This PDF file contains the front matter associated with SPIE Proceedings Volume 12844, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

Rheological properties of biological fluids are closely linked to various physiological processes. For instance, imbalances in blood viscosity are closely linked to the development of conditions such as coronary heart disease, peripheral artery diseases, stroke, and hyperviscosity syndromes. However, existing rotational-based and tube-based rheometry devices are unsuitable for measuring the viscosity of biological fluids due to the need for sample contact and cleaning the testing chamber between each sample. Moreover, not all biological fluids can be sampled in significant volumes, as is the case with blood. In this study, we present a non-contact rheometry method based on capillary waves in a shallow regime for evaluating the viscosity of thin layer fluid (sub-millimeter to micrometer depth) using acoustic radiation force-based optical coherence elastography (ARF-OCE) and compared with theoretical simulations.

Brillouin spectroscopy is increasingly employed for biomedical research and recent advances such as line-scanning configurations further widen the scope of possible measurements. Current spectrometer technologies have limitations which are prohibitive for certain applications. We propose a novel hybrid VIPA-etalon cascade scheme compatible with both confocal and line-scanning geometries. A single VIPA is followed immediately with one or multiple essentially parallel-oriented standard Fabry-Perot etalons of approximately matching thickness. The cascaded etalons preserve near-unity throughput while increasing the contrast by more than 20 dB per stage. Other advantages include simplicity, compactness, and wavelength flexibility. We present simulations of the hybrid VIPA-etalon scheme and an experimental proof-of-concept measurement.

Due to anisotropic fibril orientations across the cornea, different phase retardation is superimposed on the reflection, depending on the imaging area. As polarized light is commonly used in Brillouin microscopy to maximize the collection of the scattered light, polarization mismatch can strongly reduce the intensity of Brillouin signal, making the deep stroma undetectable. Therefore, a quarter wave plate was added to compensate corneal phase retardation according to the intensity of Brillouin scattering, aiming at getting a clear depth-dependent profile of Brillouin shifts. The compensated Brillouin profile showed similarity with profiles measured at points with limited phase retardation impact, 60 MHz smaller than the profile without compensation.

Evaluating cellular biomechanics is key to understanding cellular mechanotransduction and the correlation between forces and tissue longitudinal modulus during development. Due to sub-optimal measurement techniques, this relation is poorly understood. Brillouin microscopy is a non-invasive, high-resolution, and all-optical imaging modality capable of mapping tissue longitudinal modulus. We developed a high-resolution Brillouin microscopy system to image the biomechanical characteristics of zebrafish zygotes at cell stage of 0.2 hours post fertilization (hpf), cleavage period at cell stages of 1 and 1.75 hpf, blastula period at cell stage of 3 hpf, and gastrula period at 4.3-6 hpf during development. The resulting images were able to discern the individual cells as they duplicated during development and measure cellular changes in longitudinal modulus during these stages. The results are promising and demonstrate the potential of Brillouin microscopy for revealing the biomechanical properties of the zebrafish at early developmental stages with cellular resolution.

Phenylthiourea (PTU) is often used to block pigmentation and make zebrafish completely transparent for easy optical imaging. PTU inhibits melanogenesis by inhibiting tyrosinase. Although the PTU is commonly used, it does have some side effects. PTU at a concentration of 0.2 mM (0.003%) significantly reduces the zebrafish eye size due to the inhibition of thyroid hormone production. Furthermore, low levels of thyroid hormones in zebrafish increase the stiffness of the intervertebral joints, altering their swimming behavior. The aim of this study was to assess the structural modifications and biomechanical properties of 5-day post-fertilization (dpf) zebrafish eyes after being exposed to PTU using optical coherence tomography and reverberant optical coherence elastography, respectively. Wild-type zebrafish (n=3), treated with PTU (0.2 mM), were compared with non-treated zebrafish (n=3). The results show a significant reduction (p=0.02) in the mean eye diameter of the fishes treated with PTU (312.66 ± 8.71 μm) versus the non-treated group (340.18 ± 4.38 μm). On the other hand, the non-treated group showed a significantly slower (p=0.02) shear wave speed (0.97 ± 0.12 m/s) compared with the PTU-treated group (2.65 ± 0.51 m/s), indicating that PTU induces a biomechanical change in the stiffness of the developing eye. PTU is a potent inhibitor of the pigmentation of zebrafish; however, it can also severely affect its biomechanical properties, specifically eye development, reducing eye diameter and increasing its stiffness.

Liquid to gas phase transition of dye-loaded PFC nanodroplets (nanobombs, NBs) can be facilitated by the optical absorption of energy of laser pulse. Activation of NBs with laser pulse can produce highly localized longitudinal shear waves (LSW). The advent of LSW elastography has enhanced the ability to measure depth-dependent tissue elasticity. Highly localized NB-induced LSWs propagate through the tissue depth and can discriminate the tissue elasticity gradients along the depth. In this study, we explore the capability of the NB-induced LSWs in discriminating the elasticity properties of multilayered tissue-mimicking phantoms. The NB present in the middle layer of the test phantoms produced LSWs upon the pulse laser excitation, which can provide elasticity information in the sample depth where the NBs are located and the elasticity of layers of the sample on top and bottom of the NB layer.

Assessing corneal biomechanical properties may be important for the diagnosis of ocular diseases as the mechanical properties of the cornea change during disease development. Elastography is a technique to image the mechanical properties of tissues by applying a mechanical load to the tissue and measuring the resultant displacement using available imaging techniques like magnetic resonance or ultrasound imaging. The measured displacement is then translated to mechanical properties. Heartbeat optical coherence elastography (Hb-OCE) is a completely passive elastography technique, which uses physiological perturbations naturally present in the body in lieu of active tissue stimulation sources. In this work, we demonstrate the first use of 3D Hb-OCE to measure the biomechanical properties of the cornea in 3D in an ex vivo porcine eye. Measurements were taken on whole porcine eye globes using a Fourier Domain Mode Locked swept source laser-based OCT system with a volume rate of 4.2 Hz. The strain in the cornea was measured between successive B-scans during a simulated ocular pulse, and each scan was stacked together to obtain 3D volumetric strain due to the simulated heartbeat. This technique may potentially enable full volumetric analysis of corneal mechanical properties completely passively. Future work will focus on 3D evaluation of customized corneal crosslinking and in vivo translation of the 3D Hb-OCE technique.

The mechanical properties of the sclera are important for the integrity of the eye. Because of the distribution of collagen fibers, the sclera is structurally anisotropic, with highly aligned fibers at the limbus and more unaligned fibers in the posterior region. Noninvasive measurements to quantify structural anisotropy remain a challenge. In this study, multimeridian air-coupled ultrasonic optical coherence elastography (ACUS-OCE) was used to assess mechanical anisotropy at different locations of the sclera and the cornea in ex vivo rabbit eyes. The ACUS-OCE consists of a 200 kHz aircoupled ultrasonic transducer connected to a phase-sensitive swept source OCT system for non-contact excitation of surface waves in ocular tissue. Rabbit eyes (n=7) were measured at a constant intraocular pressure while the system generated elastic surface waves at the corneal and four scleral sites (superior/inferior temporal and superior/inferior nasal). Multi-meridian acquisition configuration allowed estimation of the phase speed at 16 angles (8 meridians) of each location, from which tissue anisotropy was calculated. Assessment of an anisotropic parameter showed that the sclera is at least 3 times (p≤.002) more anisotropic than the cornea. The results indicate a stiffer anterior sclera than the cornea and posterior sclera, and a greater scleral anisotropy than the corneal anisotropy.

Systemic sclerosis (SSc) is a rare, chronic autoimmune disease characterized by fibrosis and vascular abnormalities in multiple organs, including skin, lungs, and gastrointestinal tract. Early and accurate diagnosis of SSc is essential for timely intervention and improved patient outcomes. More than 90% of patients with SSc have fibrosis in the skin that manifests as mechanical changes in the skin. The modified Rodnan Skin Score (mRSS) is the gold standard for assessing skin involvement in SSc but has high inter-observer variability. The other widely used method is ultrasound imaging. Ultrasound elastography is emerging as a useful technique for assessing SSc, but its accuracy is still under investigation. Even though the results from these studies provide a quantitative assessment of SSc, ultrasound elastography has many limits, such as the lack of high-resolution performance to detect SSc-affected skin structural and elastic characteristics simultaneously. Optical coherence elastography (OCE) is an established technique to assess the mechanical properties of tissues noninvasively and quantitatively with high resolution. We measured the mechanical properties of skin in 45 patients (36 SSc and 9 matched controls) using a home-built swept source OCE setup that uses air-coupled acoustic radiation force for tissue excitation. The measurements were performed at three locations on each arm: the proximal phalanx of the third finger, the second intermetacarpal space, and the dorsal forearm midline. The OCE results were compared to mRSS and clinical ultrasound elastography, which a trained physician performed. Our results show that OCE outperformed ultrasound elastography with a higher correlation with mRSS.

Optical elastography techniques are rapidly emerging as preferred methods of measuring tissue mechanical properties due to a variety of benefits, such as resolution, speed, and noninvasive imaging. However, there remains a lack of research on the inter-operability of these methods, which makes inter-study and inter-method mechanical comparisons difficult. Therefore, this work aims to compare measurements obtained by various optical coherence elastography (OCE) techniques, Brillouin spectroscopy, and ultrasound-based shear wave elastography (USE) to the gold standard of uniaxial mechanical testing. This study utilized three sets of tissue-mimicking silicone phantoms with varying elasticities. We compared measurements of common optical elastography methods, including air-pulse OCE, aircoupled ultrasound OCE, reverberant OCE, and compression OCE, along with ultrasound elastography and Brillouin spectroscopy. The measurements from these methods were compared to the gold standard of uniaxial mechanical testing. The results of the quasistatic methods, i.e., mechanical testing and compression OCE, showed very good agreement for all three samples. The dynamic wave-based OCE and USE methods also had good inter-agreement, showing the inter-operability of air-pulse, air-coupled, and reverberant OCE and USE. Additionally, Brillouin spectroscopy measurements yielded the Brillouin frequency shift, which was able to discriminate all three sample sets. These results are the first step of a more robust framework for studying the relationship between mechanical measurements performed by various excitation methods.

In this paper, we applied Brillouin microscopy for the measurement of the lipid molecule properties in the liver tissues and successfully visualized the two-dimensional distribution of Brillouin shift in the liver tissue. Our results may lead to new insights into developing early diagnosis and elucidation of the mechanism of non-alcoholic fatty disease.

Optical coherence elastography (OCE), the elastography extension of optical coherence tomography (OCT), has been proposed to quantify the biomechanical properties of ocular tissues (e.g., cornea and sclera) for early detection of different diseases, such as keratoconus and cataract. In wave-based OCE, various tissue stimulation methods have been demonstrated to induce waves in ocular tissues. Acoustic radiation force (ARF) is commonly used as a non-contact excitation source with tightly controlled stimulation parameters for various tissues, including the cornea and crystalline lens. However, ARF’s reliance on tightly focusing acoustic pressure within the tissue raises concerns about potential tissue damage. The aim of this study was to assess the safety of ARF-OCE on freshly enucleated ex vivo porcine eyes and investigate the ability of safe acoustic pressures to produce detectable displacements for OCE. In this study, the maximum value for ophthalmic acoustic pressure set by the Food and Drug Administration (FDA) was set as the 100% threshold in our assessment, and it was determined using a needle hydrophone. OCT and confocal microscopy were used to assess the integrity of the porcine crystalline lens before and after ARF-based OCE experiments. The maximum ARF intensity allowed by the FDA produced detectable wave propagation on the crystalline lens without damaging the lens.

The need to characterize the mechanical properties of cells led biologists to promote spectroscopy based on Brillouin scattering. This rapidly required a balance between two important criteria: on the one hand, the acquisition time for individual spectra, and on the other, sufficient contrast to observe low-intensity signals.

Spectrometers based on VIPA (Virtually imaged phased array) give the possibility of obtaining the full spectrum in just a few hundred milliseconds, with contrasts that make it possible to study biological samples.

The addition of a Lyot mask therefore makes it possible, at low cost, to suppress part of the background of the recorded signal, using the geometry of the pattern formed by the two Rayleigh peak suppression slits. This addition, if optimally placed, enables effective transmission to be maintained for Brillouin shift measurement, while gaining 25 dB in contrast.

Thus, the configuration presented here, combining a trick for reducing specular reflection at the sample/substrate interface and a device for obtaining measurements under good conditions, makes it possible to envisage combining this spectroscope with other spectroscopy.

In this study, a method for depth-resolved internal stress field measurement of soft materials using swept-source polarization-sensitive optical coherence tomography (SS-PS-OCT) was proposed. An SS-PS-OCT system based on circularly polarization state was constructed. A depth-resolved dispersion compensation technique was used to improve image quality and the contrast of phase retardation fringes. The stress measurement theory for PS-OCT was derived. A rubber specimen and the double-layer thermoplastic elastomer (TPE) specimen were measured. Stress magnitude results showed good agreement between PS-OCT and tensile measurements with an error of less than 5%. The principal stress direction results showed an error of less than 6%. The PS-OCT-based method for measuring internal stress fields has the potential for measuring the internal mechanical properties of soft materials.

The utilization of lasers in dentistry expands greatly in recent years. For instance, fs-lasers are effective for both drilling and caries prevention, while cw-lasers are useful for adhesive hardening. A cutting-edge application of lasers in dentistry is the debonding of veneers. While there are pre-existing tools for this purpose, there is still potential for improvement. Initial efforts to investigate laser assisted debonding mechanisms with measurements of the optical and mechanical properties of teeth and prosthetic ceramics are presented. Preliminary tests conducted with a laser system used for debonding that is commercially available showed differences in the output power set at the systems console to that at specified distances from the handpiece. Furthermore, the optical properties of the samples (human teeth and ceramics) were characterised. The optical properties of the ceramics should closely resemble those of teeth in terms of look and feel, but they also influence the laser assisted debonding technique and thus must be taken into account. In addition first attempts were performed to investigate the mechanical properties of the samples by means of pump-probe-elastography under a microscope. By analyzing the sample surface up to 20 ns after a fs-laser pulse impact, pressure and shock waves could be detected, which can be utilized to determine the elastic constants of specific materials. Together such investigations are needed to shape the basis for a purely optical approach of debonding of veneers utilizing acoustic waves.