Quantitative evaluation of biomechanical properties of optic nerve head by using acoustic radiation force optical coherence elastography.

Significance: Previous studies have demonstrated that the biomechanical properties of the optic nerve head (ONH) are associated with a variety of ophthalmic diseases; however, they have not been adequately studied. Aim: We aimed to obtain a two-dimensional (2D) velocity distribution image based on the one-to-one correspondence between velocity values and position using the acoustic radiation force optical coherence elastography (ARF-OCE) technique combined with a 2D phase velocity algorithm. Approach: An ARF-OCE system has the advantages of non-invasive detection, high resolution, high sensitivity, and high-speed imaging for quantifying the biomechanical properties of the ONH at different intraocular pressures (IOPs) and detection directions. The 2D phase velocity algorithm is used to calculate the phase velocity values at each position within the imaging region, and then the 2D velocity distribution image is realized by mapping the velocity values to the corresponding structure based on the one-to-one relationship between velocity and position. The elasticity changes can be read directly according to the quantitative relationship between Lamb wave velocity and Young's modulus. Results: Our quantitative results show that the phase velocity and Young's modulus of the ONH increase by 32.50% and 129.44%, respectively, with increasing IOP, which is in general agreement with the results of previous studies, but they did not produce large fluctuations with the constant change of the ONH direction. These results are consistent with the changes of elastic information in the 2D velocity distribution image. Conclusions: The results suggest that the ARF-OCE technology has great potential in detecting the biomechanical properties of the ONH at different IOPs and directions, and thus may offer the possibility of clinical applications.

Supershear Rayleigh wave imaging for quantitative assessment of biomechanical properties of brain using air-coupled optical coherence elastography.

Recently, supershear Rayleigh waves (SRWs) have been proposed to characterize the biomechanical properties of soft tissues. The SRWs propagate along the surface of the medium, unlike surface Rayleigh waves, SRWs propagate faster than bulk shear waves. However, their behavior and application in biological tissues is still elusive. In brain tissue elastography, shear waves combined with magnetic resonance elastography or ultrasound elastography are generally used to quantify the shear modulus, but high spatial resolution elasticity assessment in 10 µm scale is still improving. Here, we develop an air-coupled ultrasonic transducer for noncontact excitation of SRWs and Rayleigh waves in brain tissue, use optical coherent elastography (OCE) to detect, and reconstruct the SRW propagation process; in combing with a derived theoretical model of SRWs on a free boundary surface, we quantify the shear modulus of brain tissue with high spatial resolution. We first complete validation experiments using a homogeneous isotropic agar phantom, and the experimental results clearly show the SRW is 1.9649 times faster than the bulk shear waves. Furthermore, the propagation velocity of SRWs in both the frontal and parietal lobe regions of the brain is all 1.87 times faster than the bulk shear wave velocity. Finally, we evaluated the anisotropy in different brain regions, and the medulla oblongata region had the highest anisotropy index. Our study shows that the OCE system using the SRW model is a new potential approach for high-resolution assessment of the biomechanical properties of brain tissue.

Noncontact longitudinal shear wave imaging for the evaluation of heterogeneous porcine brain biomechanical properties using optical coherence elastography.

High-resolution quantification of heterogeneous brain biomechanical properties has long been an important topic. Longitudinal shear waves (LSWs) can be used to assess the longitudinal Young's modulus, but contact excitation methods have been used in most previous studies. We propose an air-coupled ultrasound transducer-based optical coherence elastography (AcUT-OCE) technique for noncontact excitation and detection of LSWs in samples and assessment of the nonuniformity of the brain's biomechanical properties. The air-coupled ultrasonic transducer (AcUT) for noncontact excitation of LSWs in the sample has a center frequency of 250 kHz. Phase-resolved Doppler optical coherence tomography (OCT) was used to image and reconstruct the propagation behavior of LSWs and surface ultrasound waves at high resolution. An agar phantom model was used to verify the feasibility of the experimental protocol, and experiments with ex vivo porcine brain samples were used to assess the nonuniformity of the brain biomechanical properties. LSWs with velocities of 0.83 ± 0.11 m/s were successfully excited in the agar phantom model. The perivascular elastography results in the prefrontal cortex (PFC) of the ex vivo porcine brains showed that the Young's modulus was significantly higher in the longitudinal and transverse directions on the left side of the cerebral vessels than on the right side and that the Young's modulus of the PFC decreased with increasing depth. The AcUT-OCE technique, as a new scheme for LSW applications in in vivo elastography, can be used for noncontact excitation of LSWs in brain tissue and high-resolution detection of heterogeneous brain biomechanical properties.

Novel acoustic radiation force optical coherence elastography based on ultrasmall ultrasound transducer for biomechanics evaluation of in vivo cornea.

We developed a novel acoustic radiation force optical coherence elastography (ARF-OCE) based on an ultrasmall ultrasound transducer for quantitative biomechanics evaluations of in vivo cornea. A custom single-sided meta-ultrasonic transducer with an outer diameter of 1.8 mm, focal spot diameter of 1.6 mm, central frequency of 930 kHz, and focal length of 0.8 mm was applied to excite the sample. The sample arm of the ARF-OCE system employed a three-dimensional printed holder that allowed for ultrasound excitation and ARF-OCE detection. The phase-resolved algorithm was combined with a Lamb wave model to depth-resolved evaluate corneal biomechanics after keratoconus and cross-linking treatments (CXL). The results showed that, compare to the healthy cornea, the Lamb wave velocity was significantly reduced in the keratoconus, increased in the cornea after CXL, and increased with cross-linked irradiation energy in the cornea. These results indicated the good clinical translation potential of the proposed novel ARF-OCE.

Retrieval of sound-velocity profile in ocean by employing Brillouin scattering LiDAR.

Accurate remote sensing of the sound velocity profile of the upper-ocean mixed layers is of major important in oceanography, especially in underwater acoustic communication. However, the existing technologies cannot realize fast and real-time detection on sound velocity profile, a cost efficiency, flexibility, and real-time remote sensing technique is still highly urgent. In this paper, we propose a novel approach based on stimulated Brillouin scattering (SBS) LiDAR for retrieving the sound velocity profile. The sound velocity profiles in the upper-ocean mixed layer of South China Sea were retrieved theoretically and experimentally. We simulated the sound velocity profile of the upper-ocean mixed layer in South China Sea by using the Del Grosso algorithm and the data of temperature, salinity, depth selected from the World Ocean Atlas 2018 (WOA18). We designed a special ocean simulation system to measure the sound velocity in seawater with different temperatures, salinities, and pressures through measuring the frequency shift of SBS. Based on the measured sound velocities, we built a retrieval equation to express the sound velocity as a function of temperature, salinity, and pressure. Then, we retrieved the sound velocity profile of the upper-ocean mixed layer of South China Sea by using the retrieval equation. The results show that the retrieved sound velocity profile is good agreement with the theoretical simulation, and the difference between them is approximately 1∼2 m/s. Also, we have analyzed the differences between the theoretical simulation and experimental measurement. This work is essential to future application for remote sensing the sound velocity distribution profiles of the upper-ocean mixed layers by using the Brillouin LiDAR technique.

Rapid fabrication of sub-micron scale functional optical microstructures on the optical fiber end faces by DMD-based lithography.

The rapid development of optical fiber application systems puts forward higher requirements for the miniaturization and integration of optical fiber devices. One promising solution is to integrate diffractive optical microstructures on the end faces of optical fibers. However, rapid microfabrication on such tiny and irregular substrates is a challenge. In recent years, Femtosecond laser polymerization technology has become an effective solution to the challenge, which can be flexibly applied for the fabrication of complex 3D microstructures with ultra-high resolution. When the demand for the lithography resolution is not very high, other microfabrication methods with a lower technical threshold may be developed for achieving a balance between fabrication precision, cost and efficiency. In this paper, we report a Digital Micromirror Device (DMD) based lithography method dedicated to the fabrication of functional optical microstructures on the optical fiber end faces. Especially, it's also applicable to single-mode fibers (SMFs). By the projection via a 40x objective lens, the fabrication resolution of 0.405 µm was achieved within an exposure area of 209.92 µm × 157.44 µm. We evaluated the microfabrication results by the photomicrographs and the optical diffraction modulation effects of the functional optical microstructures. This method provides a new idea for fabricating both hybrid optical fiber devices and SMF devices, and it may be an alternative method for resolving the conflict between the precision, the cost and the efficiency.

Optical microfiber sensor for detection of Ni2+ ions based on ion imprinting technology.

The detection of ultralow heavy metal ion concentration is highly significant for protecting human health and maintaining the stability of the ecological environment. Herein, a microfiber interferometer chemical sensor for the detection of Ni2+ ions was proposed and experimentally demonstrated. The microfiber sensor was coated with an ion-imprinted chitosan polymer using Ni2+ as the template ion. Experimental results demonstrated a high sensitivity of 0.0454 nm nM-M for detect-ing Ni2+ in the range of 10 nM to 100 nM, and a limit of detection as low as 6.5 nM was achieved. The microfiber sensor was verified using two different non-template heavy ions, Cu2+ and Cr3+, and was determined to be highly selective to Ni2+. Furthermore, the regeneration characteristics of the sensor were experimentally assessed by three repeated adsorption-desorption cycles, and the results showed that the microfiber sensor achieved good stability without a significant loss in sensitivity. Besides, the detecting tests of Ni2+ in lake water and industrial sewage samples demonstrated the sensor's practical application. This proposed sensor has the advantages of simple configuration, high selectivity and sensitivity, fast response, and the ability to serve as a platform for water safety monitoring and remote sensing.

In Vivo Evaluation of the Effects of SMILE with Different Amounts of Stromal Ablation on Corneal Biomechanics by Optical Coherence Elastography.

This work aims to depth-resolved quantitatively analyze the effect of different stromal ablation amounts on the corneal biomechanical properties during small incision lenticule extraction (SMILE) using optical coherence elastography (OCE). A 4.5-MHz ultrasonic transducer was used to excite elastic waves in the corneal tissue. The OCE system combined with the antisymmetric Lamb wave model was employed to achieve a high-resolution, high-sensitivity, and depth-resolved quantitative detection of the corneal Young's modulus. Eighteen rabbits were randomly divided into three groups; each group had six rabbits. The first and second groups underwent -3D and -6D SMILE surgeries, and the third group was the control group, respectively. Young's modulus of the corneal cap and residual stromal bed (RSB) were both increased after SMILE, which shared the stress under intraocular pressure (IOP). Furthermore, the Young's modulus of both the corneal cap and RSB after 3D SMILE group were significantly lower than that in the -6D group, which indicated that the increases in the post-operative corneal Young's modulus were positively correlated with the amount of stromal ablation. The OCE system for quantitative spatial characterization of corneal biomechanical properties can provide useful information on the extent of safe ablation for SMILE procedures.

Quantification of biomechanical properties of human corneal scar using acoustic radiation force optical coherence elastography.

Biomechanical properties of corneal scar are strongly correlated with many corneal diseases and some types of corneal surgery, however, there is no elasticity information available about corneal scar to date. Here, we proposed an acoustic radiation force optical coherence elastography system to evaluate corneal scar elasticity. Elasticity quantification was first conducted on ex vivo rabbit corneas, and the results validate the efficacy of our system. Then, experiments were performed on an ex vivo human scarred cornea, where the structural features, the elastic wave propagations, and the corresponding Young's modulus of both the scarred region and the normal region were achieved and based on this, 2D spatial distribution of Young's modulus of the scarred cornea was depicted. Up to our knowledge, we realized the first elasticity quantification of corneal scar, which may provide a potent tool to promote clinical research on the disorders and surgery of the cornea.

Spatial confinement effects of laser-induced breakdown spectroscopy at reduced air pressures.

Spatial confinement is a simple and cost-effective method for enhancing signal intensity and improving the detection sensitivity of laser-induced breakdown spectroscopy (LIBS). However, the spatial confinement effects of LIBS under different pressures remains a question to be studied, because the pressure of the ambient gas has a significant influence on the temporal and spatial evolution of plasma. In this study, spatial confinement effects of LIBS under a series of reduced air pressures were investigated experimentally, and the plasma characteristics under different air pressures were studied. The results show that the reduced air pressure can lead to both earlier onset and weakening of the enhancement effect of the spatial confinement on the LIBS line intensity. When the air pressure drops to 0.1 kPa, the enhancement effect of the emission intensity no longer comes from the compression of the reflected shock wave on the plasma, but from the cavity's restriction of the plasma expansion space. In conclusion, the enhancement effect of spatial confinement technology on the LIBS is still effective when the pressure is reduced, which further expands the research and application field of spatial confinement technology.

Influence of temperature-salinity-depth structure of the upper-ocean on the frequency shift of Brillouin LiDAR.

Brillouin-based LiDAR is an alternative remote sensing technique for measuring the distribution profiles of temperature, salinity, and sound speed in the upper ocean mixed layer. Its principle is based on the dependence of Brillouin frequency shift on the temperature, salinity, and depth of ocean. Therefore, it is necessary to investigate the effect of various seawater parameters on Brillouin frequency shift for ocean remote sensing by using the Brillouin LiDAR. Here we theoretically and experimentally investigate the influence of temperature, salinity, and pressure (depth) of seawater on Brillouin frequency shift in the upper ocean for the first time. Numerical simulations of the distribution profiles of temperature, salinity, and Brillouin frequency shift in the upper-ocean mixed layers of East China Sea and South China Sea were performed, respectively, by employing the Brillouin equations and the World Ocean Atlas 2018 (WOA18). A special ocean simulation system was designed to carry out the stimulated Brillouin scattering (SBS) experiments for validating the numerical simulations. The results show that the seawater temperature is the most important factor for the Brillouin frequency shift in the upper-ocean mixed layer compared with the salinity and pressure. At the same salinity and pressure, the frequency shift increases by more than 10 MHz for every 1 °C increase in temperature. Also, the differences of Brillouin frequency shift between experimental and theoretical values at the same parameter conditions were analyzed. The experimental results coincide well with the theoretical simulations. This work is essential to future applications of Brillouin LiDAR in remote sensing of the temperature, salinity, or sound velocity profiles of ocean.

Effects of temperature and pressure on the threshold value of SBS LIDAR in seawater.

Effects of temperature and pressure on the threshold value of stimulated Brillouin scattering (SBS) in seawater were analyzed theoretically and experimentally. Theoretically, the change of threshold value of SBS versus the ocean depth was simulated based on the International Thermodynamic Equation of Seawater-2010 (TEOS-10) and the World Ocean Atlas 2013 (WOA13). Experimentally, an ocean temperature and pressure simulator (OTPS) was designed to measure the threshold value of SBS through simulating the changes of temperature and pressure of seawater in 0∼1000 meters. The theoretical and experimental results exhibit that the threshold value of SBS decreases with the increase of temperature at the same seawater pressure and increases with the increase of pressure at the same seawater temperature. The results imply that the SBS process is more likely to occur in upper seawater of lower-latitude areas. The theoretical and experimental results are helpful for remote sensing in ocean using the SBS LIDAR.

Quantification of iris elasticity using acoustic radiation force optical coherence elastography.

Careful quantification of the changes in biomechanical properties of the iris can offer insight into the pathophysiology of some ocular diseases. However, to date there has not been much information available regarding this subject because clinical detection for iris elasticity remains challenging. To overcome this limitation, we explore, for the first time to our knowledge, the potential of measuring iris elasticity using acoustic radiation force optical coherence elastography (ARF-OCE). The resulting images and shear wave propagation, as well as the corresponding shear modulus and Young's modulus from ex vivo and in vivo rabbit models confirmed the feasibility of this method. With features of noninvasive imaging, micrometer-scale resolution, high acquisition speed and real-time processing, ARF-OCE is a promising method for reconstruction of iris elasticity and may have great potential to be applied in clinical ophthalmology with further refinement.

Stimulated Brillouin scattering in combination with visible absorption spectroscopy for authentication of vegetable oils and detection of olive oil adulteration.

Vegetable oils provide high nutritional value in the human diet. Specifically, extra virgin olive oil (EVOO) possesses a higher price than that of other vegetable oils. Adulteration of pure EVOO with other types of vegetable oils has attracted increasing attentions. In this work, a stimulated Brillouin scattering (SBS) combined with visible absorption spectroscopy method is proposed for authentication of vegetable oils and detection of olive oil adulteration. The results provided here have demonstrated that the different vegetable oils and adulteration oils exhibit significant differences in normalized absorbance values of two relevant wavelengths (455 and 670 nm) and frequency shifts of SBS. The normalized absorbance values of all spectra at the two relevant wavelengths of 670 nm and 455 nm linearly decrease with the increase of the adulteration concentration. The Brillouin frequency shifts exponentially increase with the increase of the adulteration concentration. Due to non-destructive and requiring no sample pretreatment procedure, this method can be effectively employed for authentication and detection of oils adulteration.

Stimulated scattering effects in gold-nanorod-water samples pumped by 532 nm laser pulses.

Stimulated scattering in gold-nanorod-water samples has been investigated experimentally. The scattering centers are impurity particles rather than the atoms or molecules of conventional homogeneous scattering media. The pump source for exciting stimulated scattering is a pulsed and narrow linewidth second-harmonic Nd: YAG laser, with 532 nm wavelength, ~8 ns pulse duration, and 10 Hz repetition rate. Experimental results indicate that SMBS, SBS and STRS can be generated in gold-nanorod-water samples under appropriate pump and absorption conditions. The incident pump energy has to be larger than a certain threshold value before stimulated scattering can be detected. The absorption coefficient of samples at 532 nm wavelength depends on the one of characteristic absorption bands of gold nanorods located around 530 nm. A critical absorption coefficient can be determined for the transition from SBS to STRS. Also, the spectral-line-broadening effects of STRS have been observed, the line-shape presents a pseudo-Voigt profile due to the random thermal motion of molecules and strong particle collision.

Experimental investigation on the competition between wideband stimulated Brillouin scattering and forward stimulated Raman scattering in water.

The utilization of a simple focused optical cell to bring to light the competition between wideband stimulated Brillouin scattering (WSBS) and forward stimulated Raman scattering (FSRS) is investigated experimentally. A pulsed, wide bandwidth second-harmonic Nd:YAG laser is used as the pump source. We found that, the competition between WSBS and FSRS is an alternate process, which one dominated depends on the linewidth and energy of the pump laser, focal length, and optical breakdown.