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Biomedical subjects

I Céspedes

Publications and source records attributed to I Céspedes.

13 recordsLinked to original sources
PubMed1999

Echo decorrelation estimated from signal powers.

Volume flow can be estimated from the decorrelation of radiofrequency (RF) intravascular ultrasound signals. The method is based on a rather time-consuming process that measures the decorrelation slope from a time signal sequence. To improve the speed of flow processing, a more efficient way of estimating the flow velocity from the ratio between the power of the temporal averaged signal and the mean signal power is described in this paper. The relationship between the signal power-ratio index and the decorrelation slope was analyzed and tested using computer-simulated data. Volumetric flow data obtained with the power-ratio method were compared to those derived from the decorrelation slope in five patients. Results of the comparison studies indicate that no significant differences in flow measurements were found between the two methods, but the power-ratio method is able to improve the processing speed significantly.

Blood Flow Velocity↗
PubMed1998

Blood flow imaging and volume flow quantitation with intravascular ultrasound.

Current intravascular ultrasound techniques produce real-time imaging of a vessel cross-section with a scan plane approximately normal to blood flow. When a cluster of randomly distributed blood particles moves across the ultrasound beam, the received echo signals decorrelate as a function of time. This phenomenon may be used to estimate blood velocities by measuring the decorrelation rate from a sequence of blood scattering signals. A decorrelation-based method for measuring local blood velocity and quantifying volume flow from cross-sectional radio frequency intravascular echo signals was developed. Serial in vitro measurements were performed with a flow phantom to test the principle of the proposed velocity estimation method. An in vivo pig experiment was carried out to study the feasibility of applying this method in clinical settings. Preliminary results of this study indicate that the proposed decorrelation method is able to extract cross-sectional velocity data and volumetric flow both in vitro and in vivo.

Animals↗
PubMed1996

Noise reduction in elastograms using temporal stretching with multicompression averaging.

Elastography uses estimates of the time delay (obtained by cross-correlation) to compute strain estimates in tissue due to quasistatic compression. Because the time delay estimates do not generally occur at the sampling intervals, the location of the cross-correlation peak does not give an accurate estimate of the time delay. Sampling errors in the time-delay estimate are reduced using signal interpolation techniques to obtain subsample time-delay estimates. Distortions of the echo signals due to tissue compression introduce correlation artifacts in the elastogram. These artifacts are reduced by a combination of small compressions and temporal stretching of the postcompression signal. Random noise effects in the resulting elastograms are reduced by averaging several elastograms, obtained from successive small compressions (assuming that the errors are uncorrelated). Multicompression averaging with temporal stretching is shown to increase the signal-to-noise ratio in the elastogram by an order of magnitude, without sacrificing sensitivity, resolution or dynamic range. The strain filter concept is extended in this article to theoretically characterize the performance of multicompression averaging with temporal stretching.

Algorithms↗
PubMed1996

Reduction of stress nonuniformities by apodization of compressor displacement in elastography.

Elastography is a method for imaging the elastic properties of compliant tissues that produces gray-scale strain or elasticity images called elastograms. The method is based on external tissue compression, with ultrasonic detection of local target displacements and subsequent computation of strain profiles along the compression axis. The internal strain variations are a result of the tissue elasticity variations and the applied deformation or compression. A number of mechanical artifacts that appear in elastograms have been identified. One such artifact appears as the result of a nonuniform stress distribution under the compressors used, including darkening (low stress) of the central region and brightening (high stress) of the peripheral regions under the compressor. On an elastogram, these areas may be misinterpreted as being respectively harder and softer than the rest of the target. In this article, a displacement apodization method for the minimization of this artifact is discussed, and its effects are studied using finite element simulations. When the isometric compression of standard elastography was replaced by an apodized displacement profile calculated from reciprocity conditions, a significant improvement in stress uniformity under the compressor was achieved.

Artifacts↗
PubMed1995

Fundamental mechanical limitations on the visualization of elasticity contrast in elastography.

Elastography is a new ultrasonic imaging technique that produces images (elastograms) of the elastic properties of complaint tissue. To determine the Young's modulus it is necessary to measure or estimate any five of seven relevant variables. In elastography, the measured quantity is the normal strain component in the direction of the applied load, and the three normal components of stress may be estimated using the modified Love's analytical models while assuming a value close to 0.5 (incompressible) for Poisson's ratio. The distribution of Young's moduli can thus be computed and displayed in the form of two-dimensional images called elastrograms. The analytical models used for the estimation of the three normal components of stress assume that the target is semi-infinite and homogeneous in composition. The objective of this article is to determine some of the errors associated with the assumption of homogeneity of the target. Experiments using computer simulations were performed to study the efficiency with which elastograms display the contrast in the Young's modulus of a lesion or target, with respect to its background under certain conditions. It was observed (using the definition of contrast-transfer efficiency of elastography as the ratio of the elasticity contrast as measured from an elastogram, to the true contrast) that elastograms were consistently efficient in quantitatively depicting the elasticity contrast of hard lesions; however, they showed suboptimal contrast-transfer efficiency in cases of soft lesions in a hard background. In general, elastograms are efficient in displaying the elasticity contrast of hard or soft lesions which have a low contrast level with respect to the surroundings, irrespective of their size and location.

Algorithms↗
PubMed1995

Methods for estimation of subsample time delays of digitized echo signals.

Time delay estimation (TDE) is commonly performed in practice by crosscorrelation of digitized echo signals. Since time delays are generally not integral multiples of the sampling period, the location of the largest sample of the crosscorrelation function (ccf) is an inexact estimator of the location of the peak. Therefore, one must interpolate between the samples of the ccf to improve the estimation precision. Using theory and simulations, we review and compare the performance of several methods for interpolation of the ccf. The maximum likelihood approach to interpolation is the application of a reconstruction filter to the discrete ccf. However, this method can only be approximated in practice and can be computationally intensive. For these reasons, a simple method is widely used that involves fitting a parabola (or other curve) to samples of the ccf in the neighborhood of its peak. We describe and compare two curve-fitting methods: parabolic and cosine interpolation. Curve-fitting interpolation can yield biased time-delay estimates, which may preclude the use of these methods in some applications. The artifactual effect of these bias errors on elasticity imaging by elastography is discussed. We demonstrate that reconstructive interpolation is unbiased. An iterative implementation of the reconstruction procedure is proposed that can reduce the computation time significantly.

Computer Simulation↗
PubMed1993

Elastography: elasticity imaging using ultrasound with application to muscle and breast in vivo.

Changes in tissue elasticity are generally correlated with its pathological state. In many cases, despite the difference in elasticity, the small size of a lesion or its location deep in the body preclude its detection by palpation. In general, such a lesion may or may not possess echogenic properties that would make it ultrasonically detectable. Elastography is an ultrasonic method for imaging the elasticity of compliant tissues. The method estimates the local longitudinal strain of tissue elements by ultrasonically assessing the one dimensional local displacements. This information can be combined with first order theoretical estimates of the local stress to yield a quantitative measure of the local elastic properties of tissue. The elasticity information is displayed in the form of a gray scale image called an elastogram. An experimental system for elastography in phantoms based on a single element transducer has been described previously [1]. Here we introduce a new elastography system based on a linear array transducer that is suitable for in vivo scanning. We describe tissue mimicking phantom experiments and preliminary in vivo breast and muscle elastograms confirming the feasibility of performing elastography in vivo. An elastogram of a breast containing an 8 mm palpable cancer nodule clearly shows the lesion. Elastograms and their corresponding sonograms show some similarities and differences in the depiction of tissue structures.

Adult↗
PubMed1993

Reduction of image noise in elastography.

Elastography is a method for imaging the elastic properties of compliant tissues which produces gray scale elasticity images called elastograms. The elastograms of phantoms with homogeneous elastic properties exhibit a noisy appearance. We demonstrate that this noisy appearance of the elastograms is due to the nonstationary relationship between the pre- and postcompression signals that results in an artifactual modulation of the strain estimates by the amplitude variations of the envelope of the rf signal. We have identified two methods to reduce the strain modulation artifact. The first method consists of reducing the signal amplitude swings within the observation windows by logarithmically or otherwise compressing the rf signal. The sensitivity of this method to amplitude compression strength and the ability to reduce the noise in the elastograms without affecting the spatial resolution are investigated through simulations. The second method to reduce the strain modulation artifact consists of temporal stretching of the signal obtained after physical compression to approximate the shape of the signal obtained before compression. In this paper, we discuss the first method. The results show that significant improvement in image noise can be obtained with logarithmic amplitude compression. This improvement is obtained in conjunction with improved spatial resolution.

Computer Simulation↗
PubMed1992

On the feasibility of pulse-echo speed of sound estimation in small regions: simulation studies.

Computer simulations are used to study the feasibility of the estimation of sound speed in small regions with precision better than 1% using the Beam Tracking method. The speed of sound is estimated in a 10-mm by 10-mm region by considering a number of parallel tracks confined to the small region. The transducer focusing and the step sizes for the tracking and tracked transducers required to extract the maximum amount of uncorrelated data from the 10-mm by 10-mm region is evaluated. The results show that the speed of sound can be estimated with error less than 1% in a small region using a typical medical transducer. The statistical comparison of estimates in small areas with different speed of sound is also considered.

Acoustics↗
PubMed1992

Correction of diffraction errors in attenuation estimation with Dynamic Beam Translation.

We describe computer simulations for the estimation of the attenuation coefficient in scattering media using the Dynamic Beam Translation (DBT) method. DBT refocuses a variable aperture transducer to maintain the focal characteristics of the transducer at different depths in order to reduce diffraction errors. The efficacy of DBT is evaluated for transducers with different focal powers and under the influence of phase aberration due to the body wall. DBT shows robustness in the elimination of diffraction errors and resistance to the effects of aberration.

Computer Simulation↗
PubMed1991

Elastography: a quantitative method for imaging the elasticity of biological tissues.

We describe a new method for quantitative imaging of strain and elastic modulus distributions in soft tissues. The method is based on external tissue compression, with subsequent computation of the strain profile along the transducer axis, which is derived from cross-correlation analysis of pre- and post-compression A-line pairs. The strain profile can then be converted to an elastic modulus profile by measuring the stresses applied by the compressing device and applying certain corrections for the nonuniform stress field. We report initial results of several phantom and excised animal tissue experiments which demonstrate the ability of this technique to quantitatively image strain and elastic modulus distributions with good resolution, sensitivity and with diminished speckle. We discuss several potential clinical uses of this technique.

Abdomen↗
PubMed1990

Diffraction correction methods for pulse-echo acoustic attenuation estimation.

We describe computer simulations and water tank experiments for the estimation of the attenuation coefficient in scattering media. The efficacies of the Axial Beam Translation (ABT) and Inverse Diffraction Filtering (IDF) methods in reducing diffraction errors in such estimates are compared throughout the radiation field of a plane transducer. The effect of phase aberration due to the body wall and the effect of the scattering properties of the media are considered. Consistent improvement in the estimation due to ABT is demonstrated as well as unreliable improvement due to IDF which is sensitive to the presence of phase aberration and to changes in scattering.

Computer Simulation↗