Jean-Gabriel Minonzio

Combined estimation of thickness and velocities using ultrasound guided waves: a pioneering study on in vitro cortical bone samples

This paper reports for the first time on inverse estimation of several bone properties from guided-wave measurements in human bone samples. Previously, related approaches have focused on ultrasonic estimation of a single bone property at a time. The method is based on two steps: the multi-Lamb mode response is analyzed using the singular value decomposition signal processing method recently introduced in the field, then an identification procedure is run to find thickness and anisotropic elastic properties of the considered specimen. Prior to the measurements on bone, the method is validated on cortical bone-mimicking phantoms. The repeatability and the trueness of the estimated parameters on bone-mimicking phantoms were found around a few percent. Estimation of cortical thickness on bone samples was in good agreement with cortical thickness derived from high-resolution peripheral quantitative computed tomography data analysis of the samples.

In vivo measurements of guided waves at the forearm

Because of the recognized importance of cortical bone in osteoporotic fracture risk, efforts have been made to improve its measurement. A step forward towards the ultrasonic characterization of cortical bone has been made recently with reports showing that cortical bone behaves as a waveguide (WG) for ultrasound. Measurements of the guided modes dispersion relationships, together with appropriate waveguide modeling have the potential for providing estimates of strength-related factors such as stiffness (which is largely determined by cortical porosity) and cortical thickness (Ct.Th). Cortical bone is readily accessible for measurements at the radius using the axial transmission (AT). The guided waves spectrum is used to estimate Ct.Th, based on a comparison of measured and model-predicted dispersion curves. The post-processing relies on finding the optimal pairing of measured and predicted dispersion curves from a free transverse isotropic plate model, assuming a parametrization of the elastic tensor. The present work presents the first in vivo ultrasound-based estimate of Ct.Th US in healthy subjects.

Genetic algorithms-based inversion of multimode guided waves for cortical bone characterization

Recent progress in quantitative ultrasound has exploited the multimode waveguide response of long bones. Measurements of the guided modes, along with suitable waveguide modeling, have the potential to infer strength-related factors such as stiffness (mainly determined by cortical porosity) and cortical thickness. However, the development of such model-based approaches is challenging, in particular because of the multiparametric nature of the inverse problem. Current estimation methods in the bone field rely on a number of assumptions for pairing the incomplete experimental data with the theoretical guided modes (e.g. semi-automatic selection and classification of the data). The availability of an alternative inversion scheme that is user-independent is highly desirable. Thus, this paper introduces an efficient inversion method based on genetic algorithms using multimode guided waves, in which the mode-order is kept blind. Prior to its evaluation on bone, our proposal is validated using laboratory-controlled measurements on isotropic plates and bone-mimicking phantoms. The results show that the model parameters (i.e. cortical thickness and porosity) estimated from measurements on a few ex vivo human radii are in good agreement with the reference values derived from x-ray micro-computed tomography. Further, the cortical thickness estimated from in vivo measurements at the third from the distal end of the radius is in good agreement with the values delivered by site-matched high-resolution x-ray peripheral computed tomography.

A method for the measurement of dispersion curves of circumferential guided waves radiating from curved shells: experimental validation and application to a femoral neck mimicking phantom.

Our long-term goal is to develop an ultrasonic method to characterize the thickness, stiffness and porosity of the cortical shell of the femoral neck, which could enhance hip fracture risk prediction. To this purpose, we proposed to adapt a technique based on the measurement of guided waves. We previously evidenced the feasibility of measuring circumferential guided waves in a bone-mimicking phantom of a circular cross-section of even thickness. The goal of this study is to investigate the impact of the complex geometry of the femoral neck on the measurement of guided waves. Two phantoms of an elliptical cross-section and one phantom of a realistic cross-section were investigated. A 128-element array was used to record the inter-element response matrix of these waveguides. This experiment was simulated using a custom-made hybrid code. The response matrices were analyzed using a technique based on the physics of wave propagation. This method yields portions of dispersion curves of the waveguides which were compared to reference dispersion curves. For the elliptical phantoms, three portions of dispersion curves were determined with a good agreement between experiment, simulation and theory. The method was thus validated. The characteristic dimensions of the shell were found to influence the identification of the circumferential wave signals. The method was then applied to the signals backscattered by the superior half of constant thickness of the realistic phantom. A cut-off frequency and some portions of modes were measured, with a good agreement with the theoretical curves of a plate waveguide. We also observed that the method cannot be applied directly to the signals backscattered by the lower half of varying thicknesses of the phantom. The proposed approach could then be considered to evaluate the properties of the superior part of the femoral neck, which is known to be a clinically relevant site.

Dispersive Radon transform

Dispersion results in the spreading and overlapping of the wave-packets, which often limits the capability of signal interpretation; on the other hand, such a phenomenon can also be used for structure or media evaluation. In this study, the authors propose an original dispersive Radon transform (DRT), which is formulated as integration transform along a set of dispersion curves. Multichannel dispersive signals of each individual mode can be concentrated to a well localized region in the DRT domain. The proposed DRT establishes the sparse projection of the dispersive components and provides an efficient solution for mode separation, noise filtering, and missing data reconstruction. Particularly the DRT method allows projecting the temporal signals of dispersive waves on the space of parameters of interest, which can be used to solve the inverse problem for waveguide or media property estimation. The least-square procedure and sparse scheme of the DRT are introduced. A high-resolution DRT is designed based on an iterative reweighting inversion scheme, which resembles the infinite-aperture velocity gather. The proposed method is applied by analyzing ultrasonic guided waves in plate-like structures and in a human radius specimen. The results suggest that the DRT method can significantly enhance the interpretation of dispersive signals.

Multisite ultrasound axial transmission study in postmenopausal women using optimized first arriving signal velocity measurements

The objective of this study is to assess the clinical performance of a novel ultrasound bidirectional axial transmission (BDAT) prototype. The speed of sound of the first arriving signal (VFAS) was measured in the radius and tibia of 58 postmenopausal females. An optimized VFAS measurement protocol was introduced in order to reduce the clinical measurement time and increase the robustness in particular for patients with pronounced body mass index (BMI). The optimized protocol was efficient and showed excellent discrimination performance: the highest area under the ROC curve (AUC) was found for VFAS radius (OR=2.60; AUC=0.89). In comparison, dual energy x-ray absorptiometry (DXA) values of the lumbar spine (OR=1.89; AUC=0.87) exhibited comparable fracture discrimination performance compared to the ultrasonic method.

An anisotropic bilayer model to gain insight into in-vivo guided wave measurements

The influence of soft-tissue on guided wave measurements for the assessment of cortical bone properties is not fully understood yet. To improve our understanding of the soft-tissue influence on the guided modes propagation characteristics, a series of assemblies of soft tissue- and bone-mimicking phantoms were ultrasonically investigated using the so-called axial transmission technique. For this purpose, an anisotropic bilayer model was developed and inserted in a model-based inverse problem procedure to infer the properties of both the solid and fluid layers. Finally, the potential of this modeling is evaluated on a few in-vivo measurements at the forearm.

The elastic properties of human cortical bone measured by resonant ultrasound spectroscopy at multiple skeletal sites

Human cortical bone is an anisotropic material, although isotropic stiffness is generally assumed in most finite element analysis. Detailed information about the anisotropic stiffness at a mesoscopic (mm) scale would improve our understanding of bones macroscopic mechanical properties. In this work, we report on the anisotropic stiffness of human cortical bone from different sites, and the variation in anisotropy seen.

Dispersion characteristics of the flexural wave assessed using low frequency (50–150 kHz) point-contact transducers: A feasibility study on bone-mimicking phantoms

&NA; Guided waves‐based techniques are currently under development for quantitative cortical bone assessment. However, the signal interpretation is challenging due to multiple mode overlapping. To overcome this limitation, dry point‐contact transducers have been used at low frequencies for a selective excitation of the zeroth order anti‐symmetric Lamb A0 mode, a mode whose dispersion characteristics can be used to infer the thickness of the waveguide. In this paper, our purpose was to extend the technique by combining a dry point‐contact transducers approach to the SVD‐enhanced 2‐D Fourier transform in order to measure the dispersion characteristics of the flexural mode. The robustness of our approach is assessed on bone‐mimicking phantoms covered or not with soft tissue‐mimicking layer. Experiments were also performed on a bovine bone. Dispersion characteristics of measured modes were extracted using a SVD‐based signal processing technique. The thickness was obtained by fitting a free plate model to experimental data. The results show that, in all studied cases, the estimated thickness values are in good agreement with the actual thickness values. From the results, we speculate that in vivo cortical thickness assessment by measuring the flexural wave using point‐contact transducers is feasible. However, this assumption has to be confirmed by further in vivo studies. HighlightsDry point‐contact transducers generate selectively the zero order anti‐symmetric Lamb A0 mode.Combining dry point‐contact transducer approach to the SVD‐enhanced 2‐D Fourier transform.Correct thickness estimation despite the presence of a soft tissue‐mimicking layer on top of the plates.

Multichannel wideband mode-selective excitation of ultrasonic guided waves in long cortical bone

Ultrasonic guided waves have been proposed as a promising means to evaluate the long cortical bone. However, the complete extraction of all dispersion curves is still challenging. Aiming to enhance the signal-to-noise-ratio (SNR) and improving the ambiguity of mode identification, we propose a multichannel wideband mode selective excitation method. The method was tested on an aluminum plate and a bone-mimicking plate. The results illustrated that the proposed method allows the selective excitation of the desired modes and enhances the mode amplitudes, which could be helpful to the multimodal guided waves based assessment of long cortical bone.