Biomedical subjects

B Audoin

Publications and source records attributed to B Audoin.

16 recordsLinked to original sources

PubMed2006

Acoustic waves generated by a laser line pulse in a hollow cylinder.

A theoretical solution is proposed to predict acoustic waves generated in a homogeneous and isotropic hollow cylinder by a laser line source under either ablation or thermoelastic regime. The Fourier series expansion is introduced for one spatial coordinate to solve this transient response problem. Theoretical displacements are obtained in both regimes for aluminum hollow cylinders with various thickness including a rod of the same size. The corresponding displacements are observed experimentally by the laser ultrasonic technique. Agreement has been found in the time arrival, shape and relative amplitude of surface waves and various longitudinal and transverse bulk waves. These acoustic waves are further identified by the ray trajectory analysis. This work will be helpful when dealing with the inverse problem of the nondestructive evaluation of hollow cylindrical parts.

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PubMed2006

Identification of laser-generated ultrasounds in the response of the cylinder over time and space.

A theoretical solution is presented to identify laser-generated ultrasounds in the transient response over time and space of a cylinder impacted by a laser line pulse. Theoretical radial displacements at various observation angles are obtained for an aluminum cylinder under thermoelastic regime. The corresponding displacements are observed experimentally by the laser ultrasonic technique. Good agreement is found in the time arrival, shape and relative amplitude of surface waves and various longitudinal and transverse bulk waves. These laser-generated ultrasounds are further identified by the ray trajectory analysis. This work will be helpful for the inverse problem of the nondestructive evaluation of cylinder parts.

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PubMed2006

The transient response of a transversely isotropic cylinder under a laser point source impact.

The transient response of a transversely isotropic cylinder under a laser point source impact is solved theoretically. The radial displacement generated by the laser under the ablation regime is numerically calculated by introducing Fourier series expansion and two-dimensional Fourier transform. The validity of this theoretical solution is demonstrated on a fiber reinforced composite cylinder with a strong anisotropy. Experimental displacements are detected at the cylinder surface by the laser ultrasonic technique, and are analyzed by the ray trajectories. Corresponding theoretical displacements are calculated numerically and compared to the experimental signals. Good agreement is found. The diffraction effect caused by the cusp is observed in both theory and experiment.

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PubMed2006

Numerical analysis of bulk conical waves in anisotropic cylinders; application to stiffness tensor measurement.

A point source-point receiver technique, based on laser generation and laser detection of acoustic waves, allows determination of mechanical properties of an anisotropic cylinder. The nature of the material and the geometry of the sample give a dispersive behaviour to the diffracted waves and make the acoustic signature difficult to interpret. To overpass the intricacies, wave fronts (conical waves in the volume and helical waves on the surface) are synthesized from signals provided by scanning the primitive line of the cylinder with a laser point source. In order to distinguish between direct bulk conical waves and other contributions in the acoustic response, some considerations on line surface waves and on reflected bulk conical waves are supplied. The identification of the stiffness tensor components, based on the inversion of the bulk waves phase velocities, is applied to signals simulated for a composite material.

Journal Article↗

PubMed2006

On the use of laser-ultrasonics technique to excite selectively cylinder acoustic resonances.

Acoustic propagation in the transverse plane of a cylinder is considered in this paper. The acoustic source is a line along the axis z of the cylinder coordinates, obtained by focusing the beam of a pulsed laser on the surface of the cylinder. Point detection is performed with a laser interferometer. By adequately combining elementary signals measured for a large number of relative source-receiver positions, a signal is synthesized containing the basic acoustic resonances of the cylinder. Various elementary signal combinations allow us to select the observed acoustic modes related to Whispering Gallery modes and to Rayleigh surface waves. The changes in the signal spectrum are discussed. These acoustic modes are experimentally selected and observed at ultrasonic frequencies for the first time.

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PubMed2006

Picosecond ultrasonics time resolved spectroscopy using a photonic crystal fiber.

We present a broadband picosecond ultrasonics time resolved spectroscopy. Detection of picosecond coherent acoustic phonons using a wavelength continuum generation in a photonic crystal fiber (PCF) with femtosecond laser pulses is developed. Measurements are performed for selected wavelengths of a broad wavelength probe pulse within a bandwidth of 250 nm with an 825 nm center wavelength on two samples made of tungsten and of gallium arsenide.

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PubMed2006

Acoustic waves generated by a laser point pulse in a transversely isotropic cylinder.

A three-dimensional (3D) model is presented to predict the acoustic waves generated by a laser point pulse in a transversely isotropic cylinder. The Fourier series expansion and the two-dimensional Fourier transform are introduced to calculate the 3D transient response under either the ablation or the thermoelastic generation. The presented physical model and the numerical inverse scheme are applied to a fiber reinforced composite cylinder with a strong anisotropy. Experimental radial displacements of the cylinder surface are detected by the laser ultrasonic technique and analyzed by the ray trajectories for both generation regimes. Corresponding theoretical displacements are obtained numerically and compared to the experimental signals. Good agreement is found between theoretical and experimental results. The focusing effects that anisotropy gives rise to are observed in both theory and experiment under either regime.

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PubMed2005

Generation and detection of shear acoustic waves in metal submicrometric films with ultrashort laser pulses.

We present experimental and calculational results demonstrating the thermoelastic generation of shear acoustic waves using femtosecond laser pulses in submicrometric isotropic aluminum films. We show that the generation of the shear waves is correlated to the reduction of the width of the optoacoustic source on the surface. The presence of shear waves is related to acoustic diffraction and acoustic mode conversion at the thin film interfaces.

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PubMed2005

Functional MRI study of PASAT in normal subjects.

The paced auditory serial addition test (PASAT) is routinely used to evaluate the cognitive part of the multiple sclerosis functional composite (MSFC) score, the new reference index of patient disability. PASAT is sensitive to subtle cognitive impairment related to MS, although the cognitive components of this test still remain unclear. In order to better characterize brain systems involved during this complex task, functional magnetic resonance imaging (fMRI) experiments were conducted during PASAT in a population of ten normal subjects. The paradigm consisted of a series of 61 single-digit numbers delivered every 3 s. After each number, subjects were asked to overt vocalize the result of the addition of the two last numbers heard. A control task consisting of the repetition of the same series of single-digit numbers was used. Statistical group analysis was performed using the random effect procedure (SPM 99). Cortical activation was observed in the left prefrontal cortex, the supplementary motor area, the lateral premotor cortex, the cingulate gyrus, the left parietal lobe, the left superior temporal gyrus, the left temporal pole, and visual associative areas. fMRI activations underlying PASAT were consistent with an involvement of verbal working memory and the semantic memory retrieval network which could be related to arithmetic fact retrieval. This study on normal subjects could provide a base for the understanding of the potential abnormal cortical activation in MS patients performing this test for a cognitive evaluation.

Adult↗

PubMed2005

Identification of laser generated acoustic waves in the two-dimensional transient response of cylinders.

The published model [Appl. Phys. Lett. 82, 4379-4381 (2003)] for the two-dimensional transient wave propagation in a cylinder is modified to avoid the inherited integration of the numerical inverse scheme. The Fourier series expansion is introduced for one spatial coordinate to resolve the transient response problem: theoretical radial displacements in either the ablation or the thermoelastic regime are obtained with little numerical noise and short computation time. The normal mode expansion method fails to deliver results with the same accuracy. Acoustic waves are fully identified by the ray trajectory analysis. These identified waves are further verified on the experimental results observed with the laser ultrasonic technique.

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PubMed2004

Modulation of effective connectivity inside the working memory network in patients at the earliest stage of multiple sclerosis.

fMRI and structural equation modeling (SEM) were used to study effective connectivity inside the working memory network in patients at the earliest stage of multiple sclerosis (MS), while performing paced auditory serial addition test (PASAT), a sensitive task to reveal subtle cognitive impairments related to working memory and information speed processing. The path model used for SEM included bilateral connections between left and right BA 46, left and right BA 40, left and right anterior cingulate cortex (ACC), left BA 44 and left BA 40, right BA 44 and right BA 40, and unidirectional ipsilateral connections from BA 46 to BA 44, from ACC to BA 46, and from ACC to BA 44. Experimental data from the two groups fit accurately the working memory model, in patients [chi20(2) = 13, P = 0.877] as well as in controls [chi20(2) = 13.54, P = 0.853]. The omnibus test indicated a significant difference of model fits in patients and in controls [chi40(2) = 160.07, P < 0.0001]. Connectivity strengths from right BA 46 to left BA 46, from left ACC to left BA 46 were lower in patients than in controls, and higher from right ACC to right BA 46, from left to right and from right to left ACC (stacked model). Effective connectivity inside the working memory network appears altered in patients at the earliest stage of MS. Modulation of effective connectivity is present in patients inside the executive subsystems of working memory, and could be related to adaptive cognitive control processes that may limit the clinical manifestation of MS.

Acoustic Stimulation↗

PubMed2003

MRI/MRS of corpus callosum in patients with clinically isolated syndrome suggestive of multiple sclerosis.

Atrophy of corpus callosum (CC) related to axonal loss has previously been observed in patients at the early stage of clinically definite multiple sclerosis (CDMS). Atrophy increases with the progression of the disease. Nevertheless, no data concerning the onset of atrophy of CC are currently available. The purpose of this study is to determine if damage in callosal tissue was present at the earliest stage of MS, in a subgroup of patients presenting with a clinically isolated syndrome suggestive of MS (CISSMS), fulfilling the dissemination in space criteria according to McDonald. Magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) techniques were applied to measure CC volume, magnetization transfer ratio (MTR), mean diffusivity (MD), N-acetyl aspartate/choline-containing compounds (NAA/Cho) ratio, N-acetyl aspartate/total creatine (NAA/Cr) ratio and Cho/Cr ratio inside the CC of 46 CISSMS patients and 24 sex- and age-matched controls. No atrophy of CC was observed in the CISSMS group. CC of patients was characterized by decreased MTR and increased MD. No change in the NAA/Cr ratio was observed while the NAA/Cho ratio decreased and Cho/Cr ratio increased in the splenium and the central anterior part of CC. These abnormalities were present in patients with, but also without, macroscopic lesions inside the CC. Our results indicate that diffuse structural and metabolic changes, which may be interpreted as representing predominantly myelin pathology, occur in the CC at the earliest stage of MS before any atrophy is detected.

Adult↗

PubMed2003

[Melanocytic meningitis and large congenital melanocytic naevus: neurocutaneous melanosis].

Neurological symptoms in a patient with large congenital melanocytic naevus are highly suggestive of cerebromeningeal melanoma metastasis. The presence of melanocytic cells in cerebrospinal fluid confirms this diagnosis If their malignant nature is shared with cutaneous naevocytic cells. Conversely, neurocutaneous melanosis is diagnosed when benign melanocytosis meningitis is found in patients with multiple and/or large congenital melanocytic naevus, whether cutaneous naevus cells are benign or not, or when cerebrospinal fluid cells are malignant with benign cutaneous melanocytic naevus. We report the case of a young man aged 19 presenting with multiple and large congenital melanocytic naevus who experienced transcient neurological signs and increased intracranial pressure. Cerebral neuroimaging evoked meningeal infiltration which benign melanocytic nature was supposed on CSF analysis and confirmed by necropsy findings, only 3 month after neurological onset, leading to neurocutaneous melanosis diagnosis. This rare neuroectodermal dysembryoplasia finds expression in various neurological signs, depending on patient's age and leptomeningeal and/or cerebral proliferation localization. Lumbar puncture, cerebral scanography and MRI may help diagnosis, but only histological examination can prove neurocutaneous melanosis, more often by necropsy because of poor prognosis.

Adult↗

PubMed2002

Non-destructive evaluation of composite materials with ultrasonic waves generated and detected by lasers.

Measurement of the stiffness properties of composite materials with laser generated and detected ultrasound requires proper understanding of waves emanating from a line or point source in anisotropic and viscoelastic media. The paper briefly presents calculation results of waves radiated by such a source through or at the surface of a composite plate. Dispersion is represented as well as the multiple wave arrivals connected with the folded shape of the quasi-shear ray surface. Moreover, internal diffraction at the cusp edges is properly depicted. An identification method with specific signal processing have been used to measure the stiffness coefficients of composite materials. From group velocity data, the stiffness tensor of materials showing an orthorhombic symmetry can be identified. The stiffness tensor changes induced by elevated temperatures in a composite material were then measured. An alternative approach was developed which allows to measure the phase velocities of waves generated with laser line sources. The material characterisation reliability is then improved. Moreover, the method can be used in practical cases where the front side of the structure only is accessible with the experimental devices. Despite reflection at the rear interface of transient divergent waves which ray surfaces may contain caustics, this inverse problem can be solved in a simple and efficient manner.

Journal Article↗

PubMed2002

Identification of complex stiffness tensor from waveform reconstruction.

An inverse method is proposed in order to determine the viscoelastic properties of composite-material plates from the plane-wave transmitted acoustic field. Analytical formulations of both the plate transmission coefficient and its first and second derivatives are established, and included in a two-step inversion scheme. Two objective functions to be minimized are then designed by considering the well-known maximum-likelihood principle and by using an analytic signal formulation. Through these innovative objective functions, the robustness of the inversion process against high level of noise in waveforms is improved and the method can be applied to a very thin specimen. The suitability of the inversion process for viscoelastic property identification is demonstrated using simulated data for composite materials with different anisotropy and damping degrees. A study of the effect of the rheologic model choice on the elastic property identification emphasizes the relevance of using a phenomenological description considering viscosity. Experimental characterizations show then the good reliability of the proposed approach. Difficulties arise experimentally for particular anisotropic media.

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PubMed2001

Elastic constants determination of anisotropic materials from phase velocities of acoustic waves generated and detected by lasers.

This paper describes a method which allows the synthesis of acoustic plane waves by means of laser ultrasonic techniques. When a laser line source radiates at the surface of an anisotropic material, intricate effects are observed on signals obtained by transmission experiments. The material characterization from such a set of signals is not a trivial task since it requires correctly selecting the data necessary for the elastic constants determination, by distinguishing them from all the parasitic phenomena. In the present paper, it is shown that a judicious sum of signals obtained for various positions of the line source at the surface of a sample leads to the synthesis of an acoustic plane wave for which the odd phenomena disappear. Moreover, by applying a constant delay between the various signals, acoustic plane waves can be synthesized with varied refracted angles. One of the advantages of this technique is to offer convenient access to the phase velocity, providing an easier determination of the elastic constants. This method is successively applied to recover the stiffness coefficients of a silicon crystal and of a composite material. The good agreement between the so-recovered elastic constants and values given by other processes shows the contribution of such a method in the field of material characterization.

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