This PDF file contains the front matter associated with SPIE Proceedings Volume 12381, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

Rheological properties of biological fluids are closely linked with various physiological processes. Capillary waves are associated with rheological properties of fluids such as viscosity and surface tension. The phase velocity of capillary waves is a primary parameter for measuring rheological properties of fluids. For a fluid layer in a shallow fluid regime (the fluid depth is smaller than 0.05 times the wavelength), capillary waves play an important role especially in cell and molecular biology. Therefore, evaluating phase velocity of thin layers of a fluid is a key mechanism for understanding the rheological properties of the fluids in small scale. However, evaluating phase velocity of thin layer fluids with a non-contact has not been widely reported, and is challenging by using existing rotational-based and tube-based rheometry devices. Here we first report that phase velocities of capillary waves on thin layer fluids in shallow fluid regime can be determined. The acoustic radiation force (ARF) was used to create capillary waves on the thin layer fluids and a spectral domain optical coherence tomography (SD-OCT) was used to detect particle motions of the waves. The experimental results were compared with the theoretical analysis. A 7.5 MHz single element transducer was used to produce the ARF to create capillary waves. The phase velocity of capillary waves on thin layer fluids were successfully determined by using the proposed elastography technique with the non-contact fashion, which paves the way for measuring viscosity of thin layer fluids in our near future study.

The term "elastography" covers a dynamic and expanding set of diagnostic imaging techniques which probe the biomechanical properties of tissues. This overview covers some of the major approaches that have evolved and their role in improving clinical diagnoses. A deeper level of study is also emerging linking our estimates of viscoelasticity to the multiscale structure and composition of living tissue in normal and diseased states. These studies can be undertaken at the highest spatial resolution with optical techniques, and examples in cornea, brain, and skin will be covered.

Systemic Sclerosis (SSc) is a chronic disease of autoimmune etiology that causes vasomotor disturbance, fibrosis, and atrophy of skin, underlying tissue, muscles, and internal organs. It manifests in about 30 people per million every year, and there are an estimated three million cases worldwide. Currently, SSc is assessed using the modified Rodnan Skin Score (mRSS), which is a manual palpation test on 17 sites that requires extensive physician training and experience for accurate assessments. Unfortunately, mRSS has very high inter-observer variability and is subjective. Optical coherence elastography (OCE) is a well-established technique for assessing the mechanical properties of tissues with sub-millimeter spatial resolution. In this work, OCE was used to non-invasively assess the mechanical properties of mouse skin in vivo. OCE measurements were performed on 3 groups of mice, (1) control group that was injected with PBS, (2) SSc group that was injected with bleomycin (BLM) to induce SSc, and (3) treatment group that was first injected with BLM and then injected with imatinib, which is postulated to reduce disease in SSc. The wave speed in BLM-SSc skin was significantly higher than that of normal skin (p<0.05). The wave speed in murine skin in the treatment group was slightly lower as compared to the BLM-SSc skin, but the difference was not significant. These results demonstrate the ability of OCE to monitor SSc disease and treatment response and support further evaluation of this platform in monitoring SSc in the clinic.

The long-term aim of this project is to establish optical coherence elastography for tumor delineation in the field of neurosurgery. Because of the challenging highly viscoelastic properties of brain tissue, we developed a new Air-Jet based excitation source. With pulse duration of up to 700 ms and real time force measurement, this novel system allows the sample to reach a semi-steady state. In parallel with a 3.2 MHz swept-source optical coherence tomography system over 800 line scans are acquired over the whole sample excitation process. The phase data is extracted, unwrapped and the displacement per pixel is calculated. This system enables the measurement of mechanical properties like stiffness and Young’s modulus, similar to the standard indentation measurement. As well as viscoelastic properties i.e. relaxation times, in non-contact. The first processing step is to split the excitation progression into three main time ranges: the high dynamic, the steady state, and the viscoelastic range. In each range typical features of the displacement curve are extracted for every pixel in the B-scan. For those features, various mechanical parameters are calculated mainly, the stiffness and Young’s modulus and stored as feature matrices. The results are processed, visualized and overlaid with either the OCT intensity image or the histological sections. Strain stress curves are generated for some selected positions in the B-scan leading to a specific viscoelastic hysteresis. The feature matrices will be utilized as a fingerprint for each tissue, and are the first step for an AI based classification of the tissue.

The fundamental physiological function of the iris is to control the amount of light entering the eye, which requires the coordinated constriction or dilation of the pupil, affected by two antagonistic muscles, namely the sphincter and radial muscles. Disorders of the iris, including these muscles, may lead to ocular pathologies, such as primary angle-closure glaucoma. Here, we assessed the regional biomechanical properties of the iris using phase-sensitive optical coherence elastography (PhS-OCE) to quantify the shear wave speed arising from perturbations generated using an acoustic radiation force (ARF) transducer of resonant frequency 3.5 MHz and focal length 19 mm. We determined regional shear wave speeds by tracking elastic wave propagation in ex vivo porcine irides. Results showed that the mean shear wave speed in the pupillary zone (~2.1 m/s) was consistently greater than in the ciliary zone (~1.87 m/s). These findings indicate that the mechanical properties of the iris exhibit regional heterogeneity, which may be related to the microstructure of the iris (muscle locations/extent) and intrinsic elastic properties.

The multiple scattering (MS) process affects the spectroscopic investigation and the optical imaging of opaque samples. In Brillouin spectroscopy, MS affects the extraction of reliable micromechanical parameters inducing the ill definition of the exchanged wavevector of the scattering process, q. Here, we propose a new experimental method called Polarization Gated Brillouin Spectroscopy (PG-BS) able to disentangle the MS and the ballistic contributions. The results obtained on milk, used as benchmark material, demonstrate both the capability and easy applicability of the proposed method. Exploiting PG-BS for different biological materials can open the route to new frontiers in Brillouin imaging of opaque samples.

The study of variation in biomechanical properties is a significant diagnostic tool for examining malignancy in tissues. Elastography is a potential technique that is used for qualitative and quantitative assessment of abnormal tissues. Optical coherence elastography is a highly efficient and promising technique for inspecting soft biological tissues with microscale resolution. In the present work, we report the elastographic measurement of tissue-mimicking phantoms of varying stiffness using reflection mode holographic imaging. The sinusoidal surface excitation is produced by an electromechanical actuator on the sample over a set of frequencies and the phase map of the surface wave is reconstructed using a phase-shifting algorithm. The preliminary results are well correlated with previously reported literature. The current work has high potential applications for quantifying the biomechanical properties of in-vivo and ex-vivo tissues in clinical practice.

Optical coherence elastography (OCE) has been used successfully for characterizing changes tissue mechanical properties particularly in breast tissue and the eye. Many dynamic ultrasound shear wave elastography (SWE) methods have been developed over the past three decades that use propagating waves with different dynamic excitations. We have successfully translated excitation and analysis methods from SWE for applications using OCE. We report here recent developments that utilize focused ultrasound to produce acoustic radiation force or mechanical vibration. We have explored characterizing the rheological properties such as surface tension and viscosity of various fluids. Additionally, we have applied these OCE methods to soft tissues such as blood clots, aorta samples, and porcine kidneys. These techniques have opened new areas for tissue characterization that take advantage of the sensitivity and resolution of optical coherence tomography and the strengths of wave-based approaches for quantifying material properties.

The biomechanical properties of the human skin are intrinsically correlated with changes associated with pathological conditions, aging, and hydration. Quantitative measurements can improve diagnostic tools, treatments, and cosmetic product evaluation. Using optical coherence elastography (OCE), an emerging imaging modality combining optical coherence tomography (OCT) with a localized excitation source to induce mechanical disturbances, a quantitative evaluation of tissue biomechanics can be achieved. OCE complements the structural information with elasticity data to attain a complete overview of skin status.

In this study, we employed a home-built OCE system, combining a swept-source OCT system with a piezoelectric actuator for tissue displacement, to evaluate changes to the skin biomechanical properties due to the application of an anti-aging cream. Skin elasticity was monitored for a total of five weeks. Anti-aging cream was applied daily for four weeks. OCE measurements continued for one additional week to assess the effect of cream application interruption. Three female volunteers were included in this proof-of-principle investigation. Their counter-arm was used as control. Although no statistical significance was reached, a decrease in skin Young’s modulus was observed with the cream application, indicating an increase in skin elasticity.

Reverberant optical coherence elastography (RevOCE) relies on multiple external excitation sources, such as mechanical actuators or vibrators, to produce random waves propagating in arbitrary directions. In this work, a preliminary study on acoustic beam manipulation to produce multi-focal acoustic radiation force is introduced using a planar ultrasound transducer and a 3D-printed acoustic lens array for RevOCE. An unfocused acoustic beam generated by an ultrasound transducer with 1 MHz central frequency and 38-mm element diameter was coupled to seven uniformly distributed acoustic focusing lenses. The spatial distribution of the acoustic field at the focal plane was measured with a needle hydrophone. The effectiveness of the system in generating reverberant shear wave fields was assessed by performing RevOCE imaging of tissue-mimicking gelatin phantoms. The multi-focus acoustic lens-transducer system was coupled with a phase-sensitive optical coherence tomography (PhS-OCT) system. The RevOCE measurement was conducted by sending ten cycles of a tone burst at 2 kHz. The measured acoustic pressure field showed that the array of concave spherical acoustic lenses spatially distributed the acoustic energy into multiple focal spots in the desired focal plane. Furthermore, RevOCE imaging in tissue-mimicking phantom indicated the effectiveness of the acoustic lens-transducer system in inducing reverberant shear wave fields for probing mechanical properties.

Embryo development is driven by several substantial events that cause structural modifications during growth. The progressive changes in the embryo during this important period cause simultaneous alterations of its biomechanical properties. Understanding the structural modifications and changes in stiffness during embryo development is important for comprehending its growth and potentially detecting congenital diseases. The aim of this study is to map the biomechanical properties of embryos in 3D during development. Murine embryos at gestational day (GD) 11 were imaged using 3D Reverberant optical coherence elastography (Rev-OCE). The embryos were placed on a glass window, which was vibrated by an actuator at 1 kHz. In addition to providing the vibration, the glass window also enabled imaging in the common path configuration, which eliminated environmental noise and improved the displacement sensitivity of the system to sub-nanometer levels. The results showed the structural changes and the differences in stiffness in the embryo. The stiffness of the embryos from GD 11 showed stiffer areas along the developing spinal cord. Combining high-resolution OCT with elastography allowed us to understand the structural and biomechanical changes in the embryo during its development. Thus, this study provides important insights into embryo mechanical properties, which could serve as a potential biomarker for deficiencies in embryo development. Our future work is focused on imaging embryos at different stages as well as studying mutant models of congenital diseases, such as neural tube defects.

Measuring corneal biomechanical properties provides important diagnostic information regarding tissue health and for evaluating the outcomes of therapies. Heartbeat OCE (Hb-OCE), a truly passive elastography technique, utilizes the pulsatile nature of the intraocular pressure (IOP) to quantify corneal stiffness completely noninvasively and with no external excitation. In this work, we utilized whole rabbit eye globes to quantify the displacement and strain in the cornea by fluctuating the IOP ex-vivo. We also approximated the non-linearity of the stiffness of the cornea by performing HbOCE measurements at various baseline IOPs. The results show an expected increase in corneal stiffness as the baseline IOP increased, demonstrating the effectiveness of Hb-OCE as a tool for measuring corneal biomechanical properties completely noninvasively and with no external excitation.

Assessing corneal biomechanical properties is useful for the diagnosis of ocular disease and for monitoring therapeutic intervention. Here, we demonstrate how two different forms of elastography can be combined to perform a multimodal biomechanical analysis of the cornea. Heartbeat optical coherence elastography (Hb-OCE) and compression elastography were combined to measure the mechanical properties of rabbit corneas in vivo. The results demonstrate that the difference in corneal stiffness measured by Hb-OCE and compression OCE is statistically insignificant, suggesting that both techniques could potentially be used interchangeably to measure corneal stiffness.

Measuring the mechanical properties of the cornea can help understand the structure and physiology of the eye, early detection of disease, and evaluation of therapy outcomes. In this work, we investigate the effect of collagen XII deficiency on the stiffness of the murine cornea using a multimodal approach for biomechanical analysis. Wave-based optical coherence elastography (OCE), heartbeat OCE, and Brillouin microscopy were all utilized to assess the mechanical properties of wild-type and collagen XII deficient ex vivo murine corneas as a function of IOP. All three techniques show that collagen XII deficiency leads to a dramatic decrease in corneal stiffness. Future work will investigate how these measurement techniques can be translated for in vivo assessment of corneal elasticity to understand the contribution of various proteins to corneal structural and mechanical integrity.