Large depth-high resolution full 3D imaging of the anterior segments of the eye using high speed optical frequency domain imaging

Robotic-arm-assisted flexible large field-of-view optical coherence tomography

Optical coherence tomography (OCT) is a three-dimensional non-invasive high-resolution imaging modality that has been widely used for applications ranging from medical diagnosis to industrial inspection. Common OCT systems are equipped with limited field-of-view (FOV) in both the axial depth direction (a few millimeters) and lateral direction (a few centimeters), prohibiting their applications for samples with large and irregular surface profiles. Image stitching techniques exist but are often limited to at most 3 degrees-of-freedom (DOF) scanning. In this work, we propose a robotic-arm-assisted OCT system with 7 DOF for flexible large FOV 3D imaging. The system consists of a depth camera, a robotic arm and a miniature OCT probe with an integrated RGB camera. The depth camera is used to get the spatial information of targeted sample at large scale while the RGB camera is used to obtain the exact position of target to align the image probe. Eventually, the real-time 3D OCT imaging is used to resolve the relative pose of the probe to the sample and as a feedback for imaging pose optimization when necessary. Flexible probe pose manipulation is enabled by the 7 DOF robotic arm. We demonstrate a prototype system and present experimental results with flexible tens of times enlarged FOV for plastic tube, phantom human finger, and letter stamps. It is expected that robotic-arm-assisted flexible large FOV OCT imaging will benefit a wide range of biomedical, industrial and other scientific applications.

Long scan depth optical coherence tomography on imaging accommodation: impact of enhanced axial resolution, signal-to-noise ratio and speed

BACKGROUND

Spectral domain optical coherence tomography (SD-OCT) was a useful tool to study accommodation in human eye, but the maximum image depth is limited due to the decreased signal-to-noise ratio (SNR). In this study, improving optical resolutions, speeds and the SNR were achieved by custom built SD-OCT, and the evaluation of the impact of the improvement during accommodation was investigated.

METHODS

Three systems with different spectrometer designs, including two Charge Coupled Device (CCD) cameras and one Complementary Metal-Oxide-Semiconductor Transistor (CMOS) camera, were tested. We measured the point spread functions of a mirror at different positions to obtain the axial resolution and the SNR of three OCT systems powered with a light source with a 50 nm bandwidth, centered at a wavelength of 840 nm. Two normal subjects, aged 26 and 47, respectively, and one 75-year-old patient with an intraocular lens implanted were imaged.

RESULTS

The results indicated that spectrometers using cameras with 4096 camera pixels optimized the axial resolutions, due to the use of the full spectrum provided by the light source. The CCD camera system with 4096 pixels had the highest SNR and the best image quality. The system with the CMOS camera with 4096 pixels had the highest speed but had a compromised SNR compared to the CCD camera with 4096 pixels.

CONCLUSIONS

Using these three OCT systems, we imaged the anterior segment of the human eye before and after accommodation, which showed similar results among the different systems. The system using the CMOS camera with an ultra-long scan depth, high resolution and high scan speed exhibited the best overall performance and therefore was recommended for imaging real-time accommodation.

Fiber-based polarization-sensitive OCT for birefringence imaging of the anterior eye segment

We demonstrate a prototype system of polarization-sensitive optical coherence tomography (PS-OCT) designed for clinical studies of the anterior eye segment imaging. The system can measure Jones matrices of the sample with depth-multiplexing of two orthogonal incident polarizations and polarization-sensitive detection. An optical clock is generated using a quadrature modulator and a logical circuit to double the clock frequency. Systematic artifacts in measured Jones matrices are theoretically analyzed and numerically compensated using signals at the surface of the sample. Local retardation images of filtering blebs after trabeculectomy show improved visualization of subconjunctival tissue, sclera, and scar tissue of the bleb wall in the anterior eye segment.

Noninvasive imaging and measurement of accommodation using dual-channel SD-OCT

PURPOSE

To investigate the feasibility and practicality of real-time noninvasive imaging and measurement of ocular accommodation (0-5D with one diopter step) using dual-channel, dual-focus spectral domain optical coherence tomography (SD-OCT).

MATERIALS AND METHODS

A custom-built, dual-channel, dual-focus SD-OCT was used to image the anterior segment, including the cornea, iris, and anterior and posterior lens surfaces. Three consecutive measurements were taken in each accommodative session for each subject. Changes in ocular dimensions during accommodation were calculated based on the acquired SD-OCT images. A Friedman test was used to test sensitivity of ocular dimension changes per diopter of accommodation.

RESULTS

With accommodation, anterior chamber depth (ACD), curvature radius of both anterior (RAL) and posterior (RPL) lens surfaces, and pupil diameter (PD) decreased significantly, whereas lens thickness (LT) increased significantly (p < 0.05, Friedman test). Ocular dimensions tended to change according to the increasing of accommodative stimulus, as did a backward movement of the posterior lens surface.

CONCLUSIONS

SD-OCT, via extended imaging depth through a dual-channel, dual-focus approach, is a feasible and practical modality for noninvasive imaging and measurement of ocular accommodation.

Simultaneous real-time imaging of the ocular anterior segment including the ciliary muscle during accommodation

We demonstrated a novel approach of imaging the anterior segment including the ciliary muscle using combined and synchronized two spectral domain optical coherence tomography devices (SD-OCT). In one SD-OCT, a Complementary Metal-Oxide-Semiconductor Transistor (CMOS) camera and an alternating reference arm was used to image the anterior segment from the cornea to the lens. Another SD-OCT for imaging the ciliary muscle was equipped with a light source with a center wavelength of 1,310 nm and a bandwidth of 75 nm. Repeated measurements were performed under relaxed and 4.00 D accommodative stimulus states in six eyes from 6 subjects. We also imaged dynamic changes in the anterior segment in one eye during accommodation. The biometry of the anterior segment and the ciliary muscle was obtained. The combined system appeared to be capable to simultaneously real-time image the biometry of the anterior segment, including the ciliary muscle, in vivo during accommodation.

Accommodative changes in human eye observed by Kitasato anterior segment optical coherence tomography

PURPOSE

To study accommodative changes in the human lens using swept-source optical coherence tomography (Kitasato anterior segment OCT/KAs-OCT), which can image the whole anterior segment of the eye.

METHODS

Thirty-five healthy subjects (mean age 41 years, range 13-79 years) were recruited. Using KAs-OCT, we measured the curvature of the anterior (ASC) and posterior surfaces (PSC), the thickness (LT) of the lens and the anterior chamber depth (ACD) in response to far (0.4 D) and near (10 D) accommodative stimuli.

RESULTS

In response to accommodative stimuli (0.4/10 D), the mean values ± standard deviations were: radius of ASC, 9.72 ± 2.53/7.84 ± 1.85 mm (Wilcoxon ranked-sign test, p < 0.0001); radius of PSC, 5.06 ± 0.71/4.70 ± 0.76 mm (p = 0.0012); LT, 3.86 ± 0.77/4.00 ± 0.76 mm (p < 0.0001); ACD, 2.72 ± 0.61/2.61 ± 0.54 mm (p = 0.0002). The rate of accommodation-associated changes in ASC, LT, and ACD showed significant correlation with aging (Pearson correlation coefficient: r = -0.725, p < 0.0001; r = -0.626, p = 0.0001; r = -0.720, p < 0.0001, respectively), but there was no such correlation in PSC (r = -0.064, p = 0.401).

CONCLUSION

The radius of ASC and PSC decreased with accommodation, and the rates of changes in ASC were larger than those in PSC.

Anterior segment biometry during accommodation imaged with ultralong scan depth optical coherence tomography

PURPOSE

To measure by ultralong scan depth optical coherence tomography (UL-OCT) dimensional changes in the anterior segment of human eyes during accommodation.

DESIGN

Evaluation of diagnostic test or technology.

PARTICIPANTS

Forty-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.

METHODS

A 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.

MAIN OUTCOME MEASURES

Anterior 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).

RESULTS

Repeated 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).

CONCLUSIONS

The 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.

FINANCIAL DISCLOSURE(S)

The authors have no proprietary or commercial interest in any of the materials discussed in this article.

Wound architecture of clear corneal incision with or without stromal hydration observed with 3-dimensional optical coherence tomography

PURPOSE

To evaluate wound architectures of a clear corneal incision and the duration of stromal edema caused by intentional hydration in cataract surgery using 3-dimensional (3-D) cornea and anterior segment optical coherence tomography (OCT).

DESIGN

Prospective, randomized study.

METHODS

On 30 eyes of 23 patients, cataract surgery was performed through a clear corneal incision created with a 2.4-mm blade. After confirming the water tightness of the clear corneal incision at the end of surgery, 15 randomly selected eyes received stromal hydration, and the remaining 15 eyes did not. Using the 3-D cornea and anterior segment optical coherence tomography, wound architecture was assessed 1 day, 1 week, and 2 weeks after surgery.

RESULTS

There was a statistically significant difference in corneal thickness at the clear corneal incision between eyes with and without stromal hydration 1 day and 1 week after surgery (P < .001 and P < .05, Mann-Whitney U test), but not at 2 weeks after surgery. On day 1, gaping at the epithelial side was seen in 6.7% (2 eyes), gaping at the endothelial side in 30% (9 eyes), misalignment of the roof and floor of incision in 40% (12 eyes), and local detachment of Descemet membrane in 36.7% (11 eyes). These imperfections improved with time.

CONCLUSIONS

Using the 3-D cornea and anterior segment optical coherence tomography, detailed architectures of the clear corneal incision were investigated. It was found that the effect of stromal hydration lasted for at least 1 week after surgery.

Full-range imaging of eye accommodation by high-speed long-depth range optical frequency domain imaging

We describe a high-speed long-depth range optical frequency domain imaging (OFDI) system employing a long-coherence length tunable source and demonstrate dynamic full-range imaging of the anterior segment of the eye including from the cornea surface to the posterior capsule of the crystalline lens with a depth range of 12 mm without removing complex conjugate image ambiguity. The tunable source spanned from 1260 to 1360 nm with an average output power of 15.8 mW. The fast A-scan rate of 20,000 per second provided dynamic OFDI and dependence of the whole anterior segment change on time following abrupt relaxation from the accommodated to the relaxed status, which was measured for a healthy eye and that with an intraocular lens.

3D refraction correction and extraction of clinical parameters from spectral domain optical coherence tomography of the cornea

Capable of three-dimensional imaging of the cornea with micrometer-scale resolution, spectral domain-optical coherence tomography (SDOCT) offers potential advantages over Placido ring and Scheimpflug photography based systems for accurate extraction of quantitative keratometric parameters. In this work, an SDOCT scanning protocol and motion correction algorithm were implemented to minimize the effects of patient motion during data acquisition. Procedures are described for correction of image data artifacts resulting from 3D refraction of SDOCT light in the cornea and from non-idealities of the scanning system geometry performed as a pre-requisite for accurate parameter extraction. Zernike polynomial 3D reconstruction and a recursive half searching algorithm (RHSA) were implemented to extract clinical keratometric parameters including anterior and posterior radii of curvature, central cornea optical power, central corneal thickness, and thickness maps of the cornea. Accuracy and repeatability of the extracted parameters obtained using a commercial 859nm SDOCT retinal imaging system with a corneal adapter were assessed using a rigid gas permeable (RGP) contact lens as a phantom target. Extraction of these parameters was performed in vivo in 3 patients and compared to commercial Placido topography and Scheimpflug photography systems. The repeatability of SDOCT central corneal power measured in vivo was 0.18 Diopters, and the difference observed between the systems averaged 0.1 Diopters between SDOCT and Scheimpflug photography, and 0.6 Diopters between SDOCT and Placido topography.

Ultra high-speed swept source OCT imaging of the anterior segment of human eye at 200 kHz with adjustable imaging range

We present an application of in vivo anterior segment imaging of the human eye with an ultrahigh speed swept source OCT instrument. For this purpose, a dedicated OCT system was designed and constructed. This instrument enables axial zooming by automatic reconfiguration of spectral sweep range; an enhanced imaging range mode enables imaging of the entire anterior segment while a high axial resolution mode provides detailed morphological information of the chamber angle and the cornea. The speed of 200,000 lines/s enables high sampling density in three-dimensional imaging of the entire cornea in 250 ms promising future applications of OCT for optical corneal topography, pachymetry and elevation maps. The results of a preliminary quantitative corneal analysis based on OCT data free form motion artifacts are presented. Additionally, a volumetric and real time reconstruction of dynamic processes, like pupillary reaction to light stimulus or blink-induced contact lens movements are demonstrated.

Dual channel dual focus optical coherence tomography for imaging accommodation of the eye

A dual channel dual focus spectral-domain optical coherence tomography was developed for imaging the accommodation of the eye in real time. The system can provide simultaneous cross-sectional imaging of all the surfaces of the anterior segment of the eye including the cornea, anterior chamber, anterior and posterior surfaces of the crystalline lens. Thus, the modification of the curvatures of the anterior and posterior surfaces of the crystalline lens and the dimensions of the anterior segment of the eye with accommodation can be calculated. The system was successfully tested in imaging accommodation. The preliminary results demonstrated the feasibility of this novel approach.

Anterior ocular biometry using 3-dimensional optical coherence tomography

PURPOSE

To evaluate anterior ocular biometry by comparing the measurements of central corneal thickness (CCT) and anterior chamber depth (ACD) with 3-dimensional corneal and anterior segment optical coherence tomography (CAS-OCT) and other methods.

DESIGN

Cross-sectional study.

PARTICIPANTS

Forty eyes of 40 normal subjects.

METHODS

The CCT was measured by 4 methods (CAS-OCT, Scheimpflug camera, scanning-slit topography, and ultrasonic pachymetry), and the ACD was measured by 3 methods (CAS-OCT, Scheimpflug camera, and scanning-slit topography). The anterior chamber volume (ACV) was calculated with CAS-OCT. Repeatability and reproducibility of CAS-OCT measurements were evaluated.

MAIN OUTCOME MEASUREMENTS

The CCT and ACD were measured and compared between devices. The ACV was calculated with CAS-OCT. Coefficient of variation and intraclass correlation coefficient (ICC) were calculated to evaluate repeatability and reproducibility of CAS-OCT measurements.

RESULTS

The mean CCT was 547.0+/-39.0, 590.0+/-39.8, 525.0+/-45.0, and 545.0+/-40.3 microm with CAS-OCT, Scheimpflug camera, scanning-slit topography, and ultrasonic pachymetry, respectively. Significant differences were observed among the 4 methods (P<0.0001; 1-way analysis of variance [ANOVA]). The CCT measured with Scheimpflug camera was significantly larger than those measured with the other methods (P<0.0001; Bonferroni multiple comparison), but there was no significant difference among the other 3 methods. The mean ACD was 3.01+/-0.47, 3.04+/-0.52, and 2.88+/-0.50 mm with CAS-OCT, Scheimpflug camera, and scanning-slit topography, respectively. There was no significant difference among the 3 methods (P = 0.678; 1-way ANOVA). A significant linear correlation in CCT and ACD were observed between CAS-OCT and other methods (all P<0.0001). The mean ACV calculated with CAS-OCT was 169.7+/-23.1 mm(3). The coefficient of repeatability and reproducibility of CCT and ACD measurements were <5% and ICCs were >0.98.

CONCLUSIONS

The CCT measurements were comparable among CAS-OCT, ultrasonic pachymetry, and scanning-slit topography, but Scheimpflug camera yielded a significantly higher CCT value. There was no significant difference in ACD measurements among CAS-OCT, Scheimpflug camera, and scanning-slit topography. The ACV was noninvasively measured by CAS-OCT.

FINANCIAL DISCLOSURE(S)

The authors have no proprietary or commercial interest in any materials discussed in this article.

Anterior segment imaging with Spectral OCT system using a high-speed CMOS camera

We describe a new ultrahigh speed Spectral OCT instrument making use of a CMOS camera and demonstrate high quality in vivo imaging of the anterior segment of the human eye. The high flexibility of the designed imaging system allows a wide range of imaging protocols. Two- and three-dimensional high quality OCT images of the cornea, the anterior chamber and the crystalline lens are presented. A high acquisition rate, up to 135,000 A-scans/second enables three-dimensional reconstruction of the anterior segment during lenticular accommodation, blinking and pupillary reaction to light stimulus. We demonstrate OCT tomographic real time imaging of the lens dynamics during accommodation and high quality OCT cross-sectional images of the entire anterior segment of the eye from the cornea up to posterior part of the crystalline lens.

Investigation of post-glaucoma-surgery structures by three-dimensional and polarization sensitive anterior eye segment optical coherence tomography

A sequential case series of post-glaucoma-surgery structures examined by three-dimensional corneal and anterior eye segment optical coherence tomography (3D-CASOCT) and 3D polarization sensitive CASOCT (PS-CASOCT) is presented. A total of 5 patients who underwent glaucoma surgery were included in this study. Of these, 1, 1, and 3 patient underwent trabeculotomy, laser iridotomy, and trabeculectomy respectively. One patient each who had undergone trabeculotomy or laser iridotomy was examined using a prototype 3D-CASOCT. This prototype is based on swept-source OCT technology, uses a probe beam with a center wavelength of 1.31 microm, and has an axial resolution of 11.6 microm and a scanning speed of 20,000 A lines/s. All 3 patients who underwent trabeculectomy were examined by PS-CASOCT, which has similar specifications to those of 3DCASOCT, measures the depth-resolved birefringence of a specimen, and yields conventional OCT images. Detailed 3D visualization of the incision site of trabeculotomy and the ablation site of laser iridotomy was achieved using 3D-CASOCT. PS-CASOCT revealed, in addition to the structural details, the birefringent properties of the tissues of the trabeculectomy bleb. Some blebs showed abnormal birefringence in the conjunctiva and in a remnant fluid pool. This may indicate the existence of fibrosis in these regions. Both 3D-CASOCT and PS-CASOCT provide clinically significant information for the postoperative assessment of structures created during glaucoma surgery. Interactive 3D datasets of all cases are provided for interactive clinical review. Complex raw 3D OCT volumes are also provided as a reference dataset for the development of PS-OCT algorithms.

Detailed visualization of the anterior segment using fourier-domain optical coherence tomography

OBJECTIVE

To study details of the anterior chamber drainage angle using Fourier-domain optical coherence tomography in healthy subjects and patients with angle abnormalities.

METHODS

A high-speed anterior segment optical coherence tomography prototype was developed using a 1310-nm-wavelength swept light source. Six healthy subjects and 6 patients with glaucoma were imaged in an observational cross-sectional study.

RESULTS

Schlemm's canal and the trabecular meshwork were visualized in all of the patients. Fifteen-millimeter scans enabled entire anterior segment visualization providing configuration details of the iris with respect to the angle. Four-millimeter scans permitted detailed views of the angle configuration and its structures. Volumetric imaging was possible and Schlemm's canal was visualized along part of its circumference.

CONCLUSION

Anterior segment Fourier-domain optical coherence tomography permits detailed noncontact imaging of the angle and its structures, providing a tool to improve our understanding of the pathogenesis of narrow-angle glaucoma.