Ex vivo cortical porosity and thickness predictions at the tibia using full-spectrum ultrasonic guided-wave analysis

Abstract

Summary The estimation of cortical thickness (Ct.Th) and porosity (Ct.Po) at the tibia using axial transmission ultrasound was successfully validated ex vivo against site-matched micro-computed tomography. The assessment of cortical parameters based on full-spectrum guided-wave analysis might improve the prediction of bone fractures in a cost-effective and radiation-free manner. Purpose Cortical thickness (Ct.Th) and porosity (Ct.Po) are key parameters for the identification of patients with fragile bones. The main objective of this ex vivo study was to validate the measurement of Ct.Po and Ct.Th at the tibia using a non-ionizing, low-cost, and portable 500-kHz ultrasound axial transmission system. Additional ultrasonic velocities and site-matched reference parameters were included in the study to broaden the analysis. Methods Guided waves were successfully measured ex vivo in 17 human tibiae using a novel 500-kHz bi-directional axial transmission probe. Theoretical dispersion curves of a transverse isotropic free plate model with invariant matrix stiffness were fitted to the experimental dispersion curves in order to estimate Ct.Th and Ct.Po. In addition, the velocities of the first arriving signal ( υ FAS ) and A 0 mode ( υ A0 ) were measured. Reference Ct.Po, Ct.Th, and vBMD were obtained from site-matched micro-computed tomography. Scanning acoustic microscopy (SAM) provided the acoustic impedance of the axial cortical bone matrix. Results The best predictions of Ct.Po (R 2 = 0.83, RMSE = 2.2%) and Ct.Th (R 2 = 0.92, RMSE = 0.2 mm, one outlier excluded) were obtained from the plate model. The second best predictors of Ct.Po and Ct.Th were vBMD (R 2 = 0.77, RMSE = 2.6%) and υ A0 (R 2 = 0.28, RMSE = 0.67 mm), respectively. Conclusions Ct.Th and Ct.Po were accurately predicted at the human tibia ex vivo using a transverse isotropic free plate model with invariant matrix stiffness. The model-based predictions were not further enhanced when we accounted for variations in axial tissue stiffness as reflected by the acoustic impedance from SAM. Electronic supplementary material The online version of this article (10.1007/s11657-019-0578-1) contains supplementary material, which is available to authorized users.

In Vivo Measurements of Cortical Thickness and Porosity at the Proximal Third of the Tibia Using Guided Waves: Comparison with Site-Matched Peripheral Quantitative Computed Tomography and Distal High-Resolution Peripheral Quantitative Computed Tomography

“…Thus, to improve robustness, we chose to estimate y A0 in a lower and narrower frequency range (0.4À0.5 MHz, Fig. 2b) compared with what we used ex vivo (0.5À0.8 MHz) (Schneider et al 2019). Consequently, we did not observe any y A0 failure case and our in vivo reproducibility (1.1%) was superior to that of Moilanen et al (2008), 2.3%.…”

“…To optimize the excitation of guided waves, we decreased the central frequency to 500 kHz and slightly increased the probe dimensions. The novel probe was used previously ex vivo on 17 tibiae and could successfully predict Ct.Po (R 2 = 0.83, root mean square error [RMSE] = 2.2%) and Ct.Th (R 2 = 0.57, RMSE 0.37 mm) measured by site-matched micro-CT (Schneider et al, 2019). The aim of this study was to test the 500-kHz probe for the first time in vivo at the tibia in a small group of patients.…”

Estimation of Cortical Bone Microstructure From Ultrasound Backscatter

“…Nineteen left tibia bones from human cadavers (6 male, 13 female, age: 83.7 ± 8.4 years, range: 69 -94 years) described in previous studies [1,15,35] were used for the ex-vivo measurements. Specimens were received without soft tissue and distal ends (cut off at approximately 50 %) and stored at −20 °C.…”

“…Citation information: DOI 10.1109/TUFFC.2020.3033050, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control TUFFC-10513-2020 2 reference values of porosity (R 2 = 0.79) and pore size (R 2 = 0.81) obtained from microcomputed tomography. In an ex-vivo BDAT-US study [15], we could predict Ct.Po from dispersive guided waves with high accuracy and precision (R² = 0.83, RMSE = 2.2 %). It is known that biomechanical parameters, such as yield stress and elastic modulus are inversely related with cortical pore diameter, but a positive relationship with pore density has been shown [16].…”

“…At the lower end, the acoustic pulse length must be short enough to allow a temporal separation of backscattered signals from specular reflections occurring at periosteal and endosteal bone interfaces. Thickness values at typical measurement sites in radius and tibia bones are in the range between 1-6 mm [3,14,15,31]. The minimum pulse duration is limited by the acoustic wavelength.…”