Biomedical subjects

Pascal Laugier

Publications and source records attributed to Pascal Laugier.

28 records · Page 2Linked to original sources

PubMed2004

Three-dimensional simulations of ultrasonic axial transmission velocity measurement on cortical bone models.

The ultrasonic axial transmission technique, used to assess cortical shells of long bones, is investigated using numerical simulations based on a three-dimensional (3D) finite difference code. We focus our interest on the effects of 3D cortical bone geometry (curvature, cortical thickness), anisotropy, and microporosity on speed of sound (SOS) measurements for different frequencies in the MHz range. We first show that SOS values measured on tubular cortical shells are identical to those measured on cortical plates of equal thickness. Anisotropy of cortical bone is then shown to have a major impact on SOS measurement as a function of cortical thickness. The range of SOS values measured on anisotropic bone is half the range found when bone is considered isotropic. Dependence of thickness occurs for cortical shell thinner than 0.5 x lambda(bone) in anisotropic bone (lambda(bone): wavelength in bone), whereas it occurs for cortical shell thinner than lambda(bone) when anisotropy is neglected. Sensitivity of SOS along the bone axis to intracortical microporosity is shown to be approximately -20 m s(-1) per percent of porosity. Using homogenized porous bone, we finally show that the cortical depth that contributes to lateral wave SOS measurement is approximately 1-1.5 mm for frequencies ranging from 500 kHz to 2 MHz under classical in vivo measurement conditions.

Algorithms↗

PubMed2004

High-resolution ultrasonography for analysis of age- and disease-related cartilage changes.

Because of their limited spatial resolution, current clinical noninvasive imaging modalities (radiography, computed tomography, conventional echography, and magnetic resonance imaging) are able to detect only the late stages of the cartilage degradation. To detect early lesions and follow their evolution in time with imaging, higher resolution is necessary. Recent work suggest that high-frequency ultrasound may serve as a useful means for the investigation of cartilage matrix structural changes occurring under various experimental and clinical circumstances, like the growing process and osteoarthritis. In this chapter, an experimental 50-100-MHz ultrasound scanner is described for high-resolution echographic imaging of articular cartilage. The procedures of data acquisition and signal processing are detailed for the quantitative evaluation of ultrasonic reflection and backscatter coefficients, which have been reported to be sensitive to subtle surface and internal disease-related alterations. Further technological developments and miniaturization of the echographic probes may lead to extension of this technique to the study of living small animals or to the clinical field in combination with conventional arthoscopy.

Aging↗

PubMed2004

Segmentation of quantitative ultrasonographic images of the calcaneus using elastic deformation of the flexible Fourier contour.

OBJECTIVE: This study investigated a new technique for automatic model-based segmentation of broadband ultrasound attenuation (BUA) images of the calcaneus. We determined whether this technique was able to improve osteoporotic fracture discrimination. METHODS: The segmentation process included 2 major steps: a model-building stage and the automatic segmentation of new image data sets via an elastic deformation of contour models. Broadband ultrasound attenuation was then averaged within the final contour (BUAwhole). The results of the segmentation were validated on a database of 256 patients by comparison of the clinical results obtained with the automatic circular region of interest (BUAcirc) currently implemented on a commercially available ultrasonography unit. All patients were selected by the same physician, who assessed that the fractures were caused by bone fragility on the basis of the circumstances under which fractures occurred and radiologic data. RESULTS: Short-term reproducibility assessed in 49 women was 3.5% and 3.98% for BUAcirc and BUAwhole, respectively. Both BUAcirc (age-adjusted T score, -3.78; P < .0005; age-adjusted odds ratio, 1.92; 95% confidence interval, 1.34-2.75; area under the receiver operating characteristic curve, 0.70) and BUAwhole (age-adjusted T score,-2.73; P < .01; age-adjusted odds ratio, 1.57; 95% confidence interval, 1.12-2.21; area under the curve, 0.67) performed equally well in discriminating healthy postmenopausal patients (n = 150) from those with fractures (n = 60). CONCLUSIONS: Fully automatic segmentation by parametrically deformable elastic models for contour using Fourier descriptors can be achieved with reasonable reproducibility and fracture risk prediction. The method is similar to existing methods (automatic circular region of interest); however, the new contour-based region of interest allows more flexible region of interest geometries and placement and potential adaptation to individual anatomy. The method could also possibly be extended to quantitative ultrasonographic imaging at different skeletal sites.

Adult↗

PubMed2004

Singular spectrum analysis applied to backscattered ultrasound signals from in vitro human cancellous bone specimens.

Mean scatterer spacing (MSS) holds particular promise for the detection of changes in quasiperiodic tissue microstructures such as may occur during development of disease in the liver, spleen, or bones. Many techniques that may be applied for MSS estimation (temporal and spectral autocorrelation, power spectrum and cepstrum, higher order statistics, and quadratic transformation) characterize signals that contain a mixture of periodic and nonperiodic contributions. In contrast, singular spectrum analysis (SSA), a method usually applied in nonlinear dynamics, first identifies components of signals corresponding to periodic structures and, second, identifies dominant periodicity. Thus, SSA may better separate periodic structures from nonperiodic structures and noise. Using an ultrasound echo simulation model, we previously demonstrated SSA's potential to identify MSS of structures in quasiperiodic scattering media. The current work aims to observe the behavior of MSS estimation by SSA using ultrasound measurements in phantom materials (two parallel, nylon-line phantoms and four foam phantoms of different densities). The SSA was able to estimate not only the nylon-line distances but also nylon-line thickness. The method also was sensitive to the average pore-size differences of the four sponges. The algorithms then were applied to characterize human cancellous bone microarchitectures. Using 1-MHz center-frequency, radio-frequency ultrasound signals, MSS was measured in 24 in vitro bone samples and ranged from 1.0 to 1.7 mm. The SSA MSS estimates correlate significantly to MSS measured independently from synchrotron microtomography, r2 = 0.68. Thus, application of SSA to backscattered ultrasound signals seems to be useful for providing information linked to tissue microarchitecture that is not evident from clinical images.

Aged↗

PubMed2003

Effects of antiinflammatory drugs on arthritic cartilage: a high-frequency quantitative ultrasound study in rats.

OBJECTIVE: To evaluate the ability of 55-MHz quantitative ultrasound (US) to detect the in vivo effects of experimental arthritis, as well as those of two antiinflammatory drugs, naproxen (NPX) and dexamethasone (DEX), on cartilage and subchondral bone. METHODS: Arthritis was induced in both knees of 108 rats by intraarticular injection of zymosan (ZYM). Two groups of arthritic rats (n = 36 per group) were treated daily with either NPX (10 mg/kg/day) or DEX (0.1 mg/kg/day). Using a 3-dimensional US microscope, patellae were explored in vitro on days 5, 14, and 21 after injections. US assessment included the analysis of quantitative indices of local modifications involving cartilage and bone: integrated reflection coefficient (IRC) from the cartilage surface and apparent integrated backscatter from the cartilage internal structure (cartilage matrix) (AIB(cartilage)) and the cartilage-bone interface (AIB(bone)). RESULTS: ZYM induced articular surface fibrillation that resulted in a decrease in IRC at all times (P < 0.02) and in an increase in AIB(bone) on days 5 and 14 (P < 0.005). Fibrillation was not changed by NPX administration, while it disappeared following DEX treatment. Cartilage-bone interface alterations were prevented by DEX and partially compensated for by NPX. Cartilage matrix echogenicity decreased with time in all groups due to maturation (P < 0.05), except in DEX-treated rats. CONCLUSION: Quantitative 55 MHz US allowed detection of early cartilage and bone lesions due to experimental arthritis, and also allowed detection of the effects of antiinflammatory drugs. NPX seemed to have an effect on subchondral bone lesions, but not on cartilage. DEX appeared to repair articular surface and bone, but prevented animal growth and cartilage maturation.

Animals↗

PubMed2003

Parametric analysis of carotid plaque using a clinical ultrasound imaging system.

We evaluated quantitative ultrasonic methods for assessment of carotid plaque content. In vitro measurements of fixed, carotid plaque specimens obtained by surgical endarterectomy were performed using a clinical Philips HDI 5000 imaging system connected to a radiofrequency (RF) signal-acquisition system. We acquired RF signals and grey-scale images from carotid specimens (n = 17) and a tissue-mimicking reference phantom. Imaged plaque sections were then classified according to histology. Parametric images were constructed from the integrated backscatter (IBS), and the midband, slope and intercept values of a straight-line fit to the apparent backscatter transfer function. Analysis was performed on 82 regions-of-interest (ROIs). The IBS values for collagen, lipid and hemorrhage plaques were 5.8 +/- 5.4, 3.9 +/- 3.7, 2.8 +/- 2.2 dB, respectively. Midband and IBS parameter images exhibited good agreement in morphology with histology, whereas the slope and intercept parameter images were noisy. Mean IBS, midband, and grey-scale values of complex plaques were found to be statistically different (p < 0.05) from lipid, hemorrhage and fibrolipid plaques. The bias and limits of agreement (1.3 +/- 4.9 dB) between the grey-scale and IBS methods, however, indicated that the two methods were not interchangeable. Results indicate necessary improvements, such as reduction of large measurement variances and identification of robust parameters, that will permit multiparametric characterization of carotid plaque under in vivo conditions.

Carotid Arteries↗

PubMed2003

Prediction of frequency-dependent ultrasonic backscatter in cancellous bone using statistical weak scattering model.

The goal of this study was to propose a model for the ultrasonic frequency-dependent backscatter coefficient in cancellous bone. This model allows us to address the inverse problem and to predict the mean trabecular thickness. A weak scattering model is used and the backscatter coefficient is expressed in terms of an autocorrelation function of the medium. Different autocorrelation functions (Gaussian, exponential and densely populated media) were used to compute the backscatter coefficient and comparison is made with experimental data for 19 specimens and for frequency ranging from 0.4 to 1.2 MHz. For each specimen, a nonlinear regression was performed and the mean trabecular thickness is estimated. Experimental data and theoretical predictions were averaged over the 19 specimens. A good agreement between experimental data and predictions was found for both the magnitude and the frequency-dependence of the backscatter coefficient. We also found a good agreement between the experimental mean trabecular thickness (Tb. Th = 130 +/- 6.5 micro m) derived from the analysis of bone 3-D microarchitecture using high-resolution microtomography and theoretical predictions (d(Gauss) = 140 +/- 10 micro m, d(exponential) = 153 +/- 12.5 micro m and d(dense) = 138 +/- 6.5 micro m). These results open interesting prospects for the estimation of the mean trabecular thickness from in vivo measurements.

Acoustics↗

PubMed2003

Optimization of attenuation estimation in reflection for in vivo human dermis characterization at 20 MHz.

In vivo skin attenuation estimators must be applicable to backscattered radio frequency signals obtained in a pulse-echo configuration. This work compares three such estimators: short-time Fourier multinarrowband (MNB), short-time Fourier centroid shift (FC), and autoregressive centroid shift (ARC). All provide estimations of the attenuation slope (beta, dB x cm(-1) x MHz(-1)); MNB also provides an independent estimation of the mean attenuation level (IA, dB x cm(-1)). Practical approaches are proposed for data windowing, spectral variance characterization, and bandwidth selection. Then, based on simulated data, FC and ARC were selected as the best (compromise between bias and variance) attenuation slope estimators. The FC, ARC, and MNB were applied to in vivo human skin data acquired at 20 MHz to estimate betaFC, betaARC, and IA(MNB), respectively (without diffraction correction, between 11 and 27 MHz). Lateral heterogeneity had less effect and day-to-day reproducibility was smaller for IA than for beta. The IA and betaARC were dependent on pressure applied to skin during acquisition and IA on room and skin-surface temperatures. Negative values of IA imply that IA and beta may be influenced not only by skin's attenuation but also by structural heterogeneity across dermal depth. Even so, IA was correlated to subject age and IA, betaFC, and betaARC were dependent on subject gender. Thus, in vivo attenuation measurements reveal interesting variations with subject age and gender and thus appeared promising to detect skin structure modifications.

Adult↗

PubMed2003

Prediction of backscatter coefficient in trabecular bones using a numerical model of three-dimensional microstructure.

A model of ultrasonic backscattering for cancellous bone saturated by water is proposed. This model assumes that scattering is caused by the solid trabeculae and describes the cancellous bone as a weak scattering medium. The backscatter coefficient is related to the spatial Fourier transform of bone microarchitecture and to the density and compressibility fluctuations between the solid trabeculae and the saturating fluid. The computations of the model make use of three-dimensional numerical images of bone microarchitecture, obtained by tomographic reconstructions with a 10 microm spatial resolution. With this model, the predictions of the frequency dependence and of the magnitude of the backscatter coefficient are reasonably accurate. The theoretical predictions are compared to experimental data obtained on 19 specimens. An accuracy error of approximately 1 dB was found (difference between the averaged experimental values and theoretical predictions). One limit of the model may come from inaccurate values of trabecular bone characteristics needed for the computations (density and longitudinal velocity), which are yet to be precisely determined for human trabecular bone. However, the model is only slightly sensitive to variations of bone material properties. It was found that an accuracy error of 2.2 dB at maximum resulted from inaccurate a priori values of bone material properties. A computation of the elastic mean free path in the medium suggests that multiple scattering plays a minor role in the working frequency bandwidth (0.4-1.2 MHz). It follows from these results that a weak scattering medium model may be appropriate to describe scattering from trabecular bone.

Aged↗

PubMed2002

Effect of bone cortical thickness on velocity measurements using ultrasonic axial transmission: a 2D simulation study.

In recent years, quantitative ultrasound (QUS) has played an increasing role in the assessment of bone status. The axial transmission technique allows to investigate skeletal sites such as the cortical layer of long bones (radius, tibia), inadequate to through-transmission techniques. Nevertheless, the type of propagation involved along bone specimens has not been clearly elucidated. Axial transmission is investigated here by means of two-dimensional simulations at 1 MHz. We focus our interest on the apparent speed of sound (SOS) of the first arriving signal (FAS). Its dependence on the thickness of the plate is discussed and compared to previous work. Different time criteria are used to derive the apparent SOS of the FAS as a function of source-receiver distance. Frequency-wave number analysis is performed in order to understand the type of propagation involved. For thick plates (thickness>lambdabone, longitudinal wavelength in bone), and for a limited range of source-receiver distances, the FAS corresponds to the lateral wave. Its velocity equals the longitudinal bulk velocity of the bone. For plate thickness less than lambdabone, some plate modes contribute to the FAS, and the apparent SOS decreases with the thickness in a way that depends on both the time criterion and on the source-receiver distance. The FAS corresponds neither to the lateral wave nor to a single plate mode. For very thin plates (thickness< lambdabone/4), the apparent SOS tends towards the velocity of the lowest order symmetrical vibration mode (S0 Lamb mode).

Humans↗