This conference presentation was prepared for SPIE BiOS, 2023.

Optical coherence elastography (OCE) is an image formation tool used to retrieve the mechanical properties of biological tissues. OCE images are formed from the acquisition of two optical coherence tomography (OCT) images of the sample subjected to different states of mechanical loading. Conventional models for strain retrieval in OCE estimate the strain from the phase difference between the two OCT B-scans, the sensitivity of which are limited by approximations and phase wrapping. Furthermore, their performance decreases as strain increases. We present a deep learning model for OCE which overcomes these problems, and give examples to compare it with existing methods.

Compression optical coherence elastography depends heavily on strain retrieval from maps of displacement due to mechanical loading. Displacement data is derived from the phase difference between OCT images and is highly oscillatory due to speckle. This limits strain sensitivity and signal to noise ratio. We present an alternative approach to strain retrieval that overcomes this speckle induced limitation by determining the unique spectral domain transformation that maps unloaded A-scans to the loaded A-scans, exactly, for regions of constant strain. Our novel method of strain retrieval has a higher sensitivity and signal to noise ratio than existing approaches.

Dynamic optical coherence elastography (OCE) tracks mechanical wave propagation in the subsurface region of tissue to map its shear modulus. For bulk shear waves, the lateral resolution of the reconstructed modulus map (i.e., elastographic resolution) can approach that for OCT, typically a few tens of microns. However, skin, cornea and many other tissues are layered or bounded leading to the formation of guided mechanical waves. We performed numerical simulations and acoustic micro-tapping experiments to show that in bounded media, the elastographic resolution cannot reach the OCT structural resolution and is mainly defined by the thickness of the bounded tissue layer.

Multiplexing the spectral analysis of Brillouin scattering to many spatial points of a sample was recently demonstrated as a powerful way to improve Brillouin microscopy image acquisiton speeds. However, this approach is currently limited to line-scan Brillouin microscopes because spectral analysis is performed via etalon spectrometers where only one spatial direction can be used for spectral analysis. To enable 2D spectral multiplexing, we propose replacing etalons with monochromators based on laser-induced circular dichroism in atomic-vapors for Brillouin analysis. This enables single shot imaging at a single frequency, while the full Brillouin dataset is built scanning the monochromator transmission window.

Brillouin microscopy provides a non-invasive method for quantifying the mechanical properties of biological materials with diffraction-limited resolution. Since the signal of spontaneous Brillouin scattering is very weak, high laser power and ultrasensitive detection are usually required. Here, we report a new approach to enhance the signal of Brillouin microscopy by recycling the power of the illumination beam. With a multipass illumination geometry, we obtained about 3.75 times enhancement of the Brillouin signal under the same input laser power. We will present the details of the optical design as well as the experiment.

Impulsive stimulated Brillouin microscopy (ISBS) is a technique that promises to enable fast measurements of the viscoelastic properties of biological samples. Its big advantage compared to spontaneous Brillouin arises from the capability to manipulate the signal-to-noise ratio by the choice of the system parameters. In this contribution, we will present a thorough analysis of the signal generation dependencies and the resulting implications on the spatiotemporal resolution. First measurements on blood, water and hydrogels underline the potential of the technique and show that ISBS can be an important alternative for selected biomedical applications.

Tissue function is dependent on its mechanical properties. Characterizing the mechanical properties of tissue on the micro-scale will enable an improved understanding of its physiological function and aid to identify disease at an early stage. In this study, we present a tension-based optical coherence elastography that maps the micro-scale strain tensor resulting from tensile loading to study the mechanics of load-bearing tissues (e.g., tendon) that routinely resist tensile loading. We demonstrate this technique through experiments of phantom and tendon tissue, presenting the micro-scale mechanical contrast arising from the local structural and mechanical heterogeneity of samples under tensile loading.

Hepatocellular carcinoma is the most common type of primary liver cancer, which mainly develops from cirrhosis. Although it is known that cirrhosis increases the elasticity of liver, the possible role of increased microenvironment elasticity in deriving hepatocellular carcinoma is not understood, due to the paucity of micro-scale elastography techniques. We propose quantitative micro-elastography for the mechanical assessment of liver micro-structures during chronic injury in mouse models. Our results show a significant increase in the mean elasticity and elasticity heterogeneity caused by advanced chronic liver injury, and a microscale correlation between the elasticity variation and common pathological features such as fibrosis.

The nucleus is the largest organelle in the cell. When deformed with techniques like Atomic Force Microscopy or micropipette aspiration, the nucleus appears to be elastic and much stiffer than the cytoplasm. Whether the nucleus behaves like a stiff elastic object when shaped by cellular forces and on physiological time scales, such as during migration through confining channels, is not clear. Here I will discuss our efforts to understand nuclear mechanics in cell migration. I will present live cell imaging experiments that reveal surprising nuclear mechanical behaviors such as drop-like deformation. I will show how the nucleus is likely shaped by viscous coupling between the nucleus and the cytoplasm rather than static cytoskeletal stresses. I will conclude with an example of high content imaging of cancer nuclear morphology for drug discovery applications.

Assessment of the mechanical behavior of adherent cells currently lacks a commercial solution. Mechanical behavior is projected to play a key role in development, vascular diseases, cancer progression, and tissue engineering. Microscopes customized using a single-board computing ecosystem have created an opportunity to assess cellular mechanics quantitatively. The success of such customized technology solutions can be ensured with unified and cross-platform toolkits that enable guided and programmable data acquisition, data analysis, and results visualization. To address this need, we developed an Integrative Toolkit to Analyze Cellular Signals (iTACS), which can be obtained from https://github.com/IntegrativeMechanobiologyLaboratory/iTACS.

Biological tissues exhibit distinct viscoelastic behavior across multiple frequency scales. Biophysical interactions between cells and extracellular matrix across this spectrum play an important role in governing many pathophysiological processes. We implemented Laser Speckle Microrheology (LSM) to map and measure frequency-dependent viscous and elastic moduli in tumor specimens and ECM constructs up to the sub-MHz regime. We identified distinct frequency-dependent responses in both elasticity and viscosity across multiple regimes, lending a unique source of micromechanical contrast in tissues. Thus, micromechanical spectroscopy with LSM may provide invaluable biomechanical insights that are inaccessible when solely characterizing elasticity over a limited frequency scale.

Altered tissue stiffness has raised as both the cause and the consequence of breast tumorigenesis. Imaging the heterogeneous mechanical properties of tissue microenvironment provides crucial insights into micro-mechanical attributes of aggressive behavior. Here, we exploit laser Speckle rHEologicAl micRoscopy (SHEAR), to map the shear viscoelastic modulus, G*(x,y,ω), of excised breast specimens. Results demonstrate that micromechanical entropy of the tumor microenvironment is significantly reduced in aggressive triple-negative tumors compared to other molecular subtypes. These findings highlight the prognostic yield of tumor micromechanical entropy, measured by SHEAR.

Collagen crosslinking and degradation have been associated with changes in tissue stiffness. Although femtosecond laser irradiation might induce either collagen crosslinking or collagen degradation, the photomodulation effects are still unclear. Our goal was to characterize effects of photomodulation. We found that collagen crosslinking and degradation were related to the number of treatments, and photomodulation-involved reactants diffuse. The precision of photomodulation was at the micrometer level in both lateral and axial directions. Polarized light microscopy shows no change in collagen architecture in the treatment boundary and without the production of damaging thermoacoustic effects.

Purposed at validating the hypothesis that overly stiff sclera undermines the passive and adaptive mechanisms of the aqueous outflow pathway in regulating IOP, we combined photoacoustic microscopy (PAM) and finite element analysis (FEA) technologies to resolve and quantify the strains in the aqueous veins and surrounding perilimbal sclera in human and porcine eyes at high resolution in 3D in our previous study. In this study, we introduced large dynamic range of scleral stiffness in intact porcine eyes by crosslinking and observed the correlations between the principal strains in sclera and aqueous veins during IOP elevations, and between the principal strains and the steady state IOP. The results showed strong correlations in both cases.

Neural tube closure is a complex process driven by mechanical forces, but this process can be disturbed leading to development defects. So, to understand the interplay between forces and tissue stiffness during neurulation, we developed a multimodal Brillouin microscopy and optical coherence system (OCT). OCT provides structural guidance while mapping the biomechanical properties of embryonic neural tube using Brillouin microscopy. 3D-OCT, 2D-OCT, and 2D-Brillouin images of Mthfd1l and Fuz knockout mouse embryos at gestation days 9.5 and 10.5 were acquired. Our results show overall decrease in the stiffness of homozygotic knockout neural tube tissues compared to the wildtype.

Brillouin microscopy provides information about the longitudinal modulus (M’) of materials; however, in cell and tissue biomechanics the preferred information for mechanical characterizations is the Shear Modulus (G’). These two quantities have been empirically correlated, but this correlation is ill-defined due to the different underlying physics corresponding to these moduli. Using hydrogels as models, we hypothesize and demonstrate that the empirical connection between the two moduli is due to underlying biochemical and physical local packaging.

This conference presentation was prepared for SPIE BiOS, 2023.

Quantification of fibrosis is critical for the management of inflammatory bowel disease. In this study, photoacoustic (PA) strain imaging were used to estimate intestinal stiffness during the progression of intestinal fibrosis in 23 rabbits in vivo. The tissue was then harvested to measure the young’s modulus ex vivo. Collagen-to-Hb ratio measured using spectroscopic PA imaging was also recorded. Results show that PA-strain is positively correlated to Young’s Modulus with a correlation coefficient of 0.81. PA-strain distinguishes the low histological fibrosis (0-2) and high histological fibrosis (3-5) significantly (p-value<0.001). Collagen-to-Hb ratio and PA-strain are highly correlated with the histological fibrosis (0-5) with correlation of 0.67 and 0.64, respectively.

In this work, we present the development of a compact, wireless imaging probe using a cost effective camera based optical elastography technique, stereoscopic optical palpation, towards intraoperative tumour assessment for breast cancer surgery. We demonstrate the working principle of this probe and test its capability of tumour margin assessment on freshly excised tissues. With further development, this probe holds the potential to be used as a real time cancer imaging tool that can help surgeons more effectively remove cancer during the operation, reducing the need for follow-up surgery. The probe has the potential to be used in rural and remote areas.

OCT has enabled high resolution imaging of the subsurface microstructure of tissues. However, existing systems lack the ability to robustly measure biomechanical contrast. To image stiffness, reverberant elastography uses correlations of the velocity field and curve fitting to their expected functional shape to measure shear wave number. Prior work has assumed that raster scan systems cannot achieve spatial coherence, so current methods require reproducible synchronization of the excitation with imaging. We demonstrate through simulation and phantom imaging that spatial coherence does indeed exist within a single B-scan. Furthermore, leveraging the displacement field also allows us to overcome temporal coherence limitations.

The measurement of ocular biomechanics has a fundamental role in the diagnostics of ocular pathologies, and the monitoring of corneal treatments. In this work, we present success cases of clinical translation of wave-based optical coherence elastography to map the elasticity of human corneas in the following applications: (1) diagnosis and staging of keratoconus; (2) evaluation of the biomechanical impact of keratoconus treatments (UV-crosslinking, intrastromal ring segments, and corneal transplants); and (3) assessment of elastic changes in corneas after LASIK refractive surgery. This work contributes towards the development of a more effective diagnosis and customized treatment of eye diseases that will support ophthalmologists during their clinical decision-making.