Noncontact Elastic Wave Imaging Optical Coherence Elastography for Evaluating Changes in Corneal Elasticity Due to Crosslinking

Detecting emergence of ruptures in individual layers of the stretched intestinal wall using optical coherence elastography: A pilot study

We report a new application of compression optical coherence elastography (C-OCE) to monitor the emergence of ruptures in individual layers of longitudinally stretched small-intestine walls using tissue samples (n = 36) from nine minipigs. Before stretching, C-OCE successfully estimated stiffness for each intestine-wall layer: longitudinal muscular layer with serosa, circumferential muscular layer, submucosa and mucosa. In stretched samples, C-OCE clearly visualized initial stiffening in both muscular layers. By 25% elongation, a sharp stiffness decrease for the longitudinal muscular layer, indicated emergence of tears in all samples. With further stretching, for most samples, ruptures emerged in the circumferential muscular layer and submucosa, while mucosa remained undamaged. Histology confirmed the OCE-revealed damaging and absence of tissue damage for ~15% elongation. Thus, C-OCE has demonstrated a high potential for determining the safety tissue-stretching threshold which afterward may be used intraoperatively to prevent rupture risk in intestinal tissues stretched during various diagnostic/therapeutic procedures.

Spatial mapping of corneal biomechanical properties using wave-based optical coherence elastography

Quantifying the mechanical properties of the cornea can provide valuable insights into the occurrence and progression of keratoconus, as well as the effectiveness of corneal crosslinking surgery. This study presents a non-contact and non-invasive wave-based optical coherence elastography system that utilizes air-pulse stimulation to create a two-dimensional map of corneal elasticity. Homogeneous and dual concentration phantoms were measured with the sampling of 25 × 25 points over a 6.6 × 6.6 mm2 area, to verify the measurement capability for elastic mapping and the spatial resolution (0.91 mm). The velocity of elastic waves distribution of porcine corneas before and after corneal crosslinking surgery were further mapped, showing a significant change in biomechanics in crosslinked region. This system features non-invasiveness and high resolution, holding great potential for application in ophthalmic clinics.

In situ assessment of lens elasticity with noncontact optical coherence elastography

Lens biomechanics has great potential for application in clinical diagnostics and treatment monitoring of presbyopia and cataracts. However, current approaches to lens elastography do not meet the desired safety or sensitivity for clinical application. In this regard, we propose a noncontact optical coherence elastography (OCE) method to facilitate quantitative in situ imaging of lens elasticity. Elastic waves induced by air-pulse stimulation on the limbus propagate to the lens and are then imaged using custom-built swept-source optical coherence tomography to obtain the elastic wave velocity and Young's modulus. The proposed OCE method was first validated by comparing the results of in situ and in vitro measurements of porcine lenses. The results demonstrate that the Young's modulus measured in situ was highly consistent with that measured in vitro and had an intraclass correlation coefficient of 0.988. We further investigated the elastic changes induced by cold storage and microwave heating. During 36-hour cold storage, the mean Young's modulus gradually increased (from 5.62 ± 1.24 kPa to 11.40 ± 2.68 kPa, P < 0.0001, n = 9) along with the formation of nuclear opacities. 15-second microwave heating caused a greater increase in the mean Young's modulus (from 6.86 ± 1.21 kPa to 25.96 ± 8.64 kPa, P < 0.0025, n = 6) without apparent cataract formation. Accordingly, this study reports the first air-pulse OCE measurements of in situ lenses, which quantified the loss of lens elasticity during simulated cataract development with good repeatability and sensitivity, thus enhancing the potential for adoption of lens biomechanics in the clinic.

Multimodal Heartbeat and Compression Optical Coherence Elastography for Mapping Corneal Biomechanics

The biomechanical properties of the cornea have a profound influence on the health, structural integrity, and function of the eye. Understanding these properties may be critical for diagnosis and identifying disease pathogenesis. This work demonstrates how two different elastography techniques can be combined for a multimodal approach to measuring corneal biomechanical properties. Heartbeat optical coherence elastography (Hb-OCE) and compression OCE were performed simultaneously to measure the stiffness of the cornea in an in vivo rabbit model. Measurements were further performed after collagen crosslinking to demonstrate how the combined technique can be used to measure changes in corneal stiffness and map mechanical contrast. The results of this work further suggest that measurements from Hb-OCE and compression OCE are comparable, meaning that Hb-OCE and compression OCE may be used interchangeably despite distinct differences in both techniques.

Assessing corneal cross-linking with reverberant 3D optical coherence elastography

SIGNIFICANCE

Corneal cross-linking (CXL) is a well-known procedure for treating certain eye disorders such as keratoconus. However, characterization of the biomechanical changes in the cornea as a result of this procedure is still under active research. Specifically, there is a clinical need for high-resolution characterization of individual corneal layers.

AIM

A high-resolution elastography method in conjunction with a custom optical coherence tomography system is used to track these biomechanical changes in individual corneal layers. Pre- and post-treatment analysis for both low-dose and high-dose CXL experiments are performed.

APPROACH

A recently developed elastography technique that utilizes the theory of reverberant shear wave fields, with optical coherence tomography as the modality, is applied to pig corneas ex vivo to evaluate elasticity changes associated with corneal CXL. Sets of low-dose and high-dose CXL treatments are evaluated before and after treatments with three pairs of pig corneas per experiment.

RESULTS

The reverberant three-dimensional (3D) optical coherence elastography (OCE) technique can identify increases in elasticity associated with both low-dose and high-dose CXL treatments. There is a notable graphical difference between low-dose and high-dose treatments. In addition, the technique is able to identify which layers of the cornea are potentially affected by the CXL procedure and provides insight into the nonlinearity of the elasticity changes.

CONCLUSIONS

The reverberant 3D OCE technique can identify depth-resolved changes in elasticity of the cornea associated with CXL procedures. This method could be translated to assess and monitor CXL efficacy in various clinical settings.

Surface wave elastography using high speed full-field optical interferometry

The assessment of mechanical stiffness is an essential diagnostic tool for investigating the biomechanical properties of biological tissues. Surface wave elastography (SWE) is an emerging technique to quantify elastic properties of tissues in clinical diagnosis. High-speed optical imaging combined with SWE has enormous potential in quantifying the elastic properties of tissues at microscale resolutions. In this study, we implement surface wave elastography using high-speed optical interferometry to characterize the elastic properties of tissue-mimicking phantoms andex-vivonative caprine liver tissue by imaging the surface wave induced by an electromechanical actuator. The sinusoidal mechanical excitations ranging from 120 Hz to 1.2 kHz on the surface of tissues are captured using a high-speed camera with a frame rate of 4 kHz at micrometer resolutions. The surface wavefront reconstruction is performed using a phase-shifting algorithm and linear regression is used to calculate the surface wave velocity. The mechanical stiffness estimated from the optical system is compared with the results of mechanical compression testing measurements. The results from this multimodal platform combining optical interferometry and vibrational spectroscopy using SWE are highly promising towards a non-invasive or minimally invasive imaging forin-vivoandex-vivomechanical characterization of tissues with future clinical applications.

Wave-based optical coherence elastography: The 10-year perspective

After 10 years of progress and innovation, optical coherence elastography (OCE) based on the propagation of mechanical waves has become one of the major and the most studied OCE branches, producing a fundamental impact in the quantitative and nondestructive biomechanical characterization of tissues. Preceding previous progress made in ultrasound and magnetic resonance elastography; wave-based OCE has pushed to the limit the advance of three major pillars: (1) implementation of novel wave excitation methods in tissues, (2) understanding new types of mechanical waves in complex boundary conditions by proposing advance analytical and numerical models, and (3) the development of novel estimators capable of retrieving quantitative 2D/3D biomechanical information of tissues. This remarkable progress promoted a major advance in answering basic science questions and the improvement of medical disease diagnosis and treatment monitoring in several types of tissues leading, ultimately, to the first attempts of clinical trials and translational research aiming to have wave-based OCE working in clinical environments. This paper summarizes the fundamental up-to-date principles and categories of wave-based OCE, revises the timeline and the state-of-the-art techniques and applications lying in those categories, and concludes with a discussion on the current challenges and future directions, including clinical translation research.

Quantitative compressive optical coherence elastography using structural OCT imaging and optical palpation to measure soft contact lens mechanical properties

In this study, the principle of 'optical palpation' was applied to a compression optical coherence elastography (OCE) method using spectral domain optical coherence tomography (OCT). Optical palpation utilizes a compliant transparent material of known mechanical properties, which acts as a stress sensor, in order to derive the mechanical properties of a sample material under examination. This technique was applied to determine the mechanical properties of soft contact lenses, with one lens being used as the compliant stress sensor and the other as the sample under investigation to extract the mechanical properties. This compliant stress sensor allowed for the stress of the compression to be measured without the use of a force sensor. The strain of the materials was measured through an automatic boundary segmentation that tracks the material thickness (of the sensor and the sample) during compression through sequential structural OCT images. A total of five contact lens combinations were tested, using three separate commercially available contact lenses with unique mechanical properties. Various combinations of contact lens materials were used to further validate the technique. The Young's modulus derived from this method was compared to nominal manufacturer's values. Both accuracy and repeatability were assessed, with highly accurate measurements obtained, with a percentage difference between the nominal and experimentally derived Young's modulus being less than 6% for all the tested combinations as well as providing a Young's modulus that was not statistically significant different (p > 0.01) to the nominal value. The results demonstrate the potential of optical palpation in OCE to accurately measure the mechanical properties of a material without the use of sophisticated electronics to capture the stress of the sample. These findings have potential to be translated into a method for tissue mechanical testing with ex vivo and in vivo clinical applications.

2-D Ultrasonic Array-Based Optical Coherence Elastography

Acoustic radiation force optical coherence elastography (ARF-OCE) has been successfully implemented to characterize the biomechanical properties of soft tissues, such as the cornea and the retina, with high resolution using single-element ultrasonic transducers for ARF excitation. Most currently proposed OCE techniques, such as air puff and ARF, have less capability to control the spatiotemporal information of the induced region of deformation, resulting in limited accuracy and low temporal resolution of the shear wave elasticity imaging. In this study, we propose a new method called 2-D ultrasonic array-based OCE imaging, which combines the advantages of 3-D dynamic electronic steering of the 2-D ultrasonic array and high-resolution optical coherence tomography (OCT). The 3-D steering capability of the 2-D array was first validated using a hydrophone. Then, the combined 2-D ultrasonic array OCE system was calibrated using a homogenous phantom, followed by an experiment on ex vivo rabbit corneal tissue. The results demonstrate that our newly developed 2-D ultrasonic array-based OCE system has the capability to map tissue biomechanical properties accurately, and therefore, has the potential to be a vital diagnostic tool in ophthalmology.

Advances in multimodal imaging in ophthalmology

Multimodality ophthalmic imaging systems aim to enhance the contrast, resolution, and functionality of existing technologies to improve disease diagnostics and therapeutic guidance. These systems include advanced acquisition and post-processing methods using optical coherence tomography (OCT), combined scanning laser ophthalmoscopy and OCT systems, adaptive optics, surgical guidance, and photoacoustic technologies. Here, we provide an overview of these ophthalmic imaging systems and their clinical and basic science applications.

In Vivo Human Corneal Shear-wave Optical Coherence Elastography

SIGNIFICANCE

A novel imaging technology, dynamic optical coherence elastography (OCE), was adapted for clinical noninvasive measurements of corneal biomechanics.

PURPOSE

Determining corneal biomechanical properties is a long-standing challenge. Elasticity imaging methods have recently been developed and applied for clinical evaluation of soft tissues in cancer detection, atherosclerotic plaque evaluation, surgical guidance, and more. Here, we describe the use of dynamic OCE to characterize mechanical wave propagation in the human cornea in vivo, thus providing a method for clinical determination of corneal biomechanical properties.

METHODS

High-resolution phase-sensitive optical coherence tomography imaging was combined with microliter air-pulse tissue stimulation to perform dynamic elasticity measurements in 18 eyes of nine participants. Low-pressure (0.1 mmHg), spatiotemporally discreet (150 μm, 800 μs) tissue stimulation produced submicron-scale tissue deformations that were measured at multiple positions over a 1-mm2 area. Surface wave velocity was measured and used to determine tissue stiffness. Elastic wave propagation velocity was measured and evaluated as a function of IOP and central corneal thickness.

RESULTS

Submicron corneal surface displacement amplitude (range, 0.005 to 0.5 μm) responses were measured with high sensitivity (0.24 nm). Corneal elastic wave velocity ranged from 2.4 to 4.2 m/s (mean, 3.5; 95% confidence interval, 3.2 to 3.8 m/s) and was correlated with central corneal thickness (r = 0.64, P < .001) and IOP (r = 0.52, P = .02).

CONCLUSIONS

Phase-sensitive optical coherence tomography imaging combined with microliter air-pulse mechanical tissue stimulation has sufficient detection sensitivity to observe submicron elastic wave propagation in corneal tissue. These measurements enable in vivo corneal stiffness determinations that will be further studied for use with disease detection and for monitoring clinical interventions.

A review of imaging modalities for detecting early keratoconus

OBJECTIVES

Early identification of keratoconus is imperative for preventing iatrogenic corneal ectasia and allowing for early corneal collagen cross-linking treatments to potentially halt progression and decrease transplant burden. However, early diagnosis of keratoconus is currently a diagnostic challenge as there is no uniform screening criteria. We performed a review of the current literature to assess imaging modalities that can be used to help identify subclinical keratoconus.

METHODS

A Pubmed database search was conducted. We included primary and empirical studies for evaluating different modalities of screening for subclinical keratoconus.

RESULTS

A combination of multiple imaging tools, including corneal topography, tomography, Scheimpflug imaging, anterior segment optical coherence tomography, and in vivo confocal microscopy will allow for enhanced determination of subclinical keratoconus. In patients who are diagnostically borderline using a single screening criteria, use of additional imaging techniques can assist in diagnosis. Modalities that show promise but need further research include polarization-sensitive optical coherence tomography, Brillouin microscopy, and atomic force microscopy.

CONCLUSIONS

Recognition of early keratoconus can reduce risk of post-refractive ectasia and reduce transplantation burden. Though there are no current uniform screening criterion, multiple imaging modalities have shown promise in assisting with the early detection of keratoconus.

Heartbeat OCE: corneal biomechanical response to simulated heartbeat pulsation measured by optical coherence elastography

SIGNIFICANCE

It is generally agreed that the corneal mechanical properties are strongly linked to many eye diseases and could be used to assess disease progression and response to therapies. Elastography is the most notable method of assessing corneal mechanical properties, but it generally requires some type of external excitation to induce a measurable displacement in the tissue.

AIM

We present Heartbeat Optical Coherence Elastography (Hb-OCE), a truly passive method that can measure the elasticity of the cornea based on intrinsic corneal displacements induced by the heartbeat.

APPROACH

Hb-OCE measurements were performed in untreated and UV-A/riboflavin cross-linked porcine corneas ex vivo, and a distinct difference in strain was detected. Furthermore, a partially cross-linked cornea was also assessed, and the treated and untreated areas were similarly distinguished.

RESULTS

Our results suggest that Hb-OCE can spatially map displacements in the cornea induced by small fluctuations in intraocular pressure, similar to what is induced by the heartbeat.

CONCLUSIONS

The described technique opens the possibility for completely passive and noncontact in vivo assessment of corneal stiffness.

Clinical Corneal Optical Coherence Elastography Measurement Precision: Effect of Heartbeat and Respiration

Purpose

Normal physiological movements (e.g., respiration and heartbeat) induce eye motions during clinical measurements of human corneal biomechanical properties using optical coherence elastography (OCE). We quantified the effects of respiratory and cardiac-induced eye motions on clinical corneal OCE measurement precision and repeatability.

Methods

Corneal OCE was performed using low-force, micro-air-pulse tissue stimulation and high-resolution phase-sensitive optical coherence tomography (OCT) imaging. Axial surface displacements of the corneal apex were measured (M-mode) at a 70-kHz sampling rate and three different stimulation pressures (20-60 Pa). Simultaneously, the axial corneal position was tracked with structural OCT imaging, while the heartrate and respiration were monitored over a 90 second period.

Results

Respiratory- and cardiac-induced eye motions have distinctly lower frequency (0.1-1 Hz) and much greater amplitude (up to ± 50 µm movements) than air-pulse-induced corneal tissue deformations (∼250 Hz, <1 µm). The corneal displacements induced during OCE measurements in vivo were -0.41 ± 0.06 µm (n = 22 measurements, coefficient of variation [CV]: 14.6%) and -0.44 ± 0.07 µm (n = 50 measurements, CV: 15.9%), respectively, from two human subjects at 40 Pa stimulation pressure. Observed variation in corneal tissue displacements were not associated with tissue stimulation magnitude, or the amplitude of physiologically induced axial eye motion.

Conclusions

The microsecond timescale and submicron tissue displacements observed during corneal OCE measurements are separable from normal involuntary physiological movements, such as the oculocardiac pulse and respiratory movements.

Translational Relevance

This work advances innovations in biomedical imaging and engineering for clinical diagnostic applications for soft-tissue biomechanical testing.

Multimodal quantitative optical elastography of the crystalline lens with optical coherence elastography and Brillouin microscopy

Assessing the biomechanical properties of the crystalline lens can provide crucial information for diagnosing disease and guiding precision therapeutic interventions. Existing noninvasive methods have been limited to global measurements. Here, we demonstrate the quantitative assessment of the elasticity of crystalline lens with a multimodal optical elastography technique, which combines dynamic wave-based optical coherence elastography (OCE) and Brillouin microscopy to overcome the drawbacks of individual modalities. OCE can provide direct measurements of tissue elasticity rapidly and quantitatively, but it is a challenge to image transparent samples such as the lens because this technique relies on backscattered light. On the other hand, Brillouin microscopy can map the longitudinal modulus with micro-scale resolution in transparent samples. However, the relationship between Brillouin-deduced modulus and Young's modulus is not straightforward and sample dependent. By combining these two techniques, we can calibrate Brillouin measurements with OCE, based on the same sample, allowing us to completely map the Young's modulus of the crystalline lens. The combined system was first validated with tissue-mimicking gelatin phantoms of varying elasticities (N = 9). The OCE data was used to calibrate the Brillouin shift measurements and subsequently map the Young's modulus of the phantoms. After validation, OCE and Brillouin measurements were performed on ex-vivo porcine lenses (N = 6), and the Young's modulus of the lenses was spatially mapped. The results show a strong correlation between Young's moduli measured by OCE and longitudinal moduli measured by Brillouin microscopy. The correlation coefficient R was 0.98 for the phantoms and 0.94 for the lenses, respectively. The mean Young's modulus of the anterior and posterior lens was 1.98 ± 0.74 kPa and 2.93 ± 1.13 kPa, respectively, and the Young's modulus of the lens nucleus was 11.90 ± 2.94 kPa. The results presented in this manuscript open a new way for truly quantitative biomechanical mapping of optically transparent (or low scattering) tissues in 3D.

Assessment of corneal viscoelasticity using elastic wave optical coherence elastography

The corneal viscoelasticity have great clinical significance, such as the early diagnosis of keratoconus. In this work, an analysis method which utilized the elastic wave velocity, frequency and energy attenuation to assess the corneal viscoelasticity is presented. Using phase-resolved optical coherence tomography, the spatial-temporal displacement map is derived. The phase velocity dispersion curve and center frequency are obtained by transforming the displacement map into the wavenumber-frequency domain through the 2D fast Fourier transform (FFT). The shear modulus is calculated through Rayleigh wave equation using the phase velocity in the high frequency. The normalized energy distribution is plotted by transforming the displacement map into the spatial-frequency domain through the 1D FFT. The energy attenuation coefficient is derived by exponential fitting to calculate the viscous modulus. Different concentrations of tissue-mimicking phantoms and porcine corneas are imaged to validate this method, which demonstrates that the method has the capability to assess the corneal viscoelasticity.

Simultaneously imaging and quantifying in vivo mechanical properties of crystalline lens and cornea using optical coherence elastography with acoustic radiation force excitation

The crystalline lens and cornea comprise the eye's optical system for focusing light in human vision. The changes in biomechanical properties of the lens and cornea are closely associated with common diseases, including presbyopia and cataract. Currently, most in vivo elasticity studies of the anterior eye focus on the measurement of the cornea, while lens measurement remains challenging. To better understand the anterior segment of the eye, we developed an optical coherence elastography system utilizing acoustic radiation force excitation to simultaneously assess the elasticities of the crystalline lens and the cornea in vivo. A swept light source was integrated into the system to provide an enhanced imaging range that covers both the lens and the cornea. Additionally, the oblique imaging approach combined with orthogonal excitation also improved the image quality. The system was tested through first ex vivo and then in vivo experiments using a rabbit model. The elasticities of corneal and lens tissue in an excised normal whole-globe and a cold cataract model were measured to reveal that cataractous lenses have a higher Young's modulus. Simultaneous in vivo elasticity measurements of the lens and cornea were performed in a rabbit model to demonstrate the correlations between elasticity and intraocular pressure and between elasticity and age. To the best of our knowledge, we demonstrated the first in vivo elasticity of imaging of both the lens and cornea using acoustic radiation force-optical coherence elastography, thereby providing a potential powerful clinical tool to advance ophthalmic research in disorders affecting the lens and the cornea.

In vivo evaluation of corneal biomechanical properties by optical coherence elastography at different cross-linking irradiances

Corneal collagen cross-linking (CXL) strengthens the biomechanical properties of damaged corneas. Quantifying the changes of stiffness due to different CXL protocols is difficult, especially in vivo. A noninvasive elastic wave-based optical coherence elastography system was developed to construct in vivo corneal elasticity maps by excitation of air puff. Biomechanical differences were compared for rabbit corneas given three different CXL protocols while keeping the total energy delivered constant. The Young’s modulus was weaker in corneas treated with higher irradiance levels over shorter durations, and a slight increase of Young’s modulus was present in all groups one week after the recovery process. Due to the noninvasive nature and minimal force to generate corneal elastic waves, this technique has the potential for early detection and treatment of corneal diseases in clinic.

Spatial resolution in dynamic optical coherence elastography

Dynamic optical coherence elastography (OCE) tracks elastic wave propagation speed within tissue, enabling quantitative three-dimensional imaging of the elastic modulus. We show that propagating mechanical waves are mode converted at interfaces, creating a finite region on the order of an acoustic wavelength where there is not a simple one-to-one correspondence between wave speed and elastic modulus. Depending on the details of a boundary’s geometry and elasticity contrast, highly complex propagating fields produced near the boundary can substantially affect both the spatial resolution and contrast of the elasticity image. We demonstrate boundary effects on Rayleigh waves incident on a vertical boundary between media of different shear moduli. Lateral resolution is defined by the width of the transition zone between two media and is the limit at which a physical inclusion can be detected with full contrast. We experimentally demonstrate results using a spectral-domain OCT system on tissue-mimicking phantoms, which are replicated using numerical simulations. It is shown that the spatial resolution in dynamic OCE is determined by the temporal and spatial characteristics (i.e., bandwidth and spatial pulse width) of the propagating mechanical wave. Thus, mechanical resolution in dynamic OCE inherently differs from the optical resolution of the OCT imaging system.

Longitudinal elastic wave imaging using nanobomb optical coherence elastography

Wave-based optical coherence elastography (OCE) is a rapidly emerging technique for elasticity assessment of tissues having high displacement sensitivity and simple implementation. However, most current noncontact wave excitation techniques are unable to target a specific tissue site in 3D and rely on transversal scanning of the imaging beam. Here, we demonstrate that dye-loaded perfluorocarbon nanoparticles (nanobombs) excited by a pulsed laser can produce localized axially propagating longitudinal shear waves while adhering to the laser safety limit. A phase-correction method was developed and implemented to perform sensitive nanobomb elastography using a ∼1.5  MHz Fourier domain mode-locking laser. The nanobomb activation was also monitored by detecting photoacoustic signals. The highly localized elastic waves detected by the nanobomb OCE suggest the possibility of high-resolution 3D elastographic imaging.

Assessment of the changes in corneal biomechanical properties after collagen cross-linking in patients with keratoconus

Purpose

To assess the changes in biomechanical properties of the cornea after treatment of keratoconus patients with UV-A/riboflavin corneal collagen cross-linking (CXL) using Corvis ST (Oculus, Wetzlar, Germany) and Ocular Response Analyzer (ORA; Reichert Ophthalmic Instruments, Inc., Buffalo, NY, USA) devices.

Methods

In this prospective, observational case series, 48 eyes from 48 consecutive patients with progressive keratoconus were enrolled. Patients with history or signs of ocular disorders other than keratoconus, previous eye surgery, systemic diseases, or inability to cooperate with any measurement device were excluded. Corvis ST and ORA images were obtained at baseline and 4 months after CXL. The primary outcome measures comprised Corvis ST corneal biomechanical factors [time of highest concavity (T), time of applanation 1 (T1), time of applanation 2 (T2), length of applanation 1 (L1), length of applanation 2 (L2), velocity of applanation 1 (V1), velocity of applanation 2 (V2), deformation amplitude (DA), peak distance (PD), and radius (R)] and the ORA parameters [corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann-related IOP (IOPg), cornea-compensated IOP (IOPcc), and waveform score (WS)].

Results

The mean [± standard deviation (SD)] age of patients was 20 ± 5 years, and 27 (56%) were male. At baseline, the averages of the refraction, mean keratometry, and keratometric astigmatism were −3.0 ± 1.8 diopter (D), 47.0 ± 1.8 D, and 3.5 ± 1.5 D, respectively. According to Corvis ST, L2 increased from 0.83 ± 0.25 mm at baseline to 1.15 ± 0.57 mm after CXL; and V2 decreased from −0.81 ± 0.08 to −0.94 ± 0.26 m/s (P = 0.001 and P = 0.032, respectively). ORA parameters showed significant decrease in the CRF (from 7.82 ± 1.72 to 7.21 ± 1.05 mmHg; P = 0.036) and increase in the WS (from 4.58 ± 2.55 to 6.12 ± 1.92; P = 0.002).

Conclusions

According to in vivo observation with Corvis ST and ORA, CXL induces significant changes in corneal biomechanical properties in cases with keratoconus. The parameters with significant changes (L2 and V2) may reflect increased stiffness of the treated cornea. The importance of such observations should be elucidated in future studies.

Theory and design of Schwarzschild scan objective for Optical Coherence Tomography

Optical coherence elastography (OCE) is one form of multi-channel imaging that combines high-resolution optical coherence tomography (OCT) imaging with mechanical tissue stimulation. This combination of structural and functional imaging can require additional space to integrate imaging capabilities with additional functional elements (e.g., optical, mechanical, or acoustic modulators) either at or near the imaging axis. We address this challenge by designing a novel scan lens based on a modified Schwarzchild objective lens, comprised of a pair of concentric mirrors with potential space to incorporate additional functional elements and minimal compromise to the available scan field. This scan objective design allows perpendicular tissue-excitation and response recording. The optimized scan lens design results in a working distance that is extended to ~140 mm (nearly 2x the focal length), an expanded central space suitable for additional functional elements (>15 mm in diameter) and diffraction-limited lateral resolution (19.33 μm) across a full annular scan field ~ ± 7.5 mm to ± 12.7 mm.

Live human assessment of depth-dependent corneal displacements with swept-source optical coherence elastography

Purpose

To assess depth-dependent corneal displacements in live normal subjects using optical coherence elastography (OCE).

Methods

A corneal elastography method based on swept-source optical coherence tomography (OCT) was implemented in a clinical prototype. Low amplitude corneal deformation was produced during OCT imaging with a linear actuator-driven lens coupled to force transducers. A cross-correlation algorithm was applied to track frame-by-frame speckle displacement across horizontal meridian scans. Intra-measurement force and displacement data series were plotted against each other to produce local axial stiffness approximations, k, defined by the slope of a linear fit to the force/displacement data (ignoring non-axial contributions from corneal bending). Elastographic maps displaying local k values across the cornea were generated, and the ratio of mean axial stiffness approximations for adjacent anterior and posterior stromal regions, k_a/k_p, was calculated. Intraclass correlation coefficients (ICC) were used to estimate repeatability.

Results

Seventeen eyes (ten subjects) were included in this prospective first-in-humans translational study. The ICC was 0.84. Graphs of force vs. displacement demonstrated that, for simultaneously acquired measurements involving the same applied force, anterior stromal displacements were lower (suggesting stiffer behavior) than posterior stromal displacements. Mean k_a was 0.016±0.004 g/mm and mean k_p was 0.014±0.004 g/mm, giving a mean k_a/k_p ratio of 1.123±0.062.

Conclusion

OCE is a clinically feasible, non-invasive corneal biomechanical characterization method capable of resolving depth-dependent differences in corneal deformation behavior. The anterior stroma demonstrated responses consistent with stiffer properties in compression than the posterior stroma, but to a degree that varied across normal eyes. The clinical capability to measure these differences has implications for assessing the biomechanical impact of corneal refractive surgeries and for ectasia risk screening applications.

Ultrasonic Microelastography to Assess Biomechanical Properties of the Cornea

OBJECTIVE

To both qualitatively and quantitatively investigate corneal biomechanical properties through an ultrasonic microelastography imaging system, which is potentially useful in the diagnosis of diseases, such as keratoconus, postrefractive keratectasia, and tracking treatment such as cross-linking surgery.

METHODS

Our imaging system has a dual-frequency configuration, including a 4.5 MHz ring transducer to push the tissue and a confocally aligned 40 MHz needle transducer to track micron-level displacement. Two-dimensional/three-dimensional acoustic radiation force impulse (ARFI) imaging and Young's modulus in the region of interest were performed on ex vivo porcine corneas that were either cross-linked using formalin solution or preloaded with intraocular pressure (IOPs) from 5 to 30 mmHg.

RESULTS

The increase of corneal stiffness and the change in cross-linked volume following formalin crosslinking could be precisely observed in the ARFI images and reflected by the reconstructed Young's modulus while the B-mode structural images remained almost unchanged. In addition, the relationship between the stiffness of the cornea and IOPs was investigated among 12 porcine corneas. The corneal stiffness is significantly different at various IOPs and has a tendency to become stiffer with increasing IOP.

CONCLUSION

Our results demonstrate the principle of using ultrasonic microelastography techniques to image the biomechanical properties of the cornea. Integrating high-resolution ARFI imaging labeled with reconstructed Young's modulus and structural imaging of the cornea can potentially lead to a routinely performed imaging modality in the field of ophthalmology.

Biomechanical assessment of myocardial infarction using optical coherence elastography

Myocardial infarction (MI) leads to cardiomyocyte loss, impaired cardiac function, and heart failure. Molecular genetic analyses of myocardium in mouse models of ischemic heart disease have provided great insight into the mechanisms of heart regeneration, which is promising for novel therapies after MI. Although biomechanical factors are considered an important aspect in cardiomyocyte proliferation, there are limited methods for mechanical assessment of the heart in the mouse MI model. This prevents further understanding the role of tissue biomechanics in cardiac regeneration. Here we report optical coherence elastography (OCE) of the mouse heart after MI. Surgical ligation of the left anterior descending coronary artery was performed to induce an infarction in the heart. Two OCE methods with assessment of the direction-dependent elastic wave propagation and the spatially resolved displacement damping provide complementary analyses of the left ventricle. In comparison with sham, the infarcted heart features a fibrotic scar region with reduced elastic wave velocity, decreased natural frequency, and less mechanical anisotropy at the tissue level at the sixth week post-MI, suggesting lower and more isotropic stiffness. Our results indicate that OCE can be utilized for nondestructive biomechanical characterization of MI in the mouse model, which could serve as a useful tool in the study of heart repair.

Optical coherence elastography in ophthalmology

Optical coherence elastography (OCE) can provide clinically valuable information based on local measurements of tissue stiffness. Improved light sources and scanning methods in optical coherence tomography (OCT) have led to rapid growth in systems for high-resolution, quantitative elastography using imaged displacements and strains within soft tissue to infer local mechanical properties. We describe in some detail the physical processes underlying tissue mechanical response based on static and dynamic displacement methods. Namely, the assumptions commonly used to interpret displacement and strain measurements in terms of tissue elasticity for static OCE and propagating wave modes in dynamic OCE are discussed with the ultimate focus on OCT system design for ophthalmic applications. Practical OCT motion-tracking methods used to map tissue elasticity are also presented to fully describe technical developments in OCE, particularly noting those focused on the anterior segment of the eye. Clinical issues and future directions are discussed in the hope that OCE techniques will rapidly move forward to translational studies and clinical applications.

Investigating Elastic Anisotropy of the Porcine Cornea as a Function of Intraocular Pressure With Optical Coherence Elastography

PURPOSE

To evaluate the elastic anisotropy of porcine corneas at different intraocular pressures (IOPs) using a noncontact optical coherence elastography (OCE) technique.

METHODS

A focused air-pulse induced low amplitude (≤ 10 µm) elastic waves in fresh porcine corneas (n = 7) in situ in the whole eye globe configuration. A home-built phase-stabilized swept source optical coherence elastography (PhS-SSOCE) system imaged the elastic wave propagation at different stepped radial directions. A closed-loop feedback system was used to artificially control the IOP and the OCE measurements were repeated as the IOP was incrementally increased from 15 to 30 mm Hg in 5-mm Hg increments.

RESULTS

The OCE measurements demonstrated that the stiffness of the cornea increased as a function of IOP and elastic anisotropy of the cornea became more pronounced at higher IOPs. The standard deviation of the modified planar anisotropy coefficient increased from 0.72 ± 0.42 at an IOP of 15 mm Hg to 1.58 ± 0.40 at 30 mm Hg.

CONCLUSIONS

The presented noncontact OCE method was capable of detecting and assessing the corneal elastic anisotropy as a function of IOP. Due to the noninvasive nature and small amplitude of the elastic wave, this method may be able to provide further information about corneal health and integrity in vivo. [J Refract Surg. 2016;32(8):562-567.].

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.

Diffuse shear wave imaging: toward passive elastography using low-frame rate spectral-domain optical coherence tomography

Optical coherence tomography (OCT) can map the stiffness of biological tissue by imaging mechanical perturbations (shear waves) propagating in the tissue. Most shear wave elastography (SWE) techniques rely on active shear sources to generate controlled displacements that are tracked at ultrafast imaging rates. Here, we propose a noise-correlation approach to retrieve stiffness information from the imaging of diffuse displacement fields using low-frame rate spectral-domain OCT. We demonstrated the method on tissue-mimicking phantoms and validated the results by comparison with classic ultrafast SWE. Then we investigated the in vivo feasibility on the eye of an anesthetized rat by applying noise correlation to naturally occurring displacements. The results suggest a great potential for passive elastography based on the detection of natural pulsatile motions using conventional spectral-domain OCT systems. This would facilitate the transfer of OCT-elastography to clinical practice, in particular, in ophthalmology or dermatology.

Optical coherence elastography assessment of corneal viscoelasticity with a modified Rayleigh-Lamb wave model

The biomechanical properties of the cornea play a critical role in forming vision. Diseases such as keratoconus can structurally degenerate the cornea causing a pathological loss in visual acuity. UV-A/riboflavin corneal collagen crosslinking (CXL) is a clinically available treatment to stiffen the cornea and restore its healthy shape and function. However, current CXL techniques do not account for pre-existing biomechanical properties of the cornea nor the effects of the CXL treatment itself. In addition to the inherent corneal structure, the intraocular pressure (IOP) can also dramatically affect the measured biomechanical properties of the cornea. In this work, we present the details and development of a modified Rayleigh-Lamb frequency equation model for quantifying corneal biomechanical properties. After comparison with finite element modeling, the model was utilized to quantify the viscoelasticity of in situ porcine corneas in the whole eye-globe configuration before and after CXL based on noncontact optical coherence elastography measurements. Moreover, the viscoelasticity of the untreated and CXL-treated eyes was quantified at various IOPs. The results showed that the stiffness of the cornea increased after CXL and that corneal stiffness is close to linear as a function of IOP. These results show that the modified Rayleigh-Lamb wave model can provide an accurate assessment of corneal viscoelasticity, which could be used for customized CXL therapies.