Estimation of Cortical Bone Microstructure From Ultrasound Backscatter

Abstract

Multi-channel pulse-echo ultrasound using linear arrays and single-channel data acquisition systems opens new perspectives for the evaluation of cortical bone. In combination with spectral backscatter analysis, it can provide quantitative information about cortical microstructural properties. We present a numerical study, based on the finite-difference time-domain (FDTD) method, to estimate the backscatter cross-section of randomly distributed circular pores in a bone matrix. A model that predicts the backscatter coefficient using arbitrary pore diameter distributions was derived. In an ex-vivo study on 19 human tibia bones (6 males, 13 females, 83.7 ± 8.4 years), multidirectional ultrasound backscatter measurements were performed using an ultrasound scanner equipped with a 6-MHz 128-element linear array with sweep motor control. A normalized depth-dependent spectral analysis was performed to derive backscatter and attenuation coefficients. Site-matched reference values of tissue acoustic impedance Z, cortical thickness Ct.Th, pore density Ct.Po.Dn, porosity Ct.Po and characteristic parameters of the pore diameter (Ct.Po.Dm) distribution were obtained from 100-MHz scanning-acoustic microscopy images. Proximal femur areal bone mineral density (aBMD), stiffness S and ultimate force Fu from the same donors were available from a previous study. All pore structure and material properties could be predicted using linear combinations of backscatter parameters with median to high accuracy (0.28  adjusted R²  0.59). The combination of cortical thickness and backscatter parameter provided similar or better prediction accuracies than aBMD. For the first time, a method for the non-invasive assessment of the pore diameter distribution in cortical bone by ultrasound is proposed. The combined assessment of cortical thickness, sound velocity, and pore size distribution in a mobile, non-ionizing measurement system could have a major impact to prevent osteoporotic fractures.

Cortical pore size distribution and viscoelastic tibia properties discriminate fragility fractures independent of bone mineral density

“…In line with the well-known microstructural deteriorations induced by OP, both HR-pQCT and CortBS revealed predominantly features describing the asymmetry of the cortical pore size distribution rather parameters describing the mean pore-tissue volume fraction as factors associated with fragility fractures. Moreover, frequency-dependent ultrasound attenuation, which is determined by both structural and viscoelastic tissue properties 24 was found to be significantly altered in subjects with fragility fractures.…”

“…Thereby, the tibia midshaft provides a much larger and anatomically more invariant tissue volume for cortical bone microstructural characterization than the distal shaft. Recent ex-vivo studies provided evidence that structural deterioration at this measurement site is associated with reduced proximal femur strength 32 and that the biomarkers assessed by CortBS combined with Ct.Th provide superior predictions of proximal femur stiffness and strength compared to aBMD 24 . Ultrasound can be transmitted most easily to and along bone at the facies medialis of the tibia midshaft, where the periosteum is covered by a thin layer of soft tissue only.…”

“…The measurement principle has been described in detail previously 24 . Briefly, a medical ultrasound scanner SonixTOUCH equipped with a SonixDAQ single-channel data acquisition system and a 4DL14-5/38 3-D linear array transducer (Ultrasonix, Richmond, Canada) was used.…”

“…We have recently developed theoretical cortical bone backscatter model (CortBS) and an ultrasonic multi-angle 3-D acquisition and data processing scheme to assess viscoelastic and microstructural properties in cortical bone 24 . The method measures the frequency-dependent attenuation and backscatter coefficients (f) and BSC(f) at the tibia and retrieves the cortical pore size distribution Ct.Po.Dm.D by fitting a theoretical backscatter coefficient to the measured BSC(f).…”

“…From the normalized depth-dependent backscatter spectrum (NDS) (c), the depth and frequency ranges of 1-3 mm and 4-9 MHz, respectively, are used to derive the attenuation and backscatter coefficients (f) (d) and BSC(f) (e). By fitting model-based backscatter coefficients24 to the measured BSC(f), the pore diameter distribution is estimated (f). Panels(e-f) show representative (f), and BSC(f), and Ct.Po.Dm.D data for one subject with (ID43; T-scoreTotal = -1.7) and one without fragility fractures (ID10; T-scoreTotal = -3.5).…”