Biomedical subjects

Tomy Varghese

Publications and source records attributed to Tomy Varghese.

At least 19 recordsLinked to original sources

PubMed2007

Improvement of elastographic displacement estimation using a two-step cross-correlation method.

The cross-correlation algorithm used to compute the local strain components for elastographic imaging requires a minimum radio-frequency data segment length of around 10 wavelengths to obtain accurate and precise strain estimates with a reasonable signal-to-noise ratio. Shorter radio-frequency data segments generally introduce increased estimation errors as the information content in the data segment reduces. However, shorter data segments and increased overlaps are essential to improve the axial resolution in the strain image. In this paper, we propose a two-step cross-correlation technique that enables the use of window lengths on the order of a single wavelength to provide displacement and strain estimates with similar noise properties as those obtained with a 10 wavelength window. The first processing step utilizes a window length on the order of 10 wavelengths to obtain coarse displacement estimates between the pre- and post-compression radio frequency data frames. This coarse displacement is then interpolated and utilized as the initial guess-estimate for the second cross-correlation processing step using the smaller window. This step utilizes a single wavelength window to improve the axial resolution in strain estimation, without significantly compromising the noise properties of the image. Simulation and experimental results show that the signal-to-noise and contrast-to-noise ratio estimates improve significantly at the smaller window lengths with the two-step processing when compared with the use of a similar sized window in the currently utilized single window method.

Elasticity↗

PubMed2006

Two-dimensional multi-level strain estimation for discontinuous tissue.

A large number of the strain estimation methods presented in the literature are based on the assumption of tissue continuity that establishes a continuous displacement field. However, in certain locations in the body such as the arteries in vivo scanning may produce displacement fields that are discontinuous between the two walls of the artery. Many of the displacement or strain estimators fail when the displacement fields are discontinuous. In this paper, we present a new 2D multi-level motion or displacement tracking method for accurate estimation of the strain in these situations. The final high-resolution displacement estimate is obtained using two processing steps. The first step involves an estimation of a coarse displacement estimate utilizing B-mode or envelope signals. To reduce computational time, the coarse displacement estimates are obtained starting from down-sampled B-mode pre- and post-compression image pairs using a pyramidal processing approach. The coarse displacement estimate obtained from the B-mode data is used to guide the final 2D cross-correlation computations on radio-frequency (RF) data. Results from finite element simulations and in vivo experimental data demonstrate the feasibility of this approach for imaging tissue with discontinuous displacement fields.

Algorithms↗

PubMed2006

Contrast-transfer improvement for electrode displacement elastography.

Electrode displacement elastography is a strain imaging method that can be used for in-vivo imaging of radiofrequency ablation-induced lesions in abdominal organs such as the liver and kidney. In this technique, tissue motion or deformation is introduced by displacing the same electrode used to create the lesion. Minute displacements (on the order of a fraction of a millimetre) are applied to the thermal lesion through the electrode, resulting in localized tissue deformation. Ultrasound echo signals acquired before and after the electrode-induced displacements are then utilized to generate strain images. However, these local strains depend on the modulus distribution of the tissue region being imaged. Therefore, a quantitative evaluation of the conversion efficiency from modulus contrast to strain contrast in electrode-displacement elastograms is warranted. The contrast-transfer efficiency is defined as the ratio (in dB) of the observed elastographic strain contrast and the underlying true modulus contrast. It represents a measure of the efficiency with which elastograms depict the underlying modulus distribution in tissue. In this paper, we develop a contrast-transfer efficiency formalism for electrode displacement elastography (referred to as contrast-transfer improvement). Changes in the contrast-transfer improvement as a function of the underlying true modulus contrast and the depth of the inclusion in the simulated phantom are studied. We present finite element analyses obtained using a two-dimensional mechanical deformation and tissue motion model. The results obtained using finite element analyses are corroborated using experimental analysis and an ultrasound simulation program so as to incorporate noise artifacts.

Algorithms↗

PubMed2006

Frequency-dependent complex modulus of the uterus: preliminary results.

The frequency-dependent complex moduli of human uterine tissue have been characterized. Quantification of the modulus is required for developing uterine ultrasound elastography as a viable imaging modality for diagnosing and monitoring causes for abnormal uterine bleeding and enlargement, as well assessing the integrity of uterine and cervical tissue. The complex modulus was measured in samples from hysterectomies of 24 patients ranging in age from 31 to 79 years. Measurements were done under small compressions of either 1 or 2%, at low pre-compression values (either 1 or 2%), and over a frequency range of 0.1-100 Hz. Modulus values of cervical tissue monotonically increased from approximately 30-90 kPa over the frequency range. Normal uterine tissue possessed modulus values over the same range, while leiomyomas, or uterine fibroids, exhibited values ranging from approximately 60-220 kPa.

Adult↗

PubMed2006

Stability of heterogeneous elastography phantoms made from oil dispersions in aqueous gels.

A set of five tissue-mimicking phantoms with cylindrical inclusions were produced for assessing long-term stability of geometry and elastic properties and assessing accuracy of determination of elastic properties. The base aqueous materials were either gelatin or a mixture of agar and gelatin. Stiffness was controlled by selection of the volume percent consisting of microscopic safflower oil droplets. Cylinder diameters remained unchanged within 1% or 2% over many months. Strain ratios from elastograms of the phantoms were stable over many months, implying that elastic contrasts were also stable. Test samples, called production samples, for measurement of Young's moduli were made at the time of manufacture of each phantom and were stored separately from one another. Each production sample was homogeneous and consisted of either inclusion material or background material. For all five phantoms, it was found that the elastic contrast computed using Young's modulus values determined using the production samples accurately represented the true elastic contrasts in the corresponding phantom. This finding was established by the fact that the (true) elastic contrasts determined using samples excised from the phantoms themselves agreed with the elastic contrasts obtained using the homogeneous production samples.

Agar↗

PubMed2006

Segmentation of elastographic images using a coarse-to-fine active contour model.

Delineation of radiofrequency-ablation-induced coagulation (thermal lesion) boundaries is an important clinical problem that is not well addressed by conventional imaging modalities. Elastography, which produces images of the local strain after small, externally applied compressions, can be used for visualization of thermal coagulations. This paper presents an automated segmentation approach for thermal coagulations on 3-D elastographic data to obtain both area and volume information rapidly. The approach consists of a coarse-to-fine method for active contour initialization and a gradient vector flow, active contour model for deformable contour optimization with the help of prior knowledge of the geometry of general thermal coagulations. The performance of the algorithm has been shown to be comparable to manual delineation of coagulations on elastograms by medical physicists (r = 0.99 for volumes of 36 radiofrequency-induced coagulations). Furthermore, the automatic algorithm applied to elastograms yielded results that agreed with manual delineation of coagulations on pathology images (r = 0.96 for the same 36 lesions). This algorithm has also been successfully applied on in vivo elastograms.

Algorithms↗

PubMed2006

Anthropomorphic breast phantoms for testing elastography systems.

Two equivalent anthropomorphic breast phantoms were constructed, one for use in ultrasound elastography and the other in magnetic resonance (MR) elastography. A complete description of the manufacturing methods is provided. The materials used were oil-in-gelatin dispersions, where the volume percent oil differentiates the materials, primarily according to Young's moduli. Values of Young's moduli are in agreement with in vitro ranges for the corresponding normal and abnormal breast tissues. Ultrasound and nuclear magnetic resonance (NMR) properties are reasonably well represented. Phantoms of the type described promise to aid researchers who are developing hardware and software for elastography. Examples of ultrasound and MR elastograms of the phantoms are included to demonstrate the utility of the phantoms. Also, the level of stability of elastic properties of the component materials is quantified over a 15-month period. Such phantoms can serve as performance-assessing intermediaries between simple phantoms (consisting, for example, of homogeneous cylindrical inclusions in a homogeneous background) and a full-scale clinical trial. Thus, premature clinical trials may be avoided.

Anthropometry↗

PubMed2006

Elastographic measurements of in-vivo radiofrequency ablation lesions of the kidney.

BACKGROUND AND PURPOSE: Elastography may prove useful for monitoring radiofrequency ablative (RFA) therapy because heat-ablated tissues are more elastic than untreated tissues. Herein, we report our initial evaluations of the reliability of elastography for delineating thermal-lesion boundaries at the time of RFA of porcine kidneys. MATERIALS AND METHODS: In-vivo RFA was performed on 20 kidneys from 10 40-kg female pigs. Elastography was performed at the time of surgery and after 48 hours. The imaging plane was perpendicular to the axis of the RF electrode so that the ablated region was around the center of the plane. Measurements of the sections representing the same image plane used for elastography were taken at pathologic examination and compared with the measurements obtained from the elastograms. RESULTS: We found a statistically significant correlation between elastography and pathology measurements with respect to the area and volume estimates (r = 0.9302 and r = 0.953, respectively). Overall, elastography slightly underestimated the lesion size, as judged by the digitalized pathologic images, a finding consistent with previous reports. CONCLUSION: We found a correlation between the area and volume estimates of thermal lesions that were based on elastographic images and the measurements from gross pathologic dimensions. A significant limitation of renal RFA is the inaccuracy of current imaging modalities to provide real-time monitoring, and elastography may prove to be reliable for delineating the resulting thermal lesions.

Animals↗

PubMed2006

The Ultrasonix 500RP: a commercial ultrasound research interface.

Unlike researchers in magnetic resonance imaging who have considerable access to high level tools and to data at a very basic level on their scanners, those involved with ultrasound have found little in the way of meaningful and widespread access to even the most basic echo signals in their clinical systems. Interest has emerged, however, in ultrasound research interfaces on commercial scanners to provide access to raw ultrasound data and control of basic research functions. This paper describes initial experience gained on one such ultrasound system. The Ultrasonix 500RP system provides research access to the data at multiple points in the signal processing chain and allows control over most imaging parameters. The Ultrasonix system allows for three methods of research control. One is implemented along with the standard clinical imaging software using "mouseover" screens on the periphery of the application window. These screens are configured by the user to display various signal processing variables, which can be modified in real time. Second, the system can be controlled via a user-written remote control client application interacting through the clinical exam software. Lastly, the user can write a complete application which initializes the basic ultrasound module but need not use the Ultrasonix clinical exam software. All of the modes can be done locally on the scanner itself or via a network, and are based on software developed in C++ with libraries supplied with the scanner. Two examples are presented in this paper from the evaluation of the system in "real world" applications. Measurements of absolute backscatter coefficients and attenuation coefficients versus frequency are shown and elastograms utilizing spatial compounding are described.

Biomedical Research↗

PubMed2006

Spatial-angular compounding for elastography using beam steering on linear array transducers.

Spatial-angular compounding is a new technique that enables the reduction of noise artifacts in ultrasound elastography. Under this method, compounded elastograms are obtained from a spatially weighted average of local strain estimated from radio frequency (rf) echo signals acquired at different insonification angles. In previous work, the acquisition of the rf signals was performed through the lateral translation of a phased-array transducer. Clinical applications of angular compounding would, however, require the utilization of beam steering on linear-array transducers to obtain angular data sets, which is more efficient than translating phased-array transducers. In this article, we investigate the performance of angular compounding for elastography by using beam steering on a linear-array transducer. Quantitative experimental results demonstrate that spatial angular compounding provides significant improvement in both the elastographic signal-to-noise ratio and the contrast-to-noise ratio. For the linear array transducer used in this study, the optimum angular increment is around 1.5 degrees-3.75 degrees, and the maximum angle that can be used in angular compounding should not exceed 10 degrees.

Artifacts↗

PubMed2006

Correlation analysis of the beam angle dependence for elastography.

Signal decorrelation is a major source of error in the displacements estimated using correlation techniques for elastographic imaging. Previous papers have addressed the variation in the correlation coefficient as a function of the applied compression for a finite window size and an insonification angle of zero degrees. The recent use of angular beam-steered radio-frequency echo signals for spatial angular compounding and shear strain estimation have demonstrated the need for understanding signal decorrelation artifacts for data acquired at different beam angles. In this paper, we provide both numerical and closed form theoretical solutions of the correlation between pre- and post-compression radio-frequency echo signals acquired at a specified beam angle. The expression for the correlation coefficient obtained is a function of the beam angle and the applied compression for a finite duration window. Accuracy of the theoretical results is verified using tissue-mimicking phantom experiments on a uniformly elastic phantom using beam-steered data acquisitions on a linear array transducer. The theory predicts a faster decorrelation with changes in the beam or insonification angle for longer radio-frequency echo signal segments and at deeper locations in the medium. Theoretical results provide useful information for improving angular compounding and shear strain estimation techniques for elastography.

Elasticity↗

PubMed2006

Evaluation of acoustic wave propagation velocities in the ocular lens and vitreous tissues of pigs, dogs, and rabbits.

OBJECTIVE: To evaluate propagation velocity of acoustic waves through the lens and vitreous body of pigs, dogs, and rabbits and determine whether there were associations between acoustic wave speed and age, temperature, and time after enucleation. SAMPLE POPULATION: 9 pig, 40 dog, and 20 rabbit lenses and 16 pig, 17 dog, and 23 rabbit vitreous bodies. PROCEDURE: Acoustic wave velocities through the ocular structures were measured by use of the substitution technique. RESULTS: Mean sound wave velocities in lenses of pigs, dogs, and rabbits were 1,681, 1,707, and 1,731 m/s, respectively, at 36 degrees C. Mean sound wave velocities in the vitreous body of pigs, dogs, and rabbits were 1,535, 1,535, and 1,534 m/s, respectively, at 38 degrees C. The sound wave speed through the vitreous humor, but not the lens, increased linearly with temperature. An association between wave speed and age was observed in the rabbit tissues. Time after enucleation did not affect the velocity of sound in the lens or vitreous body. The sound wave speed conversion factors for lenses, calculated with respect to human ocular tissue at 36 degrees C, were 1.024, 1.040, and 1.055 for pig, dog, and rabbit lenses, respectively. CONCLUSIONS AND CLINICAL RELEVANCE: Conversion factors for the speed of sound through lens tissues are needed to avoid underestimation of the thickness of the lens and axial length of the eye in dogs during comparative A-mode ultrasound examinations. These findings are important for accurate calculation of intraocular lens power required to achieve emmetropia in veterinary patients after surgical lens extraction.

Aging↗

PubMed2006

Initial clinical experience imaging scatterer size and strain in thyroid nodules.

OBJECTIVE: This article describes a new research ultrasound scanner that can be programmed to produce elastograms and backscatter parametric images in real time. Its performance was evaluated in a clinical setting. METHODS: Radio frequency data were acquired from 13 patients with thyroid nodules and from 4 normal thyroids, along with reference phantom data. Scatterer size was deduced by measuring the backscatter versus frequency and fitting data to a model. Strain was obtained by a cross-correlation method, comparing precompression and postcompression radio frequency signals. Scatterer size contrast was defined as the observed contrast between the "normal" and "abnormal" tissue in the same gland or, when considering diffuse conditions, by comparing with normal values. Strain contrast was estimated if abnormal and normal tissue was captured in the same palpation, that is, excluding diffuse disease, which was the case for 9 subjects. RESULTS: On scatterer size images, 4 nodules exhibited positive contrast versus the adjacent normal parenchyma, indicating larger scatterers. Five nodules were isoechoic, and 4 had negative contrast. Four nodules exhibited positive strain contrast, indicating that they were softer than the normal parenchyma. Two nodules had the same brightness, and 3 were darker than the background thyroid tissue on strain images. CONCLUSIONS: Contrast was observed between nodules and thyroid parenchymal tissue for both types of parametric images. Further work is needed to determine whether the diagnostic importance of these parameters in characterizing thyroid nodules might be worthwhile. Both modes must be of a sufficient frame rate to provide real-time feedback to operators, which will require further work.

Adult↗

PubMed2005

Spherical lesion phantoms for testing the performance of elastography systems.

A set of three cubic one-litre phantoms containing spherical simulated lesions was produced for use in comparing lesion detection performance of different elastography systems. The materials employed are known to be stable in heterogeneous configurations regarding geometry and elastic contrast identical with (storage modulus of lesion material) / (storage modulus of background material), and regarding ultrasound and NMR properties. The materials mimic soft tissues in terms of elastic, ultrasound and NMR properties. Each phantom has only one value of elastic contrast (3.3, 4.6 or 5.5) and contains arrays of 1.6 mm, 2 mm, 3 mm and 4 mm diameter spherical simulated lesions. All the spheres of a given diameter are arranged in a regular array with coplanar centres. Elastograms of an array made with ultrasound allow determination of the depth range over which lesions of that diameter and elastic contrast can be detected. Two phantoms are made from agar-plus-gelatin-based materials, and one is made from oil-in-gelatin dispersions. The methods for producing the phantoms are described in detail. Lesion detection performances for two ultrasound systems, both operating at about 7.5 MHz and focused at about 5 cm, were quantified with distinctions between the two systems demonstrated. Neither system was capable of detecting any of the 1.6 mm lesions. Phantoms such as these should be useful in research labs that are refining hardware and/or software for elastography.

Agar↗

PubMed2005

Tissue-mimicking agar/gelatin materials for use in heterogeneous elastography phantoms.

Five 9 cm x 9 cm x 9 cm phantoms, each with a 2-cm-diameter cylindrical inclusion, were produced with various dry-weight concentrations of agar and gelatin. Elastic contrasts ranged from 1.5 to 4.6, and values of the storage modulus (real part of the complex Young's modulus) were all in the soft tissue range. Additives assured immunity from bacterial invasion and can produce tissue-mimicking ultrasound and NMR properties. Monitoring of strain ratios over a 7 to 10 month period indicated that the mechanical properties of the phantoms were stable, allowing about 1 month for the phantom to reach chemical equilibrium. The only dependable method for determining the storage moduli of the inclusions is to make measurements on samples excised from the phantoms. If it is desired to produce and accurately characterize a phantom with small inclusions with other shapes, such as an array of small spheres, an auxiliary phantom with the geometry of the cylindrical inclusion phantoms or the equivalent should be made at the same time using the same materials. The elastic contrast can then be determined using samples excised from the auxiliary phantom. A small increase of about 10% in volume of the cylindrical inclusions occurred-a tolerable increase. Interestingly, the smallest increase (about 5%) occurred in the phantom with the largest elastic contrast.

Adhesives↗

PubMed2005

Monitoring stiffness changes in lesions after radiofrequency ablation at different temperatures and durations of ablation.

The variations in the stiffness or stiffness contrast of lesions resulting from radiofrequency (RF) ablation of canine liver tissue at different temperatures and for different ablation durations at a specified temperature are analyzed. Tissue stiffness, in general, increases with temperature; however, an anomaly exists around 80 degrees C, where the stiffness of the lesion is lower than that of the lesion ablated at 70 degrees C. On the other hand, the stiffness increases monotonically with the duration of ablation. Plots illustrating the ratio of mean strains in normal canine liver tissue to mean strains in ablated thermal lesions demonstrate the variation in the stiffness contrast of the thermal lesions. The contrast-to-noise ratio (CNRe) of the lesions, which serves as an indicator of the detectability of the lesions under the different experimental imaging conditions described above, is also presented. The results presented in this paper show that the elastographic depiction of stiffer thermal lesions is better, in terms of the CNRe parameter. An important criterion in the elastographic depiction of RF-ablated regions of tissue is the trade-off between ablation temperature and duration of ablation. Tissue necrosis can occur either by ablating tissue to high temperatures for short durations or to lower temperatures for longer durations. In this paper, we attempt to characterize the elastographic depiction of thermal lesions under these different experimental conditions. This paper provides results that may be utilized by practitioners of RF ablation to decide the ablation temperature and duration, on the basis of the strain images of normal liver tissue and ablated thermal lesions discussed in this paper.

Animals↗

PubMed2005

Improvements in elastographic contrast-to-noise ratio using spatial-angular compounding.

Spatial-angular compounding is a new technique developed for improving the signal-to-noise ratio (SNR) in elastography. Under this method, elastograms of a region-of-interest (ROI) are obtained from a spatially weighted average of local strain estimated along different insonification angles. In this article, we investigate the improvements in the strain contrast and contrast-to-noise ratio (CNR) of the spatially compounded elastograms. Spatial angular compounding is also applied and evaluated in conjunction with global temporal stretching. Quantitative experimental results obtained using a single-inclusion tissue-mimicking phantom demonstrate that the strain contrast reduces slightly but the CNR improves by around 8 to 13 dB. We also present experimental spatial angular compounding results obtained from an in vitro thermal lesion in canine liver tissue embedded in a gelatin phantom that demonstrate the improved visual characteristics (due to the improved CNR) of the compound elastogram. The experimental results provide guidelines for the practical range of maximum insonification angles and estimates of the optimum angular increment.

Animals↗

PubMed2005

A general solution for catheter position effects for strain estimation in intravascular elastography.

Intravascular ultrasound (US) elastography reveals the elastic properties of vascular tissue and plaque. However, misalignment of the US catheter in the vessel lumen can cause incorrect strain estimation in intravascular US elastography caused by strain projection artifacts. In this paper, we present a general theoretical solution where the impact of catheter eccentricity, tilt and noncoplanar errors on the strain estimates are derived. Appropriate corrections to strain estimates can then be applied with prior knowledge of the catheter position information to reduce the strain projection artifacts. Simulations using a frequency-domain-based algorithm that models intravascular US imaging before and after a specified deformation are presented. The simulations are used to verify the theoretical derivations for two displacement situations (linear and nonlinear) under intraluminal pressure, with and without stress decay. The linear displacement case demonstrates that the correction factor is dependent only on the angle between the US beam and the cross-sectional plane of the vessel. For the nonlinear displacement case, where a l/r stress decay in the displacement is modeled, the correction factor becomes a more complicated function of the azimuthal angle.

Artifacts↗