Optical elastography offers a rich body of imaging capabilities that can serve as a bridge between organ-level medical elastography and single-molecule biophysics. We review the methodologies and recent developments in optical coherence elastography, Brillouin microscopy, optical microrheology, and photoacoustic elastography. With an outlook toward maximizing the basic science and translational clinical impact of optical elastography technologies, we discuss potential ways that these techniques can integrate not only with each other, but also with supporting technologies and capabilities in other biomedical fields. By embracing cross-modality and cross-disciplinary interactions with these parallel fields, optical elastography can greatly increase its potential to drive new discoveries in the biomedical sciences as well as the development of novel biomechanics-based clinical diagnostics and therapeutics.

Quantitative characterisation of micro-scale mechanical properties of the extracellular matrix (ECM) and dynamic cell-ECM interactions can significantly enhance fundamental discoveries and their translational potential in the rapidly growing field of mechanobiology. However, quantitative 3D imaging of ECM mechanics with cellular-scale resolution and dynamic monitoring of cell-mediated changes to pericellular viscoelasticity remain a challenge for existing mechanical characterisation methods. Here, we present light-sheet photonic force optical coherence elastography (LS-pfOCE) to address this need by leveraging a light-sheet for parallelised, non-invasive, and localised mechanical loading. We demonstrate the capabilities of LS-pfOCE by imaging the micromechanical heterogeneity of fibrous collagen matrices and perform live-cell imaging of cell-mediated ECM micromechanical dynamics. By providing access to 4D spatiotemporal variations in the micromechanical properties of 3D biopolymer constructs and engineered cellular systems, LS-pfOCE has the potential to drive new discoveries in mechanobiology and contribute to the development of novel biomechanics-based clinical diagnostics and therapies.

Viscoelastic transformation of tissue drives aberrant cellular functions and is an early biomarker of disease pathogenesis. Tissues scale a range of viscoelastic moduli, from biofluids to bone. Moreover, viscoelastic behavior is governed by the frequency at which tissue is probed, yielding distinct viscous and elastic responses modulated over a wide frequency band. Existing tools do not quantify wideband viscoelastic spectra in tissues, leaving a vast knowledge gap. We present wideband laser speckle rheological microscopy (WB-SHEAR) that reveals elastic and viscous response over sub-megahertz frequencies previously not investigated in tissue. WB-SHEAR uses an optical, noncontact approach to quantify wideband viscoelastic spectra in specimens spanning a range of moduli from low-viscosity fibrin to highly elastic bone. Via laser scanning, micromechanical imaging is enabled to access wideband viscoelastic spectra in heterogeneous tumor specimens with high spatial resolution (25 micrometers). The ability to interrogate the viscoelastic landscape of diverse biospecimens could transform our understanding of mechanobiological processes in various diseases.

Optical microscopy suffers from multiple scattering (MS), which limits the optical imaging depth into scattering media. We previously demonstrated aberration-diverse optical coherence tomography (AD-OCT) for MS suppression, based on the principle that for datasets acquired with different aberration states of the imaging beam, MS backgrounds become decorrelated while single scattering (SS) signals remain correlated, so that a simple coherent average can be used to enhance the SS signal over the MS background. Here, we propose a space/spatial-frequency domain analysis framework for the investigation of MS in OCT, and apply the framework to compare AD-OCT (using astigmatic beams) to standard Gaussian-beam OCT via experiments in scattering tissue phantoms. Utilizing this framework, we found that increasing the astigmatic magnitude produced a large drop in both MS background and SS signal, but the decay experienced by the MS background was larger than the SS signal. Accounting for the decay in both SS signal and MS background, the overall signal-to-background ratio (SBR) of AD-OCT was similar to the Gaussian control after about 10 coherent averages, when deeper line foci was positioned at the plane-of-interest and the line foci spacing was smaller than or equal to 80 µm. For an even larger line foci spacing of 160 µm, AD-OCT resulted in a lower SBR than the Gaussian-beam control. This work provides an analysis framework to gain deeper levels of understanding and insights for the future study of MS and MS suppression in both the space and spatial-frequency domains.

Fibrin plays an important role in both immune response and pathogen virulence during bacterial infection. Pathogens such as staphylococci interact with fibrin through dynamic processes, involving binding, entrapment, and release from fibrin scaffolds, through which they experience an evolving cascade of nano- to microscale dynamics that span broad timescales. Yet, the biophysical dynamics that unfold between invading bacteria and host fibrin are not well understood. Here, we present a non-invasive optical approach based on speckle fluctuation to characterize the multiscale dynamics of microparticles in fibrin scaffolds. Using microparticles of varying sizes and surface chemistry to emulate spherical, non-motile bacterial cells, we demonstrate real-time monitoring of bacteria-fibrin interactions during fibrin clot formation and fibrinolysis in purified fibrins scaffolds and clinical plasma clots. Our approach circumvents the need for sophisticated position tracking equipment, making it potentially applicable to a broad range of experimental systems for biophysical investigation of bacteria-extracellular network interactions.

Joint tissues rely on a finely tuned interplay between solid (elastic) and fluid (viscous) constituents to support their mechanical function. Degradation of these constituents due to disease or trauma disrupts the elastic and viscous susceptibility of joint tissues, directly impairing joint function. We present Speckle rHEologicAl micRoscopy (SHEAR) to investigate the viscoelastic behavior of intact joint tissues without making contact or otherwise manipulating the sample, offering a promising non-destructive imaging tool for assessing mechanical function in degenerative joint disease. SHEAR demonstrated strong correlation to conventional rheometer in healthy swine knee joint tissues. In swine cartilage with focal lesions and surgically excised cartilage specimens from human patients undergoing knee replacement, SHEAR revealed transformation in both the elastic and viscous landscape of the tissue. SHEAR has the potential to provide a powerful tool for biomechanical analysis of joint structures and pave new paths for improved diagnosis and therapy of degenerative joint disease.