In Vivo Human Corneal Shear-wave Optical Coherence Elastography

Dynamic evaluation of corneal cross-linking and osmotic diffusion effects using optical coherence elastography

Dynamic deformation events induced by osmosis or photochemical stiffening substantially influence geometrical and mechanical assessments in post-mortem corneas, therefore need to be carefully monitored in experimental settings. In this study, we employed optical coherence elastography (OCE) to quantify dynamic deformation processes at high resolution in freshly enucleated porcine corneas. Osmotic effects were studied by immerging n = 9 eyes in preservation media of three different tonicities. Dynamic processes underlying corneal cross-linking (CXL) were studied by subjecting n = 6 eyes to standard Dresden treatment, while three control groups were used. The entire procedures were performed under an OCE setup during up to 80 min, acquiring a volumetric scan every 20 s. Changes in OCE-derived axial deformations were incrementally calculated between consecutive scans. Preservation conditions had a strong influence on the observed strain patterns, which were consistent with the tonicity of the medium (swelling in hypotonic, deswelling in hypertonic environment). In the CXL group, we observed deswelling of the anterior stroma 10 min after starting the UV irradiation, which was not observed in any control group (p = 0.007). The presented results proved OCE to be a valuable technique to quantify subtle dynamic biomechanical alterations in the cornea resulting from CXL and preservation solutions.

In-vivo high-frequency quantitative ultrasound-derived parameters of the anterior sclera correlated with level of myopia and presence of staphyloma

BACKGROUND

A high-frequency point-of-care (POC) ultrasound instrument was used to evaluate the microstructural and biomechanical properties of the anterior sclera in vivo using parameters computed from quantitative ultrasound (QUS) methods.

METHODS

In this cross-sectional study, both eyes of 85 enrolled patients were scanned with the POC instrument and ultrasound data were processed to obtain QUS parameters. Pearson correlation and multi-linear regression were used to identify relationships between QUS parameters and refractive error (RE) or axial length. After categorising eyes based on RE, binary support vector machine (SVM) classifiers were trained using the QUS or ophthalmic parameters (anterior chamber depth, central corneal thickness, corneal power, and intraocular pressure) to classify each eye. Classifier performance was evaluated by computing the area under the receiver-operating characteristic curve (AUC).

RESULTS

Individual QUS parameters correlated with RE and axial length (p < 0.05). Multi-linear regression revealed significant correlation between the set of QUS parameters and both RE (R = 0.49, p < 0.001) and axial length (R = 0.46, p = 0.001). Classifiers trained with QUS parameters achieved higher AUC (𝑝 = 0.06) for identifying myopic eyes (AUC = 0.71) compared to classifiers trained with ophthalmic parameters (AUC = 0.63). QUS-based classifiers attained the highest AUC when identifying highly myopic eyes (AUC = 0.77).

CONCLUSIONS

QUS parameters correlate with progressing myopia and may be indicative of myopia-induced microstructural and biomechanical changes in the anterior sclera. These methods may provide critical clinical information complementary to standard ophthalmic measurements for predicting myopia progression and risk assessment for posterior staphyloma formation.

Non-contact elasticity contrast imaging using photon counting

Significance

Tissues' biomechanical properties, such as elasticity, are related to tissue health. Optical coherence elastography produces images of tissues based on their elasticity, but its performance is constrained by the laser power used, working distance, and excitation methods.

Aim

We develop a new method to reconstruct the elasticity contrast image over a long working distance, with only low-intensity illumination, and by non-contact acoustic wave excitation.

Approach

We combine single-photon vibrometry and quantum parametric mode sorting (QPMS) to measure the oscillating backscattered signals at a single-photon level and derive the phantoms' relative elasticity.

Results

We test our system on tissue-mimicking phantoms consisting of contrast sections with different concentrations and thus stiffness. Our results show that as the driving acoustic frequency is swept, the phantoms' vibrational responses are mapped onto the photon-counting histograms from which their mechanical properties-including elasticity-can be derived. Through lateral and longitudinal laser scanning at a fixed frequency, a contrast image based on samples' elasticity can be reliably reconstructed upon photon level signals.

Conclusions

We demonstrated the reliability of QPMS-based elasticity contrast imaging of agar phantoms in a long working distance, low-intensity environment. This technique has the potential for in-depth images of real biological tissue and provides a new approach to elastography research and applications.

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.

Optical-resolution photoacoustic microelastography system for elasticity mapping: Phantom study and practical application

Elastography is a noninvasive technique for characterizing the mechanical properties of biological tissues. Conventional methods have limitations in resolution and sensitivity, hindering disease detection in clinical diagnostics. To address these issues, this study developed an optical-resolution photoacoustic microelastography (OR-PAME) system. Using an agar tissue phantom with varying agar concentrations and contrast agents, PAME evaluated elasticity distribution under compression in both lateral and axial dimensions. It indirectly measured elastic properties by correlating photoacoustic responses, temporal lags, and induced displacement. We also applied the system to the study of the distribution of elastic characteristics of the liver tissue after ablation, which confirmed the potential of OR-PAME in the study of elastic characteristics. Quantitative analysis showed greater lateral displacement in regions with reduced agar concentrations, indicating decreased stiffness. PAME also detected vertical displacement along the axial plane, validating its efficacy in elastographic imaging. By improving resolution and penetration, PAME provides superior visualization of elasticity distribution. Its methodology correlates microstructural alterations with tissue biomechanics, holding potential implications in medical diagnostics.

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.

Multi-modal imaging for the detection of early keratoconus: a narrative review

Keratoconus is a common progressive corneal disorder that can be associated with significant ocular morbidity. Various corneal imaging techniques have been used for the diagnosis of established cases. However, in the early stages of the disease, which include subclinical keratoconus and forme fruste keratoconus, detection of such cases can be challenging. The importance of detecting such cases is very important because early intervention can halt disease progression, improve visual outcomes and prevent postrefractive surgery ectasia associated with performing corneal refractive procedures in such patients. This narrative review aimed to examine several established and evolving imaging techniques for the detection of early cases of keratoconus. The utilization of combinations of these techniques may further increase their diagnostic ability.

In Vivo Optical Coherence Elastography Unveils Spatial Variation of Human Corneal Stiffness

OBJECTIVE

The mechanical properties of corneal tissues play a crucial role in determining corneal shape and have significant implications in vision care. This study aimed to address the challenge of obtaining accurate in vivo data for the human cornea.

METHODS

We have developed a high-frequency optical coherence elastography (OCE) technique using shear-like antisymmetric (A0)-mode Lamb waves at frequencies above 10 kHz.

RESULTS

By incorporating an anisotropic, nonlinear constitutive model and utilizing the acoustoelastic theory, we gained quantitative insights into the influence of corneal tension on wave speeds and elastic moduli. Our study revealed significant spatial variations in the shear modulus of the corneal stroma on healthy subjects for the first time. Over an age span from 21 to 34 (N = 6), the central corneas exhibited a mean shear modulus of 87 kPa, while the corneal periphery showed a significant decrease to 44 kPa. The central cornea's shear modulus decreases with age with a slope of -19 +/- 8 kPa per decade, whereas the periphery showed non-significant age dependence. The limbus demonstrated an increased shear modulus exceeding 100 kPa. We obtained wave displacement profiles that are consistent with highly anisotropic corneal tissues.

CONCLUSION

Our approach enabled precise measurement of corneal tissue elastic moduli in situ with high precision (<7%) and high spatial resolution (<1 mm). Our results revealed significant stiffness variation from the central to peripheral corneas.

SIGNIFICANCE

The high-frequency OCE technique holds promise for biomechanical evaluation in clinical settings, providing valuable information for refractive surgeries, degenerative disorder diagnoses, and intraocular pressure assessments.

Acute alcohol consumption modulates corneal biomechanical properties as revealed by optical coherence elastography

Acute alcohol ingestion has been found to impact visual functions, including eye movement, but its effects on corneal biomechanical properties remain unclear. This study aimed to investigate the influence of acute alcohol consumption on corneal biomechanical properties using optical coherence elastography (OCE). An air-coupled ultrasound transducer induced elastic waves in mice corneas in vivo, and a high-resolution phase-sensitive optical coherence tomography (OCT) system tracked the mechanical waves to quantify the elastic wave speed. In vivo measurements were performed on three groups of age- and gender-matched mice: control, placebo (administered saline), and alcohol (administered ethanol) groups. Longitudinal measurements were conducted over a one-hour period to assess acute temporal changes in wave speeds, which are associated with inherent biomechanical properties of the cornea. The results showed a significant decrease in wave speed for the alcohol group after 10 min of ingestion in comparison to pre-ingestion values (p = 0.0096), whereas the temporal wave speed changes for the placebo group were statistically insignificant (p = 0.076). In contrast, the control group showed no significant changes in elastic wave speed and corneal thickness. Furthermore, a significant difference was observed between the wave speeds of the placebo and alcohol groups at each measurement time point between 10 and 50 min (p < 0.05), though both groups exhibited a similar trend in corneal thickness change. The findings of this study have important implications for clinical assessments and research in corneal disorders, highlighting the potential of OCE as a valuable tool for evaluating such changes.

Dual-channel air-pulse optical coherence elastography for frequency-response analysis

Microliter air-pulse optical coherence elastography (OCE) has recently been proposed for the characterization of soft-tissue biomechanics using transient, sub-nanometer to micrometer-scale natural frequency oscillations. However, previous studies have not been able to provide real-time air-pulse monitoring during OCE natural frequency measurement, which could lead to inaccurate measurement results due to the unknown excitation spectrum. To address this issue, we introduce a dual-channel air-pulse OCE method, with one channel stimulating the sample and the other being simultaneously measured with a pressure sensor. This allows for more accurate natural frequency characterization using the frequency response function, as proven by a comprehensive comparison under different conditions with a diverse range of excitation spectra (from broad to narrow, clean to noisy) as well as a diverse set of sample response spectra. We also demonstrate the capability of the frequency-response analysis in distinguishing samples with different stiffness levels: the dominant natural frequencies increased with agar concentrations (181-359 Hz, concentrations: 1-2%, and maximum displacements: 0.12-0.47 µm) and intraocular pressures (IOPs) for the silicone cornea (333-412 Hz, IOP: 5-40 mmHg, and maximum displacements: 0.41-0.52 µm) under a 200 Pa stimulation pressure. These frequencies remained consistent across different air-pulse durations (3 ms to 35 ms). The dual-channel OCE approach that uses transient, low-pressure stimulation and high-resolution imaging holds the potential to advance our understanding of sample frequency responses, especially when investigating delicate tissues such as the human cornea in vivo.

Viscoelasticity quantification of cancerous tongue using intraoral optical coherence elastography: a preliminary study

Quantifying the biomechanical properties of the tongue is significant for early diagnosis of tongue carcinoma. Therefore, an intraoral optical coherence elastography system based on a miniature probe was proposed here to evaluate the viscoelasticity of in vivo tongue for the first time. Results of experiments with Sprague-Dawley rats indicate that considerable elasticity diversity occurred between cancerous and normal tongues, and the corresponding ratio of their Young's modulus was evaluated to be 3.74. It is also found that, viscosity in diseased tissue is smaller than that in normal tissue. Additionally, healthy, transitional and cancerous regions in the cancerous tongue can be distinguished easily by calculating viscoelasticity characteristics. Based on this preliminary attempt, our method with advantages of noninvasive, high-resolution, high-sensitivity and real-time detection and convenient operation may have good potential to become a useful tool for tongue carcinoma assessment after further optimization.

Quantitative assessment of corneal elasticity distribution after FS-LASIK using optical coherence elastography

Quantifying corneal elasticity after femtosecond laser-assisted in situ keratomileusis (FS-LASIK) procedure plays an important role in improving surgical safety and quality, since some latent complications may occur ascribing to changes in postoperative corneal biomechanics. Nevertheless, it is suggested that current research has been severely constrained due to the lack of an accurate quantification method to obtain postoperative corneal elasticity distribution. In this paper, an acoustic radiation force optical coherence elastography system combined with the improved phase velocity algorithm was utilized to realize elasticity distribution images of the in vivo rabbit cornea after FS-LASIK under various intraocular pressure levels. As a result, elasticity variations within and between the regions of interest could be identified precisely. This is the first time that elasticity imaging of in vivo cornea after FS-LASIK surgery was demonstrated, and the results suggested that this technology may hold promise in further exploring corneal biomechanical properties after refractive surgery.

Simultaneous tensile and shear measurement of the human cornea in vivo using S0- and A0-wave optical coherence elastography

Understanding corneal stiffness is valuable for improving refractive surgery, detecting corneal abnormalities, and assessing intraocular pressure. However, accurately measuring the elastic properties, specifically the tensile and shear moduli that govern mechanical deformation, has been challenging. To tackle this issue, we have developed guided-wave optical coherence elastography that can simultaneously excite and analyze symmetric (S0) and anti-symmetric (A0) elastic waves in the cornea at around 10 kHz frequencies, enabling us to extract tensile and shear properties from measured wave dispersion curves. We verified the technique using elastomer phantoms and ex vivo porcine corneas and investigated the dependence on intraocular pressure using acoustoelastic theory that incorporates corneal tension and a nonlinear constitutive tissue model. In a pilot study involving six healthy human subjects aged 31 to 62, we measured shear moduli (Gzx) of 94±20 kPa (mean±standard deviation) and tensile moduli (Exx) of 4.0±1.1 MPa at central corneas. Our preliminary analysis of age-dependence revealed contrasting trends: -8.3±4.5 kPa/decade for shear and 0.30±0.21 MPa/decade for tensile modulus. This OCE technique has the potential to become a highly useful clinical tool for the quantitative biomechanical assessment of the cornea. STATEMENT OF SIGNIFICANCE: This article reports an innovative elastography technique using two guided elastic waves, demonstrating the measurement of both tensile and shear moduli in human cornea in vivo with unprecedented precision. This technique paves the way for comprehensive investigations into corneal mechanics and holds clinical significance in various aspects of corneal health and disease management.

Corneal stress-strain index in myopic Indian population

AIM

The purpose is to study the corneal stress-strain index (SSI) in myopic refractive error among Indian subjects.

METHODS

A retrospective study where young myopic subjects aged between 11 and 35 years who had undergone corneal biomechanics assessment using Corvis ST between January 2017 and December 2021 were enrolled. Subjects with central corneal thickness (CCT) <500 μ, intraocular pressure (IOP) >21 mmHg, history of any systemic and ocular disease or any previous ocular surgery, high astigmatism, corneal disease such as keratoconus were excluded. Subjects with missing data or having poor quality scan were excluded. Corneal biomechanical properties and corneal SSI were assessed using Corvis ST. For statistical purposes, eyes were divided into four different groups and were analyzed using one-way ANOVA.

RESULTS

Nine hundred and sixty-six myopic eyes with mean ± standard deviation age, IOP, and CCT of 26.89 ± 4.92 years, 16.94 ± 2.00 mmHg, and 540.18 ± 25.23 microns, respectively, were included. There were 311, 388, 172, and 95 eyes that were low, moderate, severe, and extreme myopic. Deformation amplitude ratio at 1 mm and 2 mm were similar across different myopic groups. A significant increase in max inverse radius, ambrosia relational thickness, biomechanically corrected IOP, integrated radius was noted with an increase in myopic refractive error. Corvis biomechanical index, corneal SSI was found to be decreased significantly with an increase in myopic refractive error. We noted a significant positive association between myopic refractive error and SSI (P < 0.001).

CONCLUSION

Corneal SSI was found to be reduced in extreme myopic eyes.

Nonlinear fourth-order elastic characterization of the cornea using torsional wave elastography

Measuring the mechanical nonlinear properties of the cornea remains challenging due to the lack of consensus in the methodology and in the models that effectively predict its behaviour. This study proposed developing a procedure to reconstruct nonlinear fourth-order elastic properties of the cornea based on a mathematical model derived from the theory of Hamilton et al. and using the torsional wave elastography (TWE) technique. In order to validate its diagnostic capability of simulated pathological conditions, two different groups were studied, non-treated cornea samples (n=7), and ammonium hydroxide ([Formula: see text]) treated samples (n=7). All the samples were measured in-plane by a torsional wave device by increasing IOP from 5 to 25 mmHg with 5 mmHg steps. The results show a nonlinear variation of the shear wave speed with the IOP, with higher values for higher IOPs. Moreover, the shear wave speed values of the control group were higher than those of the treated group. The study also revealed significant differences between the control and treated groups for the Lamé parameter [Formula: see text] (25.9-6.52 kPa), third-order elastic constant A (215.09-44.85 kPa), and fourth-order elastic constant D (523.5-129.63 kPa), with p-values of 0.010, 0.024, and 0.032, respectively. These findings demonstrate that the proposed procedure can distinguish between healthy and damaged corneas, making it a promising technique for detecting diseases associated with IOP alteration, such as corneal burns, glaucoma, or ocular hypertension.

Optical coherence elastography and its applications for the biomechanical characterization of tissues

The biomechanical characterization of the tissues provides significant evidence for determining the pathological status and assessing the disease treatment. Incorporating elastography with optical coherence tomography (OCT), optical coherence elastography (OCE) can map the spatial elasticity distribution of biological tissue with high resolution. After the excitation with the external or inherent force, the tissue response of the deformation or vibration is detected by OCT imaging. The elastogram is assessed by stress-strain analysis, vibration amplitude measurements, and quantification of elastic wave velocities. OCE has been used for elasticity measurements in ophthalmology, endoscopy, and oncology, improving the precision of diagnosis and treatment of disease. In this article, we review the OCE methods for biomechanical characterization and summarize current OCE applications in biomedicine. The limitations and future development of OCE are also discussed during its translation to the clinic.

Optomechanical assessment of photorefractive corneal cross-linking via optical coherence elastography

Purpose: Corneal cross-linking (CXL) has recently been used with promising results to positively affect corneal refractive power in the treatment of hyperopia and mild myopia. However, understanding and predicting the optomechanical changes induced by this procedure are challenging. Methods: We applied ambient pressure modulation based optical coherence elastography (OCE) to quantify the refractive and mechanical effects of patterned CXL and their relationship to energy delivered during the treatment on porcine corneas. Three different patterned treatments were performed, designed according to Zernike polynomial functions (circle, astigmatism, coma). In addition, three different irradiation protocols were analyzed: standard Dresden CXL (fluence of 5.4 J/cm2), accelerated CXL (fluence of 5.4 J/cm2), and high-fluence CXL (fluence of 16.2 J/cm2). The axial strain distribution in the stroma induced by ocular inflation (Δp = 30 mmHg) was quantified, maps of the anterior sagittal curvature were constructed and cylindrical refraction was assessed. Results: Thirty minutes after CXL, there was a statistically significant increase in axial strain amplitude (p < 0.050) and a reduction in sagittal curvature (p < 0.050) in the regions treated with all irradiation patterns compared to the non-irradiated ones. Thirty-6 hours later, the non-irradiated regions showed compressive strains, while the axial strain in the CXL-treated regions was close to zero, and the reduction in sagittal curvature observed 30 minutes after the treatment was maintained. The Dresden CXL and accelerated CXL produced comparable amounts of stiffening and refractive changes (p = 0.856), while high-fluence CXL produced the strongest response in terms of axial strain (6.9‰ ± 1.9‰) and refractive correction (3.4 ± 0.9 D). Tripling the energy administered during CXL resulted in a 2.4-fold increase in the resulting refractive correction. Conclusion: OCE showed that refractive changes and alterations in corneal biomechanics are directly related. A patient-specific selection of both, the administered UV fluence and the irradiation pattern during CXL is promising to allow customized photorefractive corrections in the future.

Simultaneous tensile and shear measurement of the human cornea in vivo using S0- and A0-wave optical coherence elastography

Understanding corneal stiffness is valuable for improving refractive surgery, detecting corneal abnormalities, and assessing intraocular pressure. However, accurately measuring the elastic properties, particularly the tensile and shear moduli that govern mechanical deformation, has been challenging. To tackle this issue, we have developed guided-wave optical coherence elastography that can simultaneously excite and analyze symmetric (S0) and anti-symmetric (A0) elastic waves in the cornea at frequencies around 10 kHz and allows us to extract tensile and shear properties from measured wave dispersion curves. By applying acoustoelastic theory that incorporates corneal tension and a nonlinear constitutive tissue model, we verified the technique using elastomer phantoms and ex vivo porcine corneas and investigated the dependence on intraocular pressure. For two healthy human subjects, we measured a mean tensile modulus of 3.6 MPa and a mean shear modulus of 76 kPa in vivo with estimated errors of < 4%. This technique shows promise for the quantitative biomechanical assessment of the cornea in a clinical setting.

Corneal Surface Wave Propagation Associated with Intraocular Pressures: OCT Elastography Assessment in a Simplified Eye Model

Assessing corneal biomechanics in vivo has long been a challenge in the field of ophthalmology. Despite recent advances in optical coherence tomography (OCT)-based elastography (OCE) methods, controversy remains regarding the effect of intraocular pressure (IOP) on mechanical wave propagation speed in the cornea. This could be attributed to the complexity of corneal biomechanics and the difficulties associated with conducting in vivo corneal shear-wave OCE measurements. We constructed a simplified artificial eye model with a silicone cornea and controllable IOPs and performed surface wave OCE measurements in radial directions (54-324°) of the silicone cornea at different IOP levels (10-40 mmHg). The results demonstrated increases in wave propagation speeds (mean ± STD) from 6.55 ± 0.09 m/s (10 mmHg) to 9.82 ± 0.19 m/s (40 mmHg), leading to an estimate of Young's modulus, which increased from 145.23 ± 4.43 kPa to 326.44 ± 13.30 kPa. Our implementation of an artificial eye model highlighted that the impact of IOP on Young's modulus (ΔE = 165.59 kPa, IOP: 10-40 mmHg) was more significant than the effect of stretching of the silicone cornea (ΔE = 15.79 kPa, relative elongation: 0.98-6.49%). Our study sheds light on the potential advantages of using an artificial eye model to represent the response of the human cornea during OCE measurement and provides valuable insights into the impact of IOP on wave-based OCE measurement for future in vivo corneal biomechanics studies.

In vivo corneal elastography: A topical review of challenges and opportunities

Clinical measurement of corneal biomechanics can aid in the early diagnosis, progression tracking, and treatment evaluation of ocular diseases. Over the past two decades, interdisciplinary collaborations between investigators in optical engineering, analytical biomechanical modeling, and clinical research has expanded our knowledge of corneal biomechanics. These advances have led to innovations in testing methods (ex vivo, and recently, in vivo) across multiple spatial and strain scales. However, in vivo measurement of corneal biomechanics remains a long-standing challenge and is currently an active area of research. Here, we review the existing and emerging approaches for in vivo corneal biomechanics evaluation, which include corneal applanation methods, such as ocular response analyzer (ORA) and corneal visualization Scheimpflug technology (Corvis ST), Brillouin microscopy, and elastography methods, and the emerging field of optical coherence elastography (OCE). We describe the fundamental concepts, analytical methods, and current clinical status for each of these methods. Finally, we discuss open questions for the current state of in vivo biomechanics assessment techniques and requirements for wider use that will further broaden our understanding of corneal biomechanics for the detection and management of ocular diseases, and improve the safety and efficacy of future clinical practice.

In Vivo Biomechanical Measurements of the Cornea

In early corneal examinations, the relationships between the morphological and biomechanical features of the cornea were unclear. Although consistent links have been demonstrated between the two in certain cases, these are not valid in many diseased states. An accurate assessment of the corneal biomechanical properties is essential for understanding the condition of the cornea. Studies on corneal biomechanics in vivo suggest that clinical problems such as refractive surgery and ectatic corneal disease are closely related to changes in biomechanical parameters. Current techniques are available to assess the mechanical characteristics of the cornea in vivo. Accordingly, various attempts have been expended to obtain the relevant mechanical parameters from different perspectives, using the air-puff method, ultrasound, optical techniques, and finite element analyses. However, a measurement technique that can comprehensively reflect the full mechanical characteristics of the cornea (gold standard) has not yet been developed. We review herein the in vivo measurement techniques used to assess corneal biomechanics, and discuss their advantages and limitations to provide a comprehensive introduction to the current state of technical development to support more accurate clinical decisions.

Recent advances in optical elastography and emerging opportunities in the basic sciences and translational medicine [Invited]

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

Deep Tissue Characterization with Optical Coherence Elastography: A Comparison of Different Methods

The measurement of the biomechanical properties of the skin is of great interest since these properties play an important role in the development of several diseases such as skin cancer and systemic sclerosis. In this direction, several diagnostic tools have been developed to analyze the mechanical properties of the skin. Optical coherence elastography (OCE) is one of the emerging imaging techniques used for the characterization of the mechanical properties of the tissue quantitatively. In systemic sclerosis patients, the measurement of the mechanical properties of the deeper skin layers is desirable compared to the superficial layers. There are several variants of OCE that exist, but it is still not clear which method is more suitable for the measurement of the mechanical properties of the deeper tissue. In this work, we tested three common methods, the pulsed excitation method, the continuous wave excitation method, and the resonant frequency method, for the measurement of the mechanical properties of the deeper layers in the tissue. We found out that the pulsed wave excitation method provides the most reliable measurements in the shortest possible time compared to the other two methods.

Effect of corneal collagen crosslinking on viscoelastic shear properties of the cornea

The cornea is responsible for most of the refractive power in the eye and acts as a protective layer for internal contents of the eye. The cornea requires mechanical strength for maintaining its precise shape and for withstanding external and internal forces. Corneal collagen crosslinking (CXL) is a treatment option to improve corneal mechanical properties. The primary objective of this study was to characterize CXL effects on viscoelastic shear properties of the porcine cornea as a function of compressive strain. For this purpose, corneal buttons were prepared and divided into three groups: control group (n = 5), pseudo-crosslinked group (n = 5), and crosslinked group (n = 5). A rheometer was used to perform dynamics torsional shear experiments on corneal disks at different levels of compressive strain (0%-40%). Specifically, strain sweep experiments and frequency sweep tests were done in order to determine the range of linear viscoelasticity and frequency dependent shear properties, respectively. It was found that the shear properties of all samples were dependent on the shear strain magnitude, loading frequency, and compressive strain. With increasing the applied shear strain, all samples showed a nonlinear viscoelastic response. Furthermore, the shear modulus of samples increased with increasing the frequency of the applied shear strain and/or increasing the compressive strain. Finally, the CXL treatment significantly increased the shear storage and loss moduli when the compressive strain was varied from 0% to 30% (p < 0.05); larger shear moduli were observed at compressive 40% strain but the difference was not significant (P = 0.12).

High-frequency ultrasound detects biomechanical weakening in keratoconus with lower stiffness at higher grade

In vivo biomechanical characterization of the cornea remains a challenge. We have developed a high-frequency ultrasound elastography method, the ocular pulse elastography (OPE), to measure corneal axial displacement (CAD) induced by the ocular pulse. Here we compared CAD and a stiffness index derived from CAD between keratoconus patients and normal controls. We also explored the trend of these parameters with keratoconus grade. Twenty normal subjects and twenty keratoconus patients were recruited in this study. Corneal topography, tomography, intraocular pressure (IOP) and ocular pulse amplitude (OPA) were obtained in each measured eye. The cornea’s heartbeat-induced cyclic axial displacements were measured by high-frequency (50 MHz) ultrasound. A corneal stiffness index (CSI) was derived from CAD normalized against OPA. CAD and CSI were compared between normal and keratoconus groups, and across keratoconus grades. Keratoconus corneas had significantly greater CAD and lower CSI than normal controls (p’s<0.01). Both parameters correlated strongly with grade, in which CAD increased significantly (p = 0.002) and CSI decreased significantly (p = 0.011) with grade. These results suggested a biomechanical weakening in keratoconus which worsens at higher disease severity. This study also demonstrated the ability of high-frequency ultrasound elastography to provide a safe, quick, and accurate evaluation of the cornea’s biomechanical condition in vivo. The OPE-measured biomechanical metrics, when integrated with existing diagnostic criteria, may aid the decision-making in the early and definitive diagnosis and staging of keratoconus.

Differences of Corneal Biomechanics Among Thin Normal Cornea, Forme-Fruste Keratoconus, and Cornea After SMILE

Background: To compare the corneal biomechanics of thin normal cornea (TNC) with thinnest corneal thickness (TCT) (≤500 µm), forme-fruste keratoconus (FFKC) and cornea after small incision lenticule extraction (Post-SMILE) had their central corneal thickness (CCT) matched by Corneal Visualization Scheimpflug Technology (Corvis ST).

Methods: CCT were matched in 23 eyes with FFKC, 23 eyes by SMILE in 3 months post-operatively, and 23 TNC eyes. The differences in corneal biomechanics by Corvis ST among the three groups were compared.

Results: There was no significant difference in CCT among the three groups, and the biomechanically corrected intraocular pressure (bIOP) did not differ significantly among the three groups (all p &gt; 0.05). There were significant differences in most DCR parameters between pre- and post-operatively (all p &lt; 0.05). Compared with TNC, the values of corneal deflection amplitude during the first applanation (A1DA), length at the first applanation (A1L), corneal deflection amplitude during the second applanation (A2DA), and maximum deformation amplitude (DA) decreased in 3 months after SMILE (all p &lt; 0.05), these values increased in the FFKC (all p &lt; 0.05).

Conclusion: The majority of the DCR parameters were different among the three groups. The parameters A1DA, A1L, A2DA, and DA may be different between TNC and Post-SMILE, TNC and FFKC, and Post-SMILE and FFKC.

Micron-scale hysteresis measurement using dynamic optical coherence elastography

We present a novel optical coherence elastography (OCE) method to characterize mechanical hysteresis of soft tissues based on transient (milliseconds), low-pressure (<20 Pa) non-contact microliter air-pulse stimulation and micrometer-scale sample displacements. The energy dissipation rate (sample hysteresis) was quantified for soft-tissue phantoms (0.8% to 2.0% agar) and beef shank samples under different loading forces and displacement amplitudes. Sample hysteresis was defined as the loss ratio (hysteresis loop area divided by the total loading energy). The loss ratio was primarily driven by the sample unloading response which decreased as loading energy increased. Samples were distinguishable based on their loss ratio responses as a function loading energy or displacement amplitude. Finite element analysis and mechanical testing methods were used to validate these observations. We further performed the OCE measurements on a beef shank tissue sample to distinguish the muscle and connective tissue components based on the displacement and hysteresis features. This novel, noninvasive OCE approach has the potential to differentiate soft tissues by quantifying their viscoelasticity using micron-scale transient tissue displacement dynamics. Focal tissue hysteresis measurements could provide additional clinically useful metrics for guiding disease diagnosis and tissue treatment responses.

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.

Spatial Assessment of Heterogeneous Tissue Natural Frequency Using Micro-Force Optical Coherence Elastography

Analysis of corneal tissue natural frequency was recently proposed as a biomarker for corneal biomechanics and has been performed using high-resolution optical coherence tomography (OCT)-based elastography (OCE). However, it remains unknown whether natural frequency analysis can resolve local variations in tissue structure. We measured heterogeneous samples to evaluate the correspondence between natural frequency distributions and regional structural variations. Sub-micrometer sample oscillations were induced point-wise by microliter air pulses (60–85 Pa, 3 ms) and detected correspondingly at each point using a 1,300 nm spectral domain common path OCT system with 0.44 nm phase detection sensitivity. The resulting oscillation frequency features were analyzed via fast Fourier transform and natural frequency was characterized using a single degree of freedom (SDOF) model. Oscillation features at each measurement point showed a complex frequency response with multiple frequency components that corresponded with global structural features; while the variation of frequency magnitude at each location reflected the local sample features. Silicone blocks (255.1 ± 11.0 Hz and 249.0 ± 4.6 Hz) embedded in an agar base (355.6 ± 0.8 Hz and 361.3 ± 5.5 Hz) were clearly distinguishable by natural frequency. In a beef shank sample, central fat and connective tissues had lower natural frequencies (91.7 ± 58.2 Hz) than muscle tissue (left side: 252.6 ± 52.3 Hz; right side: 161.5 ± 35.8 Hz). As a first step, we have shown the possibility of natural frequency OCE methods to characterize global and local features of heterogeneous samples. This method can provide additional information on corneal properties, complementary to current clinical biomechanical assessments, and could become a useful tool for clinical detection of ocular disease and evaluation of medical or surgical treatment outcomes.

Introduction to optical coherence elastography: tutorial

Optical coherence elastography (OCE) has seen rapid growth since its introduction in 1998. The past few decades have seen tremendous advancements in the development of OCE technology and a wide range of applications, including the first clinical applications. This tutorial introduces the basics of solid mechanics, which form the foundation of all elastography methods. We then describe how OCE measurements of tissue motion can be used to quantify tissue biomechanical parameters. We also detail various types of excitation methods, imaging systems, acquisition schemes, and data processing algorithms and how various parameters associated with each step of OCE imaging can affect the final quantitation of biomechanical properties. Finally, we discuss the future of OCE, its potential, and the next steps required for OCE to become an established medical imaging technology.

Optical coherence elastography for assessing the influence of intraocular pressure on elastic wave dispersion in the cornea

The cornea is a highly specialized organ that relies on its mechanical stiffness to maintain its aspheric geometry and refractive power, and corneal diseases such as keratoconus have been linked to abnormal tissue stiffness and biomechanics. Dynamic optical coherence elastography (OCE) is a clinically promising non-contact and non-destructive imaging technique that can provide measurements of corneal tissue stiffness directly in vivo. The method relies on the concepts of elastography where shear waves are generated and imaged within a tissue to obtain mechanical properties such as tissue stiffness. The accuracy of OCE-based measurements is ultimately dependent on the mathematical theories used to model wave behavior in the tissue of interest. In the cornea, elastic waves propagate as guided wave modes which are highly dispersive and can be mathematically complex to model. While recent groups have developed detailed theories for estimating corneal tissue properties from guided wave behavior, the effects of intraocular pressure (IOP)-induced prestress have not yet been considered. It is known that prestress alone can strongly influence wave behavior, in addition to the associated non-linear changes in tissue properties. This present study shows that failure to account for the effects of prestress may result in overestimations of the corneal shear moduli, particularly at high IOPs. We first examined the potential effects of IOP and IOP-induced prestress using a combination of approximate mathematical theories describing wave behavior in thin plates with observations made from data published in the OCE literature. Through wave dispersion analysis, we deduce that IOP introduces a tensile hoop stress and may also influence an elastic foundational effect that were observable in the low-frequency components of the dispersion curves. These effects were incorporated into recently developed models of wave behavior in nearly incompressible, transversely isotropic (NITI) materials. Fitting of the modified NITI model with ex vivo porcine corneal data demonstrated that incorporation of the effects of IOP resulted in reduced estimates of corneal shear moduli. We believe this demonstrates that overestimation of corneal stiffness occurs if IOP is not taken into consideration. Our work may be helpful in separating inherent corneal stiffness properties that are independent of IOP; changes in these properties and in IOP are distinct, clinically relevant issues that affect the cornea health.

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.

Temporal phase evolution OCT for measurement of tissue deformation in the human retina in-vivo

We demonstrate the use of temporal phase evolution (TPE-) OCT methods to evaluate retinal tissue deformation in-vivo over time periods of several seconds. A custom built spectral domain (SD)-OCT system with an integrated retinal tracker, ensuring stable imaging with sub-speckle precision, was used for imaging. TPE-OCT measures and images phase differences between an initial reference B-scan and each of the subsequent B-scans of the evaluated temporal sequence. In order to demonstrate the precision and repeatability of the measurements, retinal nerve fiber (RNF) tissue deformations induced by retinal vessels pulsating with the heartbeat were analyzed in several healthy subjects. We show TPE maps (M-scans of phase evolution as a function of position along B-scan trace vs. time) of wrapped phase data and corresponding deformation maps in selected regions of the RNF layer (RNFL) over the course of several cardiac cycles. A reproducible phase pattern is seen at each heartbeat cycle for all imaged volunteers. RNF tissue deformations near arteries and veins up to ∼ 1.6 µm were obtained with an average precision for a single pixel of about 30 nm. Differences of motion induced by arteries and veins are also investigated.

Customized eye modeling for optical quality assessment in myopic femto-LASIK surgery

Refractive surgery is recognized as an effective method for myopia treatment, but it can induce night vision disturbances such as glare. We present an eye modeling method for the optical quality assessment in response to the structural changes in the eyes by femto-LASIK surgery. Customized eye models were built from the measurements of 134 right eyes pre- and post-operatively. Optical performance was evaluated using spot diagrams, point spread functions (PSFs), modulation transfer functions (MTFs), and chromatic aberrations at various fields (0°-30°), different pupil diameters (2-6 mm), and initial myopias (- 1.25 to - 10.5 D). Pupil size and initial myopia are the two major factors that affect visual performance of post-operative eyes. The results of spot diagrams, PSFs, and MTFs indicated that post-operative visual performance deteriorated as the visual field and pupil size increased, and it was significantly influenced by initial myopia. Post-operative chromatic aberrations were also affected by initial myopia. As pupil size increased, the post-operative longitudinal chromatic aberrations tended to decrease slightly, while the transverse chromatic aberrations remained similar. The use of eye modeling for refractive surgery assessment could possibly provide a more personalized surgical approach, could improve the prediction accuracy of refractive surgery outcomes, and promote the invention and development of better surgical methods.

In vivo human corneal natural frequency quantification using dynamic optical coherence elastography: Repeatability and reproducibility

Reliable and quantitative assessment of corneal biomechanics is important for the detection and treatment of corneal disease. The present study evaluates the repeatability and reproducibility of a novel optical coherence tomography (OCT)-based elastography (OCE) method for in vivo quantification of corneal natural frequency in 20 normal human eyes. Sub-micron corneal oscillations were induced by repeated low-force (13 Pa) microliter air pulses at the corneal apex and were observed by common-path phase-sensitive OCT imaging adjacent to a measurement region of 1-6.25 mm². Corneal natural frequencies were quantified using a single degree of freedom model based on the corneal oscillations. Corneal natural frequencies ranged from 234 to 277 Hz (coefficient of variation: 3.2%; n = 286 for a 2.5 × 2.5 mm² area; time: 28.6 s). The same natural frequencies can be acquired using a smaller sampling size (n = 9 for 1 mm²) and a shorter time (0.9 s). Spatial distribution and local changes in natural frequencies can be distinguished using denser sampling (e.g., 26 × 41 points for 2.5 × 5 mm²). This novel optical method demonstrates highly repeatable and reliable in vivo measurements of human corneal natural frequencies. While further studies are required to fully characterize anatomical and structural dependencies, this method may be complementary to the current OCE methods used to estimate Young's modulus from strain- or shear-wave-based measurements for the quantitative determination of corneal biomechanics.