Although Quantitative Ultrasound (QUS) has been shown to reliably predict osteoporotic fracture risk, to date it has not provided an accurate estimate of bone mineral density; hence, the associated WHO criteria for osteopenia and osteoporosis cannot be applied. Langton has proposed that ultrasound propagation through cancellous bone may be considered as an array of parallel sonic-rays, the transit-time of each determined by the corresponding proportion of bone and marrow propagated. This concept has led to the development of ultrasound transit time spectroscopy (UTTS) to estimate solid (bone) volume fraction (SVF).

QUS assessment of the calcaneus within the heel incorporates overlying soft-tissues that will significantly shift the transit time spectrum and correspondingly underestimate SVF. We investigated the potential to compensate for overlying soft-tissues and thereby provide a more accurate estimation of SVF.

Four cylindrical replica cancellous bone samples, with flat-parallel cortex discs on opposite faces, were studied; with varying thicknesses of overlying soft-tissues replicated by water. Apparent transit time spectra were derived via signal deconvolution of through-transmission ultrasound signals. Additional pulse-echo signals were utilised for both ultrasound transducers to measure the thickness of water overlying the replica cortices. Solid volume fraction was calculated from the transit time spectra (UTTS-SVF), with and without overlying water compensation, and compared with μCT measured SVF (μCT-SVF). Although variable water-thickness provided very similar transit time spectral formats, they exhibited correspondingly extended time-shifts. The overall agreement between UTTS-SVF with μCT-SVF increased from 8.8% to 92.7% when overlying water thickness compensation was implemented. It is therefore suggested that ultrasound transit time spectroscopy has the potential to provide a reliable in-vivo estimate of BMD and facilitation of WHO T-scores for routine clinical assessment of osteoporosis.