Acoustic radiation force optical coherence elastography for evaluating mechanical properties of soft condensed matters and its biological applications

Evaluating mechanical properties of biological soft tissues and viscous mucus is challenging because of complicated dynamic behaviors. Soft condensed matter models have been successfully used to explain a number of dynamical behaviors. Here, we reported that optical coherence elastography (OCE) is capable of quantifying mechanical properties of soft condensed matters, micellar fluids. A 7.5 MHz focused transducer was utilized to generate acoustic radiation force exerted on the surface of soft condensed matters in order to produce Rayleigh waves. The waves were recorded by optical coherence tomography (OCT). The Kelvin‐Voigt model was adopted to evaluate shear modulus and loss modulus of soft condensed matters. The results reported that various concentrations of micellar fluids can provide reasonable ranges of elasticity from 65.71 to 428.78 Pa and viscosity from 0.035 to 0.283 Pa·s, which are close to ranges for actual biological samples, like mucus. OCE might be a promising tool to differentiate pathologic mucus samples from healthy cases as advanced applications in the future.     

In vivo noninvasive measurement of spatially resolved corneal elasticity in human eyes using Lamb wave optical coherence elastography

Current elastography techniques are limited in application to accurately assess spatially resolved corneal elasticity in vivo for human eyes. The air‐puff optical coherence elastography (OCE) with an eye motion artifacts correction algorithm is developed to distinguish the in vivo cornea vibration from the eye motion and visualize the Lamb wave propagation clearly in healthy subjects. Based on the Lamb wave model, the phase velocity dispersion curve in the high‐frequency is calculated to obtain spatially resolved corneal elasticity accurately with high repeatability. It is found that the corneal elasticity has regional variations and is correlated with intraocular pressure, which suggests that the method has the potential to provide noninvasive measurement of spatially resolved corneal elasticity in clinical practice.     

Relaxation time constant based optical coherence elastography

This study aimed at visualizing relative relaxation time constant (RTC) in soft tissue by using optical coherence elastography (OCE). We proposed a forced vibration model as a theoretical base to express RTC using axial gradient of periodic vibration phase captured by phase sensitive optical coherence tomography (PhS‐OCT). Validation of the model had been accomplished by experiments with isotropic and double‐layered phantoms. A fresh chicken breast sample treated with focused ultrasound was prepared to test performance of the RTC‐OCE in real tissue. All results were cross‐validated with indentation test and traditional strain‐based elastography. This study first utilized RTC mapping in 2D and 3D that covers the information of both elasticity and viscosity. The generated RTC mapping revealed the same mechanical difference internal sample which is correlated with conventional strain mapping. RTC mapping is potentially to be served as new biomarker for disease diagnosis in the future.     

Strain and elasticity imaging in compression optical coherence elastography: The two-decade perspective and recent advances

Quantitative mapping of deformation and elasticity in optical coherence tomography has attracted much attention of researchers during the last two decades. However, despite intense effort it took ~15 years to demonstrate optical coherence elastography (OCE) as a practically useful technique. Similarly to medical ultrasound, where elastography was first realized using the quasi-static compression principle and later shear-wave-based systems were developed, in OCE these two approaches also developed in parallel. However, although the compression OCE (C-OCE) was proposed historically earlier in the seminal paper by J. Schmitt in 1998, breakthroughs in quantitative mapping of genuine local strains and the Young's modulus in C-OCE have been reported only recently and have not yet obtained sufficient attention in reviews. In this overview, we focus on underlying principles of C-OCE; discuss various practical challenges in its realization and present examples of biomedical applications of C-OCE. The figure demonstrates OCE-visualization of complex transient strains in a corneal sample heated by an infrared laser beam.     

Optimal stimulation frequency for vibrational optical coherence elastography

Vibrational optical coherence elastography (OCE) is a promising tool for extracting the mechanical property of soft tissue. Purpose of this study is focusing on settling the optimal frequency range for vibrational OCE with evenly distributed stress filed. A finite element model of 2% agar phantom was built by ANSYS with a vibration stimulation frequency range from 200 to 3000 Hz. Practical experiments were carried out for cross‐validation with the same frequencies and sample. Lateral and horizontal stress filed distributions under different frequencies were mathematically evaluated by coefficient of variance and degree of linearity. Results from simulation and practical experiment cross‐validated each other and 1000 Hz was set as the maximum ideal frequency for vibrational OCE, while the minimum frequency is set by theoretical calculation with a result of 250 Hz. An ex vivo biological sample was utilised to testify performance of vibrational OCE with excitation frequencies in and out of concluded optimal range, which showed that stiffness was better mapped out in optimal frequency range.     

Vascular elasticity measurement of the great saphenous vein based on optical coherence elastography

Vascular elasticity is important in physiological and clinical problems. The mechanical properties of the great saphenous vein (GSV) deserve attention. This research aims to measure the radial elasticity of ex vivo GSV using the optical coherence elasticity (OCE). The finite element model of the phantom is established, the displacement field is calculated, the radial mechanical characteristics of the simulation body are obtained. Furthermore, we performed OCE on seven isolated GSVs. The strain field is obtained by combining the relationship between strain and displacement to obtain the radial elastic modulus of GSVs. In the phantom experiment, the strain of the experimental region of interest is mainly between 0.1 and 0.4, while the simulation result is between 0.06 and 0.40. The radial elastic modulus of GSVs ranged from 3.83 kPa to 7.74 kPa. This study verifies the feasibility of the OCE method for measuring the radial elastic modulus of blood vessels.     

Interplay of temperature,thermal‐stresses and strains in laser‐assisted modification of collagenous tissues: Speckle‐contrast and OCT‐based studies

Moderate heating of collagenous tissues such as cartilage and cornea by infrared laser irradiation can produce biologically nondestructive structural rearrangements and relaxation of internal stresses resulting in the tissue reshaping. The reshaping results and eventual changes in optical and biological properties of the tissue strongly depend on the laser‐irradiation regime. Here, a speckle‐contrast technique based on monochromatic illumination of the tissue in combination with strain mapping by means of optical coherence elastography (OCE) is applied to reveal the interplay between the temperature and thermal stress fields producing tissue modifications. The speckle‐based technique ensured en face visualization of cross correlation and contrast of speckle images, with evolving proportions between contributions of temperature increase and thermal‐stresses determined by temperature gradients. The speckle‐technique findings are corroborated by quantitative OCE‐based depth‐resolved imaging of irradiation‐induced strain‐evolution. The revealed relationships can be used for real‐time control of the reshaping procedures (e.g., for laser shaping of cartilaginous implants in otolaryngology and maxillofacial surgery) and optimization of the laser‐irradiation regimes to ensure the desired reshaping using lower and biologically safer temperatures. The figure of waterfall OCE‐image demonstrates how the strain‐rate maximum arising in the heating‐beam center gradually splits and drifts towards the zones of maximal thermal stresses located at the temperature‐profile slopes.     

Diurnal variation of corneal elasticity in healthy young human using air-puff optical coherence elastography

Due to the disruption of intraocular pressure (IOP) and central corneal thickness (CCT), diurnal variation in normal young human corneal elasticity is not clear. Using the custom-built air-puff optical coherence elastography, one eye of 21 normal subjects is enrolled randomly to measure the central corneal elasticity, IOP, and CCT in different time points within a day. Based on the multi-level model, the corneal elastic modulus is found to have a linear positive relation with IOP (P < .01) but not CCT (P = .175) and time point (P = .174–.686). A new indicator, corneal elasticity change rate, is proposed to present the magnitude of corneal elasticity change caused by 1 mmHg IOP, which can correct the interference effect of IOP. The results show that the corneal elasticity in the normal young human does not have the characteristics of diurnal variation under IOP control. Furthermore, IOP plays an important role in the corneal elasticity, and corneal elasticity change rate can increase the comparability of results between individuals.     

Three‐dimensional static optical coherence elastography based on inverse compositional Gauss‐Newton digital volume correlation

The three‐dimensional (3D) mechanical properties characterization of tissue is essential for physiological and pathological studies, as biological tissue is mostly heterogeneous and anisotropic. A digital volume correlation (DVC)‐based 3D optical coherence elastography (OCE) method is developed to measure the 3D displacement and strain tensors. The DVC algorithm includes a zero‐mean normalized cross‐correlation criterion‐based coarse search regime, an inverse compositional Gauss‐Newton fine search algorithm and a local ternary quadratic polynomial fitting strain calculation method. A 3D optical coherence tomography (OCT) scanning protocol is proposed through theoretical analysis and experimental verification. Measurement errors of the DVC‐based 3D OCE method are evaluated to be less than 2.0 μm for displacements and 0.30% for strains by rigid body motion experiments. The 3D displacements and strains of a phantom and a specimen of chicken breast tissue under compression are measured. Results of the phantom show a good agreement with theoretical analysis and tensile testing. The strains of the chicken breast tissue indicate anisotropic biomechanical properties. This study provides an effective method for 3D biomechanical property studies of soft tissue and improves the development of 3D OCE techniques.     

Translational optical coherence elastography for assessment of systemic sclerosis

Systemic sclerosis (SSc‐scleroderma) is an autoimmune disorder with high mortality rate that results in excessive accumulation of collagen in the skin and internal organs. Currently, the modified Rodnan Skin Score (mRSS) is the gold standard for evaluating the dermal thickening due to SSc. However, mRSS has noticeable inter‐ and intra‐observer variabilities as quantified by the interclass correlation coefficient (ICC: 0.6‐0.75). In this work, optical coherence elastography (OCE) combined with structural optical coherence tomography (OCT) image analysis was used to assess skin thickness in 12 SSc patients and healthy volunteers. Inter‐ (ICC: 0.62‐0.99) and intra‐observer (ICC > 0.90) assessment of OCT/OCE showed excellent reliability. Clinical assessments, including histologically assessed dermal thickness (DT), mRSS, and site‐specific mRSS (SMRSS) were also performed for further validation. The OCE and OCT results from the forearm demonstrated the highest correlation (OCE: 0.78, OCT: 0.65) with SMRSS. Importantly, OCE and OCT had stronger correlations with the histological DT (OCT: r = .78 and OCE: r = .74) than SMRSS (r = .57), indicating the OCT/OCE could outperform semi‐quantitative clinical assessments such as SMRSS. Overall, these results demonstrate that OCT/OCE could be useful for rapid, noninvasive and objective assessments of SSc onset and monitoring skin disease progression and treatment response.     

Simplifying the assessment of human breast cancer by mapping a micro‐scale heterogeneity index in optical coherence elastography

Surgical treatment of breast cancer aims to identify and remove all malignant tissue. Intraoperative assessment of tumor margins is, however, not exact; thus, re‐excision is frequently needed, or excess normal tissue is removed. Imaging methods applicable intraoperatively could help to reduce re‐excision rates whilst minimizing removal of excess healthy tissue. Optical coherence elastography (OCE) has been proposed for use in breast‐conserving surgery; however, intraoperative interpretation of complex OCE images may prove challenging. Observations of breast cancer on multiple length scales, by OCE, ultrasound elastography, and atomic force microscopy, have shown an increase in the mechanical heterogeneity of malignant breast tumors compared to normal breast tissue. In this study, a micro‐scale mechanical heterogeneity index is introduced and used to form heterogeneity maps from OCE scans of 10 ex vivo human breast tissue samples. Through comparison of OCE, optical coherence tomography images, and corresponding histology, malignant tissue is shown to possess a higher heterogeneity index than benign tissue. The heterogeneity map simplifies the contrast between tumor and normal stroma in breast tissue, facilitating the rapid identification of possible areas of malignancy, which is an important step towards intraoperative margin assessment using OCE.

     

Handheld volumetric manual compression‐based quantitative microelastography

Compression optical coherence elastography (OCE) typically requires a mechanical actuator to impart a controlled uniform strain to the sample. However, for handheld scanning, this adds complexity to the design of the probe and the actuator stroke limits the amount of strain that can be applied. In this work, we present a new volumetric imaging approach that utilizes bidirectional manual compression via the natural motion of the user's hand to induce strain to the sample, realizing compact, actuator‐free, handheld compression OCE. In this way, we are able to demonstrate rapid acquisition of three‐dimensional quantitative microelastography (QME) datasets of a tissue volume (6 × 6 × 1 mm³) in 3.4 seconds. We characterize the elasticity sensitivity of this freehand manual compression approach using a homogeneous silicone phantom and demonstrate comparable performance to a benchtop mounted, actuator‐based approach. In addition, we demonstrate handheld volumetric manual compression‐based QME on a tissue‐mimicking phantom with an embedded stiff inclusion and on freshly excised human breast specimens from both mastectomy and wide local excision (WLE) surgeries. Tissue results are coregistered with postoperative histology, verifying the capability of our approach to measure the elasticity of tissue and to distinguish stiff tumor from surrounding soft benign tissue.     

Mesoscopic fluorescence lifetime imaging: Fundamental principles,clinical applications and future directions

Fluorescence lifetime imaging (FLIm) is an optical spectroscopic imaging technique capable of real-time assessments of tissue properties in clinical settings. Label-free FLIm is sensitive to changes in tissue structure and biochemistry resulting from pathological conditions, thus providing optical contrast to identify and monitor the progression of disease. Technical and methodological advances over the last two decades have enabled the development of FLIm instrumentation for real-time, in situ, mesoscopic imaging compatible with standard clinical workflows. Herein, we review the fundamental working principles of mesoscopic FLIm, discuss the technical characteristics of current clinical FLIm instrumentation, highlight the most commonly used analytical methods to interpret fluorescence lifetime data and discuss the recent applications of FLIm in surgical oncology and cardiovascular diagnostics. Finally, we conclude with an outlook on the future directions of clinical FLIm.     

Characterization of optical clearing mechanisms in muscle during treatment with glycerol and gadobutrol solutions

The recent increasing interest in the application of radiology contrasting agents to create transparency in biological tissues implies that the diffusion properties of those agents need evaluation. The comparison of those properties with the ones obtained for other optical clearing agents allows to perform an optimized agent selection to create optimized transparency in clinical applications. In this study, the evaluation and comparison of the diffusion properties of gadobutrol and glycerol in skeletal muscle was made, showing that although gadobutrol has a higher molar mass than glycerol, its low viscosity allows for a faster diffusion in the muscle. The characterization of the tissue dehydration and refractive index matching mechanisms of optical clearing was made in skeletal muscle, namely by the estimation of the diffusion coefficients for water, glycerol and gadobutrol. The estimated tortuosity values of glycerol (2.2) and of gadobutrol (1.7) showed a longer path-length for glycerol in the muscle.     

High‐intensity‐focused ultrasound and phase‐sensitive optical coherence tomography for high resolution surface acoustic wave elastography

Elastography has the ability of quantitatively evaluating the mechanical properties of soft tissue; thus it is helpful for diagnosis and treatment monitoring of many diseases, for example, skin diseases. Surface acoustic waves (SAWs) have been proven to be a non‐invasive, non‐destructive method for accurate characterization of tissue elastic properties. Current SAW elastography using high‐energy laser pulse or mechanical shaker still have some problems. In order to improve SAW elastography in medical application, a new technique was proposed in this paper, which combines high‐intensity‐focused ultrasound as a SAWs impulse inducer and phase‐sensitive optical coherence tomography as a SAWs detector. A 2% agar‐agar phantom and ex‐vivo porcine skin were tested. The data were processed by a new algorithm based on the Fourier analysis. The results show that the proposed method has the capability of quantifying the elastic properties of soft tissue‐mimicking materials. The lateral resolution of the elastogram has been significantly improved and the different layers in heterogeneous material could also been distinguished. Our improved technique of SAW elastography has a large potential to be widely applied in clinical use for skin disease diagnosis and treatment monitoring.      

Front Cover: Label‐free spectroscopic tissue characterization using fluorescence excitation‐scanning spectral imaging (J. Biophotonics 2/2020)

A false‐colored and merged image of fresh, ex vivo rat kidney acquired using an excitation‐scanning hyperspectral imaging system. The spectral image was acquired using excitation wavelengths from 360 to 550 nm. Colors represent principal components extracted from a spectral image cube featuring no added labels or markers. Further details can be found in the article by Peter F. Favreau, Joshua A. Deal, Bradley Harris, et al. ( e201900183 ).

     

An optical system for noninvasive microscopy of psoriatic mice in vivo

Psoriasis is a chronic inflammatory skin disease involved with both complex morphological changes of skin and immune processes. The clinical diagnostics and research of psoriasis often require invasive biopsy which lacks their real-time dynamics in vivo. Here we report a noninvasive microscopic system developed by combining in vivo fluorescent microscopy, optical clearing, and immunolabeling to enable real-time imaging of immune cells and cytokines in blood flow in psoriatic animal models. The vascular morphology and time-lapse kinetics of interleukin (IL)-23, IL-17, tumor necrosis factor-α, and CD4+ cells in blood are captured at submicron resolution through the thickening epidermis and opaque scales during the development of psoriasis in vivo. Our data suggest IL-23 recruits CD4+ cells to release IL-17 in blood that further leaks out in the psoriatic skin area. This optical system enables noninvasive and real-time assessment of immune molecules and cells in vivo, providing good potential for medical researches on psoriasis.     

Brillouin microscopy for the evaluation of hair micromechanics and effect of bleaching

Brillouin microscopy is a new form of optical elastography and an emerging technique in mechanobiology and biomedical physics. It was applied here to map the viscoelastic properties of human hair and to determine the effect of bleaching on hair properties. For hair samples, longitudinal measurements (i.e. along the fibre axis) revealed peaks at 18.7 and 20.7 GHz at the location of the cuticle and cortex, respectively. For hair treated with a bleaching agent, the frequency shifts for the cuticle and cortex were 19.7 and 21.0 GHz, respectively, suggesting that bleaching increases the cuticle modulus and—to a minor extent—the cortex modulus. These results demonstrate the capability of Brillouin spectroscopy to address questions on micromechanical properties of hair and to validate the effect of applied treatments.     

Optical coherence hyperspectral microscopy with a single supercontinuum light source

In the paper, we have developed an optical coherence hyperspectral microscopy with a single supercontinuum light source. The microscopy consists of optical coherence tomography (OCT) and hyperspectral imaging (HSI), which can visualize the structural and functional characteristics of biological tissues. The 500 to 700 nm band is selected for HSI and OCT imaging, where HSI enables imaging of oxygen saturation and hemoglobin (Hb) content, while OCT acquires structural characteristics to assess the morphology of biological tissues. The system performance of the optical coherence hyperspectral microscopy is verified by normal mice ears, and the practical applications of the microscopy is further performed in 4T1 and inflammation Balb/c mice ears in vivo. The experimental results demonstrate that the microscopy has potential to provide complementary information for clinical applications.     

Is oblique scanning laser ophthalmoscope applicable to human ocular optics? A feasibility study using an eye model for volumetric imaging

Oblique scanning laser ophthalmoscopy (oSLO) is a novel imaging modality to provide volumetric retinal imaging without depth sectioning over a large field of view (FOV). It has been successfully demonstrated in vivo in rodent eyes for volumetric fluorescein angiography (vFA). However, engineering oSLO for human retinal imaging is challenging because of the low numerical aperture (NA) of human ocular optics. To overcome this challenge, we implement optical designs to (a) increase the angle of the intermediate image under Scheimpflug condition, and (b) expand the magnification in the depth dimension with cylindrical lens to enable sufficient sampling density. In addition, we adopt a scanning‐and‐descaning strategy, resulting in a compact oSLO system. We experimentally show that the current setup can achieve a FOV of ~3 × 6 × 0.8 mm³, and the transverse and axial resolutions of 7 and 41 μm, respectively. This feasibility study serves an important step for future in vivo human retinal imaging.