Françoise Peyrin

Observation of microstructure and damage in materials by phase sensitive radiography and tomography

The novel possibilities of phase feature detection in radiography at a third generation synchrotron radiation source are used to image, both in projection and in computed tomography, a cracked silicon single crystal and metal matrix composites strained in tension. Through an instrumentally very simple technique, based on Fresnel diffraction, phase jumps related to the interface between the matrix and the reinforcing phases of the composites are detected even when these phases show very similar x-ray attenuation. Strain-induced cracks with openings below the micrometer range are also visible through the phase modulation they introduce, illustrating the potential of the technique for assessing damage in materials with improved resolution and sensitivity.

A synchrotron radiation microtomography system for the analysis of trabecular bone samples.

X-ray computed microtomography is particularly well suited for studying trabecular bone architecture, which requires three-dimensional (3-D) images with high spatial resolution. For this purpose, we describe a three-dimensional computed microtomography (microCT) system using synchrotron radiation, developed at ESRF. Since synchrotron radiation provides a monochromatic and high photon flux x-ray beam, it allows high resolution and a high signal-to-noise ratio imaging. The principle of the system is based on truly three-dimensional parallel tomographic acquisition. It uses a two-dimensional (2-D) CCD-based detector to record 2-D radiographs of the transmitted beam through the sample under different angles of view. The 3-D tomographic reconstruction, performed by an exact 3-D filtered backprojection algorithm, yields 3-D images with cubic voxels. The spatial resolution of the detector was experimentally measured. For the application to bone investigation, the voxel size was set to 6.65 microm, and the experimental spatial resolution was found to be 11 microm. The reconstructed linear attenuation coefficient was calibrated from hydroxyapatite phantoms. Image processing tools are being developed to extract structural parameters quantifying trabecular bone architecture from the 3-D microCT images. First results on human trabecular bone samples are presented.

Quantification of the degree of mineralization of bone in three dimensions using synchrotron radiation microtomography.

The availability of three-dimensional measuring techniques coupled to specific image processing methods opens new opportunities for the analysis of bone structure. In particular, synchrotron radiation microtomography may provide three-dimensional images with spatial resolution as high as one micrometer. Moreover, the use of a monoenergetic synchrotron beam, which avoids beam-hardening effects, allows quantitative measurements of the degree of mineralization in bone samples. Indeed, the reconstructed gray levels of tomographic images correspond directly to a map of the linear attenuation coefficient within the sample. Since the absorption depends on the amount of mineral content, we proposed a calibration method to evaluate the three-dimensional distribution of the degree of mineralization within the sample. First a theoretical linear relationship modeling the linear attenuation coefficient as a function of the hydroxyapatite concentrations was derived. Then, an experimental validation on phantoms confirmed both the accuracy of the image processing tools and the experimental setup used. Finally, the analysis of the degree of mineralization in four iliac crest bone biopsy samples was reported. Our method was compared to the reference microradiography technique, currently used for this quantification in two dimensions. The concentration values of the degree of mineralization were found with both techniques in the range 0.5-1.6 g of mineral per cubic centimeter of bone, both in cortical and in trabecular region. The mean difference between the two techniques was around 4.7%, and was slightly higher in trabecular region than in cortical bone.

Synchrotron radiation microtomography allows the analysis of three-dimensional microarchitecture and degree of mineralization of human iliac crest biopsy specimens: effects of etidronate treatment.

Quantitative microcomputed tomography using synchrotron radiation (SR μCT) was used to assess the effects of a sequential etidronate therapy on both three‐dimensional (3D) microarchitecture and degree of mineralization of bone (DMB) in postmenopausal osteoporosis. Thirty‐two iliac crest biopsy specimens were taken from 14 patients with osteoporosis (aged 64 ± 1.8 years) before (baseline) and after 1 year of etidronate treatment, and after 2 years of treatment for four of the patients. The samples were imaged at high spatial resolution (voxel size = 10 μm) using the microtomography system developed at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. Three‐dimensional microarchitecture parameters were calculated and compared with those obtained from conventional histomorphometry. In addition, the DMB was evaluated also in 3D. No significant statistical changes regarding bone mass and structural parameters were observed in histomorphometry or 3D analyses. The distribution of the DMB in cortical and trabecular bone showed a trend to a shift toward highest mineralization values after 1 year of etidronate treatment (3.88% and 1.24% in cortical and trabecular bone, respectively). This trend was more evident after 2 years. The study also showed that SR μCT is an accurate technique and the only one for quantifying both the mineralization and the microarchitecture of bone samples at the same time in 3D.

Ultrasonic characterization of human cancellous bone using transmission and backscatter measurements: relationships to density and microstructure.

The present study was designed to evaluate the relationships between ultrasonic backscatter, density, and microarchitecture of cancellous bone. The slopes of the frequency-dependent attenuation coefficient (nBUA), ultrasound bone velocity (UBV), the frequency-averaged backscatter coefficient (BUB) were measured in 25 cylindrical cancellous bone cores. Bone mineral density (BMD) was determined using X-ray quantitative computed tomography. Microarchitecture was investigated with synchrotron radiation microtomography with an isotropic spatial resolution of 10 microm. Several microstructural parameters reflecting morphology, connectivity, and anisotropy of the specimens were derived from the reconstructed three-dimensional (3D) microarchitecture. The association of the ultrasonic variables with density and microarchitecture was assessed using simple and multivariate linear regression techniques. For all ultrasonic variables, a strong association was found with density (r = 0.84-0.90). We also found that, with the exception of connectivity, all microstructural parameters correlated significantly with density, with r values of 0.54-0.92. For most microstructural parameters there was a highly significant correlation with ultrasonic parameters (r = 0.33-0.91). However, the additional variance explained by microstructural parameters compared with the variance explained by BMD alone was small (Delta r(2) = 6% at best). In particular, no significant independent association was found between microstructure and backscatter coefficient (a microstructure-related ultrasonic parameter) after adjustment for density. The source for the unaccounted variance of quantitative ultrasound (QUS) parameters remains unknown.

Cortical Bone in the human femoral neck: Three-dimensional appearance and porosity using synchrotron radiation

A high‐resolution CT system using synchrotron radiation allowed visualization of the 3D cortical bone microarchitecture and measurement of intracortical porosity of femoral neck cortical bone specimens from 19 female cadavers imaged at 10.13‐μm resolution. 3D reconstruction of specimens showed osteonal system arrangement. Mean porosity was 15.88%. This technique will provide insights into the mechanisms involved in osteoporotic hip fractures.

Quantitative comparison of direct phase retrieval algorithms in in-line phase tomography.

A well-known problem in x-ray microcomputed tomography is low sensitivity. Phase contrast imaging offers an increase of sensitivity of up to a factor of 10(3) in the hard x-ray region, which makes it possible to image soft tissue and small density variations. If a sufficiently coherent x-ray beam, such as that obtained from a third generation synchrotron, is used, phase contrast can be obtained by simply moving the detector downstream of the imaged object. This setup is known as in-line or propagation based phase contrast imaging. A quantitative relationship exists between the phase shift induced by the object and the recorded intensity and inversion of this relationship is called phase retrieval. Since the phase shift is proportional to projections through the three-dimensional refractive index distribution in the object, once the phase is retrieved, the refractive index can be reconstructed by using the phase as input to a tomographic reconstruction algorithm. A comparison between four phase retrieval algorithms is presented. The algorithms are based on the transport of intensity equation (TIE), transport of intensity equation for weak absorption, the contrast transfer function (CTF), and a mixed approach between the CTF and TIE, respectively. The compared methods all rely on linearization of the relationship between phase shift and recorded intensity to yield fast phase retrieval algorithms. The phase retrieval algorithms are compared using both simulated and experimental data, acquired at the European Synchrotron Radiation Facility third generation synchrotron light source. The algorithms are evaluated in terms of two different reconstruction error metrics. While being slightly less computationally effective, the mixed approach shows the best performance in terms of the chosen criteria.

Three-dimensional simulation of ultrasound propagation through trabecular bone structures measured by synchrotron microtomography

Three-dimensional numerical simulations of ultrasound transmission were performed through 31 trabecular bone samples measured by synchrotron microtomography. The synchrotron microtomography provided high resolution 3D mappings of bone structures, which were used as the input geometry in the simulation software developed in our laboratory. While absorption (i.e. the absorption of ultrasound through dissipative mechanisms) was not taken into account in the algorithm, the simulations reproduced major phenomena observed in real through-transmission experiments in trabecular bone. The simulated attenuation (i.e. the decrease of the transmitted ultrasonic energy) varies linearly with frequency in the MHz frequency range. Both the speed of sound (SOS) and the slope of the normalized frequency-dependent attenuation (nBUA) increase with the bone volume fraction. Twenty-five out of the thirty-one samples exhibited negative velocity dispersion. One sample was rotated to align the main orientation of the trabecular structure with the direction of ultrasonic propagation, leading to the observation of a fast and a slow wave. Coupling numerical simulation with real bone architecture therefore provides a powerful tool to investigate the physics of ultrasound propagation in trabecular structures. As an illustration, comparison between results obtained on bone modelled either as a fluid or a solid structure suggested the major role of mode conversion of the incident acoustic wave to shear waves in bone to explain the large contribution of scattering to the overall attenuation.

Frequency dependence of ultrasonic backscattering in cancellous bone: Autocorrelation model and experimental results

The goal of this study is to model the frequency dependence of the ultrasonic backscatter coefficient in cancellous bone. A twofold theoretical approach has been adopted: the analytical theoretical model of Faran for spherical and cylindrical elastic scatterers, and the scattering model for weakly scattering medium in which the backscatter coefficient is related to the autocorrelation function of the propagating medium. The ultrasonic backscatter coefficient was measured in 19 bone specimens (human calcaneae) in the frequency range of 0.4-1.2 MHz. The autocorrelation function was computed from the three-dimensional (3D) microarchitecture measured using synchrotron radiation microtomography. Good agreement was found between the frequency dependence of the experimental (f3.38+/-0.31) and autocorrelation modeled (f3.48+/-0.26) backscatter coefficients. The results based on Faran theory (cylindrical Faran model: f2.89+/-0.06 and spherical Faran model: f3.91+/-0.04) show qualitative agreement with experimental data. The good prediction obtained by modeling the backscatter coefficient using the autocorrelation function of the medium opens interesting prospects for the investigation of the influence of bone microarchitecture on ultrasonic scattering.

Variation of Ultrasonic Parameters With Microstructure and Material Properties of Trabecular Bone: A 3D Model Simulation†‡

This study determined the influence of trabecular bone microstructure and material properties on QUS parameters using numerical simulations coupled with high‐resolution synchrotron radiation μCT.