Fourier-domain Optical Coherence Tomography Research Articles

ABC-OCT – A Cross-Platform Implementation of Real-Time Fourier-Domain Optical Coherence Tomography

ABC-OCT, Affordable B-scan Camera-based Optical Coherence Tomography, implements Fourier-Domain Optical Coherence Tomography with real-time display using cross-platform C++ and the OpenCV framework. The software can be compiled using current versions of GCC for *nix/Mac and Microsoft®Visual Studio for Windows. Full functionality of ABC-OCT needs the camera SDK from QHYCCD and a QHYCCD camera to be connected; but the code can be easily modified to support other camera drivers, as is shown by an included demo version which can use any installed webcam. The code is made available under the MIT license. The software is available from GitHub ( https://github.com/hn-88/FDOCT ). Funding statement: This work has not been funded by any grants.

Optical coherence tomography and its applications in experimental mechanics: A review

Optical coherence tomography (OCT) is a low-coherence interferometry based optical technique capable of imaging internal structures of transparent/semitransparent objects with micrometer resolution and millimeter range. It features distinct advantages of non-invasive and fast-imaging, and has been widely applied in biology, medicine, and material science. After combining with digital image/volume correlation (DIC/DVC) or phase-contrast, OCT can also be used as an elastography method, known as OCT elastography, for measuring full-field displacements and strains inside objects. Recently, OCT elastography has already been developed as an effective tool for investigating mechanical behaviors of inside bio-tissues and materials, such as cornea, skin, tumor, polymer, and polymer composite. In this work, we introduce the basic principles of OCT elastography, summarize the typical experimental mechanics applications of OCT elastography, and finally discuss the remaining problems and limitations of the technique. OCT was originally developed by D. Huang et al. in 1991. The early OCT technique detects depth information of a specimen by means of mechanical scanning, and therefore it is also called time-domain OCT (TD-OCT). Owing to the limitation of mechanical scanning manner, the imaging speed and signal-to-noise (SNR) of this first-generation OCT is relatively low, which makes it unable to meet the requirements of most measurement situations. In 1995, A. F. Fercher et al. developed another type of OCT, called Fourier-domain OCT (FD-OCT). This second-generation OCT uses parallel detection instead of mechanical scanning to improve the imaging speed and SNR, and greatly accelerated the extension of OCT application. Recently, OCT has also been combined with some other techniques and forms a lot of functional OCT methods, including Doppler OCT, polarization-sensitive OCT, OCT elastography, and spectroscopic OCT. Among them, OCT elastography can realize displacement and strain fields measurement inside objects, which is a promising technique in the field of experimental mechanics. The development of OCT elastography can be traced back to 1998, J. M. Schmitt et al. measured the in-plane and out-of-plane displacement fields inside specimens by using the combination of OCT and DIC. However, owing to the low resolution and poor SNR of the early OCT systems, the imaging quality of the method is unsatisfactory. In addition, the displacement sensitivity of the method is at sub-pixel level (micrometer or sub-micrometer level), which is insufficient for most measurement scenarios. For these reasons, DIC-based OCT elastography hasn’t been widely accepted by the researchers. In 2006, R. K. Wang et al. proposed an alternative method that combines OCT and phase-contrast for achieving OCT elastography. The method uses phase sensitive characteristic of OCT for measuring displacements, and makes the displacement sensitivity reach to nanometer level or even higher. Benefiting from the significant improved displacement sensitivity, OCT elastography has received increasing attention in the last decade. Though OCT elastography developed rapidly in recent years, there still remain some problems: (1) Current OCT elastography is only available of measuring transparent and semitransparent objects. For measuring non-transparent objects, it is helpless. Therefore, in the future work, improving penetrating capability is required for OCT elastography. (2) Despite the fact that the development of FD-OCT greatly improved the imaging speed of OCT elastography, it is still insufficient for measuring full-field deformations of inside objects dynamically. Therefore, improving the imaging speed of OCT elastography is also on demand. (3) The employment of phase-contrast can significantly improve the displacement measurement sensitivity of OCT elastography, but it also makes the method lose its in-plane displacement measurement capability. Therefore, developing a novel OCT elastography enabling both in-plane and out-of-plane displacement measurements with high sensitivity is required in the further research.

Computational aberration correction in spatiotemporal optical coherence (STOC) imaging.

Spatiotemporal optical coherence (STOC) imaging is a new technique for suppressing coherent cross talk noise in Fourier-domain full-field optical coherence tomography (FD-FF-OCT). In STOC imaging, the time-varying inhomogeneous phase masks modulate the incident light to alter the interferometric signal. Resulting interference images are then processed as in standard FD-FF-OCT and averaged incoherently or coherently to produce cross-talk-free volumetric optical coherence tomography (OCT) images of the sample. Here, we show that coherent averaging is suitable when phase modulation is performed for both interferometer arms simultaneously. We explain the advantages of coherent over incoherent averaging. Specifically, we show that modulated signal, after coherent averaging, preserves lateral phase stability, enabling computational phase correction to compensate for geometrical aberrations. Ultimately, we employ it to correct for aberrations present in the image of the photoreceptor layer of the human retina that reveals otherwise invisible photoreceptor mosaics.

Fourier-Domain OCT Imaging of the Ocular Surface and Tear Film Dynamics: A Review of the State of the Art and an Integrative Model of the Tear Behavior During the Inter-Blink Period and Visual Fixation.

In the last few decades, the ocular surface and the tear film have been noninvasively investigated in vivo, in a three-dimensional, high resolution, and real-time mode, by optical coherence tomography (OCT). Recently, OCT technology has made great strides in improving the acquisition speed and image resolution, thus increasing its impact in daily clinical practice and in the research setting. All these results have been achieved because of a transition from traditional time-domain (TD) to Fourier-domain (FD) technology. FD-OCT devices include a spectrometer in the receiver that analyzes the spectrum of reflected light on the retina or ocular surface and transforms it into information about the depth of the structures according to the Fourier principle. In this review, we summarize and provide the state-of-the-art in FD-OCT imaging of the ocular surface system, addressing specific aspects such as tear film dynamics and epithelial changes under physiologic and pathologic conditions. A theory on the dynamic nature of the tear film has been developed to explain the variations within the individual compartments. Moreover, an integrative model of tear film behavior during the inter-blink period and visual fixation is proposed.

Comparison of corneal epithelial remodeling after small incision lenticule extraction and femtosecond laser-assisted LASIK

Objective: To investigate the corneal epithelial thickness changes between small incision lenticule extraction (SMILE) and femtosecond laser-assisted LASIK (FS-LASIK), study the related factors and analyze the relationship between the changes in corneal epithelial thickness and corneal aberration. Methods: This case control study included 59 patients (118 eyes), 27 males and 32 females, aged 25.64±5.57 years, who were scheduled for SMILE or FS-LASIK for treatment of myopia with or without myopic astigmatism at the First Affiliated Hospital of Soochow University between October 2016 and May 2017. All patients were divided into two groups according to the surgery. Epithelial thickness was obtained in nine zones with Fourier-domain optical coherence tomography across a 5-mm diameter centered by the pupil before surgery and at 1 week, 1 month, 3 months and 6 months postoperatively. The observed changes were analyzed by linear-regression analysis with the central corneal thickness decrement, ablation rate and corneal aberration. Comparison between two groups was analyzed by independent sample t-test, analysis of variance and Mann-Whitney U test. Results: There was no significant difference in corneal epithelial thickness of nine zones between SMILE and FS-LASIK for low to moderate myopia (manifest refraction sphere equivalent<-6.00 diopters) and high myopia (manifest refraction sphere equivalent ≥-6.00 to -10.00 diopters), respectively, before surgery (P>0.05). At 1 month, no significant difference existed in corneal epithelial thickness increment in nine zones between FS-LASIK and SMILE for low to moderate myopia (P>0.05), but a larger increase was observed in the superior (t=2.670, P=0.010), superonasal (t=2.506, P=0.015) and nasal (t=2.831, P=0.007) zones following FS-LASIK [(4.2±3.2), (3.4±2.7) and (3.7±2.5) μm] than SMILE [(2.2±2.2), (1.6±2.5) and (1.7±2.6) μm] for high myopia. At 3 months, a larger increase in corneal epithelial thickness was observed in the superior (t=2.703, P=0.009) zone following FS-LASIK than SMILE for low to moderate myopia, and the same increase was observed in the central (t=2.660, P=0.010), superior (t=3.229, P=0.002), nasal (t=2.420, P=0.019), inferonasal (t=2.651, P=0.011), inferotemporal (t=2.153, P=0.036) and superotemporal (t=2.281, P=0.027) zones after FS-LASIK [(6.7±2.6), (5.0±2.1), (3.9±1.9), (4.8±1.8), (7.5±2.5) and (6.4±2.7) μm] than SMILE [(4.7±2.9), (2.9±2.5), (2.4±2.4), (3.3±2.2), (6.0±2.4) and (4.8±2.4) μm] for high myopia. At 6 months, a larger increase in corneal epithelial thickness was observed in the superior (t=3.340, P=0.001) and nasal (t=1.952, P=0.055) zones following FS-LASIK than SMILE for low to moderate myopia, and the same increase was observed in the superior (t=2.332, P=0.024) and inferonasal (t=2.172, P=0.034) zones after FS-LASIK than SMILEfor high myopia. The central corneal thickness decrement after SMILE was much more than FS-LASIK both in the low to moderate myopia and high myopia groups (P<0.05). The average corneal epithelial thickness increment correlated positively with the central corneal thickness decrement and ablation rate at 1 month, 3 months and 6 months postoperatively (P<0.01). For SMILE, the total higher orders aberration (0.81±0.26, 0.79±0.28 and 0.81±0.31) and spherical aberration (0.50±0.21, 0.48±0.20 and 0.52±0.23) were less than FS-LASIK (0.97±0.34, 0.97±0.33 and 0.93±0.32; 0.72±0.25, 0.66±0.30 and 0.71±0.25) at 1 month, 3 months and 6 months postoperatively (P<0.05). Furthermore, the corneal aberration increment correlated positively with the average corneal epithelial thickness increment (P<0.05). Conclusions: The corneal epithelial thickness increment after SMILE was less than FS-LASIK. SMILE had better uniformity of the corneal epithelial thickness increment at the observed zones, which may explain the finding that the postoperative spherical aberration of SMILE was less than FS-LASIK. (Chin J Ophthalmol, 2020, 56:93-102).

Dispersion, spot size and speckle noise effects in fourier-domain optical coherence tomography

In this work we present a rigorous analysis of the propagation of light wave through the sample in Fourier-domain optical coherence tomography (FD−OCT). We considered a sample with four layers having refractive index dispersion. We assumed that light wave is a Gaussian beam propagating through the sample. We consider the radius of the beam under different circumstances. Beam radius accounts for diffraction and lateral resolution. We varied different parameters like focal length, depth of focus, tilt of the sample and center wavelength and calculated beam radius under variation of these parameters. Then we considered the interferogram under dispersion, dispersion free and dispersion with speckle Rayleigh noise. We calculated resolution and shift of position of the main lobe with regard to the position in the case of dispersion free and noise free condition. We plotted family of curves for different layer interfaces.

Mechanical test study in composites using digital holographic interferometry and optical coherence tomography simultaneously.

A dual optical configuration to inspect the internal and external mechanical response of a composite specimen is presented. The inspection simultaneously uses two equally aligned optical techniques, digital holographic interferometry and Fourier domain optical coherence tomography, to retrieve surface and internal data, respectively. The sample under study is a composite specimen of poly-methyl-methacrylate reinforced with metallic particles. Two different sets of samples are analyzed to compare their mechanical behavior. A homemade, fully controlled testing machine is used to apply a controlled compression load while each technique registers an image. In this form, the surface and internal optical phase measurements are correlated to the same compression value for comparison purposes. Results for each technique are directly presented as simultaneous displacement maps, and a discussion and conclusion of this proposed dual method of inspection are presented.

Resolving absolute depth in circular-ranging optical coherence tomography by using a degenerate frequency comb.

In Fourier-domain optical coherence tomography, an interference signal is generated that spans an RF bandwidth proportional to the product of three parameters: the imaging range, the imaging speed, and the inverse of the axial resolution. Circular-ranging optical coherence tomography (CR-OCT)architectures were introduced to ease long-range imaging by decoupling the imaging range from the signal RF bandwidth. As a consequence, present CR-OCT systems resolve the relative, but not the absolute, depth location of the scatterers. We introduce here a modified implementation of CR-OCT that uses a degenerate frequency comb source that allows recovery of absolute depth information while only minimally impacting the previously described RF bandwidth compression benefits of CR. We show that this degenerate frequency comb can be created by relatively simple modifications to existing frequency comb source designs, and we present absolute ranging capabilities through imaging studies and simulations.

A Generic Framework for Fourier-Domain Optical Coherence Tomography Imaging: Software Architecture and Hardware Implementations

Fourier-domain optical coherence tomography (FD-OCT), including spectral-domain OCT (SD-OCT) and swept-source OCT (SS-OCT), allows the volumetric imaging of the tissue architecture with a faster speed and higher detection sensitivity than does time-domain OCT. Although the hardware implementations of SD-OCT and SS-OCT are different, these technologies share very similar signal processing steps for image reconstruction. In this study, we developed hardware implementations and software architectures to design a generic framework for FD-OCT. For SD-OCT systems, an external synchronization approach was used to realize a data acquisition schematic similar to that used in SS-OCT by carefully managing the timing clocks in the detection unit and for the waveform generation. In addition, by utilizing modules and factory concepts, a software engine can be developed that supports various acquisition devices and software operations or image processing functions with high operation flexibility while maintaining its robustness. Data processing and data saving were optimized using the parallel computing method with the OpenMP library and by leveraging the parallelism within the acquired data, respectively.

Ganglion Cell Complex Analysis in Glaucoma Patients: What Can It Tell Us?

Glaucoma is a group of optic neuropathies characterized by a progressive degeneration of retina ganglion cells (RGCs) and their axons that precedes functional changes detected on the visual field. The macular ganglion cell complex (GCC), available in commercial Fourier-domain optical coherence tomography, allows the quantification of the innermost retinal layers that are potentially involved in the glaucomatous damage, including the retinal nerve fiber (RNFL), ganglion cell and inner plexiform layers. The average GCC thickness and its related parameters represent a reliable biomarker in detecting preperimetric glaucomatous damage. The most accurate GCC parameters are represented by average and inferior GCC thicknesses, and they can be associated with progressive visual field loss. Although the diagnostic accuracy increases with more severe glaucomatous damage and higher signal strength values, it is not affected by increasing axial length, resulting in a more accurate discrimination of glaucomatous damage in myopic eyes with respect to the traditional RNFL thickness. The analysis of the structure-function relationship revealed a good agreement between the loss in retinal sensitivity and GCC thickness. The use of a 10-2° visual field grid, adjusted for the anatomical RGCs displacement, describes more accurately the relationship between RGCs thickness and visual field sensitivity loss.

Assessment of ocular surface response to tinted soft contact lenses with different characteristics and pigment location

This study investigated the characteristics and location of the pigment layer of commercially available tinted contact lenses and evaluated ocular surface properties. The rub-off test and anterior segment Fourier-domain optical coherence tomography were used to analyze the pigment layer of the lenses. They also completed a rating questionnaire of subjective symptom perception after wearing. Among the four groups, there were significant differences in the ocular surface properties on day 3 (p < 0.05). Most of the ocular surface properties were significantly different between baseline and after being worn on day 1, day 3 and day 7 (p < 0.05), especially for tinted contact lenses with pigment layers on the lens surface. On initial wearing of the tinted contact lenses, contact between pigment layers on the surface of the lens and ocular tissues appeared to irritate the ocular surface more than non-pigment layers on the surface. Pigment-tinted soft contact lenses with lower oxygen permeability may cause worse ischemic signs on the ocular surface. However, the ocular surface adapted gradually and the symptoms decreased after wearing the tinted contact lens.

Investigation of sputtering target re-deposits by Optical Coherence Tomography and grazing incident X-ray diffraction

The sputtering target re-deposit grown by cumulative sputtering has been investigated in this work by studying both fresh and rigorously sputtered zirconium oxide target by Fourier Domain Optical Coherence Tomography (FDOCT), a depth resolved optical imaging modality. The Optical Coherence Tomography (OCT) depth profiles have further been analysed to extract optical attenuation coefficient of sputtered target surface as well as fresh target surface. Re-deposited sputtered target surface shows larger optical attenuation coefficient in the range of 1.5–4.4 mm−1 as opposed to that of 1.1 mm−1 for fresh ZrO2 target. The same sputtered target surfaces have also been looked into by using grazing incident X-ray diffraction technique with an optimised grazing angle of incidence. A strong correlation between structural changes of re-deposited surface and the extent of light depth penetration obtained by analysing OCT depth profiles at different locations of the sputtered target was observed. The results indicate, that FDOCT can be considered to be an effective tool to probe sputtering target re-deposits, with the advantage of being faster and non-contact method.

Stable multi-megahertz circular-ranging optical coherence tomography at 1.3 µm.

In Fourier-domain optical coherence tomography (OCT), the finite bandwidth of the acquisition electronics constrains the depth range and speed of the system. Circular-ranging (CR) OCT methods use optical-domain compression to surpass this limit. However, the CR-OCT system architectures of prior reports were limited by poor stability and were confined to the 1.55 µm wavelength range. In this work, we describe a novel CR-OCT architecture that is free from these limitations. To ensure stable operation, temperature sensitive optical modules within the system were replaced; the kilometer-length fiber spools used in the stretched-pulse mode-locked (SPML) laser was eliminated in favor of a single 10 meter, continuously chirped fiber Bragg grating, and the interferometer's passive optical quadrature demodulation circuit was replaced by an active technique using a lithium niobate phase modulator. For improved imaging penetration in biological tissues, the system operating wavelength was shifted to a center wavelength of 1.29 µm by leveraging the wavelength flexibility intrinsic to CFBG-based dispersive fibers. These improvements were achieved while maintaining a broad (100 nm) optical bandwidth, a long 4 cm imaging range, and a high 7.6 MHz A-line rate. By enhancing stability, simplifying overall system design, and operating at 1.3 µm, this CR-OCT architecture will allow a broader exploration of CR-OCT in both medical and non-medical applications.

All fiber polarization insensitive detection for spectrometer based optical coherence tomography using optical switch

Polarization dependent image artifacts are common in optical coherence tomography imaging. Polarization insensitive detection scheme for swept source based optical coherence tomography systems is well established but is yet to be demonstrated for all fiber spectrometer-based Fourier domain optical coherence tomography systems. In this work, we present an all fiber polarization insensitive detection scheme for spectrometer based optical coherence tomography systems. Images from chicken breast muscle tissue were acquired to demonstrate the effectiveness of this scheme for the conventional Fourier domain optical coherence tomography system.

Agreement between Fourier-domain and swept-source optical coherence tomography used for optic nerve head measurements

To assess the correlation between optic nerve head measurements generated by Fourier-domain (FD) and swept-source (SS) optical coherence tomography (OCT) both in healthy Caucasian subjects and patients with primary open angle glaucoma (POAG). This was a cross-sectional study of the right eyes of 118 subjects. In each participant, the measurements of disc area, cup to disc ratio (CDR), vertical cup to disc ratio (VCDR), rim area and rim volume were performed consecutively by FD-OCT and then SS-OCT. Participant age, gender and spherical equivalent were also recorded. Agreement between the two devices was assessed through intraclass correlation coefficients (ICC) and Bland-Altman plots. The study sample consisted of 95 healthy eyes and 23 eyes with glaucoma. Mean participant age was 48.6±20.0 years, 54.2% were female, and mean spherical equivalent was -1.6±3.0 diopters. FD-OCT and SS-OCT measurements were respectively: mean disc area 1.79±0.3 vs 1.83±0.3 mm2 (ICC=0.71), mean CDR 0.38±0.2 vs 0.33±0.2 (ICC=0.91), mean VCDR 0.58±0.2 vs 0.52±0.2 (ICC=0.92), mean rim area 1.05±0.4mm2 vs 1.03±0.5mm2 (ICC=0.29), and mean rim volume 0.14±0.11 vs 0.21±0.17mm3 (ICC=0.53). Good agreement between the devices was noted for rim area and rim volume in glaucoma subjects (ICC=0.76 and 0.68 respectively), while weak agreement was observed for these variables in healthy subjects (ICC≤0.50). The CDR and VCDR measurements provided by FD and SS OCT showed excellent agreement for the overall sample. When the devices were used for rim measurements, agreement was excellent only in the POAG patients.

Şizofreni Hastalarında Gözün Arka Segment Yapılarının Değerlendirilmesi

Purpose: We aimed to evaluate the effects of schizophrenia on posterior ocular structures including the retinal nerve fiber layer (RNFL), central macular thickness (CMT), choroidal thickness (CT) and lamina cribrosa (LC) measurements.Methods: A total of 35 patients with schizophrenia and 35 healthy individuals were enrolled. Only the right eye of each participant was tested. RNFL thickness in all quadrants, CT, CMT, LC thickness, and LC depth (LCD) measurements were performed using Fourier domain optic coherence tomography (OCT), and each variable was compared between groups.Results: The mean age and sex distributions were similar between groups (p=0.528 and p=0.299, respectively). The average, superior, inferior, nasal, and temporal RNFL thicknesses were significantly lower in schizophrenia patients than in controls (p&amp;lt;0.001, p&amp;lt;0.001, p=0.001, p=0.002 and p&amp;lt;0.001, respectively). The mean subfoveal and perifoveal CT were not significantly different between the groups (subfoveal; p=0.676, 1.5 mm nasal; p=0.632, 3 mm nasal; p=1.000, 1.5 mm temporal; p=0.811, 3 mm temporal; p=0.145). The mean CMT was similar among the groups (p=0.678). LC thickness and LCD were not statistically significant between groups (p=0.816 and p=0.161).Conclusions: We demonstrated that RNFL thicknesses in all quadrants were significantly lower in schizophrenia patients than in control subjects. However, there were no significant differences in the CMT, perifoveal and subfoveal CT, LC thickness, and LCD between the groups. These results suggest that OCT can be used to image neuronal loss by evaluating the RNFL in patients with schizophrenia. Keywords: Schizophrenia; retinal nerve fiber layer; choroidal thickness; lamina cribrosa thickness; macular thickness

Cardiac Imaging: OCT

Abstract Introduction Intracoronary imaging by means of optimal coherence tomography (OCT) provides valuable incremental information that can be used clinically to optimize stent implantation and minimize stent-related problems. Over the past decades, OCT has progressively evolved with respect to technical performance and procedural aspects. Principle OCT is a fiberoptic wire with a rotating lens emitting near infrared (1,300 nm) light and records the light reflected from the tissue. The currently used Fourier domain-OCT allows superior image quality and faster image acquisition. OCT in Acute Coronary Syndrome (ACS) It helps in accurate detection of actual event leading to ACS among plaque rupture, plaque erosion, and calcified nodule. OCT has an advantage over intravascular ultrasound for identification of vulnerable plaque and also detection of thin cap fibroatheroma &lt; 65 µm. Clinical Utility OCT helps in diagnosis of an angiographically ambiguous lesion such as dissection, thrombus, and calcified nodule. It also helps in planning different aspects of percutaneous coronary intervention such as lesion preparation, determining stent size and length and also helps in optimization of results of stent deployment.Certain nonatherosclerotic events such as coronary arteritis can be diagnosed with OCT.Continued advancement in technology, faster image acquisition, and more sophisticated coregistered image analysis will facilitate greater adoption through ease of use and interpretation.

Full-range Fourier-domain optical coherence tomography based on Mach–Zehnder interferometer

We proposed a full-range Fourier-domain optical coherence tomography (FR-FDOCT) based on a Mach–Zehnder interferometer with a tilted mirror which could generate a series of accurate phase-delay interference signals simultaneously. The FR-FDOCT is capable of acquiring artifact-free images by the construction of both amplitude and phase of the interferogram from multiple sets of phase-delay signals. The Mach–Zehnder interferometer make the reference arm became single-way optical path which is more convenient to adjust the reference beam than the return-way optical path (which means the incident and reflected beams use the same optical path) of traditional systems. A series of phase-delay interference signals were simultaneously acquired by a home-made two-dimensional spectrometer when tilting the mirror in the reference arm. The technique was employed to measure an anterior chamber of a porcine eye with a high symmetric artifact suppression of more than 45 dB. The technology could achieve double-range images effectively without any additional scanning device. It could actively promote the innovation and development of OCT technology.

The frequency of non-pathologically thin corneas in young healthy adults.

Purpose: Measurement of normal corneal thickness and corneal epithelial thickness is important in keratorefractive surgery, glaucoma, following extended contact lens wear, and in patients with corneal disease. Clinically, a central corneal thickness less than 500 µm is considered to be moderately-to-extremely thin. The purpose of this study was to compare biological differences in patients with clinically thin compared to normal corneal thickness values in healthy young adults using Fourier domain optical coherence tomography.Patients and methods: In total, 168 eyes from 84 patients aged 19–38 years were scanned using an Avanti optical coherence tomographer. To eliminate circadian effects on corneal thickness, all patients were scanned within a 4-hour window. Corneal thickness was measured across the central 6 mm of the cornea. Total central corneal thickness, corneal epithelial thickness, and corneal stromal thickness were compared between males and females and tested for correlations with age, use of systemic hormones, degree of myopia, and corneal curvature.Results: The average central corneal thickness for males and females was 540.5±32.0 μm and 525.2±33.0 μm, respectively (P=0.020). Thirty-eight eyes had corneal thickness measurements below 500 µm; 12% (6 eyes) from males and 28% (16 eyes) from females (P=0.008). All women with corneas below 500 μm were bilaterally thin. This finding differed for men. Corneal thinning was not associated with age, use of systemic hormones, or degree of myopia. Females had steeper keratometry (K) readings (P=0.01 for flat K, P=0.002 for steep K) than males. No differences in layer offset values between normal thickness corneas and thin corneas were evident, suggesting that the reduced thickness was not pathological.Conclusion: The results of this study indicate that a subpopulation of healthy young adults have non-pathologically thin corneas, well below 500 μm; and that these thinner corneas are more frequent in females. This underscores the importance of accurate corneal thickness measurements prior to keratorefractive surgery and when evaluating intraocular pressure in glaucoma.

Application of optical coherence tomography and optical path length method for monitoring corneal thickness and refractive index change during corneal cross-linking.

Corneal cross-linking (CXL) using UVA irradiation with a riboflavin photosensitizer has emerged as a new treatment paradigm for corneal ectatic disorders. The thickness threshold for protection of intraocular structures has often been challenged with ongoing developments, and corneal thinning becomes an important safety concern, especially for patients with thin corneas. In this study with an ex vivo bovine eye model, we monitored corneal thinning and corneal refractive index changes using optical coherence tomography (OCT) integrated with an adaptation of the optical path length method. CXL experiments were performed based on the standard protocol that includes removal of the corneal epithelium to facilitate diffusion of riboflavin into the stroma. The corneal stromal thickness and group refractive index were measured by a 1310nm Fourier-domain OCT imaging system at three critical points of the procedure: immediately after epithelial removal, after 30min riboflavin instillation, and after 30min UVA irradiation with continuing instillation. We found that the refractive index of the bovine cornea changed significantly from epithelial removal to riboflavin instillation and UVA irradiation, increasing from 1.377±0.005 (mean±standard deviation) after de-epithelization to 1.387±0.003 after 30min instillation and 1.388±0.008 after subsequent irradiation. The corneas also underwent a considerable decrease (10%-20%) in stromal thickness with thinning of 95±29 μm (mean±standard deviation) after riboflavin instillation and a further decrease (∼5%) with thinning of 42±19 μm after UVA irradiation. Our study highlights the importance of corneal thickness monitoring during CXL, especially after riboflavin instillation when the decrease is the largest, to avoid delivering endothelial cytotoxic doses. An increase in refractive index heightens the concern for corneal thinning and the need for careful monitoring as a safety precaution.