This PDF file contains the front matter associated with SPIE Proceedings Volume 11645, including the Title Page, Copyright information, and Table of Contents.

It is increasingly being recognized that cells sense and respond to mechanical signals. In living organisms such signals arise from the mechanical properties of the surrounding tissue and its constituents. We are especially interested in the influence of mechanical signaling during central nervous system (CNS) development and pathologies. So far, most techniques to study the mechanical properties of the CNS relied on sacrificing animals and preparing tissue slices to provide accessible surfaces for indentation-type mechanical characterization. However, there are clear indications that both death and dissection change the mechanical properties of the tissue. To address this concern, Brillouin microscopy has emerged as a promising tool to quantitatively map the mechanical properties in 3D inside living organisms. Brillouin microscopy is advantageously paired with zebrafish larvae as an animal model that is transparent, amenable to genetic engineering and capable of striking repair of its CNS after injury. In this presentation, I will give an overview of our work illuminating the mechanical properties of the zebrafish CNS during development and repair, and our current understanding of the relevant biological mechanisms. These results start to form a solid foundation for further investigations into the mechanobiology of cell function in the CNS. Ultimately, this research could help treating previously incurable neuropathologies such as spinal cord injuries and neurodegenerative disorders.

The nucleus is the largest and probably stiffest organelle of eukaryotic cells. As such, its mechanical properties are tightly related to various cell functions. However, the mechanical behavior of the nucleus within intact cells is much less know because it is embedded in the cytoplasm thus not directly accessible by the existing contact-based technique. In this work, the regulation of nuclear mechanics was investigated by all-optical Brillouin microscopy and the results were found to be consistent with three-dimensional chemomechanical modeling. We found the intact nuclear mechanics is regulated by both the cytoskeletal networks and internal nanostructure.

We demonstrate applications of photonic-force optical coherence elastography (PF-OCE) on imaging three-dimensional (3D) micro-scale mechanical heterogeneity in collagen phantoms and cell-modified extracellular matrix. In collagen, we observed strong correlations between the measured mechanical heterogeneity and local OCT intensity in the vicinity of beads, which could be related to the presence of fibrous structures. Moreover, we quantitatively imaged micro-scale localized stiffening and increased heterogeneities in fibers surrounding an isolated NIH-3T3 fibroblast enclosed in a 2.5 mg/mL fibrin gel with a volumetric field-of-view (FOV) of ~80μm×350μm×50μm. Our results show that PF-OCE is able to characterize 3D micromechanical properties of biomaterials, and has the potential to observe dynamics of cell-ECM interactions.

Recent evidences show the growing importance of mechanical forces in directing basic processes in cellular functions through the complex scenario of mechanotransduction. In this context, however, the impact of chromatin physical state on cellular structure and function requires further study. Here, we characterized the effects of a loss-of-function mutation of MLL4, a chromatin regulator, on nuclear mechanical and structural properties. Measurements of such features in intact cells are to date challenging, as current gold-standard methods for mechanobiology cannot have access to subcellular structures: a complete characterization of their nuclear mechanical fingerprint is hence still elusive. By exploiting the innovative all-optical technique of Brillouin microscopy in our custom-made optical setup, we measured with three-dimensional sub-micrometric resolution the effects of MLL4-dependent mechanical stress on the nucleus of mesenchymal stem cells in different conditions. These results show the importance of mechanotransduction for vital cellular processes and pave the way towards the use of Brillouin microscopy as a reliable tool for medical diagnostic purposes of chromatinopathies, this mutation being involved in Kabuki syndrome, a severe genetic disease.

Brillouin is a light-matter interaction that depends on several material properties, including the refractive index and the longitudinal modulus of the sample. We developed a technique that probes two different Brillouin scattering geometries in the same volume, allowing the retrieval of the index of refraction and the speed of the acoustic phonon in a direct manner. In biological samples, these two quantities are correlated respectively to the dry mass density and the longitudinal modulus. Coupling such measurement with a confocal microscope allows mapping of these properties for samples and in modalities unavailable with previous approaches.

We developed a photoacoustic imaging (PAI) and finite element analysis (FEA) approach for characterizing the biomechanical behaviors of the aqueous veins and perilimbal sclera, and their roles in the regulation of intraocular pressure (IOP). 3D architectures of the sclera and the aqueous veins perfused with indocyanine green have been resolved by an optical resolution PAI system. The tissue ans strain fields were quantified using FEA. The performance of the proposed method has been validated by tensile test in scleral tissue. The methods have shown the capability of resolving the strain gradients at the vein-sclera interface during the manipulation of IOP.

Shear-wave elastography is based on the imaging of displacements induced by the propagation of shear-waves through a medium. A full-field off-axis digital holography setup is utilized here to image surface displacements with high sensitivity. A low frame-rate camera is combined with a stroboscopic approach to achieve propagation imaging. We present the latest results using time of flight and time-reversal-based methods to map stiffness from a propagation movie. The methods are tested on simulation images obtained using a finite difference algorithm. Experimental images with the optical setup on agarose test samples mimicking biological tissues and first results on an ex-vivo biological sample are presented.

Cell response to mechanical stimuli are essential for cell function yet not well understood. Existing rheological tools typically have limited throughput or require complex schemes. We have developed quantitative phase imaging methods which can image cell structural changes due to mechanical stimuli at the nanoscale. These methods are label free and can image cells in culture or flowing through microfluidic chips, providing high throughput measurements. We will present our holographic cytology imaging system and show its application to profiling large samples of red blood cells. Studies of cancer cells will show that refractive index variance is linked to measured cellular mechanical properties such as shear modulus. Additional applications include examining modulation of mechanical properties due to early carcinogenic events, investigating the role of specific cellular structural proteins in mechanotransduction and tracking water and ion channel regulation due to mechanical stress.

In recent years pump-probe microscopy has been used in several studies to investigate laser-material interaction processes. Thereby, acoustic waves could be imaged that were released into the vicinity of the irradiated spot. Since these waves carry information about the elastic properties of the material, the question arose how pump-probe microscopy could be utilized for elastography of biological materials. Therefore, the influence of the pump-pulse energy on the radius and thus on the speed of acoustic waves in solid and fluid media was investigated in this study.

Digital holography enables the capture of phase and amplitude from a single camera frame with high sensitivity. Displacements induced by a shear-wave diffuse field are acquired at the surface of a scattering sample. From the acquisitions, the stiffness of the sample can be mapped using a time-reversal-based algorithm. The coupling of digital holography with a time-reversal method achieves full-field quantitative elastography without the need of a controlled or synchronized source of mechanical wave. We present here the validation of the method on tissue-mimicking polymer samples and latest results on ex-vivo and in-vivo biological samples.

Non-invasive identification, understanding and evaluation of articular cartilage damage is paramount for osteoarthritis researcher and clinician alike. Using polarisation sensitive optical coherence tomography together with impact and creep load, we use a range of metrics including birefringence to detect early signs of cartilage degeneration and gain new insights into the physiology of joint tissues

Using numerical and analytical models of wave propagation in mechanically anisotropic materials, we highlight the complications associated with quantitative estimates of mechanical moduli in human skin. To obtain reliable, quantitative measurements of moduli in human skin, multiple aspects of mechanical wave propagation in structures typical of skin must be considered. Using a nearly incompressible transverse isotropic (NITI) model, preliminary measurements of both shear moduli (G and μ) in healthy in vivo human skin are presented.

We have designed and developed an air-coupled ultrasonic radiation force probe co-focused with a phase-sensitive optical coherence tomography system (ACUS-OCT). Our custom-made 1 MHz spherically focused piezo-electric transducer with a concentric circular opening of 10 mm diameter allows for the confocal micro-excitation of waves and the spatial (2D, 3D) motion measurement of tissues. Phantom studies demonstrated the capabilities of this probe to produce quasi-harmonic excitation up to 4 kHz for the generation of highly localized elastic waves. Experimental results in ocular tissues showed the highly localized 2D/3D elasticity mapping capabilities of this approach with great potential for clinical translation.

Corneal biomechanical properties can provide important information to measure tissue health. Elastography provides a useful tool to measure the biomechanical properties of tissues in vivo, but typically relies on measuring a response to external excitation. Passive elastography is a method to assess mechanical response to physiological pressures, which can be useful in the field of ophthalmology. In this work, we utilize our technique, Hb-OCE, to measure the mechanical properties of cornea in vivo. Furthermore, we were able to successfully distinguish stiffness in an untreated and crosslinked cornea using this technique, demonstrating its effectiveness as a tool for measuring corneal biomechanics.

Phase stability of an optical coherence elastography (OCE) system is the key determining factor for precision elasticity measurement. In this study, we developed an OCE system based on swept-source optical coherence tomography (SS-OCT) with a common-path configuration (SS-OCECP). Our system has a phase stability of 4.2 mrad without external stabilization or extensive post-processing, such as averaging. This phase stability allows us to detect a displacement as small as ~300 pm. We validated the SS-OCECP performance in a tissue-mimicking phantom and an in vivo rabbit model, and the results demonstrated significant improved phase stability compared to conventional SS-OCE. To the best of our knowledge, we demonstrated the first SS-OCECP system, which possess high phase stability and can be utilized to significantly improve the sensitivity of elastography.

This report presents an overview of recent progress in the development of novel OCE-based technologies enabling non-invasive morphological segmentation of tumor tissues during their natural development and response to therapies. The approach is based on preliminary quantitative assessment of characteristic elasticity ranges corresponding to the segmented morphological constituents (“quasistatic elasto-spectroscopy”). It has been found that such constituents are characterized by fairly well separated ranges of the Young’s modulus, which makes it possible to automatically segment up to 4-6 such zones in OCE-based elasticity maps. High consistency of such OCE-based procedure with conventional morphological segmentation of histological slides in terms of percentages of the segmented-zone areas has been demonstrated. Unlike invasive, time-consuming and laborious histological studies the developed OCE-based segmentation is non-invasive and can be performed even in vivo. High accuracy and sensitivity of the OCE-based-segmentation was demonstrated in vivo for two different types of model tumors (4T1 and Ct26) treated by PDT and chemotherapies. The OCE-based analysis can readily be combined with such OCT-based extensions as texture analysis and OCT-based angiography. The latter opens the possibility to combine morphological assessment with functional characterization of tumor response to therapies. OCE-based characterization of mechanical heterogeneity and elasticity spectra of tumors has also proven to be efficient for express assessment of patients’ breast-cancer subtypes. Overall, the developed OCE-based assessment in many applications suggests a rapid and in vivo feasible alternative to conventional histology and can be efficiently used in a wide range of biomedical problems.

Thromboembolism in the cerebrovasculature can cause high morbidity and mortality. Embolectomy is one of the emergency procedures performed to remove emboli; however, the approach such as aspiration or stent retriever are empirically selected. An inappropriate selection of approach can influence the success rate during embolectomy and affect levels of brain damage. Therefore, understanding the composition of clots and their mechanical properties can lead to an appropriate treatment strategy for physicians. In this study, we investigated how the composition of human clots can affect their mechanical properties as quantified using acoustic radiation force optical coherence elastography (ARF-OCE) in a non-contact manner. Five red blood cells (RBCs) concentrations were fabricated from fibrin rich (21% RBC) to RBC rich (95% RBC). A 7.5 MHz highly focused transducer was used to provide acoustic radiation force exerted on the surface of the clots and an optical coherence tomography system was used to measure the wave propagation. The study showed that the trend of the wave velocities decreased with the RBC concentration increased. The study demonstrated that ARF-OCE could be a promising tool to quantify the mechanical properties of clots to inform treatment strategy for clinical interventionalists.

We demonstrate micro-scale vibrations of the porcine corneas as the result of the stimulation of the eye with air puff. Phase-resolved swept-source optical coherence tomography instrument was used to examine the post-air-puff reaction of the corneas. The impact of the intraocular pressure on vibration amplitude and frequency was examined. The results showed characteristic modes of vibration that will be correlated with the intraocular pressure level. The presented approach represents the next step towards understanding the eye dynamics due to air-puff-based mechanical stimulation.

We present a novel Brillouin fibre probe to achieve background-free remote mapping of micromechanical properties in tissue-mimicking hydrogel-water phantom. This Brillouin imaging system is designed using a low loss hollow core optical fibre and miniature objective for efficient light delivery and scattered light collection. The system efficiency and focusing power is comparable to free space Brillouin optical systems. This demonstration paves the way towards a wide range of biomedical and bioengineering applications where non-contact, label-free and damage-free mechanical imaging is required to retrieve in situ and in vivo microscopic viscoelastic properties of tissues and biomaterials.

In my presentation, I will describe the advantages and challenges of using Brillouin microspectroscopy for imaging biological tissues and sub cellular structures. I will review the recent advances in instrumentation development, including the first quantum measurements of Brillouin spectra, and outline the potential applications where Brillouin microspectroscopy is likely to play the pivotal role in assessing local viscoelastic properties.

Brillouin microscopy has recently emerged as an all-optical, contact- and label-free technique for in vivo 3D subcellular mapping of elasticity distribution within cells and tissues [1,2]. Since spontaneous Brillouin scattering is an extremely weak process, current technologies require long acquisition times to extract the Brillouin peaks, resulting incompatible with many bioimaging applications. Here, the stimulated version of the Brillouin scattering process [3] is discussed as an approach to obtain shot-noise limited Brillouin spectra in significantly shorter acquisition times, with no need of spectral dispersive elements and with high SNR and precision. This paves the way for a high-throughput platform for mechanical signature assessment, alongside with access to additional contrast mechanisms, such as viscosity and mass density. [1] G. Scarcelli et al.,Nat Methods 12(12),1132(2015) [2] J. Zhang et al,Lab Chip 17(4),663(2017) [3] I. Remer and A. Bilenca,APL Photonics 1(6),(2016)

Brillouin microscopy is a fully-optical technique that relies on the interaction between spontaneous, thermal generated soundwaves (phonons) and monochromatic laser light. Mechanical properties of biological samples can be inferred in 3D from the spectrum of the scattered light. A state-of-the-art Brillouin microscope consists of a confocal microscope coupled with a spectrometer that features an acquisition time ~<100ms. That is few orders of magnitude higher than a fluorescence confocal microscope, making it challenging to image fast process and/or photosensitive samples. To overcome this limitation, we implemented a line-scanning approach, that allows to acquire multiple spectra in a single camera acquisition, effectively reducing the acquisition time per point by <100x. We designed the system to achieve high resolution (0.8NA) within a FOV of 200μm and use near-IR wavelength for reduced photodamage. The illumination and detection objectives are mounted in a 90 degrees inverted ‘V’ configuration, that facilitates mounting and live-imaging of small organisms (e.g. mouse embryos) in physiological conditions. We furthermore implemented a concurrent fluorescence light-sheet imaging modality into our setup to correlate the mechanical signals to their underlying molecular constituents. Moreover, we developed a GPU-enhanced, fast numerical fitting routine appropriate for the orthogonal scattering geometry and to visualize the measured spectral properties in real-time and with high localization precision. To demonstrate the capabilities of our new microscope, we imaged mouse embryo development over 48h, from 16-cells stage to late blastocyst, without observing any photodamage.

In biomedicine, the mechanical properties of cells and tissues are increasingly recognized as a diagnostic tool. Impulsive stimulated Brillouin scattering (ISBS) is promising to overcome the long measurement durations of the established (spontaneous) Brillouin microscopy. ISBS microscopy offers fast three-dimensional non-contact measurements on in vivo samples. In ISBS microscopy, a standing acoustic wave is excited by an interference fringe pattern generated by an ultra-short pulsed laser. The oscillation frequency and therefore local mechanical properties can be retrieved using a cw probe laser. We will present the possibilities, limits and trade-offs for ISBS microscopy in biomedical applications.

Brillouin microscopy has been applied successfully to in-vivo corneal imaging to understand stiffness variation along different phases of keratoconus. However, the in-vivo spatial resolution is restricted due to a conflict between a slow Brillouin acquisition rate of hundreds of milliseconds and human movement during acquisition. In this research, frequency domain optical coherence tomography and image tracking are proved to be effective methods to locate points measured by Brillouin microscopy in axial and radial directions, respectively. Results show that OCT has an accuracy of better than 2 μm while image tracking has an accuracy better than 3 μm.

Retinal diseases such as diabetic retinopathy and glaucoma are leading cause of blindness in the world. Assessing biomechanical properties of retina is very crucial since it is constantly under stress due to the vitreous humor and eye movements. Characterizing biomechanical properties of retina noninvasively is a challenge due to its location inside the eye-globe, fragility, and thin geometry. Brillouin microscopy is a noninvasive, all optical imaging technique to qualitatively map the biomechanical properties of tissues. In this work, we mapped the layer by layer distribution of biomechanical properties of retinas using Brillouin microscopy. We found that the nuclear layer was stiffer compared to other layers. Furthermore, we observed fixing the retinas with paraformaldehyde increased the retinal stiffness compared to the fresh retinas.

Acoustic Radiation Force Impulse (ARFI) imaging employs sound impulses to induce and track tissue motion that conveys mechanical property. In application to carotid atherosclerotic plaque, ARFI noninvasively delineates features that convey risk for ischemic stroke: intraplaque hemorrhage, lipid-rich necrotic core, thin or ruptured fibrous cap, diffuse collagen, and calcium deposits. This talk will feature recent advances to in vivo ARFI atherosclerosis imaging technology, including the use of machine learning for automated feature classification, which are validated histologically in patients undergoing carotid endarterectomy. New developments, including incorporating vector blood flow estimation for combined shear strain assessment, will be highlighted.

Skin aging is a complex process accompanied by alterations in skin structure and function. We have investigated the age-related changes in collagen on dermal collagen’s physical properties. Nanomechanical properties of collagen fibrils studied using atomic force microscope nano-indentation and viscoelasticity of the collagen fibers assessed by the acoustic microscopy was investigated. The increase in Young’s modulus with increased age correlated with the increase in sound speed and attenuation is probably due to a change in the density of the fibrils caused by water retention within the fibrils and cross-linkage between fibrils.

In this study, we propose to measure the spatial deformation spreading (SDS) produced by an air-pulse on the surface of tissues during the near-field regime of propagation as a metric to characterize degree of anisotropy. Experiments in isotropic tissue-mimicking phantoms and anisotropic chicken tibialis muscle were conducted using an optical coherence tomography system synchronized with a confocal air-pulse stimulation. SDS ratio measured along versus across the direction of fibers in chicken muscle agreed with the wave speed ratio taken at the same directions, demonstrating the capabilities of the air-pulse SDS technique in measuring the elastic anisotropy of transverse isotropic tissues.

An important factor in assessing tumor status for determining therapeutic and surgical tactics is the study of its morphological structure. In particular, preoperative differentiation/grading of colorectal adenocarcinomas is very important for the adjustment of neoadjuvant therapy, surgical technique choice and survival prediction. To achieve the maximum benefit and to avoid unnecessary side effects, the ability to distinguish high-grade from low-grade colorectal adenocarcinoma, as well as identification of patients with high recurrence risk before the possible initiation of treatment will increase the quality and duration of patients' life. Taking into account the peculiarities of the biology of intestinal cancer, optical coherence elastography, as a method of studying tissue structure at a shallow depth, may be promising in the study of colorectal adenocarcinoma. Possibilities of optical coherence elastography in differentiating low-grade and high-grade types of colorectal adenocarcinoma are demonstrated in this study. The reported results are based on revealed qualitative criteria for assessment of OCE images in combination with quantitative statistically significant differences in characteristic stiffness values for low-grade and high-grade colorectal adenocarcinomas.

We demonstrate Endoscopic Optical Coherence Elastography (Endo-OCE) imaging of bronchus in an artificial breathing lung model that can breathe at 10 times per minute with a tidal volume of 500 ml. A 1.2 mm motorized catheter was delivered to the distal bronchus via the working channel of a clinical bronchoscope. Endo-OCE images was acquired at 3000 frames/s during the exhalation of the "breath". The results show that the passive breathing of the lung model can induce sufficient tissue deformation (strain) for Endo-OCE imaging. The lung model can be a useful tool to validate the Endo-OCE technology.

We characterized the stiffness and anisotropy of the equatorial sclera and its dependence on intraocular pressure (IOP). Porcine eyes were tested using ultrasound elastography under IOP control. Shear waves were induced using a mechanical shaker, and an off-axis 17.8 MHz transducer used to track the wave propagation in the equatorial and anterior-posterior directions. Wave speed was measured and used to estimate directional Young’s moduli. Anisotropy was defined as the ratio of the equatorial moduli to the anterior-posterior moduli. Sclera was stiffer in the equatorial direction, with anisotropy decreasing with IOP, from, 5.1 at 10 mmHg to 3.7 at 30 mmHg.

The biomechanical characteristics of ocular tissues have a profound influence on the normal function of the human eye. We have developed noninvasive optical coherence elastography approaches to measure elastic properties of the ocular tissues. Biomechanical properties are evaluated based on developed mathematical models of the dynamic deformation of the viscoelastic medium. We have demonstrated the influence of such factors as age and intraocular pressure on the mechanical properties of the crystalline lens, cornea, and other tissues. The optical coherence elastography was demonstrated as a promising tool for noninvasive assessment of the biomechanical properties of the ocular and other tissues.

We recently proposed an alternative elasticity measurement technique based on elastic wave propagation within a single cell. At this scale, waves at a frequency rarely reached in the field of elastography (15kHz), are captured using an ultrafast camera and a microscope. This technique is based on the local measurement of the speed of a shear wave, a type of elastic wave. By assuming an infinite and homogeneous elastic medium with respect to the wavelength, the shear modulus μ (elasticity) is estimated. These latter assumptions are discussed through experiments conducted in controlled elastic solids at micro-scale. The conclusion is that wave guide effects as well as viscosity are crucial for quantitative mapping of elasticity.

When a wave field is measured within a propagative medium, it is widely accepted that the resulting image resolution depends on the measuring point density, and no longer on the wavelength. Indeed, in-situ measurements allow the near-field details needed for super-resolution to be retrieved. Rarely studied in elastography, this is supported here by experiments in ultrasounds and optics. A passive elastography imaging of two inclusions in a tissue mimicking phantom is shown with a resolution down to 1/45th of a shear wavelength.

Corneal biomechanical properties are of great importance in the structural homeostasis of the cornea, the principle focusing element of the eye. Because of the exquisite relationship between corneal geometry and optical performance, alterations in these properties have direct consequences for human visual function. The precision and safety of corneal refractive surgery procedures are highly influenced by inter-individual differences in these properties, and focal corneal weakness is a key feature of corneal ectatic diseases such as keratoconus. This presentation will highlight clinical applications of corneal elasticity imaging and advances in the implementation of an applanation-based approach to optical coherence elastography.

The purpose of this study is to evaluate the changes in elasticity of the corneo-scleral limbus when the eye-globe is subjected to different intraocular pressures. Continuous elastic waves were mechanically induced in the sclera and imaged using a phase-sensitive optical coherence tomography system. Experimental results showed a marked heterogeneity of wave speed across the corneal-limbal-scleral region, which increased as a function of IOP. The conclusion of this study reveals that the corneo-scleral limbus tissue is more mechanically nonlinear compared to the cornea and sclera. This suggests that limbus acts as a supporting and accommodating mechanical structure for the cornea.

Accurate estimates of corneal mechanical properties may improve diagnosis and treatment of many ophthalmic conditions. Recently, we introduced a nearly incompressible transversely isotropic (NITI) model based on two independent shear moduli determining tensile and out-of-plane shear behavior. Here, we directly compare acoustic micro-tapping OCE (AμT-OCE) modulus estimates with those obtained from mechanical testing on ex-vivo porcine corneas. Both OCE and mechanical testing show tensile behavior governed by a Young’s modulus on the order of MPa and out-of-plane shear behavior by a modulus on the order of tens of kPa, suggesting strong anisotropy in the cornea.

Anisotropic mechanical properties are a feature of many load-bearing tissues, such as muscles or blood vessel walls, which typically contain preferentially oriented collagen or elastin fibers embedded in a hydrated matrix. Measuring the anisotropy on a microstructural scale is important for modeling tissues' mechanical response, but has been difficult to obtain noninvasively. Here, we employ Brillouin microscopy to characterize the anisotropy of corneal tissues. We use angle-resolved Brillouin measurements to determine the longitudinal moduli of the ex vivo porcine cornea, assuming a lamellar, transverse isotropic model. We also observe significant anisotropy in Brillouin maps of the human cornea in vivo.

Optical Coherence Elastography (OCE) and a gold-standard method, oscillatory test, provide estimates of the shear modulus of a vitreous humour at different frequencies and are therefore impossible to compare. To reconcile these two methods, we investigated the frequency dependence of the shear modulus of a phantom of an extracted vitreous. The values obtained in the high frequency range with OCE were extrapolated and compared with the oscillatory test data for lower frequencies. The preliminary results show some discrepancy that might stem either from overestimation of the OCE data due to the wave excitation procedure or from a partially incomplete model.

Tissue mechanics have been widely studied for the past two decades because they are significantly associated with disease states. The variations of mechanical properties in soft tissues are considerable indicators used for clinical diagnosis and disease monitoring. Optical coherence elastography (OCE) has been extensively developed to characterize the mechanical properties of various tissues. However, these methods are generally based on time-domain data and measure the time-offlight of the localized shear wave propagations. Here, we propose a method to evaluate phase velocity in four-dimensional (4D) space (x, y, z, f), called 4D-OCE phase velocity, and applied it to a heterogeneous phantom with a 4 mm diameter inclusion. A 7.5 MHz highly focused transducer transmitting a toneburst of 500 μs was used to provide an acoustic radiation force (ARF) for transient elastography. A customized 4D acquisition pattern was developed under Microsoft Visual C++ 2019 development environment. The 3D-OCE group velocity was reconstructed to compare with 4D-OCE phase velocity as cross-verification. The experimental result from the heterogeneous phantom showed that 4D-OCE phase velocity is capable of accurately reconstructing a 4D-OCE phase velocity map and provides good contrast between two gelatin materials with different concentrations, 7% versus 14%. The proposed 4D-OCE phase velocity can provide further information to better understand the delicate change of mechanical properties of various biological tissues in each voxel over a range of temporal frequencies.

Brillouin spectroscopy has recently emerged as a valuable tool for assessing microscopic viscoelastic properties in biological tissues and cells. For many practical biomedical applications, the viscoelastic measurement techniques should be sensitive to low sample concentrations in biological media. In this report, we assess the sensitivity of a recently improved impulsive stimulated Brillouin scattering (ISBS) setup. We explored biologically relevant solutions in distilled water using citric acid, glycine, and sucrose, for which we performed Brillouin measurements. We detailed the peak fitting methodology and analyzed the Brillouin shift and linewidth as a function of concentration. We discuss the sensitivity of the ISBS setup to low concentration measurements and its implications to biological applications.

Techniques aimed at the non-invasive characterization of soft tissues according to elastic properties are rapidly evolving. Virtual touch-based elastographic methods including acoustic radiation force imaging (ARFI) and optical elastography measure the peak axial displacement (PD) and time-to-peak-displacement (TTP) of tissue in response to a localized force. These measurements have been used clinically to differentiate tissues, albeit with mixed results. However, to date, the reason has not been fully understood. In this study, we apply a novel modeling approach to explore the mechanistic link between simplistic displacement measurements and tissue viscoelasticity in the application of virtual touch-based elastographic methods to staging chronic liver disease (CLD). To our knowledge, such a study has not been reported in the literature. Specifically, a numerical screening study was first conducted to identify factors that most strongly determine PD and TTP. Response surface experimental designs were then applied to these factors to produce meta-models of expected PD and TTP probability density functions (PDFs) as functions of identified factors. Results from the screening study suggest that both PD and TTP measurements are primarily influenced by three factors: the initial Young’s modulus of the tissue, the first viscoelastic Prony series time constant, and pre-compression ap- plied during acquisition. To investigate the implications of these results, stochastic inputs for these three factors associated were used to determine a robust response surface. The identified response surface methodology can be used to determine optimal cutoff values for PD and TTP that could be used in order to stage chronic liver disease.

We present here our latest results on noise correlation based optical elastography using off-axis digital elastography. In this study, noise correlation elastography is used to access quantitative measurement of stiffness anisotropy. The principle is to numerically refocus the diffuse shear wave field at each pixel using noise-correlation algorithms. The refocusing gives not only access to the local shear wave wavelength directly related to the local shear wave speed but also to the mechanical anisotropy through the 2D shape of the refocusing. The method is validated on finite difference simulation and first experimental measure is presented.