Biomedical subjects

Frédéric Patat

Publications and source records attributed to Frédéric Patat.

10 recordsLinked to original sources

PubMed2006

In-vivo imaging of skin under stress: potential of high-frequency (20 MHz) static 2-D elastography.

The aim of this study was to evaluate the potential of high-frequency static two-dimensional (2-D) elastography for in vivo exploration of the mechanical behavior of skin. Our device was based on the combination of a 20 MHz sonographer and a patented extensiometer device able to apply calibrated uniaxial stretching of the skin. We used a new algorithm to compute elastograms that improve elastographic signal-to-noise ratio (SNRe) without sacrificing resolution. Mechanical behavior was described according to the axial strain and lateral displacements induced in the tissue. The efficacy of the strain anpolyvinyl alcohol first evaluated in polyvinyl alcohol (PVA)-cryogel phantoms. Several in vivo experiments then were conducted, mainly with the multistretching averaging method, and demonstrated the potential of this technique in the evaluation of mechanical behavior of the dermis and the hypodermis under stress.

Compressive Strength↗

PubMed2006

Two-dimensional electroacoustic model of transducer array based on 1-3 piezocomposite materials.

An analytical model is presented to achieve simultaneous prediction of the elementary electroacoustic response and directivity pattern of a one-dimensional (1-D) piezocomposite array. The theoretical approach was based on guided wave theory in a multilayered structure in which the 1-3 piezocomposite material is considered as a homogeneous piezoelectric plate. A matrix method was applied to simulate the displacement fields generated at the surface of the array when one element was excited with an electrical pulse. A test device was manufactured, then characterized through measurements of displacement performed with an interferometric laser probe when the array vibrated in air and in water. The experimental results are presented and compared with theory.

Computer Simulation↗

PubMed2005

A finite difference model for cMUT devices.

A finite difference method was implemented to simulate capacitive micromachined ultrasonic transducers (cMUTs) and compared to models described in the literature such as finite element methods. Similar results were obtained. It was found that one master curve described the clamped capacitance. We introduced normalized capacitance versus normalized bias voltage and metallization rate, independent of layer thickness, gap height, and size membrane, leading to the determination of a coupling factor master curve. We present here calculations and measurements of electrical impedance for cMUTs. An electromechanical equivalent circuit was used to perform simulations. Our experimental measurements confirmed the theoretical results in terms of resonance, anti-resonance frequencies, clamped capacitance, and electromechanical coupling factor. Due to inhomogeneity of the tested element array and strong parasitic capacitance between cells, the maximum coupling coefficient value achieved was 0.27. Good agreement with theory was obtained for all findings.

Computer Simulation↗

PubMed2005

Response of bare 1-3 piezocomposite array to localized electrical excitation.

The theoretical response of a 1-3 piezocomposite plate submitted to localized electrical excitation was studied with the theory of guided waves. The theoretical modeling was based on the global matrix method, and the piezocomposite material was considered as a homogeneous medium. To validate the theoretical results, experimental displacement measurements were performed with an interferometric probe on two piezocomposite plates, one with a single element and one with an array of electrodes. The measured response on the single-element plate was mainly supported by the S0 and S3 modes of the plate. Homogenization limits of the composite in terms of frequency and wave number are defined on the basis of data from this sample. Within these limits, the piezocomposite material operates as a homogeneous medium, and comparison between theoretical and experimental results allows the equivalent electroacoustic parameters to be evaluated. A second sample was measured to study the effects of the electrode array on the electroacoustic response of the plate. Two kinds of electrical excitation were studied.

Journal Article↗

PubMed2005

Temporal analysis of tissue displacement induced by a transient ultrasound radiation force.

One of the stress sources that can be used in dynamic elastography imaging methods is the acoustic radiation force. However, displacements of the medium induced by this stress field are generally not fully understood in terms of spatial distribution and temporal evolution. A model has been developed based on the elastodynamic Green's function describing the different acoustic waves generated by focused ultrasound. The function is composed of three terms: two far-field terms, which correspond to a purely longitudinal compression wave and a purely transverse shear wave, and a coupling near-field term which has a longitudinal component and a transverse component. For propagation distances in the shear wavelength range, the predominant term is the near field term. The displacement duration corresponds to the propagation duration of the shear wave between the farthest source point and the observation point. This time therefore depends on the source size and the local shear modulus of the tissue. Evolution of the displacement/time curve profile, which is directly linked to spatial and temporal source profiles, is computed at different radial distances, for different durations of force applications and different shear elastic coefficients. Experimental results performed with an optical interferometric method in a homogeneous tissue-mimicking phantom agreed with the theoretical profiles.

Acoustics↗

PubMed2004

Bidirectional axial transmission can improve accuracy and precision of ultrasonic velocity measurement in cortical bone: a validation on test materials.

The axial transmission technique uses a linear arrangement of ultrasonic emitters and receivers placed on a same side of a cortical bone site in contact with the skin, involving ultrasonic propagation along the axis of bone. The velocity of the waves radiated from bone has been shown to reflect bone status. The thickness and composition of soft tissue may vary along the length of the bone, between different skeletal sites, or between subjects. Hence, accurate estimates of velocity require first to eliminate the effect of the overlying soft tissue that is traversed by the ultrasound wave. To correct for such bias without measuring soft tissue properties, we designed new ultrasonic probes in the 1-2 MHz frequency range. It is based on propagation along the bone surface in two opposite directions from two sources placed on both sides of a unique group of receivers. The aim is to obtain an unbiased estimate of the velocity without any intermediate calculation of soft tissue properties, such as thickness variation or velocity. Validation tests were performed on academic material such as Perspex or aluminum. We found that head wave velocity values could be biased by more than 10% for inclination of a few degrees between the test specimen surface and the probe. On test materials, the compensation procedure implemented in our probe led to a relative precision error on velocity measurement lower than 0.2 to 0.3%. These results suggest that the correction procedure allows measuring in vivo velocities independently of soft tissue properties.

Algorithms↗

PubMed2003

High resolution processing techniques for ultrasound doppler velocimetry in the presence of colored noise. Part I: Nonstationary methods.

Real-time flow velocity measurement is a practical issue in industrial and biomedical applications. Because their good frequency resolution, parametric methods such as autoregressive (AR) modeling and time-frequency distributions (TFD) are generally preferred to Fourier analysis. However, these methods become highly inaccurate in the presence of colored noise. We review here the principal parametric and nonparametric techniques and show their limitations in the estimation of Doppler frequency in the presence of strong colored noise. Different solutions to overcome these limitations are then proposed and compared using synthetic Doppler signals with colored noise.

Algorithms↗

PubMed2002

Effect of sonication parameters on transdermal delivery of insulin to hairless rats.

Application of low-frequency ultrasound has been shown to enhance transdermal drug transport of large molecules such as insulin. In this study, we investigated the dependence of ultrasound-induced transdermal delivery of insulin on ultrasound parameters. Insulin was delivered in vivo to hairless rats using 20 kHz ultrasound applied over a range of ultrasound intensity, application time and pulse length. Change in blood glucose levels of the animals was monitored to assess insulin transport. The results showed a threshold below which no detectable changes in blood glucose level was observed for each ultrasound parameter. Moreover, our findings indicated that sonophoretic enhancement is dependent on energy dose and length of ultrasound pulse that is consistent with a cavitation-based mechanism. The more significant effect of lowering glycemia was obtained with application of less than 15 min ultrasound and was similar to subcutaneous injection of 0.5 U of insulin. Pretreatment of hairless rat skin with ultrasound followed by application of insulin resulted in no significant modification in blood glucose level, indicating that transdermal transport of insulin mainly occurred during sonication. Sonophoresis may therefore potentially be applied for non-invasive and painless delivery of insulin in the treatment of insulin-dependent diabetes.

Administration, Cutaneous↗

PubMed2002

Spatial variation of acoustic parameters in human skin: an in vitro study between 22 and 45 MHz.

This study of spatial variance of acoustic parameters was performed on eight nonfrozen samples of female abdominal skin (women 46.5 +/- 12.2 years old), obtained during plastic surgery. Intra- and interindividual variations are discussed on the basis of estimations of three acoustic parameters (slope of attenuation- beta; integrated attenuation coefficient- IAC; integrated backscattering coefficient- IBC) and one texture parameter (based on two estimators of effective density of scatterers: alpha(2) and alpha(1/2)) as a function of surface area and depth of acquisition in the frequency range 22 to 45 MHz. Values of intraindividual variations varied from 7.1% for IAC to 23.2% for IBC, and significantly decreased at a ratio between 1.2 to 2.3 when the acquisition surface area was increased from 4 mm(2) to 1 cm(2). Interindividual variations were higher than intraindividual variations, and varied from 14.2% for alpha(1/2) to 51% for IBC. The mean values (+/- SD) for all specimens combined, estimated with a large number of independent radiofrequency (RF) lines (400) and for a surface area of exploration of 4 cm(2), were 1.06 +/- 0.17 dB cm(-1) MHz(-1) for beta, 135 +/- 37 dB cm(-1) for IAC, (3.7 +/- 1.9) x 10(-2) cm(-1) sr(-1) for IBC, 1.40 +/- 0.17 scatterers/resolution cell for alpha(2) and 1.32 +/- 0.27 scatterers/resolution cell for alpha(1/2). Finally, attenuation micro(f) and backscattering coefficient sigma(b)(f) were compared to published results for the same parameters measured in human skin.

Adipose Tissue↗

PubMed2000

Potential of a high-frequency correlation method to study skin blood flow.

BACKGROUND/AIMS: The vast number of existing dedicated techniques proves that skin blood flow estimation is an unsolved problem. Specificities of cutaneous vascularization (very low blood velocity, noisy environment, complexity of the vascularization architecture) result in the unsuitability of conventional ultrasonic Doppler techniques (long acquisition time, low spatial resolution). The object here was to present a high-frequency time-domain correlation METHOD: In particular, the difficulties of adaptating this type of measurement (data processing, hardware problem) are pointed out. METHODS: Radio-frequency (RF) backscattered signals, obtained with a modified version of a home-made 20 MHz skin imaging system, are studied. Time shifts between successive windowed sections of the RF signals are determined by the mean of the cross-correlation algorithm. A realignment procedure (to remove the artefacts caused by the movements of the patient and the manipulator) and a stationary echo cancelling procedure (to remove the signals coming from the cutaneous tissues and to permit the detection of very small vessels) are used. RESULTS: In vitro results show that velocity measurements as low as 0.1 mm/s are attainable with a 80 &mgr;m axial resolution, and blood vessels of 100 &mgr;m are detectable. Our technique has also been validated by means of in vivo experiment on an erysipelas located on a human leg. In this way, a 180-&mgr;m-diameter blood vessel has been detected on a M-mode RF image and the corresponding velocity profile has been obtained. CONCLUSION: Further improvements can be expected, and the level of performance obtained in vitro in this work should be also attainable in vivo and should then provide an effective tool for skin physiology and pathology.

Journal Article↗