This PDF file contains the front matter associated with SPIE Proceedings Volume 9327, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

Many disease processes such as cancer cause profound changes in the mechanical properties of tissues. This accounts for the efficacy of palpation for detecting abnormalities and provides motivation for developing practical methods to quantitatively image tissue elasticity. Magnetic Resonance Elastography (MRE) is an emerging MRI-based technique that can quantitatively image tissue properties such as stiffness, viscosity, attenuation, and anisotropic behavior - providing access to a new range of previously unexplored tissue imaging biomarkers highly relevant in diagnostic medicine and in the emerging field of mechanobiology.

Human studies have demonstrated that it is feasible to apply MRE to quantitatively assess skeletal muscle, brain, thyroid, breast, myocardium, kidney, liver, and skin. The first established clinical application of the technology is for detection of hepatic fibrosis, which is a growing health problem and the most important precedent to primary hepatic malignancy. Growing clinical experience indicates that MRE is at least as accurate as liver biopsy for this diagnosis, while also being safer, more comfortable, and less expensive.

Preliminary studies suggest that MRE may be helpful in differentiating between benign and malignant neoplasms. New research has also shown that MRE-assessed estimates of tumor stiffness are helpful in the preoperative assessment of patients with brain tumors such as menigiomas.

We demonstrate a novel method for noninvasive elasticity mapping in three dimensions using phase stabilized swept source optical coherence elastography (PhS-SSOCE). By calculating the velocity in all radial directions from the origin of the induced shear wave, a volumetric elasticity map of the sample was generated. Due to the submicrometer spatial sensitivity of PhS-SSOCE, the loading force and the induced deformation amplitude can be minimal, thus preserving the structure and function of delicate tissues such as the cornea and sclera of the eye. Tissue mimicking agar phantoms were utilized for proof of concept testing and the results show that this method can noninvasively provide a three dimensional estimation of sample elasticity.

Tissue stiffness can be measured from the propagation speed of shear waves. Acoustic radiation force (ARF) can generate shear waves by focusing ultrasound in tissue for ~100 μs. Safety considerations and electronics abilities limit ultrasound pressures. We previously presented shear wave elastography combining ARF and phase-sensitive optical coherence tomography (PhS-OCT) [1]. Here, we use amplitude-modulated ARF to enhance shear wave signal-to-noise ratio (SNR) at low pressures. Experiments were performed on tissue-mimicking phantoms. ARF was applied using a single-element transducer, driven by a 7.5 MHz, 3-ms, sine wave modulated in amplitude by a linear-swept frequency (1 to 7 kHz). Pressures between 1 to 3 MPa were tested. Displacements were tracked using PhS-OCT and numerically compressed using pulse compression methods detailed in previous work [2]. SNR was compared to that of 200-μs bursts. Stiffness maps were reconstructed using time-of-flight computations. 200-μs bursts give barely detectable displacements at 1 MPa (3.7 dB SNR). Pulse compression gives 36.2 dB at 1.5 MPa. In all cases with detectable displacements, shear wave speeds were determined in 5%-gelatin and 10%-gelatin phantoms and compared to literature values. Applicability to ocular tissues (cornea, intraocular lens) is under investigation.

Characterization of the relationship between external pressure and blood flow is important in the examination of pressure-induced disturbance in tissue microcirculation. Optical coherence tomography (OCT) angiography is a promising imaging technique, capable of providing the noninvasive extraction of functional vessels within the skin tissue with capillary-scale resolution. Here, we present a feasibility study of OCT angiography to monitor effect of external pressures on blood perfusion in human skin tissue in vivo. Graded external pressure is loaded normal to the surface of the nailfold tissue of a healthy human. The incremental loading is applied step by step and then followed by an immediate release. Concurrent OCT imaging of the nailfold is performed during the pre/post loading. Blood perfusion images including baseline (at pre-loading) and corresponding tissue strain maps are calculated from 3D OCT dataset obtained at the different applied pressures, allowing visualization of capillary perfusion events at stressed nailfold tissue. The results indicate that the perfusion progressively decreases with the constant increase of tissue strain. Reactive hyperemia is occurred right after the removal of the pressure corresponding to quick drop of the increased strain. The perfusion is returned to the baseline level after a few minutes. These findings suggest that OCT microangiography may have great potential for quantitatively assessing tissue microcirculation in the locally pressed tissue in vivo.

We present a dual wavelength endoscope which uses Endoscopic Laser Speckle Contrast Analysis (ELASCA) with the aim to image tissue blood flow and perfusion during surgical procedures. In this study we measure speckle decorelation times, which are associated with flow, by imaging speckle patterns at a wide range of detector exposure times. In order to understand the effects of image collection efficiency and sample scattering properties, control experiments with different optical systems were performed by imaging of tissue mimicking phantoms and inferring their flow parameters.

Probing the mechanical properties of skin at high resolution could aid in the assessment of skin pathologies by, for example, detecting the extent of cancerous skin lesions and assessing pathology in burn scars. Here, we present two elastography techniques based on optical coherence tomography (OCT) to probe the local mechanical properties of skin. The first technique, optical palpation, is a high-resolution tactile imaging technique, which uses a complaint silicone layer positioned on the tissue surface to measure spatially-resolved stress imparted by compressive loading. We assess the performance of optical palpation, using a handheld imaging probe on a skin-mimicking phantom, and demonstrate its use on human skin. The second technique is a strain imaging technique, phase-sensitive compression OCE that maps depth-resolved mechanical variations within skin. We show preliminary results of in vivo phase-sensitive compression OCE on a human skin lesion.

We have estimated the micro-mechanical properties of ovarian tissue using phase-sensitive swept source optical coherence tomography. Ovary samples were mechanically excited by periodical vibration of an ultrasound transducer. The displacement and strain of the tissues were calculated during loading. Significant difference in strain was observed between the normal and malignant ovary groups, which indicates much softer and heterogeneous tissue structure for malignant ovaries. The initial results show that the phase sensitive swept source optical coherence elastography (OCE) can be an effective tool for characterization of stiffness and other micro-mechanical properties of normal and malignant ovarian tissue.

Biomechanical properties of arterial wall is crucial for understanding the changes in the cardiovascular system. Catheters are used during intravascular optical coherence tomography (IVOCT) imaging. The presence of a catheter alters the flow field, pressure distribution and frictional resistance to flow in an artery. In this paper, we first study the transmural stress distribution of the catheterized vessel. COMSOL (COMSOL 4.4) was used to simulate the blood flow induced deformation in a catheterized vessel. Blood is modeled as an incompressible Newtonian fluid. Stress distribution from an three-layer vascular model with an eccentric catheter are simulated, which provides a general idea about the distribution of the displacement and the stress. Optical coherence elastography techniques were then applied to porcine carotid artery samples to look at the deformation status of the vascular wall during saline or water injection. Preliminary simulation results show nonuniform stress distribution in the circumferential direction of the eccentrically catheterized vascular model. Three strain rate methods were tested for intravascular OCE application. The tissue Doppler method has the potential to be further developed to image the vascular wall biomechnical properties in vivo. Although results in this study are not validated quantitatively, the experiments and methods may be valuable for intravascular OCE studies, which may provide important information for cardiovascular disease prevention, diagnosis and treatment.

Optical coherence elastography (OCE) is an emerging low-coherence imaging technique that provides noninvasive assessment of tissue biomechanics with high spatial resolution. Among various OCE methods, the capability of quantitative measurement of tissue elasticity is of great importance for tissue characterization and pathology detection across different samples. Here we report a quantitative OCE technique, termed quantitative shear wave imaging optical coherence tomography (Q-SWI-OCT), which enables noncontact measurement of tissue Young’s modulus based on the ultra-fast imaging of the shear wave propagation inside the sample. A focused air-puff device is used to interrogate the tissue with a low-pressure short-duration air stream that stimulates a localized displacement with the scale at micron level. The propagation of this tissue deformation in the form of shear wave is captured by a phase-sensitive OCT system running with the scan of the M-mode imaging over the path of the wave propagation. The temporal characteristics of the shear wave is quantified based on the cross-correlation of the tissue deformation profiles at all the measurement locations, and linear regression is utilized to fit the data plotted in the domain of time delay versus wave propagation distance. The wave group velocity is thus calculated, which results in the quantitative measurement of the Young’s modulus. As the feasibility demonstration, experiments are performed on tissuemimicking phantoms with different agar concentrations and the quantified elasticity values with Q-SWI-OCT agree well with the uniaxial compression tests. For functional characterization of myocardium with this OCE technique, we perform our pilot experiments on ex vivo mouse cardiac muscle tissues with two studies, including 1) elasticity difference of cardiac muscle under relaxation and contract conditions and 2) mechanical heterogeneity of the heart introduced by the muscle fiber orientation. Our results suggest the potential of using Q-SWI-OCT as an essential tool for nondestructive biomechanical evaluation of myocardium.

Living cells possess an exquisite ability to sense and respond to physical information in their microenvironment. This ability plays a key role in many fundamentally important physiological and pathological processes. We will describe our work utilizing a variety of biophysical tools to investigate the dynamic responses of cells to mechanical stimuli and how physical cues can be employed to re-purpose and manipulate biological processes. These responses to physical cues are not simply a side-product of biology but are key components of biological and physical feedback loops that govern the life of a cell.

Particle tracking microrheology (PTM) has recently been employed as a non-destructive way to longitudinally track physical changes in 3D pancreatic tumor co-culture models concomitant with tumor growth and invasion into the extracellular matrix (ECM). While the primary goal of PTM is to quantify local viscoelasticity via the Generalized Stokes-Einstein Relation (GSER), a more simplified way of describing local tissue mechanics lies in the tabulation and subsequent visualization of the spread of probe displacements in a given field of view. Proper analysis of this largely untapped byproduct of standard PTM has the potential to yield valuable insight into the structure and integrity of the ECM. Here, we use clustering algorithms in R to analyze the trajectories of probes in 3D pancreatic tumor/fibroblast co-culture models in an attempt to differentiate between probes that are effectively constrained by the ECM and/or contractile traction forces, and those that exhibit uninhibited mobility in local water-filled pores. We also discuss the potential pitfalls of this method. Accurately and reproducibly quantifying the boundary between these two categories of probe behavior could result in an effective method for measuring the average pore size in a given region of ECM. Such a tool could prove useful for studying stromal depletion, physical impedance to drug delivery, and degradation due to cellular invasion.

Brillouin microspectroscopy is a powerful tool for elasticity-sensitive non-invasive optical imaging and sensing. However, spontaneous Brillouin spectroscopy usually requires long integration time, while its spectral quality is fundamentally limited by the optical setups. In this report, we demonstrated that nonlinear Brillouin spectroscopy based on impulsive stimulated Brillouin spectroscopy is capable of providing the necessary acquisition speed and spectral resolution of elasticity measurements. As a proof-of-principle, we demonstrated potential flow-cytometry applications.

Collagen is a long fibrous structural protein that imparts mechanical support, strength and elasticity to many tissues. The state of the tissue mechanical environment is related to tissue physiology, disease and function. In the cornea, the collagen network is responsible for its shape and clarity; disruption of this network results in degradation of visual acuity, for example in the keratoconus eye disease. The objective of the present study is to investigate the feasibility of using the endogenous fluorescence of collagen crosslinks to evaluate variations in the mechanical state of tissue, in particular, the stiffness of cornea in response to different degrees of photo-crosslinking or RGX treatment—a novel keratoconus treatment. After removing the epithelium, rabbit corneas were stained with Rose Bengal and then irradiated with a 532 nm solid-state laser. Analysis of the excitation spectra obtained by fluorescence spectroscopy shows a correlation between the fluorescence intensity at 370/460 nm excitation/emission wavelengths and the mechanical properties. In principle, it may be feasible to use the endogenous fluorescence of collagen crosslinks to evaluate the mechanical stiffness of cornea non-invasively and in situ.

Keratoconus, a structural degeneration of the cornea, is often treated with UV-induced collagen cross-linking (CXL) to increase tissue resistance to further deformation and degeneration. Optimal treatment would be customized to the individual and consider pre-existing biomechanical properties as well as the effects induced by CXL. This requires the capability to noninvasively measure corneal mechanical properties. In this study, we demonstrate the use of phase-stabilized swept source optical coherence elastography (PhS-SSOCE) to assess the relaxation rate of a deformation which was induced by a focused air-pulse in tissue-mimicking gelatin phantoms of various concentration and partially cross-linked rabbit corneas. The temporal relaxation process was utilized to estimate the Young’s modulus from a newly developed model based elasticity reconstruction method. Due to the high spatial sensitivity of PhS-SSOCE, the deformation was only a few microns. The results show that the relaxation process was successfully used to differentiate the untreated (UT) and CXL region of the cornea. The results also indicate that the CXL regions had faster relaxation rates and greater Young’s moduli than the UT regions. Therefore, this method can be used to spatially assess the stiffness of the cornea. This non-contact and noninvasive measurement technique utilizes minimal force for excitation and can be potentially used to study the biomechanical properties of ocular and other sensitive tissues.

Shear Wave Optical Coherence Elastography (SW-OCE) uses the speed of propagating shear waves to provide a quantitative measurement of localized shear modulus, making it a valuable technique for the elasticity characterization of tissues such as skin and ocular tissue. One of the main challenges in shear wave elastography is to induce a reliable source of shear wave; most of nowadays techniques use external vibrators which have several drawbacks such as limited wave propagation range and/or difficulties in non-invasive scans requiring precisions, accuracy. Thus, we propose linear phase array ultrasound transducer as a remote wave source, combined with the high-speed, 47,000-frame-per-second Shear-wave visualization provided by phase-sensitive OCT. In this study, we observed for the first time shear waves induced by a 128 element linear array ultrasound imaging transducer, while the ultrasound and OCT images (within the OCE detection range) were triggered simultaneously. Acoustic radiation force impulses are induced by emitting 10 MHz tone-bursts of sub-millisecond durations (between 50 μm – 100 μm). Ultrasound beam steering is achieved by programming appropriate phase delay, covering a lateral range of 10 mm and full OCT axial (depth) range in the imaging sample. Tissue-mimicking phantoms with agarose concentration of 0.5% and 1% was used in the SW-OCE measurements as the only imaging samples. The results show extensive improvements over the range of SW-OCE elasticity map; such improvements can also be seen over shear wave velocities in softer and stiffer phantoms, as well as determining the boundary of multiple inclusions with different stiffness. This approach opens up the feasibility to combine medical ultrasound imaging and SW-OCE for high-resolution localized quantitative measurement of tissue biomechanical property.

High-resolution elasticity mapping of tissue biomechanical properties is crucial in early detection of many diseases. We report a method of acoustic radiation force optical coherence elastography (ARF-OCE) based on the methods of vibroacoustography, which uses a dual-ring ultrasonic transducer in order to excite a highly localized 3-D field. The single element transducer introduced previously in our ARF imaging has low depth resolution because the ARF is difficult to discriminate along the entire ultrasound propagation path. The novel dual-ring approach takes advantage of two overlapping acoustic fields and a few-hundred-Hertz difference in the signal frequencies of the two unmodulated confocal ring transducers in order to confine the acoustic stress field within a smaller volume. This frequency difference is the resulting “beating” frequency of the system. The frequency modulation of the transducers has been validated by comparing the dual ring ARF-OCE measurement to that of the single ring using a homogeneous silicone phantom. We have compared and analyzed the phantom resonance frequency to show the feasibility of our approach. We also show phantom images of the ARF-OCE based vibro-acoustography method and map out its acoustic stress region. We concluded that the dual-ring transducer is able to better localize the excitation to a smaller region to induce a focused force, which allows for highly selective excitation of small regions. The beat-frequency elastography method has great potential to achieve high-resolution elastography for ophthalmology and cardiovascular applications.

A combined correlation method is introduced to optical coherence elastography for axial displacement estimation. Its performance is compared with that of amplitude correlation tracking and phase shift estimation. Relative sensitivities to small (sub-micron), and large (pixel-scale) axial displacements are analysed for a Perspex test object and gelatine phantom.

The combined correlation method exhibited good overall performance, with a larger dynamic range than phase shift estimation and higher sensitivity than amplitude correlation tracking.

A quantitative measurement of the mechanical properties of biological tissue is a useful assessment of its physiologic conditions, which may aid medical diagnosis and treatment of, e.g., scleroderma and skin cancer. Traditional elastography techniques such as magnetic resonance elastography and ultrasound elastography have limited scope of application on skin due to insufficient spatial resolution. Recently, dynamic / transient elastography are attracting more applications with the advantage of non-destructive measurements, and revealing the absolute moduli values of tissue mechanical properties. Shear wave optical coherence elastography (SW-OCE) is a novel transient elastography method, which lays emphasis on the propagation of dynamic mechanical waves. In this study, high speed shear wave imaging technique was applied to a range of soft-embalmed mouse skin, where 3 kHz shear waves were launched with a piezoelectric actuator as an external excitation. The shear wave velocity was estimated from the shear wave images, and used to recover a shear modulus map in the same OCT imaging range. Results revealed significant difference in shear modulus and structure in compliance with gender, and images on fresh mouse skin are also compared. Thiel embalming technique is also proven to present the ability to furthest preserve the mechanical property of biological tissue. The experiment results suggest that SW-OCE is an effective technique for quantitative estimation of skin tissue biomechanical status.

In this study we demonstrate the use of phase-stabilized swept-source optical coherence elastography (PhS-SSOCE) to assess the biomechanical properties of porcine corneas before and after collagen cross-linking (CXL) at different intraocular pressures by measuring the velocity of an air-pulse induced elastic wave and recovery process rate of an air-pulse induced deformation. Young’s moduli were estimated by two different methods: the shear wave equation and a newly developed elasticity reconstruction model. The results show that the corneas became stiffer after the CXL treatment, as evidenced by the increased elastic wave velocity and recovery process rate and greater Young’s modulus. This non-contact and noninvasive measurement technique utilizes minimal force for excitation (deformation less than 10 μm in amplitude) of the tissue. Thus, it can be potentially used to study the biomechanical properties of ocular and other delicate tissues.

A systematic investigation was conducted on the accuracies of four analytical methods for obtaining the elasticity of soft samples by using optical coherence elastography (OCE). The results were compared to the elasticity measured by uniaxial mechanical testing. OCE has emerged as a noninvasive method for quantifying tissue biomechanical properties with spatial resolution of a few micrometers. A proper mechanical model is required for extracting the biomechanical parameters accurately from OCE measurements. In this work, tissuemimicking agar phantoms were utilized to analyze the accuracy and feasibility of four methods for reconstructing the Young’s modulus from OCE-measured elastic wave which were induced by a focused airpulse. These reconstruction methods are: the shear wave equation (SWE), the surface wave equation (SuWE), the Rayleigh-Lamb frequency equation (RLFE), and the finite element method (FEM). The reconstructed elasticity values were also compared with uniaxial mechanical testing results. It was shown that the RLFE and the FEM are more robust in quantifying elasticity than the other simplified models. This work may provide a reference for reconstructing the biomechanical properties of tissues based on OCE measurements. Accurate reconstruction of biomechanical properties is an important issue for further developing noninvasive elastography methods.

Muscular dystrophy (MD) is a group of muscle diseases that induce weakness in skeletal muscle and cause progressive muscle degeneration. The muscular mechanical properties (i.e., viscoelasticity), however, have not been thoroughly examined before and after MD. On the other hand, Brillouin spectroscopy (BS) provides a non-invasive approach to probing the local sound speed within a small volume. Moreover, recent advances in background-free Brillouin spectroscopy enable investigators to imaging not only transparent samples, but also turbid ones. In this study, we investigated the mechanical properties of muscles while employing Drosophila model of dystroglycanopathies, human congenital muscular dystrophies resulting from abnormal glycosylation of alphadystroglycan. Specifically, we analyzed larval abdominal muscles of Drosophila with mutations in protein Omannosyltransferase (POMT) genes. As a comparison, we have also examined muscular tissues dissected from wildtype Drosophila. The Brillouin spectra were obtained by a background free VIPA (virtually imaged phased array) spectrometer described in the previous report. As a reference, the Raman spectra were also acquired for each test. Our current results indicated that POMT defects cause changes in muscle elasticity, which suggests that muscular dystrophy conditions may be also associated with abnormalities in muscle elastic properties.

Current clinical methods of reconstruction surgery involve laser reshaping of nasal cartilage. The process of stress relaxation caused by laser heating is the primary method to achieve nasal cartilage reshaping. Based on this, a rapid, non-destructive and accurate elasticity measurement would allow for a more robust reshaping procedure. In this work, we have utilized a phase-stabilized swept source optical coherence elastography (PhSSSOCE) to quantify the Young’s modulus of porcine nasal septal cartilage during the relaxation process induced by heating. The results show that PhS-SSOCE was able to monitor changes in elasticity of hyaline cartilage, and this method could potentially be applied in vivo during laser reshaping therapies.

Noninvasive high-resolution depth-resolved measurement of corneal biomechanics is of great clinical significance for improving the diagnosis and optimizing the treatment of various degenerated ocular diseases. Here, we report a micro-scale optical coherence elastography (OCE) method that enables noncontact assessment of the depthwise elasticity distribution in the cornea. The OCE system combines a focused air-puff device with phase-sensitive optical coherence tomography (OCT). Low-pressure short-duration air stream is used to load the cornea with the localized displacement at micron level. The phase-resolved OCT detection with nano-scale sensitivity probes the induced corneal deformation at various locations within a scanning line, providing the ultra-fast imaging of the corneal lamb wave propagation. With spectral analysis, the amplitude spectra and the phase spectra are available for the estimation of the frequency range of the lamb wave and the quantification of the wave propagation, respectively. Curved propagation paths following the top and bottom corneal boundaries are selected inside the cornea for measuring the phase velocity of the lamb wave at the major frequency components over the whole depths. Our pilot experiments on ex vivo rabbit eyes indicate the distinct stiffness of different layers in the cornea, including the epithelium, the anterior stroma, the posterior stroma, and the innermost region, which demonstrates the feasibility of this micro-scale OCE method for noncontact depth-resolved corneal elastography. Also, the quantification of the lamb wave dispersion in the cornea could lead to the measurement of the elastic modulus, suggesting the potential of this method for quantitative monitoring of the corneal biomechanics.

In this study, we assess the properties of the contact pressure applied by manually operated probes as a function of the operator, probe contact area, and sample stiffness. For this purpose, three different human skin-like phantoms with different well-defined mechanical properties were used. To gain relevant statistics of the contact pressure properties, the study included ten experienced probe operators that were asked to apply gentle contact pressure to the skin-like phantoms by using three different contact area probes. A novel system for rapid simultaneous acquisition of spectra and corresponding contact pressure was used to collect the relevant information. Results show that the variability of the gentle contact pressure significantly depends on the probe operator, sample stiffness and probe contact area.

The Brillouin scattering spectra of biological systems have shown to be inherently related to the intrinsic elasticity and molecular constants of tissues involved. Our approach of combining confocal microscopy and high-resolution Brillouin spectroscopy via a virtual imaging phase array enabled 10-microsecond single-pixel acquisition time without dedicated spatial filtering. Such an approach is adapted via a single-frequency fiber-coupled 780-nm wavelength laser, frequency stabilized by Rb-D2 absorption line, polarization extinction scheme, ASE filtering, heated Rb-vapor Rayleigh-scattering absorbent, and spectroscopic EMCCD camera, unified as CMS-VIPA: confocal virtual-imaging phase array microscopespectrometer. Steady strengthening of corneal bulk modulus was observed via spectral shifts of Brillouin scattering from 5.0-5.2 GHz in untreated porcine eyes to 5.7-5.9 GHz in ones cross-linked in riboflavin plus UV-A light  at 0.7-0.9 GHz level of enhancement. The cross-linking depths reaching 300400 microns were measured, as predicted by modeling. A noncontact Brillouin spectroscopic microscopy system for in-vivo corneal elasticity measurement is under development.