Meixiao Shen

Topographic Thickness of Bowman's Layer Determined by Ultra-High Resolution Spectral Domain–Optical Coherence Tomography

PURPOSEnTo characterize the thickness profile of the corneal epithelium and the Bowmans layer across the horizontal meridian.nnnMETHODSnForty-four eyes of 22 healthy subjects were investigated in this study. Ultra-high resolution anterior segment spectral domain-optical coherence tomography (SD-OCT) was used to assess the topographic thickness of the epithelium and the Bowmans layer across the cornea. Thicknesses at five locations, including the center, midperiphery, and periphery close to the limbus, on both the nasal and the temporal sides along the horizontal meridian, were analyzed.nnnRESULTSnMean epithelial thickness at the central cornea was 52.5 ± 2.4 μm. It increased gradually from the center to the periphery (P < 0.001). There was no significant difference between the nasal side and the temporal side for epithelial thickness. The central Bowmans layer thickness was 17.7 ± 1.6 μm, and it remained constant from the center to the midperiphery (P > 0.05). However, thicknesses at the nasal and temporal periphery, 20.0 ± 1.9 μm and 19.8 ± 2.2 μm, respectively, were significantly greater than the central and midperipheral thicknesses (P < 0.001). Nasal and temporal thicknesses were similar on either side of the center.nnnCONCLUSIONSnThe epithelium and the Bowmans layer were not evenly distributed across the horizontal meridian of the cornea. SD-OCT provided useful information about topographic thickness of the different corneal layers in vivo.

Ultrahigh-resolution measurement by optical coherence tomography of dynamic tear film changes on contact lenses.

Purpose. To determine the dynamic pre- and postlens tear film (PLTF and PoLTF) thicknesses by using optical coherence tomography (OCT). Methods. Ultrahigh-resolution OCT was used to image the tear film of 22 subjects before and after contact lens wear. A soft lens with 1 drop of artificial tears on its concave surface was inserted onto one randomly selected eye. OCT images were taken before insertion, immediately afterward, and every 2 minutes for 10 minutes. For the contralateral eye, the lens inserted was not prewetted on the concave surface. OCT images were taken before insertion, immediately afterward, and at 3 minutes. Then another drop was instilled, and images were taken immediately afterward and every 2 minutes for 10 minutes. Images were processed by custom software to yield tear film thickness. Results. The thickness of precorneal tear film (PCTF) was 1.9 +/- 0.9 mum. The PoLTF was visualized clearly in all cases immediately after lens insertion, with 1 drop on the lens concave surface. Through the first 6 minutes after insertion, the PoLTF was greater than the PCTF. The PLTF (n = 12) and PoLTF (n = 9) were visualized immediately after lens insertion. After 3 minutes, the PLTF in most subjects and PoLTF in all subjects were invisible. The thickness of the PLTF increased after the instillation of artificial tears, whereas the PoLTF did not at any checkpoint for 10 minutes. Conclusions. Ultrahigh-resolution OCT is a promising tool for measuring the tear film. The PoLTF did not increase after instillation of artificial tears.

Anterior Segment Biometry during Accommodation Imaged with Ultralong Scan Depth Optical Coherence Tomography

PURPOSEnTo measure by ultralong scan depth optical coherence tomography (UL-OCT) dimensional changes in the anterior segment of human eyes during accommodation.nnnDESIGNnEvaluation of diagnostic test or technology.nnnPARTICIPANTSnForty-one right eyes of healthy subjects with a mean age of 34 years (range, 22-41 years) and a mean refraction of -2.5 ± 2.6 diopters were imaged in 2 repeated measurements at minimal and maximal accommodations.nnnMETHODSnA specially adapted and designed UL-OCT instrument was used to image from the front surface of the cornea to the back surface of the crystalline lens. Custom software corrected the optical distortion of the images and yielded the biometric measurements. The coefficient of repeatability and the intraclass correlation coefficient were calculated to evaluate the repeatability and reliability.nnnMAIN OUTCOME MEASURESnAnterior segment parameters and associated repeatability and reliability upon accommodation. The dimensional results included central corneal thickness (CCT), anterior chamber depth and width (ACD, ACW), pupil diameter (PD), lens thickness (LT), anterior segment length (ASL = ACD + LT), lens central position (LCP = ACD + 1/2LT), and horizontal radii of the lens anterior and posterior surface curvatures (LAC, LPC).nnnRESULTSnRepeated measurements of each variable within each accommodative state did not differ significantly (P>0.05). The coefficients of repeatability (CORs) and intraclass correlation coefficients for CCT, ACW, ACD, LT, LCP, and ASL were excellent (1.2%- 3.59% and 0.998-0.877, respectively). They were higher for PD (18.90%-21.63% and 0.880-0.874, respectively) and moderate for LAC and LPC (34.86%-42.72% and 0.669-0.251, respectively) in the 2 accommodative states. Compared with minimal accommodation, PD, ACD, LAC, LPC, and LCP decreased and LT and ASL increased significantly at maximal accommodation (P<0.05), whereas CCT and ACW did not change (P>0.05).nnnCONCLUSIONSnThe UL-OCT measured changes in anterior segment dimensions during accommodation with good repeatability and reliability. During accommodation, the back surface of the lens became steeper as the lens moved forward.nnnFINANCIAL DISCLOSURE(S)nThe authors have no proprietary or commercial interest in any of the materials discussed in this article.

Ultra-high resolution optical coherence tomography for imaging the anterior segment of the eye.

Developments in optical coherence tomography (OCT) have expanded its clinical applications for ultra-high resolution imaging of the anterior segment of the human eye. This review presents the latest advances for imaging the anterior segment of the eye using ultra-high resolution OCT (UHR-OCT). Unique applications of UHR-OCT technology in clinical and basic scientific laboratory research are discussed and a summary of the results is provided. The authors focused on the use of UHR-OCT for imaging of tear dynamics, contact lens interactions with the corneal surface, and in vivo histological diagnosis of disorders of the cornea, as well as the future direction in this field.

Repeated measurements of the anterior segment during accommodation using long scan depth optical coherence tomography

Objectives The aim was to determine the repeatability of ultralong scan depth spectral-domain optical coherence tomography (SD-OCT) measurements of the ocular anterior segment during accommodation. Methods The center wavelength of the SD-OCT light source was 840 nm with a bandwidth of 50 nm. The ocular axial resolution of the system was approximately 6.0 &mgr;m, and the scan depth was 7.2 mm in air. Twenty eyes of 20 healthy subjects were imaged with a custom-built, ultralong scan depth SD-OCT during 2 visits. The anterior segment images were acquired during nonaccommodative and maximal accommodative states. After image processing and correction, the true values of the morphometric dimensions of the anterior eye were obtained. The variables of the two states from two visits were compared. Results For the corrected anterior segment images, the variables did not significantly differ from one visit to the next. The values of anterior chamber depth, pupil diameter, and the radius of the lens anterior surface curvature during accommodation were significantly smaller than those during the nonaccommodative state. The lens thickness significantly increased with accommodation. There was no significant difference in the posterior surface curvature of the lens between the two states. Conclusions Ultralong scan depth SD-OCT holds promise for quantifying dimensional changes in the eye during accommodation. The instrument demonstrated good repeatability of ocular anterior segment dimensional measurements during accommodation.

Characterization of Soft Contact Lens Edge Fitting Using Ultra-High Resolution and Ultra-Long Scan Depth Optical Coherence Tomography

PURPOSEnTo characterize the edge fitting of soft contact lenses using ultra-high resolution optical coherence tomography (UHR-OCT) and ultra-long scan depth optical coherence tomography (UL-OCT).nnnMETHODSnA total of 20 participants (11 men, 9 women; mean age, 32.3 years) were recruited. Four different types of soft contact lenses were randomly fitted to both eyes of each subject on two separate visits. After 30 minutes, the horizontal meridians of the corneal center, midperiphery, and limbus were imaged by UHR-OCT. UL-OCT imaged each lens in vitro and the ocular surface of a physical model eye.nnnRESULTSnAngle-edged lenses had significantly less conjunctival buildup than did round-edged lenses (P = 0.008). Limbal post-lens tear film gaps were present in 42% of the eyes, with the round-edged lenses having the most at 68%. Similarly, post-lens tear film gaps at the corneal mid-periphery were present in 47% of all eyes, with the round-edged lens having the most at 75%. Mismatches between the lens and the ocular surface were simulated based on UL-OCT images of the in vitro lenses and the model eye. The existence of tear film gaps and touching points were predicted in the simulation.nnnCONCLUSIONSnThe soft contact lens edge fitting was characterized by the conjunctival buildup and tear film gaps. Different types of contact lenses presented different levels of conjunctival buildup as well as different frequencies of tear film gaps. The findings by UHR-OCT were predicted in the simulation by UL-OCT. The application of these new technologies may open new ways of designing lenses and evaluating their fit.

SD-OCT with prolonged scan depth for imaging the anterior segment of the eye.

An 840-nm wavelength spectral domain optical coherence tomography (SD-OCT) with prolonged scan depth was developed and mounted onto a conventional slit lamp for imaging the anterior segment of the eye. X-Y cross aiming was applied to align the SD-OCT scanning position during imaging. An internal fixation target displayed on a miniature LCD monitor was provided. The SD-OCT instrument had an axial resolution of 6 μm and a prolonged scan depth of 7.2 mm. High-quality SD-OCT images, consisting of 2,048 × 2,048 pixels, were acquired of the entire anterior chamber and entire crystalline lens from a healthy subject. The entire cornea, anterior chamber angle, limbus, and iris were clearly visible. Additionally, the internal structure of crystalline lens, including anterior and posterior surfaces of the crystalline lens, capsule, nucleus, and cortex, were clearly imaged with the instrument. The system was also tested in imaging accommodation of the same eye, demonstrating the feasibility of the novel approach for evaluating presbyopia/accommodation.

Entire contact lens imaged in vivo and in vitro with spectral domain optical coherence tomography.

Objectives To demonstrate the capability of directly visualizing the entire ocular surface and the entire contact lens on the eye using spectral domain optical coherence tomography (SD-OCT). Methods A custom-built, high–speed, and high-resolution SD-OCT was developed with extended scan depth and width. The eye was imaged before and after wearing a toric soft contact lens (PureVision; Bausch & Lomb). A lubricant eye drop (Soothe; Bausch & Lomb) was instilled in the eye to enhance the image contrast on the lens. The same toric soft contact lens immersed in the contact lens solution was also imaged with a contrast enhancement medium (0.5% Intralipid). Results Cross-sectional OCT images of the entire ocular surface were acquired with high-resolution 2,048 × 2,048 pixels. Quantitative surface height map of the ocular surface was obtained from a radial scan data set containing 32 B-scans. With the contrast enhancement agent, the entire toric soft contact lens was clearly visualized, both in vitro and on the eye. The surfaces of the lens were detected and used to generate the thickness maps of the soft contact lens. Conclusions Spectral domain optical coherence tomography with extended scan depth and width is a promising tool for imaging the entire ocular surface shape and soft contact lenses. This successful demonstration suggests that the extended depth SD-OCT is effective in studying ocular surface shape and its interaction with a soft contact lens. The novel method is helpful for the evaluation of contact lens fitting and lens design.

Extended scan depth optical coherence tomography for evaluating ocular surface shape

Spectral domain optical coherence tomography (SD-OCT) with extended scan depth makes it possible for quantitative measurement of the entire ocular surface shape. We proposed a novel method for ocular surface shape measurement using a custom-built anterior segment SD-OCT, which will serve on the contact lens fitting. A crosshair alignment system was applied to reduce the misalignment and tilting of the eye. An algorithm was developed to automatically segment the ocular surface. We also described the correction of the image distortion from the segmented dataset induced by the nontelecentric scanning system and tested the accuracy and repeatability. The results showed high accuracy of SD-OCT in measuring a bicurved test surface with a maximum height error of 17.4 μm. The repeatability of in vivo measurement was also good. The standard deviations of the height measurement within a 14-mm wide range were all less than 35 μm. This work demonstrates the feasibility of using extended depth SD-OCT to perform noninvasive evaluation of the ocular surface shape.

Reduced Tear Meniscus Dynamics in Dry Eye Patients With Aqueous Tear Deficiency

PURPOSEnTo measure the tear meniscus dynamics in aqueous tear deficiency dry eye patients using optical coherence tomography.nnnDESIGNnClinical research study of a laboratory technique.nnnMETHODSnTwenty-five aqueous tear deficiency dry eye patients and 30 healthy subjects were recruited. Upper and lower tear menisci of 1 randomly selected eye of each participant were imaged during normal and delayed blinking using optical coherence tomography. Measured parameters included upper tear meniscus height and volume, lower tear meniscus height and volume, the blink outcome defined as the meniscus volume change during blink action, and open eye outcome defined as the meniscus volume change during the open eye period.nnnRESULTSnDuring normal blinking, both tear meniscus height and volume before blink in dry eye patients were significantly smaller than those in healthy subjects, except for the upper tear meniscus volume. During normal blinking, the blink outcome and open eye outcome of lower tear meniscus were significantly smaller in dry eye patients compared with healthy subjects. During delayed blinking, the upper and lower tear menisci heights and volumes significantly increased in both groups. However, dry eye patients had smaller increases than healthy subjects. During delayed blinking, the open eye outcomes of upper and lower tear menisci were smaller in dry eye patients than healthy subjects.nnnCONCLUSIONSnDry eye patients seem to have reduced tear meniscus dynamics during normal blinking and smaller increases of meniscus volume during delayed blinking. Analysis of tear meniscus dynamics may provide more insight in the altered tear system in dry eye patients.