Osteoporosis is a skeletal disorder characterized by low bone mass, which compromises its resistance and increases the risk of fractures, and is a widespread problem worldwide. Currently, the gold standard for assessing fracture risk is the measurement of the areal bone mineral density with Dual-Energy X-ray Absorptiometry. Several ultrasound techniques have been presented as alternatives. It has been shown that the estimation of cortical thickness and porosity, obtained by Bi-Directional Axial Transmission, are associated with non-traumatic fractures in postmenopausal women. Cortical parameters were derived from the comparison between experimental and theoretical guided modes. However, this model-based inverse approach tends to fail for the patients associated with poor guided mode information. A recent study has shown the potential of an automatic classification tool, Support Vector Machine, to analyze guided wave spectrum images independently of any waveguide model. The aim of this study is to explore how the classification accuracy varies with the number of features. Optimization was done using the Particle Swarm Optimization algorithm, while adjustment was made considering age, body mass index, and cortisone intake. The results show that adjusting the data and optimizing the parameters improved classification. Moreover, the number of features was reduced from 32 to 15, with 73.5% accuracy comparable to the gold standard.

Accurate measurement of cortical bone parameters may improve fracture risk assessment and help clinicians on the best treatment strategy. Patients at risk of fracture are currently detected using the current X-Ray gold standard DXA (Dual XRay Absorptiometry). Different alternatives, such as 3D X-Rays, Magnetic Resonance Imaging or Quantitative Ultrasound (QUS) devices, have been proposed, the latter having advantages of being portable and sensitive to mechanical and geometrical properties. The objective of this cross-sectional study was to evaluate the performance of an improved ultrasonic device (Bi-Directional Axial Transmission, BDAT) with older adults with or without hip fragility fracture. The device, positioned at one-third distal radius, provides two velocities denoted VFAS and VA0 as well as two parameters are obtained from an inverse approach: Ct.Th (cortical thickness) and Ct.Po (cortical porosity), along with their ratio Ct.Po/Ct.Th. The areal bone mineral density (aBMD) was obtained using DXA at the hip. Fifty two patients (40 women, 12 men) were included in this study, in connection with local health services. Age ranged from 51 to 98 years, while body mass index (BMI) ranged from 22 to 36 kg.m-2. Failure rate was found to be 4% (2 patients) lower than in previous studies, mainly due to the guiding interface improvement and quality monitoring. Two groups were considered: 41 patients without fragility fractures (68±11 years), 11 with fragility femoral fractures (80±9 years). Odds ratio (OR) and the area under the ROC curve (AUC) were calculated using multivariate Partial Least Square discrimination analyses with Leave-One-Out Cross-Validation (PLS-LOOCV). We found the combination of significant ultrasound parameters (VA0, Ct.Th, Ct.Po/Ct.Th) to be predictive for fragility femoral fractures (AUC=0.72), above aBMD total femur values (AUC=0.63). When adjusted with age and gender, values for significant parameters were equal to AUC=0.82. These promising results have to be confirmed with a larger sample size. The fracture risk assessment using this ultrasound device in older patients suggests the benefit of the affordable and transportable device for routine use, in particular for countries where DXA is not widely available.

Ultrasonic guided waves have been used for characterizing cortical bone. However, multimode ultrasound signals bring difficulty in signal processing, making it challenging to extract the first arriving signals (FAS) and the individual mode components for cortical bone characterization. To overcome such a limitation, a feasibility study of cortical bone evaluation utilizing the Radon transform based signal extraction method is introduced in this paper. Forward and inverse Radon transform pair is applied to increase the signal-to-noise ratio of the ultrasonic signals collected from a bovine bone plate. Based on the known velocity ranges, the regions of interest in the intercept-slowness domain are extracted to obtain the FAS and the fundamental flexural guided wave (A0 mode) signals. Compared to the true geometric and elastic parameters, the proposed method enables to retrieve the cortical thickness, longitudinal and transverse velocities, which suggests a potential application of bone characterization.

Estimation of key factors of fracture risk, such as cortical bone thickness and porosity, can be obtained using high resolution-peripheral quantitative computed tomography (HR-pQCT). The feasibility of in vivo cortical thickness estimation by analyzing the waveguide response of long bones using axial transmission (AT) has been previously evidenced. The aim of this study is to compare both AT parameters, i.e., cortical thickness Ct. Th and porosity Ct. Po, with HR-pQCT measurements done at both site matched (one-third distal) and conventional (distal) locations. Twenty six patients (17 females and 9 males, 28 to 87 years) underwent measurements with both devices. AT measurements were performed using a 1-MHz prototype (Azalée, Paris, France). Singular value decomposition-based approach, combined with a 2-D transverse isotropic free plate waveguide model, was used to estimate cortical thickness and porosity. Site matched measurements of cortical thickness and volumetric bone mineral density (vBMD g.cm-3) were also obtained using HR-pQCT (Scanco, Brüttisellen, Switzerland). In addition, cortical thickness and vBMD (Dcomp), corresponding to conventional clinical index measured at the distal part of the radius, were automatically provided by the device software. Highly significant Pearson correlations were found between Ct. Th AT estimates and that the values obtained with HRpQCT at the one-third distal radius (R2 = 0.84, RMSE = 0.26 mm) and at the distal part (R2 = 0.57, RMSE = 0.43 mm). A significant correlation (p < 10-5) between Ct. Po and vBMD was observed at the one-third distal radius (R2 = 0.59, RMSE = 1.8 %), while no correlation existed with Dcomp at the distal site, partly due to thin cortical thickness (<0.5 mm) limitation. This study shows the potential of AT measurements to provide in vivo cortical thickness and porosity estimates using a portable and non-ionizing device.

Mode overlapping of ultrasonic guided waves challenges the signal interpretation in the field of non-destructive testing and waveguide characterization. Dispersive Radon transform (DRT), projects the array signals from distance-time domain to parameter-intercept domain (DRT domain), has been proposed for wave mode decomposition and material properties estimation. However, limited measurements along a given range of positions, so-called finite aperture problem, result in low resolution in DRT domain. In this study, considering the sparsity of guided signals in the parameter-intercept domain, orthogonal matching pursuit (OMP) is introduced to achieve a high-resolution DRT method. Simulated results prove the OMP optimized DRT can be beneficial for ultrasonic guided mode extraction and separation.