We develop a fiber based probe that is capable of two-dimensional scanning applicable in optical coherence tomography (OCT). Based on the resonance of the fiber cantilever with asymmetry structure which has two distinguished resonant frequencies in orthogonal directions, Lissajous pattern is produced suitable for two-dimensional scanning upon a sample. Orthogonal resonances of the fiber cantilever are simultaneously excited by single piezo bender actuator with one driving signal consisting of two components corresponding to above-mentioned two resonant frequencies. By integrating a backward-placed two-dimensional position sensitive detector (PSD) into the probe, real-time lateral position of the scanning pattern is registered simultaneously for image reconstruction. Dynamical characteristics of the fiber cantilever are experimentally studied with special consideration on factors determining the resolution of the scanning pattern, including frequency and amplitude ratios between two components of the driving signal and fetching duration used for an en face image. With the developed probe implemented in our established OCT system, en face OCT images of typical samples are obtained with satisfying resolution and contrast, demonstrating the feasibility of such fiber cantilever with asymmetry structure for realizing two dimensional scanning by single actuator, potentially applicable to endoscopic OCT imaging.

We develop a high speed spectral domain optical coherence tomography (SD-OCT) system based on a custom-built spectrometer and non-uniform discrete Fourier transform (NDFT) to realize minimized depth dependent sensitivity fall-off. After precise spectral calibration of the spectrometer, NDFT of the acquired spectral data is adopted for image reconstruction. The spectrometer is able to measure a wavelength range of about 138nm with a spectral resolution of 0.0674nm at central wavelength of 835nm, corresponding to an axial imaging range of 2.56mm in air. Zemax simulations and sensitivity fall-off measurements under two alignment states of the spectrometer are given. Both theoretical simulations and experiments are done to study the depth dependent sensitivity of the developed system based on NDFT in contrast to those based on conventional discrete Fourier transform (DFT) with and without interpolation. In vivo imaging on human finger from volunteer is conducted at A-scan rate of 29 kHz and reconstruction is done based on different methods. The comparing results confirm that reconstruction method based on NDFT indeed improves sensitivity especially at large depth while maintaining the coherence-function-limited depth resolution.

Thirty-seven normal and primary open angle glaucoma (POAG) subjects were noninvasively imaged by a tailor-made real-time anterior segment swept source optical coherence tomography (SS-OCT) to demonstrate the differences of the Schlemm’s canal (SC) between POAG and normal eyes. After the cross-section images of the anterior chamber angle were acquired by SS-OCT, SC was confirmed by two independent masked observers and the average area, long diameter, and perimeter of the SC were measured. In normal subjects the circumference, area, and long diameter is $580.34±87.81μm$, $8023.89±1486.10μm2$, and $272.83±49.39μm$, respectively, and these parameters were $393.25±98.04μm$, $3941.50±1210.69μm2$, and $190.91±46.47μm$ in the POAG subjects. The area of SC in the normal ones was significantly larger than that in POAG eyes ($p<0.001$), so as the long diameter and the perimeter ($p<0.001$; $p<0.001$).

Tissue optical clearing (TOC) is helpful for reducing scattering and enhancing the penetration depth of light, and shows promising potential in optimizing optical imaging performances. A mixture of fructose with PEG-400 and thiazone (FPT) is used as an optical clearing agent in mouse dorsal skin and evaluated with OCT angiography (Angio-OCT) by quantifying optical properties and blood flow imaging simultaneously. It is observed that FPT leads to an improved imaging performance for the deeper tissues. The imaging performance improvement is most likely caused by the FPT-induced dehydration of skin, and the reduction of scattering coefficient (more than $∼40.5%$) and refractive-index mismatching (more than $∼25.3%$) in the superficial (epidermal, dermal, and hypodermal) layers. A high correlation (up to $∼90%$) between the relative changes in refractive-index mismatching and Angio-OCT signal strength is measured. The optical clearing rate is $∼5.83×10−5cm/s$. In addition, Angio-OCT demonstrates enhanced performance in imaging cutaneous hemodynamics with satisfactory spatiotemporal resolution and contrast when combined with TOC, which exhibits a powerful practical application in studying microcirculation.

<!-- *** Custom HTML *** -->We have used anterior segment swept source optical coherence tomography to measure Schlemm’s canal (SC) morphometric values in the living human eye. Fifty healthy volunteers with 100 normal eyes were measured in the nasal and temporal side. Comparison with the published SC morphometric values of histologic sections proves the reliability of our results. The statistical results show that there are no significant differences between nasal and temporal SC with respect to their diameter, perimeter, and area in our study (diameter: t=0.122, p=0.903; perimeter: t=−0.003, p=0.998; area: t=−1.169, p=0.244); further, no significant differences in SC morphometric values are found between oculus sinister and oculus dexter (diameter: t=0.943, p=0.35; perimeter: t=1.346, p=0.18; area: t=1.501, p=0.135). Glaucoma is a major blinding disease that causes irreversible bilateral blindness in at least 6.8 million people.¹ High intraocular pressure (IOP) is the leading reason for the formation of glaucoma.² Aqueous outflow (AO) is the key regulator of IOP; it operates by means of the trabecular meshwork (TM) draining into Schlemm’s canal (SC) and then to the collector channels (CC), which empty into scleral veins.³ SC is like a relay station of aqueous. Thus its size is related to outflow facility, and may be have something to do with IOP.⁴ Therefore, imaging SC and measuring its size could be an important tool for glaucoma diagnosis and treatment, but it requires resolution in the range of tens of micrometers. For this reason, studies have been limited to histologic sections or animal eye.^5,6 Allingham et al. have examined the correlation between outflow facility and morphometric values of SC in normal and primary open angle glaucoma (POAG) human donor eyes.⁵ A statistically significant correlation was found between facility and SC area, and POAG eyes had a significantly smaller SC area, SC perimeter, and SC inner wall length compared with normal eyes. It would be reasonable to assume that examination of the correlation between outflow facility and morphometric measurement of SC in normal and glaucomatous living eyes could be used as further rationale to support earlier treatment for individuals with suspected POAG. With the emergence of Fourier domain optical coherence tomography (FDOCT), the image speed and sensitivity of optimal coherence tomography (OCT) have been revolutionized in last few years and have led to successful implementation of SC imaging in vivo.^7,8 Kagemann et al. were the first to noninvasively measure the SC area using commercial 840-nm spectral domain optical coherence tomography (SDOCT; Bioptigen Inc., USA) in living human eye.⁷ By using SDOCT Doppler measurement, they accurately identified the SC, CCs, and scleral veins, and measured the morphometric values of SC area. Asrani et al. have established a set of 1.3-μm swept source OCT (SSOCT).⁸ The 1.3-μm wavelength light enhances the penetration depth and system Signal to Noise Ratio in the anterior segment. This leads to both eyes’ SC, and the TM were visualized in all of the patients with glaucoma. In this study, a 1.3-μm SSOCT was developed to image the anterior segment in living human eye. High-speed, high-resolution imaging of the ocular anterior segment in vivo was achieved by SSOCT. Both eyes of 50 healthy subjects were imaged nasally and temporally, and the cross-sectional images permit the visualization of SC in both eyes. The morphometric values of SC were calculated successfully. As is shown in Fig. 1(a), the SSOCT system is based on a swept source laser (HSL-2000; Santec). The swept source is centered at 1310 nm, with a wavelength scan range of approximately 110 nm that achieves 6.78-μm axial resolution in air theoretically. A Michelson interferometer (INT-MSI-1300; Thorlabs) with a balance detector is used to detect the signal. A laser diode with 660-nm wavelength functions as an aiming laser. A photo diode is used to detect optical power to ensure that the light incident to the cornea is safe. In order to compensate the polarization mode dispersion of the whole system, a fiber polarizer control is applied in the sample path to control the polarization. A Mach-Zehnder interferometer (INT-MZI-1300; Thorlabs) is used to calibrate the wavelength to enhance the actual axial resolution. After calibration the actual axial resolution is 8.03 μm in air, which is equal to 6.0 μm in SC because the refraction index of aqueous is 1.34. Each A-line contains 512 pixels, and the typical B-Scan image contains 800 A-lines. The system frame rate is 22.5 frames per second. The real-time frame rate reduces the influence of eye jitter. <!-- named anchor --> Fig. 1 (a) Schematic of SSOCT system; (b) structure diagram of SSOCT system. SS, swept source; C1, fiber coupler ($5/95$); $\mathrm{C}2\sim \mathrm{C}4$, fiber coupler ($50/50$); BD, balance detector; PD, photo diode; LD, laser diode; WDM, wavelength division multiplexer; CL, circulator; P, polarization controller; M, Mirror; SC, $x\text{-}y$ scanner; Ob, objective; MZI, Mach-Zehnder interferometer; MSI, Michelson interferometer; RA, reference arm; SA, sample arm; TS, 3-dimensional electric translation stage; TG, target. Furthermore, the morphometric measurement of SC is incident beam angle dependent. Therefore, in order to minimize the variation of incident beam angle, a gantry structure is utilized. As is shown in Fig. 1(b), the MSI, MZI, and reference arm are fitted in a motherboard. The gantry structure allows the object to lie down. When beginning the experiment, the fixation target turns on the LED, so the object can stare the LED. This means that all the subjects are in the state of emmetropia while imaging. The sample arm is connected to a three-dimensional electric translation stage, which is manually controlled by six buttons on the frontal control panel to adjust the imaging location and focusing position to make the scanning plane (B-scan) orthogonal to the SC orientation. To ensure the accuracy of morphometric measurement, the impact of refractive index must be considered. Therefore, the axial and transverse scan ranges of SSOCT should be calibrated carefully. The typical cross-section image consists of 800 A-lines, and each A-line contains 512 pixels. The axial scan range is 2.97 mm in air. Thus, by considering the refractive index of aqueous, each pixel represents 4.3 μm length in axial when the pixel is in inside the SC. The transverse scan range is 4.8 mm in air, so each pixel represents 6-μm length in transverse. Figure 2 shows the image of the anterior segment. As is shown in Fig. 2, the outline of SC is defined as a white slit-like structure located anterior to the peripheral part of the trabecular meshwork, which is between the sclera spur and Schwalbe’s line. When we carry out the morphometric measurement, we zoom in the image and use a polygon to draw the profile of SC manually. Then the diameter can be obtained by the farthest distance of the two pixels, the perimeter can be obtained by the sum of distances, and the area can be obtained by the polygonal area formula as Eq. (1) shows: <!-- named anchor --> Eq. (1)$$S_{\mathrm{\Omega }}=\frac{1}{2}\sum _{i=1}^M(x_i\times y_{i+1}-x_{i+1}\times y_i)d_x{d}_y,$$where $x_i$ and $y_i$ are the pixels’ coordinates, and $d_x$ and $d_y$ are the length that each pixel represented in transverse and axial, respectively. <!-- named anchor --> Fig. 2 The SSOCT image of the anterior segment in living human eye. The image of temporal side of oculus dexter: Subject 20; Age: 26; sex: male; IOP: 14 mm Hg; diameter: 271.06 μm; perimeter 555.5 μm; area: $7776{\mathrm{\mu m}}^2$. SC, Schlemm’s canal; SL, Schwalbe’s line; TM, trabecular mesh; SS, scleral spur. This research was approved by Institutional Review Board of Eye and ENT Hospital of Fudan University and followed the tenets of the Declaration of Helsinki. Informed consent was signed by each subject. Fifty healthy subjects were recruited, and the male to female ratio was $1:1$. All of the subjects were checked by frequency doubling technology perimeter and Humphrey field analyzer to make sure that the subjects did not suffer from glaucoma. The IOP of subjects in this study was in the range of 10 to 21 mm Hg (average, $15.37\pm 2.72\mathrm{mm}$ Hg). The age of subjects was in the range of 21 to 74 yr (average, $44.86\pm 17.55$). All 50 healthy subjects permitted the visualization of SC in both eyes nasally and temporally, and each side was measured twice to get the average value for further analysis. Therefore we obtained four sets of measurements. There are SC morphometric values of ODN, ODT, OSN, and OST, respectively; these are shown in Fig. 3. SPSS13.0 (SPSS Inc., Chicago, IL) was used in the analysis. A p value less than 0.05 was considered statistically significant. All statistics tests adopt two-tailed test. After Shapiro-Wilk test, normal distribution materials are described as $\text{mean}\pm \text{standard deviation}$. The statistical results of SC morphometric values are shown in Table 1. <!-- named anchor --> Fig. 3 The morphometric values of SC. (a) nasal of oculus dexter (ODN); (b) temporal of oculus dexter (ODT); (c) nasal of oculus sinister (OSN); (d) temporal $s$ of oculus sinister (OST). Circles represent the area value; squares are the perimeter value; triangles are the diameter. <!-- named anchor --> Table 1 Statistical results of normal subjects. SC diameter ($\text{Mean}\pm \mathrm{SD}$) SC perimeter ($\text{Mean}\pm \mathrm{SD}$) SC cross section area ($\text{Mean}\pm \mathrm{SD}$) ODN $270.66\pm 55.47$ $573.827\pm 95.22$ $7972.4\pm 1526.99$ ODT $269.88\pm 44.10$ $570.55\pm 81.22$ $8116\pm 1626.99$ OSN $264.12\pm 51.30$ $552.98\pm 95.00$ $7561.12\pm 1343.44$ OST $263.19\pm 47.68$ $556.32\pm 97.13$ $7904\pm 1362.56$ OD $270.27\pm 49.86$ $572.19\pm 88.06$ $8044.2\pm 1570.94$ OS $263.66\pm 46.28$ $554.65\pm 96.09$ $7732.56\pm 1357.18$ Nasal $267.39\pm 53.26$ $563.4\pm 95.7$ $7766.76\pm 1445.71$ Temporal $266.54\pm 45.83$ $563.45\pm 89.36$ $8010\pm 1496.29$ Average $266.96\pm 49.55$ $563.42\pm 92.35$ $7888.38\pm 1472.58$ The values shown are mean±SD. The values of “OD” are obtained by averaging values of each ODN and ODT values, and the values of “nasal” are obtained by averaging values of each ODN and OSN values. The same definitions are made for the values of “OS” and “temporal.” “Average” values are obtained by averaging values for each OD and OS values. Allingham et al. have carried out a detailed SC morphometric measurement among 10 normal human donor eyes, and have given out averages of the SC diameter, perimeter, and area.⁵ The SC area and diameter of donor eyes will be different with the living normal human eyes. The perimeter, on the other hand, should be close to the average measurement, because the cross-section of SC is a naturally occurring cavity. After averaging OD and OS perimeter values, the average perimeter value given out by Allingham et al. is $520\pm 114\mathrm{\mu m}$, and our result is $563.42\pm 92.35\mathrm{\mu m}$. The values are close to each other. Furthermore, Kagemann et al. obtained the average SC area of 21 normal living subjects: $7254\pm 2605{\mathrm{\mu m}}^2$ (Ref. 8). Our statistical result of averaging SC area is $7888.38\pm 1472.58{\mathrm{\mu m}}^2$. There values are also close to each other. These comparisons proved the reliability of our results. However, Kagemann et al. concluded that nasal SC areas were significantly larger than temporal ones.⁸ In our study, although a number of SC area values from several subjects (such as Subjects of 9, 10, 19, 20, 26, 27, etc.) show the significant difference between nasal and temporal side, the statistical results of 100 eyes indicate that there are no significant differences between nasal and temporal side with respect to SC diameter, perimeter, and area (diameter: $t=0.122$, $p=0.903$; perimeter: $t=-0.003$, $p=0.998$; area: $t=-1.169$, $p=0.244$). Additionally, no differences were found in the SC statistical values between OS and OD (diameter: