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1 PhASE PhotoAcoustic Schlieren Elastography Design Team Hannah A. Gibson, Will A. Goth Briana J. Moretti, Zakary T. Smith Design Advisor Prof. Gregory Kowalski MIE Department Sponsor Prof. Charles DiMarzio ECE Department Abstract The mechanical properties of the cornea are not well known, and current methods of measuring these properties are too inaccurate and invasive to be used in a clinical setting. Safely and accurately measuring elastic moduli has several applications, including disease diagnosis, benchmarking of ocular treatments, and more precise computational modeling of the eye. An alternative is introduced, using a pulsed laser to induce acoustic waves and a Schlieren microscope to observe the waves propagation. Postprocessing of the different waves velocities can yield more accurate information about the elasticity of cornea. A bench top prototype has been developed as a proof of concept that will demonstrate its potential in further developing an in vivo diagnostic tool. Tissue phantoms with known elastic moduli are used to benchmark the measurements from our system. This new technique has the potential of delivering functionally accurate mechanical properties of cornea while remaining a non-invasive process. For more information, please contact g.kowalski@neu.edu.

2 The Need for Project The PhASE system is needed to acquire accurate, noninvasive, in vivo measurements of the mechanical properties of cornea. The PhASE will determine elastic properties of transparent biomaterials, specifically corneal tissue, whose functional mechanical characteristics are difficult to determine using traditional mechanical testing. There are no current methods which can precisely measure multiple elastic moduli in a precise, non-invasive way. These properties, which are influenced by various eye conditions and diseases, could be used for pre-diagnosis, benchmarking of ocular treatments, and more accurate biomechanical modeling. The PhASE system is a novel proof of concept system that aims to deliver these properties, and pave the way for a clinical device that can aide doctors in better understanding and treatment of the eye. The Design Project Objectives and Requirements The objective is to design a system capable of stimulating and observing a mechanical response in a cornea-like tissue, and use this data to quantify its elastic properties. To validate the accuracy of the system, a tissue phantom will be tested with our system and compared to results from traditional mechanical analysis. Design Objectives The ultimate goal of the system is to better understand the fundamental elastic properties of corneal tissue, which will provide more accurate functional knowledge of the eye as a whole. These properties are the six elastic moduli, which fully describe the elastic behavior of a material in response to external forces. These moduli are interrelated such that all six can be calculated using the experimental values of only two moduli. A device capable of determining two moduli of in-vivo corneal tissue must be able to stimulate and detect two different mechanical responses of the tissue in conjunction. The objective of the PhASE system is to combine different technologies to achieve this two-fold capability, while the method of detection must produce quantifiable data to be considered a viable measurement tool. Validation of the system includes benchmarking our elastic measurements with a material of known mechanical properties. For this purpose, a tissue phantom similar to corneal tissue is desired. Design Requirements Two sub-systems must be capable of inducing and detecting a stimulus and its response in such a way that two different elastic moduli can be measured. As the cornea is a relatively small structure and the response of the material is expected to be very fast, our detection system must have high spatial and temporal resolution, while the stimulatory system must have the potential to be non-invasive.

3 Additionally, post-processing algorithms are necessary for quantitative assessment of the data in a standardized, non-biased way. Finally, the tissue phantom used in this proof of concept model needs to be optically and mechanically similar to corneal tissue so that it can be tested using our system, while remaining suitable for traditional mechanical testing to benchmark and validate our system. Design Concepts Considered Several non-contact methods of mechanical stimulation were investigated, and optical setups were mainly considered, as they require minimal contact with the sample and have high resolution. Elastic moduli relations from PhASE and Traditional Mechanical Analysis The mechanical stimulation system is designed to be as noninvasive as possible. Methods involving direct contact probes or high velocity air jets were discarded. Photoacoustic stimulation of waves has been identified as the most reliable non-contact method. The stimulation of a photoacoustic wave requires heating a small area of material quicker than the heat can dissipate through conduction. In this case, a rapid pressure differential occurs, known as thermal stress confinement. This induces the propagation of pressure and shear waves, whose velocities are related to the P-wave and Shear Moduli. The power and pulse width of the pressure wave needs to be high enough to cause a thermal stress confinement within the medium, while conversely it cannot exceed levels of heating which cause denaturation or ablation of the corneal tissue. The acoustic wave front will travel at the speed of sound in the tissue, expected to be near 1500m/s, across the 1.5cm diameter of the cornea. The detection system must have temporal resolution of around 100 nanoseconds and a spatial resolution of at least 100 micrometers to capture data at distinct instances of the wave propagation. Acoustic transducers, Optical Coherence Tomography (OCT), and Schlieren imaging were investigated as observation methods. Acoustic transducers and OCT have a high degree of temporal and spatial resolution, but require direct contact with the tissue and are only capable of detecting Shear waves, and were not considered further. Schlieren imaging is capable of detecting the additional pressure wave without need for contacting the surface of the eye. The more common transmissive-mode Schlieren imaging is the easiest to implement, but requires a light source or a detector on opposite sides of the tissue; a reflective-mode system can remain on one side of the sample, but requires costly additional components.

4 Density Gradient Candle flame Preliminary Schlieren system images of thermally induced density gradients above a candle flame The general Schlieren system must be modified to suit our needs. Firstly, it needs to achieve temporal resolution on the nanosecond scale, as previously discussed. High speed cameras are potentially capable of meeting the required sampling rate, but are extremely costly. Alternatively, a strobing method was investigated, in which waves are produced and detected at discrete time steps. Wave front propagation is then captured by changing the offset between the initial laser pulse and the image capture. Several different approaches were considered for this, including triggering the camera, mechanical shuttering systems, and strobing of the illumination source. The camera triggering was possible, but the communication speed of USB cables was found to be inadequate. Mechanical shuttering systems would need to be extremely rapid, requiring a very complicated electromechanical system. High frequency triggering of the illumination source is attractive in its simplicity, but also requires a rapid illumination rise time, indicating a design compromise was necessary for our lighting source. While certain light sources, like the tungsten lamp used in a preliminary Schlieren system, have very high luminous intensity, they are not suited to rapid triggering, as the filaments do not turn on and off quickly enough. Flash tubes and laser diodes meet both the intensity and triggering rate requirements, but have issues with focusing and unwanted diffraction effects. LED s are capable of high speed triggering and do not have problems with diffraction, but inherently have much less power and focusing capability. To analyze the data quantitatively, several different image processing algorithms were developed and tested. The algorithms differed in how they identified wave-front location based on pixel intensity, and the each algorithm s wave front location was checked by hand to ensure qualitative accuracy. To validate and our system performance before investigating the unknown properties of cornea, tissue phantoms were researched. Hydrogels, similar to the material contact lenses are made of, are most similar to the cornea, but have the same inherent problems in precise measurement as the cornea: they require constant are ill-suited to traditional mechanical analysis. Silicones are less optically and

5 mechanically similar to cornea than hydrogels, but do not degrade and can be tested intensively with traditional mechanical analysis. Recommended Design Concept The proof of concept design uses a rapidly pulsed heating laser to induce mechanical waves in a tissue phantom. A strobed-source, transmissive Schlieren imaging system is used to observe the average wave velocity, and a postprocessing algorithm results in quantitative measurements of elastic moduli. PhASE Proof of Concept Implementation Design Description The heating laser produces 10Watts at the 850nanometer wavelength, and is integrated with a high speed pulse generator to pulse for 50ns durations. The light source of the Schlieren imaging system is a high power, lumen LED with a 10ns rise time. A 100ns output signal from the pulse generator is amplified to 40W and controls the LED strobing. The output is offset from the heating pulse by 100ns. This strobed source beam is focused onto a high precision slit, and then collimated by a lens into a circular beam 2.54cm in diameter. A sample is placed in the beam path between the first lens and a second lens. After the second lens, a knife edge is placed at the focal point to block any un-modulated light propagation. Any light refracted by the wave propagation is then focused by a third lens onto a High Resolution Charge Coupled Device detector. An infrared filter ensures that only visible wavelength light from the source is imaged, and no infrared light from the heating laser is detected. The material for the tissue phantom is Sylgard 184, a clear two-part silicone. Although it is not a perfect mimic, as wave attenuation in silicone is higher than in cornea, Sylgard 184 is has more constant mechanical properties than hydrogels and is capable of analysis by traditional mechanical methods due to its durability. The image analysis algorithm produces a curve of the pixel intensity with respect to pixel location across a predefined radial vector on an image. The algorithm searches for a combination of localized maxima and minima in pixel intensity, which it then records. The average velocity of each wave is calculated by finding the distance between the wave fronts of two subsequent images, converting this pixel displacement to physical distance, and dividing this value by the time step between the two images. Experimental Investigations An initial Schlieren imaging system was built and tested using different lighting sources. A Tungsten lamp was used to confirm the capture of Schlieren images before switching to a high intensity LED.

6 PhASE image of tissue phantom with low (above) and high (below) levels of surface deformation Videos of thermal convection currents above open flame heat sources confirmed detection of density gradients, while videos of surface deformation were confirmed by probing a tissue phantom sample with tweezers. The LED was capable of capturing similar Schlieren data in constant mode, but required a longer exposure time to capture the necessary data in pulsed operation. This required an optical enclosure for the system to improve the signal to noise ratio of the image data. The pulse circuits for strobing were tested using an oscilloscope and confirmed the laser/led signal pulse widths, timing offsets, voltage levels, and amplification gains. Tissue phantom samples with different solution ratios were fabricated and tested using a Dynamic Mechanical Analyzer. The first sample, a 10:1 base to curing agent sample, had an average Young s modulus of 2.6 MP, consistent with what is found in literature, while a second 12:1 sample had an average Young s modulus of 2.4 MPa and gives us a stiffness within the range of corneal tissue. The Poisson s ratio of these samples needs to be found using a three-point bending method. Once these have been determined, the P-wave and shear moduli of the samples can be determined using elastic moduli relations. Below: Graphical representation of wave-front measurement Analytical Investigations The quantitative analysis algorithms were tested on Schlieren image data of shockwaves traveling in a saturated air environment. The algorithm predicted a P-wave velocity of 320m/s. The velocity of air in normal atmospheric conditions is 340m/s, meaning our analysis was within 6%. However, conditions in the experimental data were in saturated conditions, meaning that our system was likely more accurate than the 6% range. Key Advantages of Recommended Concept The main advantage of the PhASE system is that two elastic moduli are found, which allows complete mechanical characterization of the tissue, while the technology used has the promise of measuring functionally accurate properties while remaining non-invasive. All previous systems are only able to roughly measure one modulus, and usually greater precision requires excision of the cornea from the eye. The tissue phantom used to validate PhASE system does not degrade or change in properties over time, as excised cornea and other phantoms

7 will, and is suitable for traditional mechanical analysis. The two-part mix also allows for modulation of stiffness and a graduation of moduli to benchmark our system with. Financial Issues While only $550 was spent on the proof of concept system, a second generation, optimized system would cost approximately $25-35k. As a proof of concept, one of the aims of this project was to remain as low-cost as possible, and a total of $550 was spend in additional equipment. This would not have been possible without the generous donation of equipment from Prof. DiMarzio s Optical Science Laboratory. The overall cost of the project in a second generation model with optimized components would cost between $25,000 and $35,000. New components would include a very high speed camera for image collection, a pulsed light source with higher luminous intensity, and an optimized wavelength laser diode for photoacoustic generation. Recommended Improvements Several improvements required for in-vivo use include a reflective mode of Schlieren imaging, an optimized wavelength heating laser, a more powerful illumination source, and a higher speed camera. The PhASE system has several required modifications before it can be considered as a non-invasive diagnostic tool. In its current transmissive imaging mode, the illumination source or detector would need to be inside of the eye; a reflective Schlieren system would avoid this necessity. Additionally, the laser diode currently used to generate a pressure wave through a thermal stress confinement is not eye safe. Infrared radiation from a 1550nm laser diode is mostly absorbed by cornea, meaning less power is required for heating and almost no harmful light reaches the retina. A more powerful illumination source would also provide higher contrast imaging, as well as be necessary for reflective imaging, while a higher speed, more sensitive detector would allow the measurement to take place more quickly.