The ability of quantitative ultrasound (QUS) to discriminate osteoporotic fracture subjects was first demonstrated over 30 years ago [1]; yet it is still frequently referred to as ‘an interesting and promising technique’. A primary factor impeding utility in routine diagnosis and management of osteoporosis has been the inability to transpose QUS parameters into meaningful measures of bone quantity and bone quality. Whereas X-ray photons are significantly attenuated by calcium atoms, from which we may derive bone quantity estimates of areal and volumetric bone mineral density (BMD); ultrasound wave propagation is dependent upon relatively macroscopic properties of both bone quantity and bone quality, particularly structure. 

A new concept to describe ultrasound propagation through complex composites such as cancellous bone has recently been described [2]. It considers an array of parallel sonic rays, whose individual transit time is dependent upon the proportion of bone tissue and marrow encountered. Variation in transit time results in phase-interference induced ultrasound attenuation. A transit time spectrum (TTS) may be derived, from which bone volume fraction may be estimated [3].  Recent in-vitro work has demonstrated that the TTS may also provide a measure of cancellous bone structure; by combining TTS parameters related to both bone quantity and quality, a high correlation with mechanical integrity was observed.

It is anticipated that the transit time spectral concept will provide an accurate estimate of BMD, and hence facilitate implementation of WHO osteoporosis/osteopenia criteria, as well as predicting osteoporotic fracture risk.