Robust intravascular optical coherence elastography by line correlations

Learning-based distortion correction enables proximal-scanning endoscopic OCT elastography

Proximal rotary scanning is predominantly used in the clinical practice of endoscopic and intravascular OCT, mainly because of the much lower manufacturing cost of the probe compared to distal scanning. However, proximal scanning causes severe beam stability issues (also known as non-uniform rotational distortion, NURD), which hinders the extension of its applications to functional imaging, such as OCT elastography (OCE). In this work, we demonstrate the abilities of learning-based NURD correction methods to enable the imaging stability required for intensity-based OCE. Compared with the previous learning-based NURD correction methods that use pseudo distortion vectors for model training, we propose a method to extract real distortion vectors from a specific endoscopic OCT system, and validate its superiority in accuracy under both convolutional-neural-network- and transformer-based learning architectures. We further verify its effectiveness in elastography calculations (digital image correlation and optical flow) and the advantages of our method over other NURD correction methods. Using the air pressure of a balloon catheter as a mechanical stimulus, our proximal-scanning endoscopic OCE could effectively differentiate between areas of varying stiffness of atherosclerotic vascular phantoms. Compared with the existing endoscopic OCE methods that measure only in the radial direction, our method could achieve 2D displacement/strain distribution in both radial and circumferential directions.

Corneal elastic property investigated by terahertz technology

Terahertz (THz) spectroscopy technique has been applied in ex vivo biomechanical properties analysis of human corneas. Upon the application of light pressure on the cornea, the photo elastic birefringent effect, anisotropic deformation, thickness changes and hydration levels will contribute to the sudden phase changes of terahertz time domain signal. The shelf lifetime study shows that the phase shift is reduced and cornea loose the biomechanical properties with the increase of hydration level. Mechanical behaviors have been further studied based on the "fresh" cut corneas with the similar hydration levels. THz signal was collected by focusing inside of the cornea to avoid the phase shift due to light stress caused movement of the corneal surface. By this way, the amount of THz signal refractive index variation is correlated to the elastic property of the corneas. The correlation between the THz signal phase shift and refractive index shift due to the corneal strain can be used to derive the elastic Young's modulus. Our results demonstrated the THz spectroscopy, as a non-contact and non-invasive detection method, could be potential for understanding the mechanism of corneal deformation under the action of intraocular pressure in the physiological environment in future.

Camera-based optical palpation

Optical elastography is undergoing extensive development as an imaging tool to map mechanical contrast in tissue. Here, we present a new platform for optical elastography by generating sub-millimetre-scale mechanical contrast from a simple digital camera. This cost-effective, compact and easy-to-implement approach opens the possibility to greatly expand applications of optical elastography both within and beyond the field of medical imaging. Camera-based optical palpation (CBOP) utilises a digital camera to acquire photographs that quantify the light intensity transmitted through a silicone layer comprising a dense distribution of micro-pores (diameter, 30-100 µm). As the transmission of light through the micro-pores increases with compression, we deduce strain in the layer directly from intensity in the digital photograph. By pre-characterising the relationship between stress and strain of the layer, the measured strain map can be converted to an optical palpogram, a map of stress that visualises mechanical contrast in the sample. We demonstrate a spatial resolution as high as 290 µm in CBOP, comparable to that achieved using an optical coherence tomography-based implementation of optical palpation. In this paper, we describe the fabrication of the micro-porous layer and present experimental results from structured phantoms containing stiff inclusions as small as 0.5 × 0.5 × 1 mm. In each case, we demonstrate high contrast between the inclusion and the base material and validate both the contrast and spatial resolution achieved using finite element modelling. By performing CBOP on freshly excised human breast tissue, we demonstrate the capability to delineate tumour from surrounding benign tissue.

Photoacoustic elastography imaging: a review

Elastography imaging is a promising tool-in both research and clinical settings-for diagnosis, staging, and therapeutic treatments of various life-threatening diseases (including brain tumors, breast cancers, prostate cancers, and Alzheimer's disease). Large variation in the physical (elastic) properties of tissue, from normal to diseased stages, enables highly sensitive characterization of pathophysiological states of the diseases. On the other hand, over the last decade or so, photoacoustic (PA) imaging-an imaging modality that combines the advantageous features of two separate imaging modalities, i.e., high spatial resolution and high contrast obtainable, respectively, from ultrasound- and optical-based modalities-has been emerging and widely studied. Recently, recovery of elastic properties of soft biological tissues-in addition to prior reported recovery of vital tissue physiological information (Hb, HbO2, SO, and total Hb), noninvasively and nondestructively, with unprecedented spatial resolution (μm) at penetration depth (cm)-has been reported. Studies demonstrating that combined recovery of mechanical tissue properties and physiological information-by a single (PA) imaging unit-pave a promising platform in clinical diagnosis and therapeutic treatments. We offer a comprehensive review of PA imaging technology, focusing on recent advances in relation to elastography. Our review draws out technological challenges pertaining to PA elastography (PAE) imaging, and viable approaches. Currently, PAE imaging is in the nurture stage of its development, where the technology is limited to qualitative study. The prevailing challenges (specifically, quantitative measurements) may be addressed in a similar way by which ultrasound elastography and optical coherence elastography were accredited for quantitative measurements.

Finger-mounted quantitative micro-elastography

We present a finger-mounted quantitative micro-elastography (QME) probe, capable of measuring the elasticity of biological tissue in a format that avails of the dexterity of the human finger. Finger-mounted QME represents the first demonstration of a wearable elastography probe. The approach realizes optical coherence tomography-based elastography by focusing the optical beam into the sample via a single-mode fiber that is fused to a length of graded-index fiber. The fiber is rigidly affixed to a 3D-printed thimble that is mounted on the finger. Analogous to manual palpation, the probe compresses the tissue through the force exerted by the finger. The resulting deformation is measured using optical coherence tomography. Elasticity is estimated as the ratio of local stress at the sample surface, measured using a compliant layer, to the local strain in the sample. We describe the probe fabrication method and the signal processing developed to achieve accurate elasticity measurements in the presence of motion artifact. We demonstrate the probe's performance in motion-mode scans performed on homogeneous, bi-layer and inclusion phantoms and its ability to measure a thermally-induced increase in elasticity in ex vivo muscle tissue. In addition, we demonstrate the ability to acquire 2D images with the finger-mounted probe where lateral scanning is achieved by swiping the probe across the sample surface.

Ultrahigh-Resolution Optical Coherence Elastography Images Cellular-Scale Stiffness of Mouse Aorta

Cellular-scale imaging of the mechanical properties of tissue has helped to reveal the origins of disease; however, cellular-scale resolution is not readily achievable in intact tissue volumes. Here, we demonstrate volumetric imaging of Young's modulus using ultrahigh-resolution optical coherence elastography, and apply it to characterizing the stiffness of mouse aortas. We achieve isotropic resolution of better than 15 μm over a 1-mm lateral field of view through the entire depth of an intact aortic wall. We employ a method of quasi-static compression elastography that measures volumetric axial strain and uses a compliant, transparent layer to measure surface axial stress. This combination is used to estimate Young's modulus throughout the volume. We demonstrate differentiation by stiffness of individual elastic lamellae and vascular smooth muscle. We observe stiffening of the aorta in regulator of G protein signaling 5-deficient mice, a model that is linked to vascular remodeling and fibrosis. We observe increased stiffness with proximity to the heart, as well as regions with micro-structural and micro-mechanical signatures characteristic of fibrous and lipid-rich tissue. High-resolution imaging of Young's modulus with optical coherence elastography may become an important tool in vascular biology and in other fields concerned with understanding the role of mechanics within the complex three-dimensional architecture of tissue.

Depth-encoded optical coherence elastography for simultaneous volumetric imaging of two tissue faces

Depth-encoded optical coherence elastography (OCE) enables simultaneous acquisition of two three-dimensional (3D) elastograms from opposite sides of a sample. By the choice of suitable path-length differences in each of two interferometers, the detected carrier frequencies are separated, allowing depth-ranging from each interferometer to be performed simultaneously using a single spectrometer. We demonstrate depth-encoded OCE on a silicone phantom and a freshly excised sample of mouse liver. This technique minimizes the required spectral detection hardware and halves the total scan time. Depth-encoded OCE may expedite clinical translation in time-sensitive applications requiring rapid 3D imaging of multiple tissue surfaces, such as tumor margin assessment in breast-conserving surgery.

Optical coherence elastography - OCT at work in tissue biomechanics [Invited]

Optical coherence elastography (OCE), as the use of OCT to perform elastography has come to be known, began in 1998, around ten years after the rest of the field of elastography - the use of imaging to deduce mechanical properties of tissues. After a slow start, the maturation of OCT technology in the early to mid 2000s has underpinned a recent acceleration in the field. With more than 20 papers published in 2015, and more than 25 in 2016, OCE is growing fast, but still small compared to the companion fields of cell mechanics research methods, and medical elastography. In this review, we describe the early developments in OCE, and the factors that led to the current acceleration. Much of our attention is on the key recent advances, with a strong emphasis on future prospects, which are exceptionally bright.

Optical coherence elastography for tissue characterization: a review

Optical coherence elastography (OCE) represents the frontier of optical elasticity imaging techniques and focuses on the micro-scale assessment of tissue biomechanics in 3D that is hard to achieve with traditional elastographic methods. Benefit from the advancement of optical coherence tomography, and driven by the increasing requirements in nondestructive biomechanical characterization, this emerging technique recently has experienced a rapid development. In this paper, we start with the description of the mechanical contrast that has been employed by OCE and review the state-of-the-art techniques based on the reported applications and discuss the current technical challenges, emphasizing the unique role of OCE in tissue mechanical characterization. The position of OCE among other elastography techniques.

Photoacoustic tomography of vascular compliance in humans

Characterization of blood vessel elastic properties can help in detecting thrombosis and preventing life-threatening conditions such as acute myocardial infarction or stroke. Vascular elastic photoacoustic tomography (VE-PAT) is proposed to measure blood vessel compliance in humans. Implemented on a linear-array-based photoacoustic computed tomography system, VE-PAT can quantify blood vessel compliance changes due to simulated thrombosis and occlusion. The feasibility of the VE-PAT system was first demonstrated by measuring the strains under uniaxial loading in perfused blood vessel phantoms and quantifying their compliance changes due to the simulated thrombosis. The VE-PAT system detected a decrease in the compliances of blood vessel phantoms with simulated thrombosis, which was validated by a standard compression test. The VE-PAT system was then applied to assess blood vessel compliance in a human subject. Experimental results showed a decrease in compliance when an occlusion occurred downstream from the measurement point in the blood vessels, demonstrating VE-PAT’s potential for clinical thrombosis detection.

Quantitative elasticity measurement of urinary bladder wall using laser-induced surface acoustic waves

The maintenance of urinary bladder elasticity is essential to its functions, including the storage and voiding phases of the micturition cycle. The bladder stiffness can be changed by various pathophysiological conditions. Quantitative measurement of bladder elasticity is an essential step toward understanding various urinary bladder disease processes and improving patient care. As a nondestructive, and noncontact method, laser-induced surface acoustic waves (SAWs) can accurately characterize the elastic properties of different layers of organs such as the urinary bladder. This initial investigation evaluates the feasibility of a noncontact, all-optical method of generating and measuring the elasticity of the urinary bladder. Quantitative elasticity measurements of ex vivo porcine urinary bladder were made using the laser-induced SAW technique. A pulsed laser was used to excite SAWs that propagated on the bladder wall surface. A dedicated phase-sensitive optical coherence tomography (PhS-OCT) system remotely recorded the SAWs, from which the elasticity properties of different layers of the bladder were estimated. During the experiments, series of measurements were performed under five precisely controlled bladder volumes using water to estimate changes in the elasticity in relation to various urinary bladder contents. The results, validated by optical coherence elastography, show that the laser-induced SAW technique combined with PhS-OCT can be a feasible method of quantitative estimation of biomechanical properties.

Three-dimensional optical coherence micro-elastography of skeletal muscle tissue

In many muscle pathologies, impairment of skeletal muscle function is closely linked to changes in the mechanical properties of the muscle constituents. Optical coherence micro-elastography (OCME) uses optical coherence tomography (OCT) imaging of tissue under a quasi-static, compressive mechanical load to map variations in tissue mechanical properties on the micro-scale. We present the first study of OCME on skeletal muscle tissue. We show that this technique can resolve features of muscle tissue including fibers, fascicles and tendon, and can also detect necrotic lesions in skeletal muscle from the mdx mouse model of Duchenne muscular dystrophy. In many instances, OCME provides better or additional contrast complementary to that provided by OCT. These results suggest that OCME could provide new understanding and opportunity for assessment of skeletal muscle pathologies.

Serial biomechanical comparison of edematous, normal, and collagen crosslinked human donor corneas using optical coherence elastography

PURPOSE

To noninvasively evaluate the effects of corneal hydration and collagen crosslinking (CXL) on the mechanical behavior of the cornea.

SETTING

Cleveland Clinic Cole Eye Institute, Cleveland, Ohio, USA.

DESIGN

Experimental study.

METHODS

An optical coherence elastography (OCE) technique was used to measure the displacement behavior of 5 pairs of debrided human donor globes in 3 serial states as follows: edematous, normal thickness, and after riboflavin-ultraviolet-A-mediated CXL. During micromotor-controlled axial displacements with a curved goniolens at physiologic intraocular pressure (IOP), serial optical coherence tomography scans were obtained to allow high-resolution intrastromal speckle tracking and displacement measurements over the central 4.0 mm of the cornea.

RESULTS

With no imposed increase in IOP, the mean lateral to imposed axial displacement ratios were 0.035 μm/μm ± 0.037 (SD) in edematous corneas, 0.021 ± 0.02 μm/μm in normal thickness corneas, and 0.014 ± 0.009 μm/μm in post-CXL corneas. The differences were statistically significant (P<.05, analysis of variance) and indicated a 40% increase in lateral stromal resistance with deturgescence and a further 33% mean increase in relative stiffness with CXL.

CONCLUSIONS

Serial perturbations of the corneal hydration state and CXL had significant effects on corneal biomechanical behavior. With an axially applied stress from a nonapplanating contact lens, displacements along the direction of the collagen lamellae were 2 orders of magnitude lower than axial deformations. These experiments show the ability of OCE to quantify clinically relevant mechanical property differences under physiologic conditions.

FINANCIAL DISCLOSURES

Proprietary or commercial disclosures are listed after the references.

Three-dimensional optical coherence elastography by phase-sensitive comparison of C-scans

We present an acquisition method for optical coherence elastography (OCE) that enables acquisition of three-dimensional elastograms in 5 s, an order of magnitude faster than previously reported. In this method, based on compression elastography, the mechanical load applied to the sample is altered between acquisitions of consecutive optical coherence tomography volume scans (C-scans). The voxel-by-voxel phase difference between the volumes is used to determine the axial displacement and determining the gradient of the axial displacement versus depth gives the local axial strain. We demonstrate sub-100-microstrain sensitivity and high contrast in elastograms, acquired in 5 s, of structured phantoms and freshly excised rat muscle tissue that are comparable in strain sensitivity and dynamic range to our previously reported B-scan-based method. The much higher acquisition speed may expedite the translation of OCE to clinical and in vivo applications.

Novel methods for elasticity characterization using optical coherence tomography: Brief review and future prospects

In this paper, a brief overview of several recently proposed approaches to elastographic characterization of biological tissues using optical coherence tomography is presented. A common feature of these “unconventional” approaches is that unlike most others, they do not rely on a two-step process of first reconstructing the particle displacements and then performing its error-prone differentiation in order to determine the local strains. Further, several variants of these new approaches were proposed and demonstrated essentially independently and are based on significantly different principles. Despite the seeming differences, these techniques open up interesting prospects not only for independent usage, but also for combined implementation to provide a multifunctional investigation of elasticity of biological tissues and their rheological properties in a wider sense.

Optical measurement of arterial mechanical properties: from atherosclerotic plaque initiation to rupture

During the pathogenesis of coronary atherosclerosis, from lesion initiation to rupture, arterial mechanical properties are altered by a number of cellular, molecular, and hemodynamic processes. There is growing recognition that mechanical factors may actively drive vascular cell signaling and regulate atherosclerosis disease progression. In advanced plaques, the mechanical properties of the atheroma influence stress distributions in the fibrous cap and mediate plaque rupture resulting in acute coronary events. This review paper explores current optical technologies that provide information on the mechanical properties of arterial tissue to advance our understanding of the mechanical factors involved in atherosclerosis development leading to plaque rupture. The optical approaches discussed include optical microrheology and traction force microscopy that probe the mechanical behavior of single cell and extracellular matrix components, and intravascular imaging modalities including laser speckle rheology, optical coherence elastography, and polarization-sensitive optical coherence tomography to measure the mechanical properties of advanced coronary lesions. Given the wealth of information that these techniques can provide, optical imaging modalities are poised to play an increasingly significant role in elucidating the mechanical aspects of coronary atherosclerosis in the future.

Biomechanics of atherosclerotic coronary plaque: site, stability and in vivo elasticity modeling

Coronary atheroma develop in local sites that are widely variable among patients and are considerably variable in their vulnerability for rupture. This article summarizes studies conducted by our collaborative laboratories on predictive biomechanical modeling of coronary plaques. It aims to give insights into the role of biomechanics in the development and localization of atherosclerosis, the morphologic features that determine vulnerable plaque stability, and emerging in vivo imaging techniques that may detect and characterize vulnerable plaque. Composite biomechanical and hemodynamic factors that influence the actual site of development of plaques have been studied. Plaque vulnerability, in vivo, is more challenging to assess. Important steps have been made in defining the biomechanical factors that are predictive of plaque rupture and the likelihood of this occurring if characteristic features are known. A critical key in defining plaque vulnerability is the accurate quantification of both the morphology and the mechanical properties of the diseased arteries. Recently, an early IVUS based palpography technique developed to assess local strain, elasticity and mechanical instabilities has been successfully revisited and improved to account for complex plaque geometries. This is based on an initial best estimation of the plaque components' contours, allowing subsequent iteration for elastic modulus assessment as a basis for plaque stability determination. The improved method has also been preliminarily evaluated in patients with successful histologic correlation. Further clinical evaluation and refinement are on the horizon.

A four-criterion selection procedure for atherosclerotic plaque elasticity reconstruction based on in vivo coronary intravascular ultrasound radial strain sequences

Plaque elasticity (i.e., modulogram) and morphology are good predictors of plaque vulnerability. Recently, our group developed an intravascular ultrasound (IVUS) elasticity reconstruction method which was successfully implemented in vitro using vessel phantoms. In vivo IVUS modulography, however, remains a major challenge as the motion of the heart prevents accurate strain field estimation. We therefore designed a technique to extract accurate strain fields and modulograms from recorded IVUS sequences. We identified a set of four criteria based on tissue overlapping, RF-correlation coefficient between two successive frames, performance of the elasticity reconstruction method to recover the measured radial strain, and reproducibility of the computed modulograms over the cardiac cycle. This four-criterion selection procedure (4-CSP) was successfully tested on IVUS sequences obtained in twelve patients referred for a directional coronary atherectomy intervention. This study demonstrates the potential of the IVUS modulography technique based on the proposed 4-CSP to detect vulnerable plaques in vivo.

Intracoronary optical diagnostics current status, limitations, and potential

Optical coherence tomography (OCT), is a novel intravascular imaging modality analogous to intravascular ultrasound but uses light instead of sound. This review details the background, development, and status of current investigation using OCT, and discusses advantages, limitations, and likely future developments. It provides indications for possible future clinical use, and places OCT in the context of current intravascular imaging in what is a rapidly changing field of investigation.

Magnetic Resonance Elastography

Often compared to the practice of manual palpation, magnetic resonance elastography is an emerging technology for quantitatively assessing the mechanical properties of tissue as a basis for characterizing disease. The potential of MRE as a diagnostic tool is rooted in the fact that normal and diseased tissues often differ significantly in terms of their intrinsic mechanical properties. MRE uses magnetic resonance imaging (MRI) in conjunction with the application of mechanical shear waves to probe tissue mechanics. This process can be broken down into three essential steps: inducing shear waves in the tissue,imaging the propagating shear waves with MRI, andanalyzing the wave data to generate quantitative images of tissue stiffness MRE has emerged as a safe, reliable and noninvasive method for staging hepatic liver fibrosis, and is now used in some locations as an alternative to biopsy. MRE is also being used in the ongoing investigations of numerous other organs and tissues, including, for example, the spleen, kidney, pancreas, brain, heart, breast, skeletal muscle, prostate, vasculature, lung, spinal cord, eye, bone, and cartilage. In the article that follows, some fundamental techniques and applications of MRE are summarized.

Investigating in vivo airway wall mechanics during tidal breathing with optical coherence tomography

Optical coherence tomography (OCT) is a nondestructive imaging technique offering high temporal and spatial resolution, which makes it a natural choice for assessing tissue mechanical properties. We have developed methods to mechanically analyze the compliance of the rabbit trachea in vivo using tissue deformations induced by tidal breathing, offering a unique tool to assess the behavior of the airways during their normal function. Four-hundred images were acquired during tidal breathing with a custom-built endoscopic OCT system. The surface of the tissue was extracted from a set of these images via image processing algorithms, filtered with a bandpass filter set at respiration frequency to remove cardiac and probe motion, and compared to ventilatory pressure to calculate wall compliance. These algorithms were tested on elastic phantoms to establish reliability and reproducibility. The mean tracheal wall compliance (in five animals) was 1.3±0.3×10(-5) (mm Pa)(-1). Unlike previous work evaluating airway mechanics, this new method is applicable in vivo, noncontact, and loads the trachea in a physiological manner. The technique may have applications in assessing airway mechanics in diseases such as asthma that are characterized by significant airway remodeling.

Optical coherence tomography-current technology and applications in clinical and biomedical research

Optical coherence tomography (OCT) is a noninvasive imaging technique that provides real-time two- and three-dimensional images of scattering samples with micrometer resolution. By mapping the local reflectivity, OCT visualizes the morphology of the sample. In addition, functional properties such as birefringence, motion, or the distributions of certain substances can be detected with high spatial resolution. Its main field of application is biomedical imaging and diagnostics. In ophthalmology, OCT is accepted as a clinical standard for diagnosing and monitoring the treatment of a number of retinal diseases, and OCT is becoming an important instrument for clinical cardiology. New applications are emerging in various medical fields, such as early-stage cancer detection, surgical guidance, and the early diagnosis of musculoskeletal diseases. OCT has also proven its value as a tool for developmental biology. The number of companies involved in manufacturing OCT systems has increased substantially during the last few years (especially due to its success in opthalmology), and this technology can be expected to continue to spread into various fields of application.

Optical coherence elastography: current status and future applications

Optical coherence tomography (OCT) has several advantages over other imaging modalities, such as angiography and ultrasound, due to its inherently high in vivo resolution, which allows for the identification of morphological tissue structures. Optical coherence elastography (OCE) benefits from the superior spatial resolution of OCT and has promising applications, including cancer diagnosis and the detailed characterization of arterial wall biomechanics, both of which are based on the elastic properties of the tissue under investigation. We present OCE principles based on techniques associated with static and dynamic tissue excitation, and their corresponding elastogram image-reconstruction algorithms are reviewed. OCE techniques, including the development of intravascular- or catheter-based OCE, are in their early stages of development but show great promise for surgical oncology or intravascular cardiology applications.

Spectroscopic optical coherence elastography

We present an optical technique to image the frequency-dependent complex mechanical response of a viscoelastic sample. Three-dimensional hyperspectral data, comprising two-dimensional B-mode images and a third dimension corresponding to vibration frequency, were acquired from samples undergoing external mechanical excitation in the audio-frequency range. We describe the optical coherence tomography (OCT) signal when vibration is applied to a sample and detail the processing and acquisition techniques used to extract the local complex mechanical response from three-dimensional data that, due to a wide range of vibration frequencies, possess a wide range of sample velocities. We demonstrate frequency-dependent contrast of the displacement amplitude and phase of a silicone phantom containing inclusions of higher stiffness. Measurements of an ex vivo tumor margin demonstrate distinct spectra between adipose and tumor regions, and images of displacement amplitude and phase demonstrated spatially-resolved contrast. Contrast was also observed in displacement amplitude and phase images of a rat muscle sample. These results represent the first demonstration of mechanical spectroscopy based on B-mode OCT imaging. Spectroscopic optical coherence elastography (S-OCE) provides a high-resolution imaging capability for the detection of tissue pathologies that are characterized by a frequency-dependent viscoelastic response.

DYNAMIC OPTICAL COHERENCE ELASTOGRAPHY: A REVIEW

With the development of optical coherence tomography, the application optical coherence elastography (OCE) has gained more and more attention in biomechanics for its unique features including micron-scale resolution, real-time processing, and non-invasive imaging. In this review, one group of OCE techniques, namely dynamic OCE, are introduced and discussed including external dynamic OCE mapping and imaging of ex vivo breast tumor, external dynamic OCE measurement of in vivo human skin, and internal dynamic OCE including acoustomotive OCE and magnetomotive OCE. These techniques overcame some of the major drawbacks of traditional static OCE, and broadened the OCE application fields. Driven by scientific needs to engineer new quantitative methods that utilize the high micron-scale resolution achievable with optics, results of biomechanical properties were obtained from biological tissues. The results suggest potential diagnostic and therapeutic clinical applications. Results from these studies also help our understanding of the relationship between biomechanical variations and functional tissue changes in biological systems.

Magnetic resonance elastography: a review

Magnetic resonance elastography (MRE) is a rapidly developing technology for quantitatively assessing the mechanical properties of tissue. The technology can be considered to be an imaging-based counterpart to palpation, commonly used by physicians to diagnose and characterize diseases. The success of palpation as a diagnostic method is based on the fact that the mechanical properties of tissues are often dramatically affected by the presence of disease processes, such as cancer, inflammation, and fibrosis. MRE obtains information about the stiffness of tissue by assessing the propagation of mechanical waves through the tissue with a special magnetic resonance imaging technique. The technique essentially involves three steps: (1) generating shear waves in the tissue, (2) acquiring MR images depicting the propagation of the induced shear waves, and (3) processing the images of the shear waves to generate quantitative maps of tissue stiffness, called elastograms. MRE is already being used clinically for the assessment of patients with chronic liver diseases and is emerging as a safe, reliable, and noninvasive alternative to liver biopsy for staging hepatic fibrosis. MRE is also being investigated for application to pathologies of other organs including the brain, breast, blood vessels, heart, kidneys, lungs, and skeletal muscle. The purpose of this review article is to introduce this technology to clinical anatomists and to summarize some of the current clinical applications that are being pursued.

Biomechanical properties of in vivo human skin from dynamic optical coherence elastography

Dynamic optical coherence elastography is used to determine in vivo skin biomechanical properties based on mechanical surface wave propagation. Quantitative Young's moduli are measured on human skin from different sites, orientations, and frequencies. Skin thicknesses, including measurements from different layers, are also measured simultaneously. Experimental results show significant differences among measurements from different skin sites, between directions parallel and orthogonal to Langer's lines, and under different skin hydration states. Results also suggest surface waves with different driving frequencies represent skin biomechanical properties from different layers in depth. With features such as micrometer-scale resolution, noninvasive imaging, and real-time processing from the optical coherence tomography technology, this optical measurement technique has great potential for measuring skin biomechanical properties in dermatology.

Magnetomotive nanoparticle transducers for optical rheology of viscoelastic materials

The availability of a real-time non-destructive modality to interrogate the mechanical properties of viscoelastic materials would facilitate many new investigations. We introduce a new optical method for measuring elastic properties of samples which employs magnetite nanoparticles as perturbative agents. Magnetic nanoparticles distributed in silicone-based samples are displaced upon probing with a small external magnetic field gradient and depth-resolved optical coherence phase shifts allow for the tracking of scatterers in the sample with nanometer-scale sensitivity. The scatterers undergo underdamped oscillations when the magnetic field is applied step-wise, allowing for the measurement of the natural frequencies of oscillation of the samples. Validation of the measurements is accomplished using a commercial indentation apparatus to determine the elastic moduli of the samples. This real-time non-destructive technique constitutes a novel way of probing the natural frequencies of viscoelastic materials in which magnetic nanoparticles can be introduced.

Vulnerable atherosclerotic plaque elasticity reconstruction based on a segmentation-driven optimization procedure using strain measurements: theoretical framework

It is now recognized that prediction of the vulnerable coronary plaque rupture requires not only an accurate quantification of fibrous cap thickness and necrotic core morphology but also a precise knowledge of the mechanical properties of plaque components. Indeed, such knowledge would allow a precise evaluation of the peak cap-stress amplitude, which is known to be a good biomechanical predictor of plaque rupture. Several studies have been performed to reconstruct a Young's modulus map from strain elastograms. It seems that the main issue for improving such methods does not rely on the optimization algorithm itself, but rather on preconditioning requiring the best estimation of the plaque components' contours. The present theoretical study was therefore designed to develop: 1) a preconditioning model to extract the plaque morphology in order to initiate the optimization process, and 2) an approach combining a dynamic segmentation method with an optimization procedure to highlight the modulogram of the atherosclerotic plaque. This methodology, based on the continuum mechanics theory prescribing the strain field, was successfully applied to seven intravascular ultrasound coronary lesion morphologies. The reconstructed cap thickness, necrotic core area, calcium area, and the Young's moduli of the calcium, necrotic core, and fibrosis were obtained with mean relative errors of 12%, 4% and 1%, 43%, 32%, and 2%, respectively.

Optical micro-scale mapping of dynamic biomechanical tissue properties

Mechanical forces such as adhesion, shear stress and compression play crucial roles in tissue growth, patterning and development. To understand the role of these mechanical stimuli, it is of great importance to measure biomechanical properties of developing, engineered, and natural tissues. To enable these measurements on the micro-scale, a novel, dynamic, non-invasive, high-speed optical coherence elastography (OCE) system has been developed utilizing spectral-domain optical coherence tomography (OCT) and a mechanical wave driver. Experimental results of OCE on silicone phantoms are in good agreement with those obtained from a standardized indentation method. Using phase-resolved imaging, we demonstrate OCE can map dynamic elastic moduli of normal and neoplastic ex vivo human breast tissue with a sensitivity of 0.08%. Spatial micro-scale mapping of elastic moduli of tissue offers the potential for basic science and clinical investigations into the role biomechanics play in health and disease.