Lukasz Ambrozinski

Optical coherence elastography in ophthalmology

Abstract. Optical coherence elastography (OCE) can provide clinically valuable information based on local measurements of tissue stiffness. Improved light sources and scanning methods in optical coherence tomography (OCT) have led to rapid growth in systems for high-resolution, quantitative elastography using imaged displacements and strains within soft tissue to infer local mechanical properties. We describe in some detail the physical processes underlying tissue mechanical response based on static and dynamic displacement methods. Namely, the assumptions commonly used to interpret displacement and strain measurements in terms of tissue elasticity for static OCE and propagating wave modes in dynamic OCE are discussed with the ultimate focus on OCT system design for ophthalmic applications. Practical OCT motion-tracking methods used to map tissue elasticity are also presented to fully describe technical developments in OCE, particularly noting those focused on the anterior segment of the eye. Clinical issues and future directions are discussed in the hope that OCE techniques will rapidly move forward to translational studies and clinical applications.

Structure damage modelling for guided waves-based SHM systems testing

Structure damage modelling for the simulation of elastic wave propagation is essential for simulation-based testing of structural health monitoring (SHM) systems. Reflection, transmission and other complex phenomena related to elastic wave interaction with structure defects need to be reproduced by the simulation method to replace physical testing by numerical one and aid the design process of SHM system. Authors review commonly used methods for the simulation of elastic wave propagation pointing out their advantages and disadvantages. Subsequently methods for crack and delamination modelling are presented. Proposed models are used for numerical testing of designed SHM system. Simulation results are compared with experimental ones, showing very good agreement. Finally, structure damage models are used for testing delamination localization algorithm.

Separation of Lamb waves modes using polarization filter of 3D laser measured signals

Interpretation of Lamb waves signals can rise serious difficulties due to their multi-modal nature. Different modes propagating with different velocities can be misleading with damage reflected components. As a solution to this problem we propose a technique capable of modes separation based on a polarization filter. Both S0 and A0 Lamb modes exhibit elliptical polarization, however, their polarization parameters, i.e. the ratios of in-plane and out-of-plane displacements and phase-shifts between these components are different. Furthermore, these parameters can be considered constant in a narrow frequency band. Therefore, if the vertical and horizontal components of the wave motion are available, it is possible to apply signal processing technique referred to as oblique polarization filter. This operation is based on phase-shifts and amplifications of the in- and out-of-plane components, which results in orthogonal, linearly polarized A0 and S0 waves signals. In this paper the proposed technique will be illustrated using both numerical simulations and experimental data. The simulations of wave propagation were performed using local interaction simulation approach (LISA) assuming isotropic material. The experiments were performed using 3D laser scanning Doppler vibrometer that allowed to capture the in-plane and out-of-plane wave components.

Designing 2D arrays for SHM of planar structures: a review

Monitoring structural integrity of large planar structures that aims at detecting and localizing impact or damage at any point of the structure requires normally a relatively dense network of uniformly distributed ultrasonic sensors. 2-D ultrasonic phased arrays, due to their beam-steering capability and all azimuth angle coverage are a very promising tool for structural health monitoring (SHM) of plate-like structures using Lamb waves (LW). Linear phased arrays that have been proposed for that purpose, produce mirrored image characterized by azimuth dependent resolution, which prevents unequivocal damage localization. 2D arrays do not have this drawback and they are even capable of mode selectivity when generating and receiving LWs. Performance of 2D arrays depends on their topology as well as the number of elements (transducers) used and their spacing in terms of wavelength. In this paper we propose a consistent methodology for three-step: theoretical, numerical and experimental investigation of a diversity of 2D array topologies in SHM applications. In the first step, the theoretical evaluation is performed using frequency-dependent structure transfer function (STF). STF that defines linear propagation of different LWs modes through the dispersive medium enables theoretical investigation of the particular array performance for a predefined tone-burst excitation signal. A dedicated software tool has been developed for the numerical evaluation of 2D array directional characteristics (beampattern) in a specific structure. The simulations are performed using local interaction simulation approach (LISA), implemented using NVIDIA CUDA graphical computation unit (GPU), which enables time-efficient 3D simulations of LWs propagation. Beampatterns of a 2D array can be to some extend evaluated analytically and using numerical simulations; in most cases, however, they require experimental verification. Using scanning laser vibrometer is proposed for that purpose, in a setup where LWs, excited by PZT transmitters of the investigated array are sensed in multiple points corresponding to the locations of the 2D array receiving elements. A virtual receiving sub-array is created in this way and the performance of various array architectures in the reception mode can be evaluated experimentally without the need of physical prototype; a change of topology requires only straightforward modification of the measurement points distribution at the tested structure. For illustration, beampatterns of three symmetrical 2D topologies, i.e., circular, star-shaped and spiralshaped, will be examined in the paper and compared in terms of their beam-width and side-lobes level. The effect of apodization applied to the array elements will be also investigated.

Data fusion for compensation of temperature variations in Lamb-wave based SHM systems

Temperature variations affect Lamb wave propagation and therefore in this way they can severely limit application of baseline signals in SHM systems. Various techniques are proposed in the paper to solve this problem. New method based on an interpretation of multiple signals acquired in distinct points of the structure is introduced and compared with other widely used approaches. Data fusion is used to merge a number of methods into one with a substantially increased efficiency.

Application of artificial neural networks for damage indices classification with the use of Lamb waves for the aerospace structures

Lamb waves (LW) are used for damage detection and health monitoring due to the long range propagation ability and sensitivity to structural integrity changes as well as their robustness in different applications. However, due to the dispersive character and multimode nature of LWs, analysis of the acquired ultrasonic signals is very complex. It becomes even more difficult when applied to a complex structure, for instance, an aircraft component with riveted joints and stringers characterized by difficult geometries. Therefore, numerous approaches to the evaluation of damage indices have been proposed in the literature. In this paper, comparison a number of damage indices applied to LWs testing in aircraft aluminum panels. The damage indices, known from the literature have been selected from the application point of view. Artificial neural network has been used for the classification of fatigue cracks and artificial damages induced in the specimens taken from a real aircraft structure. Article presents results of simulation, data analysis and data classification obtained using selected and dedicated neural network. The main aim of the presented research was to develop signal processing and signal classification methods for an aircraft health monitoring system. The article presents a part of the research carried out in the project named SYMOST.

Development of Lamb Waves-Based SHM Systems

Guided waves (GW) based methods are a promising tool for structural health monitoring (SHM) of plate-like metallic and composite structures in which high safety standards are required. In this paper we present research with the aim to design and manufacture a prototype of Lamb waves (LW) SHM system. Two approaches can be applied for SHM of plate-like structures. One of them can be based on a sparse array and damage imaging involving incoherent summation of signals envelope. The second approach involves phased arrays with transducers spaced at a distance lower than half wavelength of the excited Lamb-mode. The influence of an arrays parameters on beamforming of Lamb waves is discussed in the case of linear array. It appears that an unequivocal localization of damage on a plate requires a 2D arrays topology; therefore a star-shaped active array was designed and manufactured for the developed SHM system. Two signal processing approaches were applied for that array, the standard one, based on the delay and sum (DAS) synthetic aperture focusing scheme, and the second one, using a self-focusing technique to obtain the separate images for each scatterer existing in the plate.

Bayesian parameter identification of orthotropic composite materials using Lamb waves dispersion curves measurement

This paper deals with the problem of elastic constant identification in thin plates made of orthotropic composite materials. The approach is based on the analysis of Lamb wave propagation and the related dispersion curves to find the underlying material elastic constants. In the proposed implementation a scanning laser Doppler vibrometer is used to measure Lamb wave dispersion curves. The Local Interaction Simulation Approach is used simultaneously to find a solution to a high-frequency wave propagation problem. The experimental and simulated data are combined in a Bayesian framework for parameter identification which is robust in condition of parameter, modeling and measurement uncertainty. The results are discussed and compared with the results from a deterministic optimization.

Damage localization in plates with use of the procedure based on Modal Filtration

Modal filtering has numerous applications in the analysis of object dynamics. One possible use of this technique, recently presented in the literature, is damage detection. The main idea behind it, is to compare system characteristics filtered with modal filter for damaged and undamaged state. If a structural change appears in the system the filter does not work perfectly and its output significantly differs from one obtained for the healthy structure. Method is based on the modal parameter variation due to damage but does not require the modal analysis for every diagnosis. Another advantages are: computational simplicity and robustness for ambient temperature changes. The method was extended in 2008 with the possibility of damage localization. The idea here was based on the fact that local damage disturbs mode shapes locally. Instead of one modal filter, set of local modal filters were identified and used for further processing. Area of the structure connected with the filter with the worst performance is suspected of containing the damage. The method was already presented and positively verified but only for the beam-like structures. In this paper, application for plate-like structures is shown.

Acoustic micro-tapping (AuT) for non-contact optical coherence elastography (Conference Presentation)

Optical coherence elastography (OCE) holds great promise for quantitative characterization of corneal elasticity including robust measurements of the intraocular pressure (IOP) independent of corneal mechanical properties. To translate this method into a viable clinical tool, however, requires wideband, highly accurate mechanical wave measurements using mechanical stimulation requiring no physical contact with the cornea. We have developed a method of non-contact mechanical stimulation of soft media with precise spatial and temporal shaping. We call it acoustic micro-tapping (AuT) because it employs focused, air-coupled ultrasound (US) to induce significant mechanical displacement at the boundary of a soft material using reflection-based radiation force. Combining it with high-speed, four-dimensional (three space dimensions plus time) phase-sensitive optical coherence tomography (PhS-OCT) creates a non-contact tool for high-resolution and quantitative dynamic elastography of soft tissue at near real-time imaging rates. To demonstrate this approach, we present OCE results on a porcine cornea using a homemade, focused 1 MHz air-coupled piezoelectric transducer with a matching layer to launch an US wave through air onto the sample surface. To provide an acoustic line source approximating a 1-D excitation, the transducer was made from a cylindrical segment of a piezoelectric tube. A high-speed (1.6 MHz A-Scan rate) PhS-OCT system was utilized to measure acoustic wave propagation in the cornea at different intraocular pressures (IOPs). Results from this OCE study demonstrate that an air-coupled US wave reflected from an air/tissue interface provides significant radiation force to generate displacement for elasticity imaging for full mechanical characterization of the cornea.