of transit time has significant potential for phase interference to occur if a phase-sensitive receive ultrasound transducer is implemented as in most commercial ultrasound bone analysis devices.