IEEE Trans Ultrason Ferroelectr Freq Control. 2019 Oct 24. doi: 10.1109/TUFFC.2019.2948896. [Epub ahead of print]
Most bone loss during the development of osteoporosis occurs in cortical bone at the peripheral skeleton. Decreased cortical thickness (Ct.Th) and the prevalence of large pores at the tibia are associated with reduced bone strength at the hip. Ct.Th and cortical sound velocity, i.e., a surrogate marker for changes of cortical porosity (Ct.Po), are key biomarkers for the identification of patients at high fracture risk. In this study, we have developed a method using a conventional ultrasound array transducer to determine thickness (Ct.Th) and the compressional sound velocity propagating in the radial bone direction (Ct.v11) using a refraction-corrected multi-focus imaging approach. The method was validated in-silico on porous bone plate models using a two-dimensional finite-difference time domain method and ex vivo on plate-shaped plastic reference materials and on plate-shaped cortical bovine tibia samples. Plane wave pulse-echo measurements provided reference values to assess precision and accuracy of our method. In-silico results revealed the necessity to account for inclination-dependent transmission losses at the bone surface. Moreover, a dependency of Ct.v11 on both porosity and pore density was observed. Ct.Th and Ct.v11 obtained ex vivo showed a high correlation (R2>0.99) with reference values. The ex-vivo accuracy and precision for Ct.v11 were 29.9 m/s and 0.94%, respectively, and those for Ct.Th were 0.04 mm and 1.09%, respectively. In conclusion, this numerical and experimental study demonstrates an accurate and precise estimation of Ct.Th and Ct.v11. The developed multi-focus technique may have high clinical potential to improve fracture risk prediction by using non-invasive, non-ionizing conventional ultrasound technology with image guidance.