Modern Ultrasonics: From Super-Resolution Lens Design to Intraocular Pressure Tester

The aim of this thesis is to lay the foundation for the development of a contactless laser-based tonometer. Tonometers are devices capable of measuring intraocular pressure (IOP). Monitoring intraocular pressure is important for diagnosing glaucoma, which is a condition that may result in blindness and affects 70 million people worldwide. The foundation behind the development of the proposed tonometer, rests on the foundation of four published papers. In the first paper we focused acoustic waves using cylindrical metamaterial lenses. These lenses allow focusing acoustic energy into a beam narrower than half the central wavelength of the wave package. By selecting the speed of sound ratio between the material inside the lens and the surrounding medium, as well as the lens diameter, it is possible to efficiently focus acoustic energy into a jet narrower than half the central wavelength. The generation of said jet involves the focusing of the narrow-bandwidth acoustic waves as they impinge on the lens. The lens’ cylindrical geometry allows the propagation of guided surface waves that tailor the shape of the jet. An alternative approach towards generating a narrow acoustic wave front is the use of a pinhole. A pinhole in an aluminum plate allowed us to direct a shock wave front to a phantom/eye and calculate the intraocular pressure (IOP) from the time-of-flight of the membrane waves. To detect these membrane waves, we used a laser Doppler vibrometer (LDV). To understand the challenges of interferometric measurement with an LDV we conducted a study where we mapped the acoustic field on a rotating propeller. The motivation of this study was the importance of quickly monitoring the structural integrity of propellers in situ for the safe operation of aircraft. Aircraft inspection by ultrasonic means typically involves contacting transducers featuring low spatial resolution and slow measurement times. Alternatively, laser ultrasonics allows fast characterization of materials with high resolution and in a contactless manner. The demonstration of the contactless approach detected a flaw on an aluminum propeller that rotated under stroboscopic illumination of a high-power Q-switched laser. The high-power laser generated acoustic waves that travelled through the material and their measurement by an LDV resulted in acoustic maps. The maps allowed the identification as well as reconstruction of the defect on a 3D model of the sample. We further increased the complexity of the sample from a planar propeller to a geometry closer to a human eye, a metal hemisphere. The complexity introduced by the curvature of the sample ranged from the difficulty of focusing an LDV on a curved target to acoustic resonances in the sample. The motivation for choosing these samples was to develop a method to inspect acetabular implants in a contactless manner. In a similar fashion to the propeller study, a metal hemisphere featuring a defect rotated whilst a high-power laser generated acoustic waves. The detection of these acoustic waves and mapping of the acoustic fields allowed a reconstruction of the defect on a 3D model of the hemisphere. Further increasing the complexity of the sample, we studied an ocular phantom (human eye model) as a first step before measuring porcine eyes. In the phantom, the cornea was simulated by a polymer membrane stretched over a water-filled cavity. Adding water to the cavity increases the tension of the membrane and that is equivalent to increasing the intraocular pressure (IOP). To determine the internal pressure of the phantom, an electrical spark generated a shock wave that impinged on the surface of the eye generating membrane waves. These waves propagated in the cornea and an LDV measured their amplitude and propagation time. By relating the time-of-arrival of the acoustic waves to the internal pressure of the phantom we extracted a calibration curve. We further expanded our database by measuring porcine eyes allowing us to compare the IOP readings of our method to those of the leading rebound tonometer, the iCare TA01. The development of a contactless alternative to rebound tonometers will benefit from localized actuation on the cornea by a focusing structure, such as a metamaterial lens. Such a lens would allow actuation on a predetermined spot of the cornea, thus decreasing the uncertainty of the time-of-flight estimation. Such an uncertainty would be further reduced by eye tracking such that the excitation and detection locations remain fixed. The measurement series on the propeller introduces a method for synchronizing the excitation and generation of guided waves which is further improved in the study of the metal hemisphere. An important difference between eyes and metal hemispheres is the anisotropy of the tissue. Such anisotropy introduces variations in the acoustic impedance thus modifying the propagation velocity of membrane waves propagating in the cornea. Localized guided excitation of membrane waves would aid by launching guided waves along the same path, thus decreasing the error in the estimation of the IOP. Contactless measurement of IOP is possible with the technique suggested in this study. The combination of the lessons learned together with eye-safe interferometric detection of guided waves might pave the way to safe and comfortable alternatives to the current tonometric methods.The aim of this thesis is to lay the foundation for the development of a contactless laser-based tonometer. Tonometers are devices capable of measuring intraocular pressure (IOP). Monitoring intraocular pressure is important for diagnosing glaucoma, which is a condition that may result in blindness and affects 70 million people worldwide. The foundation behind the development of the proposed tonometer, rests on the foundation of four published papers. In the first paper we focused acoustic waves using cylindrical metamaterial lenses. These lenses allow focusing acoustic energy into a beam narrower than half the central wavelength of the wave package. By selecting the speed of sound ratio between the material inside the lens and the surrounding medium, as well as the lens diameter, it is possible to efficiently focus acoustic energy into a jet narrower than half the central wavelength. The generation of said jet involves the focusing of the narrow-bandwidth acoustic waves as they impinge on the lens. The lens’ cylindrical geometry allows the propagation of guided surface waves that tailor the shape of the jet. An alternative approach towards generating a narrow acoustic wave front is the use of a pinhole. A pinhole in an aluminum plate allowed us to direct a shock wave front to a phantom/eye and calculate the intraocular pressure (IOP) from the time-of-flight of the membrane waves. To detect these membrane waves, we used a laser Doppler vibrometer (LDV). To understand the challenges of interferometric measurement with an LDV we conducted a study where we mapped the acoustic field on a rotating propeller. The motivation of this study was the importance of quickly monitoring the structural integrity of propellers in situ for the safe operation of aircraft. Aircraft inspection by ultrasonic means typically involves contacting transducers featuring low spatial resolution and slow measurement times. Alternatively, laser ultrasonics allows fast characterization of materials with high resolution and in a contactless manner. The demonstration of the contactless approach detected a flaw on an aluminum propeller that rotated under stroboscopic illumination of a high-power Q-switched laser. The high-power laser generated acoustic waves that travelled through the material and their measurement by an LDV resulted in acoustic maps. The maps allowed the identification as well as reconstruction of the defect on a 3D model of the sample. We further increased the complexity of the sample from a planar propeller to a geometry closer to a human eye, a metal hemisphere. The complexity introduced by the curvature of the sample ranged from the difficulty of focusing an LDV on a curved target to acoustic resonances in the sample. The motivation for choosing these samples was to develop a method to inspect acetabular implants in a contactless manner. In a similar fashion to the propeller study, a metal hemisphere featuring a defect rotated whilst a high-power laser generated acoustic waves. The detection of these acoustic waves and mapping of the acoustic fields allowed a reconstruction of the defect on a 3D model of the hemisphere. Further increasing the complexity of the sample, we studied an ocular phantom (human eye model) as a first step before measuring porcine eyes. In the phantom, the cornea was simulated by a polymer membrane stretched over a water-filled cavity. Adding water to the cavity increases the tension of the membrane and that is equivalent to increasing the intraocular pressure (IOP). To determine the internal pressure of the phantom, an electrical spark generated a shock wave that impinged on the surface of the eye generating membrane waves. These waves propagated in the cornea and an LDV measured their amplitude and propagation time. By relating the time-of-arrival of the acoustic waves to the internal pressure of the phantom we extracted a calibration curve. We further expanded our database by measuring porcine eyes allowing us to compare the IOP readings of our method to those of the leading rebound tonometer, the iCare TA01. The development of a contactless alternative to rebound tonometers will benefit from localized actuation on the cornea by a focusing structure, such as a metamaterial lens. Such a lens would allow actuation on a predetermined spot of the cornea, thus decreasing the uncertainty of the time-of-flight estimation. Such an uncertainty would be further reduced by eye tracking such that the excitation and detection locations remain fixed. The measurement series on the propeller introduces a method for synchronizing the excitation and generation of guided waves which is further improved in the study of the metal hemisphere. An important difference between eyes and metal hemispheres is the anisotropy of the tissue. Such anisotropy introduces variations in the acoustic impedance thus modifying the propagation velocity of membrane waves propagating in the cornea. Localized guided excitation of membrane waves would aid by launching guided waves along the same path, thus decreasing the error in the estimation of the IOP. Contactless measurement of IOP is possible with the technique suggested in this study. The combination of the lessons learned together with eye-safe interferometric detection of guided waves might pave the way to safe and comfortable alternatives to the current tonometric methods

Non-contact determination of intra-ocular pressure in an ex vivo porcine model

People suffering from glaucoma often endure high intra-ocular pressure (IOP). Methods for determining IOP either contact the eye or are unpleasant to some patients. There is therefore a need for a rapid and patient friendly non-contacting method to determine IOP. To address this need, we developed a tonometer prototype that employs spark-gap induced shock waves and a laser Doppler vibrometer (LDV) that reads the amplitude of membrane waves. The IOP was first identified from the membrane wave propagation velocity first in a custom-made ocular phantom and was then verified in ex vivo porcine eyes. The time-of-flight (TOF) of the membrane wave travelling on a hemispherical membrane was compared to reference IOP values in the sample obtained with an iCare TA01 tonometer. The shock front was characterized by high speed photography. Within one eye, the method achieved an agreement of 5 mmHg (1.96 standard deviation between the shock wave tonometer and the commercial manometer) and high method-to-method association (Pearson correlation, R-2 = 0.98). The results indicate that the presented method could potentially be developed into a non-contacting technique for measuring IOP in vivo.Peer reviewe

Non-contact damage detection on a rotating blade by Lamb wave analysis

Propeller inspection is mandatory for safe operation of aircraft. Damage evaluation on such rotating structures requires dedicated measurement techniques. In this study efforts to create a stroboscopic technique are reported. Lamb waves were excited on a rotating blade with a Q-switched Nd:YAG laser synchronized to the sample rotation, whereas the wave amplitude was obtained by a laser Doppler vibrometer. A surface breaking notch on an aluminum sample rotating at 415 rpm was detected and sized with millimeter accuracy. The technique has potential for automatic non-contacting damage detection on rotating structures such as helicopter blades and turbines.Peer reviewe

Efficient Shock Wave Coupling to a Waveguide

High-voltage induced shock waves were created in different shaped spark chambers. The generated shock waves were imaged after they had traveled through a waveguide. Imaging was done by using a stroboscopic schlieren method to determine the speed of the shock wave in air. The effect of the spark chamber shape as well as the effect of the width of the spark gap were studied. The simple cylindrical chamber had the highest efficiency of the measured geometries. A wider spark gap resulted in a higher overpressure of the shock wave.Peer reviewe

Optimizing the Shock Wave-To-Vortex Power Ratio at the End of a Waveguide

Shock waves and vortices are two different ways of transferring acoustic energy to a sample in a contactless manner. This study aims at understanding the power ratio between shock waves and vortices as they exit a wave guide. Diverging tube ends increased the ratio, whereas converging ones decreased it. The power ratios were 0.1 to 9.8. Schlieren imaging was used to quantify the power-ratios. Altering the curvature at the end of the waveguide allows optimization of the shock wave-to-vortex power ratio at the end of a waveguide.Peer reviewe

Defect localization by an extended laser source on a hemisphere

The primary goal of this study is to localize a defect (cavity) in a curved geometry. Curved topologies exhibit multiple resonances and the presence of hotspots for acoustic waves. Launching acoustic waves along a specific direction e.g. by means of an extended laser source reduces the complexity of the scattering problem. We performed experiments to demonstrate the use of a laser line source and verified the experimental results in FEM simulations. In both cases, we could locate and determine the size of a pit in a steel hemisphere which allowed us to visualize the defect on a 3D model of the sample. Such an approach could benefit patients by enabling contactless inspection of acetabular cups.Peer reviewe

Practical realization of a sub-λ/2 acoustic jet

Studies in optics and acoustics have employed metamaterial lenses to achieve sub-wavelength localization, e.g. a recently introduced concept called 'acoustojet' which in simulations localizes acoustic energy to a spot smaller than lambda/2. However previous experimental results on the acoustojet have barely reached lambda/2-wide localization. Here we show, by simulations and experiments, that a sub-lambda/2 wide localization can be achieved by translating the concept of a photonic jet into the acoustic realm. We performed nano-to macroscale molecular dynamics (MD) and finite element method (FEM) simulations as well as macroscale experiments. We demonstrated that by choosing a suitable size cylindrical lens, and by selecting the speed-of-sound ratio between the lens material(s) and the surrounding medium, an acoustic jet ('acoustic sheet') is formed with a full width at half maximum (FWHM) less than lambda/2. The results show, that the acoustojet approach can be experimentally realized with easy-to-manufacture acoustic lenses at the macroscale. MD simulations demonstrate that the concept can be extended to coherent phonons at nanoscale. Finally, our FEM simulations identify some micrometer size structures that could be realized in practice. Our results may contribute to starting a new era of super resolution acoustic imaging: We foresee that jet generating constructs can be readily manufactured, since suitable material combinations can be found from nanoscale to macroscale. Tight focusing of mechanical energy is highly desirable in e.g. electronics, materials science, medicine, biosciences, and energy harvesting.Peer reviewe

Inspecting Rotating Structures by Lamb Waves

Propeller inspection is mandatory for the safe operation of aircraft. Non-contact damage evaluation on rotating structures requires dedicated measurement techniques. We report on a non-contacting stroboscopic technique that allows inspection of rotating aluminum propellers. To excite Lamb waves we used a Q-switched Nd:YAG laser in synchrony with data acquisition by a Laser Doppler Vibrometer. We detected, sized, and imaged a surface breaking notch on a sample rotating at 415 rpm. The technique showed potential for automatic non-contacting damage detection on rotating structures such as helicopter blades and turbines. The samples, manufactured at the Department of Physics of the University of Helsinki were aluminum blades 130 mm x 76.10 mm x 4 mm in size. One of them was used as a baseline while the other one had a surface breaking rectangular defect that was 5 mm x 10 mm x 2.4 mm in size, situated 35.10 mm from the center of the propeller. The propeller was rotated by a 12 V DC motor connected to a pulse width modulation circuit based on a 555 timer integrated circuit. To detect the movement of the rotating blade, a custom made optical gate was built by using common electronics such as blue LED, a LM311 comparator and a blue enhanced photodiode. All electronics were built in the Electronics Research Laboratory of the Department of Physics. Using C++ programming language we designed an algorithm for scanning the moving target. The code was implemented into an Arduino Mega 2560 microcontroller that was connected to a computer. The user operated the computer which controlled the microcontroller via a Labview program. Two types of scans of the samples were performed. In one type the TX scanned the blade from edge to center while the LDV pickup spot was stationary at the center. From this experiment we were able to determine the distance from the notch to the center of the blade and the length of the defect from time-of-flight measurements. The second type of scan was done by situating the LDV pickup point close to the center of the blade and the excitation point at the far side of it, close to the edge. This time we did not scan with the lasers but we divided the arc path that they follow into even steps. From this experiment we were able to determine the width of the notch as well as its depth based on time-of-flight analysis. The results agree with the expected values. Finally, by doing data selection of the delayed Lamb waves with Matlab R2015 and Blender 2.77a, we were able to do image reconstruction of the notch in a 3D model of the aluminum propeller

Localization of millimeter size defects in a steel hemispherical shell

Peer reviewe