Estimation of Thickness and Speed of Sound in Cortical Bone Using Multifocus Pulse-Echo Ultrasound

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.
$\nu _{11}$
) using a refraction-corrected multifocus imaging approach. The method was validated
in-silico
on porous bone plate models using a 2-D 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, the dependence of
Ct.
$\nu _{11}$
on both porosity and pore density was observed.
Ct.Th
and
Ct.
$\nu _{11}$
obtained
ex vivo
showed a high correlation
$(R 2>0.99$
) with reference values. The
ex-vivo
accuracy and precision for
Ct.
$\nu _{11}$
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.
$\nu _{11}$
. The developed multifocus technique may have high clinical potential to improve fracture risk prediction using noninvasive and nonionizing conventional ultrasound technology with image guidance.