Fernando Zvietcovich

Crawling wave optical coherence elastography

Elastography is a technique that measures and maps the local elastic property of biological tissues. Aiming for detection of micron-scale inclusions, various optical elastography, especially optical coherence elastography (OCE), techniques have been investigated over the past decade. The challenges of current optical elastography methods include the decrease in elastographic resolution as compared with its parent imaging resolution, the detection sensitivity and accuracy, and the cost of the overall system. Here we report for the first time, we believe, on an elastography technique-crawling wave optical coherence elastography (CRW-OCE)-which significantly lowers the requirements on the imaging speed and opens the path to high-resolution and high-sensitivity OCE at relatively low cost. Methods of crawling wave excitation, data acquisition, and crawling wave tracking are presented.

Reverberant shear wave fields and estimation of tissue properties

The determination of shear wave speed is an important subject in the field of elastography, since elevated shear wave speeds can be directly linked to increased stiffness of tissues. MRI and ultrasound scanners are frequently used to detect shear waves and a variety of estimators are applied to calculate the underlying shear wave speed. The estimators can be relatively simple if plane wave behavior is assumed with a known direction of propagation. However, multiple reflections from organ boundaries and internal inhomogeneities and mode conversions can create a complicated field in time and space. Thus, we explore the mathematics of multiple component shear wave fields and derive the basic properties, from which efficient estimators can be obtained. We approach this problem from the historic perspective of reverberant fields, a conceptual framework used in architectural acoustics and related fields. The framework can be recast for the alternative case of shear waves in a bounded elastic media, and the expected value of displacement patterns in shear reverberant fields are derived, along with some practical estimators of shear wave speed. These are applied to finite element models and phantoms to illustrate the characteristics of reverberant fields and provide preliminary confirmation of the overall framework.

Comparative study of shear wave-based elastography techniques in optical coherence tomography

Abstract. We compare five optical coherence elastography techniques able to estimate the shear speed of waves generated by one and two sources of excitation. The first two techniques make use of one piezoelectric actuator in order to produce a continuous shear wave propagation or a tone-burst propagation (TBP) of 400 Hz over a gelatin tissue-mimicking phantom. The remaining techniques utilize a second actuator located on the opposite side of the region of interest in order to create three types of interference patterns: crawling waves, swept crawling waves, and standing waves, depending on the selection of the frequency difference between the two actuators. We evaluated accuracy, contrast to noise ratio, resolution, and acquisition time for each technique during experiments. Numerical simulations were also performed in order to support the experimental findings. Results suggest that in the presence of strong internal reflections, single source methods are more accurate and less variable when compared to the two-actuator methods. In particular, TBP reports the best performance with an accuracy error <4.1%. Finally, the TBP was tested in a fresh chicken tibialis anterior muscle with a localized thermally ablated lesion in order to evaluate its performance in biological tissue.

Experimental classification of surface waves in optical coherence elastography

Various types of waves are produced when a harmonic force is applied to a semi-infinite half space elastic medium. In particular, surface waves are perturbations with transverse and longitudinal components of displacement that propagate in the boundary region at the surface of the elastic solid. Shear wave speed estimation is the standard for characterizing elastic properties of tissue in elastography; however, the penetration depth of Optical Coherence Tomography (OCT) is typically measured in millimeters constraining the measurement region of interest to be near the surface. Plane harmonic Rayleigh waves propagate in solid-vacuum interfaces while Scholte waves exist in solid-fluid interfaces. Theoretically, for an elastic solid with a Poisson’s ratio close to 0.5, the ratio of the Rayleigh to shear wave speed is 95%, and 84% for the Scholte to shear wave. Our study demonstrates the evidence of Rayleigh waves propagating in the solid-air boundary of tissue-mimicking elastic phantoms. Sinusoidal tone-bursts of 400Hz and 1000 Hz were excited over the phantom by using a piezoelectric actuator. The wave propagation was detected with a phase-sensitive OCT system, and its speed was measured by tracking the most prominent peak of the tone in time and space. Similarly, this same experiment was repeated with a water interface. In order to obtain the shear wave speed in the material, mechanical compression tests were conducted in samples of the same phantom. A 93.9% Rayleigh-shear and 82.4% Scholte-Shear speed ratio were measured during experiments which are in agreement with theoretical results.

An approach to viscoelastic characterization of dispersive media by inversion of a general wave propagation model

In the characterization of elastic properties of tissue using dynamic optical coherence elastography, shear/surface waves are propagated and tracked in order to estimate speed and Young’s modulus. ...

A methodology for updating 3D solid models of complex monumental structures based on local point-based meshes

Structural changes introduced during the life of monuments contribute to produce complex geometrical configurations that cannot be properly represented in standard solid modeling systems designed for current engineering applications such as finite element analysis (FEA). Likewise, point-based 3D meshes - laser-scanner or photogrammetric -, although capable of constructing detailed representation of surfaces, cannot be used for direct application in structural analysis because they do not produce complete and unambiguous solid models. To tackle this issue, we merged these two approaches into a unified methodology capable of updating a 3D solid model, representing the entire monument as reconstructed in its presumed original configuration, with information from a 3D mesh model containing a detailed geometrical description of the recent structural state of a specific sector of the same monument. The methodology is based on a series of functions that operate in the Mesh and Solid Modeling Space. The mesh model is aligned via 3D registration and, subsequently, segmented for its conversion to a solid model. Finally, this solid updates the solid representation of the entire monument via Boolean operations. We test the procedure on the Main Platform of the Huaca de la Luna, Trujillo, Peru, one of the most important massive earthen structures of the Moche civilization. Solid models are defined in AutoCAD while 3D meshes are constructed via the photogrammetric program Agisoft PhotoScan. The results indicate that the proposed methodology is effective at transferring complex geometrical and topological features from the mesh to the solid modeling space. The updated solid model can be represented and visualized in any standard CAD software, and utilized for FEA and augmented reality applications.

Viscoelastic characterization of dispersive media by inversion of a general wave propagation model in optical coherence elastography

Determining the mechanical properties of tissue such as elasticity and viscosity is fundamental for better understanding and assessment of pathological and physiological processes. Dynamic optical coherence elastography uses shear/surface wave propagation to estimate frequency-dependent wave speed and Young’s modulus. However, for dispersive tissues, the displacement pulse is highly damped and distorted during propagation, diminishing the effectiveness of peak tracking approaches. The majority of methods used to determine mechanical properties assume a rheological model of tissue for the calculation of viscoelastic parameters. Further, plane wave propagation is sometimes assumed which contributes to estimation errors. To overcome these limitations, we invert a general wave propagation model which incorporates (1) the initial force shape of the excitation pulse in the space-time field, (2) wave speed dispersion, (3) wave attenuation caused by the material properties of the sample, (4) wave spreading caused by the outward cylindrical propagation of the wavefronts, and (5) the rheological-independent estimation of the dispersive medium. Experiments were conducted in elastic and viscous tissue-mimicking phantoms by producing a Gaussian push using acoustic radiation force excitation, and measuring the wave propagation using a swept-source frequency domain optical coherence tomography system. Results confirm the effectiveness of the inversion method in estimating viscoelasticity in both the viscous and elastic phantoms when compared to mechanical measurements. Finally, the viscoelastic characterization of collagen hydrogels was conducted. Preliminary results indicate a relationship between collagen concentration and viscoelastic parameters which is important for tissue engineering applications.

An Integrated Protocol for the Research and Monitoring of Cutaneous Leishmaniasis

Cutaneous Leishmaniasis is a skin infection which is commonly present in underdeveloped countries. The incidence is particularly high in amazonic countries of Latin-America like Brazil, Colombia and Perú and is usually reported as endemic. In Perú, more than one million people are at risk of infection and approximately 6,000 new cases are detected each year. The present work proposes to integrate a set of control, monitoring and disease quantification procedures in: (1) an automated tool to expedite the analysis in laboratories studying parasiticidal agents and (2) a non-invasive treatment monitoring protocol. The first, consists in the adaptation of an optical microscope KRUSS MBL3100 to perform a fast image capture and automated promastigotes identification. This may be of value to evaluate the effectiveness of various parasiticidal agents. The counting process is performed by an automatic segmentation in the RGB green color space discriminating elements by their area. The second, proposes a protocol for monitoring the evolution of the disease treatment divided into three stages: supra-skin modeling and reconstruction, subcutaneous exploration by textural characteristics and volumetric segmentation. This protocol is performed using a Next Engine HD 3D scanner and a Vevo Visualsonix 2100 ultrasonic scanner. The results show improvements in sample processing time, accuracy and inter- and intra-operational variability. The sensitivity and accuracy of the microscopic identification system was of 97% and 92% respectively. The exactitude and precision error was up to 2% and the sensitivity and specificity went as high as 71% in the 3D reconstruction and texture analysis respectively.

Surface Acoustic Wave Propagation Using Crawling Waves Technique in High Frequency Ultrasound

Several tropical diseases generate cutaneous lesions on the skin with different elastic properties than normal tissue. A number of non-invasive elastography techniques have been created for detecting the mechanical properties in tissue in the last decades. Quantitative information is mainly obtained by harmonic elastography, which is distinguished for producing shear wave propagation. When wave propagation is near a boundary region, surface acoustic waves (SAW) are found. This work presents crawling waves elastography technique implemented with a high-frequency ultrasound (HFUS) system for the estimation of SAW speed and its relationship with the elastic modulus. Experiments are conducted to measure SAW speed in a homogeneous phantom with a solid-water interface for a theoretical validation. Afterwards, ex-vivo experiments in thigh pork were performed to show SAW propagation in animal tissue. Preliminary results demonstrate the presence of SAW propagation in phantoms and skin tissue and how wave speed should be correctly adjusted according to the coupling media for elastography applications.

A comparative study of shear wave speed estimation techniques in optical coherence elastography applications

Optical Coherence Elastography (OCE) is a widely investigated noninvasive technique for estimating the mechanical properties of tissue. In particular, vibrational OCE methods aim to estimate the shear wave velocity generated by an external stimulus in order to calculate the elastic modulus of tissue. In this study, we compare the performance of five acquisition and processing techniques for estimating the shear wave speed in simulations and experiments using tissue-mimicking phantoms. Accuracy, contrast-to-noise ratio, and resolution are measured for all cases. The first two techniques make the use of one piezoelectric actuator for generating a continuous shear wave propagation (SWP) and a tone-burst propagation (TBP) of 400 Hz over the gelatin phantom. The other techniques make use of one additional actuator located on the opposite side of the region of interest in order to create an interference pattern. When both actuators have the same frequency, a standing wave (SW) pattern is generated. Otherwise, when there is a frequency difference df between both actuators, a crawling wave (CrW) pattern is generated and propagates with less speed than a shear wave, which makes it suitable for being detected by the 2D cross-sectional OCE imaging. If df is not small compared to the operational frequency, the CrW travels faster and a sampled version of it (SCrW) is acquired by the system. Preliminary results suggest that TBP (error < 4.1%) and SWP (error < 6%) techniques are more accurate when compared to mechanical measurement test results.