Search PubMed⌕ Search

Biomedical subjects

J Ophir

Publications and source records attributed to J Ophir.

At least 55 records · Page 3Linked to original sources
PubMed1997

Reduction of signal decorrelation from mechanical compression of tissues by temporal stretching: applications to elastography.

Elastography is based on the estimation of strain due to tissue compression. Strain is computed from the estimates of time delays between gated precompression and postcompression echo signals. Time delay estimates are obtained from the location of the peak of the crosscorrelation function between gated precompression and postcompression signals. It is of paramount importance to accurately estimate the time delays for good quality elastograms. A main source of time delay estimation (TDE) error in elasticity imaging is the decorrelation of the echo signal as a result of tissue compression (decorrelation noise). The effect of decorrelation on the mean of the crosscorrelation function and the correlation coefficient has been investigated. The expected value of the cross-correlation function between the precompression and postcompression signals was shown to be a filtered version of the autocorrelation function of the precompression signal. In this article, the effect of temporal stretching of the postcompression echo signal on the cross-correlation function will be investigated along the same line. The applied compression is assumed to be uniform; the decorrelations introduced by the lateral and elevational tissue movements are ignored. The theory predicts that if the postcompression echo signals are stretched before the TDE step, then for small strains, the cross-correlation function very closely resembles the autocorrelation function. For larger strains, correlation is improved if temporal stretching is applied. The theory is corroborated by results from simulation and homogeneous phantom experiments. Thus, the decorrelation noise in elastograms can be reduced by temporal stretching of the postcompression signal.

Elasticity↗
PubMed1997

On the use of envelope and RF signal decorrelation as tissue strain estimators.

Bamber and Bush (1995) used the correlation coefficient for freehand elasticity imaging. Varghese and Ophir (1996) found it to be a biased estimator of strain with a large variability. In this study, we systematically investigate the effect of changes in various system and processing parameters on the performance of the correlation coefficient strain estimator, and demonstrate, using simulated data, that noise and frequency-dependent attenuation can introduce variable bias in this estimator.

Computer Simulation↗
PubMed1997

The nonstationary strain filter in elastography: Part II. Lateral and elevational decorrelation.

The nonstationary evolution of the strain filter due to lateral and elevational motion of the tissue scatterers across the ultrasound beam is analyzed for the 1-D cross-correlation-based strain estimator. The effective correlation coefficient that includes the contributions due to lateral and elevational signal decorrelation is used to derate the upper bound of the signal-to-noise ratio in the elastogram (SNRe) predicted by the ideal strain filter. In the case of an elastically homogeneous target, if the transducer is on the axis of symmetry of such target in the elevational direction, the motion of the scatterers out the imaging plane is minimized. In addition, the ultrasound beam along the elevational direction is broader, allowing scatterers to stay longer within the beam during tissue compression. Under these conditions, lateral signal decorrelation becomes the primary contributor to the nonstationary behavior of the strain filter. Both the elastographic SNRe and the dynamic range are reduced, with an increase in lateral decorrelation. Finite element simulations and phantom experiments are presented in this paper to corroborate the theoretical strain filter. The nonstationary behavior of the strain filter is reduced by confining the tissue in the lateral direction (minimizing motion of tissue scatterers), thereby improving the quality of the elastogram.

Computer Simulation↗
PubMed1997

The nonstationary strain filter in elastography: Part I. Frequency dependent attenuation.

The accuracy and precision of the strain estimates in elastography depend on a myriad number of factors. A clear understanding of the various factors (noise sources) that plague strain estimation is essential to obtain quality elastograms. The nonstationary variation in the performance of the strain filter due to frequency-dependent attenuation and lateral and elevational signal decorrelation are analyzed in this and the companion paper for the cross-correlation-based strain estimator. In this paper, we focus on the role of frequency-dependent attenuation in the performance of the strain estimator. The reduction in the signal-to-noise ratio (SNRs) in the RF signal, and the center frequency and bandwidth downshift with frequency-dependent attenuation are incorporated into the strain filter formulation. Both linear and nonlinear frequency dependence of attenuation are theoretically analyzed. Monte-Carlo simulations are used to corroborate the theoretically predicted results. Experimental results illustrate the deterioration in the precision of the strain estimates with depth in a uniformly elastic phantom. Theoretical, simulation and experimental results indicate the importance of high SNRs values in the RF signals, because the strain estimation sensitivity, elastographic SNRe and dynamic range deteriorate rapidly with a decrease in the SNRs. In addition, a shift in the strain filter toward higher strains is observed at large depths in tissue due to the center frequency downshift.

Computer Simulation↗
PubMed1997

Elastography of breast lesions: initial clinical results.

PURPOSE: To determine the appearance of various breast lesions on elastograms and to explore the potential of elastography in the diagnosis of breast lesions. MATERIALS AND METHODS: A total of 46 breast lesions were examined with elastography. Patients underwent biopsy or aspiration of all lesions, revealing 15 fibroadenomas, 12 carcinomas, six fibrocystic nodules, and 13 other lesions. The elastogram was generated from radio-frequency data collected with use of a 5-MHz linear-array transducer. The elastogram and corresponding sonogram were evaluated by a single observer for lesion visualization, relative brightness, and margin definition and regularity. The sizes of the lesions at each imaging examination and at biopsy were recorded and compared. RESULTS: Softer tissues such as fat appear as bright areas on elastograms. Firm tissues, including parenchyma, cancers, and other masses, appear darker. The cancers were statistically significantly darker than fibroadenomas (P < .005) and substantially larger on the elastogram than on the sonogram. Seventy-three percent of fibroadenomas and 56% of solid benign lesions could be distinguished from cancers by using lesion brightness and size difference. Some cancers that appeared as areas of shadowing on sonograms appeared as discrete masses on elastograms. CONCLUSION: Elastography has the potential to be useful in the evaluation of areas of shadowing on the sonogram. It also may be helpful in the distinction of benign from malignant masses.

Breast Neoplasms↗
PubMed1997

Elastographic dynamic range expansion using variable applied strains.

In elastography, we want to image the entire range of stiffnesses of the elastic components found in inhomogeneous tissues. In order to achieve this, the elastographic dynamic range should equal the entire stiffness dynamic range in the target. Various sources of noise limit the dynamic range of elastography. The recently-defined strain filter concept offers an analytical and graphical way of observing these limitations. In this paper, we describe a method that achieves the expansion of the elastographic dynamic range. It involves the application of variable strains in combination with selective storage of strain data that have optimal elastographic signal-to-noise ratios. This expands the current dynamic range of elastography by orders of magnitude when compared to single compression elastography. The process is explained theoretically using the strain filter framework, and 1 D as well as 2D tissue simulations are used to corroborate the theory.

Algorithms↗
PubMed1997

A least-squares strain estimator for elastography.

A least-squares strain estimator (LSQSE) for elastography is proposed. It is shown that with such an estimator, the signal-to-noise ratio in an elastogram (SNRe) is significantly improved. This improvement is illustrated theoretically using a modified strain filter and experimentally using a homogeneous gel phantom. It is demonstrated that the LSQSE results in an increase of the elastographic sensitivity (smallest, strain that could be detected), thereby increasing the strain dynamic range. Using simulated data, it is shown that a tradeoff exists between the improvement in SNRe and the reduction of strain contrast and spatial resolution.

Connective Tissue↗
PubMed1996

Fundamental limitations on the contrast-transfer efficiency in elastography: an analytic study.

Elastography is a new ultrasonic imaging technique introduced to produce images of the Young's modulus distribution of compliant tissue. This Young's modulus distribution is derived from the ultrasonically estimated longitudinal internal strains induced by an external compression of the tissue. The displayed two-dimensional images are called elastograms. Recently, contrast-transfer efficiency, defined as the ratio of elasticity contrast as measured from elastogram to the true contrast, was used to illustrate by simulation the fundamental limitation of elastography in displaying the elastic modulus contrast of soft inclusion in a hard background and vice versa. In this paper, using a classical analytic solution of the elasticity equations derived for an infinite medium subjected to a uniaxial compression, we confirm such earlier simulations results. For this purpose we derive an analytic expression predicting the observed contrast in elastograms.

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↗
PubMed1996

Estimating tissue strain from signal decorrelation using the correlation coefficient.

A simple relationship between the correlation coefficient and the applied strain, applicable only at low strains, is presented in this article. This relationship is derived for a Gaussian modulated cosine point spread function. The performance of the strain estimator is analyzed using a theoretical expression for the correlation coefficient along with simulation and experimental results. Both the theoretical and simulation results diverge from the ideal relationship between the strain and the correlation coefficient as the applied strain is increased. Simulation results illustrate that the strain estimate obtained using the correlation coefficient is a biased estimate with a large variability. Experimental results, however, illustrate that strain estimation using the 1-D correlation coefficient estimate is applicable only at high signal-to-noise ratios in the radiofrequency signal and in the absence of lateral and elevational signal decorrelation.

Computer Simulation↗
PubMed1996

Performance optimization in elastography: multicompression with temporal stretching.

A general theoretical framework known as the strain filter has been previously used to evaluate the performance in elastography. The strain filter describes the relationship among the resolution, dynamic range, sensitivity and elastographic SNR (SNRe), and may be plotted as a graph of the upper bound of the SNRe vs. the strain experienced by the tissue, for a desired elastographic axial resolution as determined by the data window length. The ideal strain filter has an infinitely high, flat all-pass characteristic shape in the strain domain, which means that all local tissue strains are displayed in the elastogram with infinite SNRe; it also means that the strain dynamic range in the elastogram is infinite as well. Practical strain filters obtained using a single tissue compression have a bandpass characteristic shape in the strain domain, where the -3 dB width of this bandpass characteristic may be defined as the elastographic dynamic range. In this paper, we present an optimal technique for stretching multicompression elastography, practiced by selecting the optimum incremental applied strain using the strain filter. Two techniques, temporal stretching and multicompression elastography, are combined in this paper to improve elastogram quality. Stretching multicompression elastography using the optimal applied strain increment alters the shape of the strain filter from its bandpass characteristic to a more desirable high-emphasis filter. The dynamic range of optimal stretching multicompress on elastography is limited only by tissue nonlinearities. This optimal applied strain increment minimizes signal decorrelation and achieves the maximum achievable elastographic SNRe.

Elastic Tissue↗
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

Effect of topical interferon-beta on recurrence rates in genital herpes: a double-blind, placebo-controlled, randomized study.

The aim of this randomized, double-blind placebo-controlled trial was to evaluate the effect of IFN-beta cream applied at the time of recurrent eruptions of genital herpes during 6 months on the overall rate of recurrence. Therapy was initiated at the clinic for the first treated recurrence, and thereafter by the patient for early treatment of eventual subsequent eruptions. Each recurrence was ascertained at the clinic in all 35 evaluable patients. The mean recurrence rate was significantly lower in the group using IFN-beta cream than in the placebo group (p = 0.03). Complete responders without recurrence for the duration of the trial were 36.4% of all patients and 46% among women versus 15.4 and 16.6% in the placebo groups, respectively. A total of 77.3% of all patients were defined as complete or partial responders, their average recurrences/year decreasing from 11 to 2.2 (p < 0.0001). The topical episodic IFN-beta treatment was well tolerated by patients and without side effects. It is concluded that IFN-beta cream application reduces the overall rate of recurrence of genital herpes.

Administration, Topical↗
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↗
PubMed1994

Ultrasonic imaging of the stress distribution in elastic media due to an external compressor.

We describe an experimental ultrasonic method capable of imaging the two-dimensional distribution of longitudinal stress in an elastic, tissue-like material due to an external compressor of arbitrary size or shape and boundary conditions. The method involves the use of a compressor and an opposing ultrasonic transducer. Local strains are derived from the ultrasonic backscatter signals before and after compression using cross correlation analysis. The strain distribution is converted to a stress map by assuming a linear stress-strain relationship. The technique is useful for quantifying the corrections that must be made to images of the elastic modulus of tissue (elastograms) due to the effects of compressor size and shape, depth and boundary conditions. It is also useful for experimental modeling of stress distributions in elastic media.

Artifacts↗
PubMed1994

Lesion detection in simulated elastographic and echographic images: a psychophysical study.

The image quality of two ultrasonic imaging techniques was studied: conventional echography and the recently introduced elastography. The image quality was assessed by estimating the detectability of disc-shaped lesions of various sizes and contrast levels. The study was designed to verify the hypothesis that elastograms could show lesions at a higher subjective and objective level of detectability than echograms of the same object contrast. This hypothesis was adopted because homogeneous elastograms can present a higher point signal-to-noise ratio than uniform echograms. Both elastograms and echograms were generated by two-dimensional (2D) simulations. The subjective assessment was performed by psychophysical experiments using the staircase up-down method. The threshold contrast of detection for both modalities was determined at different diameters of the disc-shaped lesion. These values were used to construct the contrast-detail curves for both techniques. For identical object contrasts, elastography was found to have significantly higher detectability at all lesion diameters considered. The contrast thresholds were also used for an objective evaluation with the lesion signal-to-noise ratio. The objective measure evaluated at the subjective threshold of detection for both modalities was not found to be identical, nor constant over the range of lesion diameters as expected.

Computer Simulation↗