Ophthalmic Surgery, Lasers and Imaging Retina

Brief Report 

Enhanced Visualization of the Choroido-Scleral Interface Using Swept-Source OCT

Mehreen Adhi, MBBS; Jonathan J. Liu, MS; Ahmed H. Qavi; Ireneusz Grulkowski, PhD; James G. Fujimoto, PhD; Jay S. Duker, MD

Abstract

An accurate analysis of the thickness and volume of choroid using optical coherence tomography (OCT) requires precise visualization of the choroido-scleral interface. Results of studies that use spectral-domain OCT (SD-OCT) to look at the percent visualization of the choroido-scleral interface for accurate analysis of choroidal thickness show varying success rates. This study assessed the visualization of choroido-scleral interface in 19 healthy participants (19 eyes) prospectively recruited for consecutive high-definition raster scanning with an SD-OCT system with and without enhanced depth imaging (EDI) and a prototype long-wavelength swept-source OCT (SS-OCT) system. Choroido-scleral interface was visualized in all eyes imaged on SS-OCT, compared with 13 of 19 (68.4%) and 14 of 19 (73.6%) eyes imaged on SD-OCT without EDI ( P = .009) and with EDI ( P = .02), respectively. The prototype long-wavelength SS-OCT system, with its higher acquisition speed and deeper tissue penetration, may have better clinical utility in determining the involvement of choroid in various chorioretinal diseases.

[ Ophthalmic Surg Lasers Imaging Retina. 2013;44:S40–S42.]

From the New England Eye Center, Tufts Medical Center, Boston, Massachusetts (MA, AHQ, JSD); Department of Electrical Engineering and Computer Science, Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts (JJL, IG, JGF); and Shifa College of Medicine, Islamabad, Pakistan (AHQ).

Supported in part by a Research to Prevent Blindness Unrestricted grant to the New England Eye Center/Department of Ophthalmology, Tufts University School of Medicine, and the Massachusetts Lions Club.

Dr. Fujimoto receives royalties from intellectual property owned by Massachusetts Institute of Technology and licensed to Carl Zeiss Meditec, and has stock options in Optovue. Dr. Duker receives research support from Carl Zeiss Meditec and Optovue.

The principal investigator, Dr. Duker, has full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Address correspondence to Jay S. Duker, MD, New England Eye Center, 800 Washington Street, Boston, MA 02111; email: Jduker@tuftsmedicalcenter.org.

Received: January 28, 2013
Accepted: June 07, 2013

Abstract

An accurate analysis of the thickness and volume of choroid using optical coherence tomography (OCT) requires precise visualization of the choroido-scleral interface. Results of studies that use spectral-domain OCT (SD-OCT) to look at the percent visualization of the choroido-scleral interface for accurate analysis of choroidal thickness show varying success rates. This study assessed the visualization of choroido-scleral interface in 19 healthy participants (19 eyes) prospectively recruited for consecutive high-definition raster scanning with an SD-OCT system with and without enhanced depth imaging (EDI) and a prototype long-wavelength swept-source OCT (SS-OCT) system. Choroido-scleral interface was visualized in all eyes imaged on SS-OCT, compared with 13 of 19 (68.4%) and 14 of 19 (73.6%) eyes imaged on SD-OCT without EDI ( P = .009) and with EDI ( P = .02), respectively. The prototype long-wavelength SS-OCT system, with its higher acquisition speed and deeper tissue penetration, may have better clinical utility in determining the involvement of choroid in various chorioretinal diseases.

[ Ophthalmic Surg Lasers Imaging Retina. 2013;44:S40–S42.]

From the New England Eye Center, Tufts Medical Center, Boston, Massachusetts (MA, AHQ, JSD); Department of Electrical Engineering and Computer Science, Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts (JJL, IG, JGF); and Shifa College of Medicine, Islamabad, Pakistan (AHQ).

Supported in part by a Research to Prevent Blindness Unrestricted grant to the New England Eye Center/Department of Ophthalmology, Tufts University School of Medicine, and the Massachusetts Lions Club.

Dr. Fujimoto receives royalties from intellectual property owned by Massachusetts Institute of Technology and licensed to Carl Zeiss Meditec, and has stock options in Optovue. Dr. Duker receives research support from Carl Zeiss Meditec and Optovue.

The principal investigator, Dr. Duker, has full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Address correspondence to Jay S. Duker, MD, New England Eye Center, 800 Washington Street, Boston, MA 02111; email: Jduker@tuftsmedicalcenter.org.

Received: January 28, 2013
Accepted: June 07, 2013

Introduction

Accurate thickness and volumetric analysis of the choroid using optical coherence tomography (OCT) requires precise visualization of the choroido-scleral interface. Recent advances in spectral-domain OCT (SD-OCT), such as signal enhancement with image averaging and enhanced depth imaging (EDI), enable improved visualization of the choroid allowing measurement of choroidal thickness in healthy and diseased states. 1–5 Image averaging involves acquiring multiple frames (B-scans) at the same retinal location that are then averaged together to increase the signal-to-noise ratio. 2 EDI involves image averaging in conjunction with setting the choroid adjacent to the zero-delay position, the point of highest signal detection sensitivity on SD-OCT, allowing enhanced visualization of the choroido-scleral interface. 1

The choroido-scleral interface has been visualized to varying degrees using different SD-OCT systems and imaging techniques. Studies demonstrate that the Cirrus SD-OCT system (Carl Zeiss Meditec, Dublin, CA) and the Topcon SD-OCT system (Topcon Medical Systems, Paramus, NJ) without EDI provide accurate visualization of the choroido-scleral interface in approximately 65% to 75% of healthy and diseased eyes. 2,5,6 Recently, however, the largest study analyzing the choroid in healthy eyes used the Spectralis SD-OCT system (Heidelberg Engineering, Heidelberg, Germany) and showed successful visualization of the choroido-scleral interface for measurement of the subfoveal choroidal thickness in 93% of 3,468 healthy eyes using EDI. 7

With the advent of OCT devices that use longer wavelength, an improved visualization of choroido-scleral interface may be achieved. 8 This study aimed to determine whether a prototype long-wavelength swept-source OCT (SS-OCT) system that uses a wavelength of 1050 nm 9 can enhance the visualization of the choroido-scleral interface in healthy eyes.

Design and Methods

Participants

Nineteen healthy participants (19 eyes) were prospectively recruited at the New England Eye Center, Boston, between September and November 2012 for consecutive high-definition (HD) raster scanning with and without EDI on Cirrus HD-OCT and a prototype SS-OCT system developed at the Massachusetts Institute of Technology and deployed at the New England Eye Center. All participants had a best corrected visual acuity of 20/20 or better and underwent fundus examination to exclude existing retinal or choroidal pathology. One randomly selected eye from each participant was scanned consecutively on the two systems within 10 minutes. The order of image acquisition on the two OCT systems was random. This study was approved by the institutional review boards of Tufts Medical Center and the Massachusetts Institute of Technology and is adherent to the tenets of Declaration of Helsinki.

SS-OCT System and Scan Protocols

The SS-OCT system employed for this study 9 uses a light source with a wavelength-sweeping laser centered at 1050 nm, yielding a 5.3 μm axial resolution in tissue. It features an imaging speed of 100,000 A-scans/second, which is about 3.7 times faster than the Cirrus HD-OCT A-scan rate. The Cirrus performs imaging with light at 800 nm wavelength with a 5 μm axial resolution in tissue at an imaging speed of 27,000 A-scans/second.

The scan pattern employed on the Cirrus was a 6-mm scan acquired in 0.75 seconds, consisting of 1,000 A-scans/frame, with 20 frames averaging. All participants were scanned first without the EDI function (Cirrus non-EDI), and then using the EDI function (Cirrus EDI). Only scans with an intensity of 6/10 or greater were analyzed. A scan pattern that replicated the pattern available on the Cirrus HD-OCT was created on SS-OCT to adequately compare the two systems, such that the time of image acquisition was kept constant on both systems. Thus, the scan pattern on SS-OCT was a 6-mm scan acquired in 0.75 seconds, consisting of 1,000 A-scans/frame, and 75 frames averaging. All images were obtained by a researcher (MA) trained in acquiring OCT images using the two systems and were taken as close to the fovea as possible to ensure that the same retinal location was scanned using the two systems. Two independent observers experienced in analyzing OCT images assessed all the scans for visualization of the choroido-scleral interface. When the choroido-scleral interface was clearly visualized throughout the 6-mm scan, it was labeled as being “visible” for accurate choroidal analysis. If it was visualized within the 4.5-mm region centered on the foveal center (75% of the 6-mm scan), it was still classified as being visualized. Otherwise, it was labeled as being “not visible” for an accurate choroidal analysis.

Findings

The mean age of the study participants was 29 years (23 to 41 years). Visualization of the choroido-scleral interface was achieved in all eyes imaged on SS-OCT (19/19, 100%) with both averaging of 75 frames (B-scans) as well as averaging of 20 frames (B-scans), when compared with 13/19 (68.4%) and 14/19 (73.6%) eyes imaged using Cirrus non-EDI ( P = .009) and Cirrus EDI ( P = .02), respectively (Figures 1 and 2 ). There was a 100% agreement between the two independent observers for visualization of the choroido-scleral interface in all the images analyzed.

Bar graph showing the percentage of visualization of the choroido-scleral interface using Cirrus non-enhanced depth imaging (non-EDI), Cirrus EDI, and swept-source optical coherence tomography (SS-OCT).

Figure 1.

Bar graph showing the percentage of visualization of the choroido-scleral interface using Cirrus non-enhanced depth imaging (non-EDI), Cirrus EDI, and swept-source optical coherence tomography (SS-OCT).

Example of a healthy eye imaged consecutively using Cirrus spectral-domain optical coherence tomography (SD-OCT) and swept-source OCT (SS-OCT). The choroido-scleral interface is hardly visible on Cirrus non-enhanced depth imaging (non-EDI) except at the far temporal and nasal locations (yellow arrows) (A). The Cirrus image with EDI (B) results in a subtle enhancement of the visibility of the choroido-scleral interface (yellow arrows). The SS-OCT images show a complete visualization of the choroido-scleral interface (yellow arrows). Note that the SS-OCT image in (C) employs image averaging of 20 frames (B-scans) and that in (D) employs image averaging of 75 frames (B-scans).

Figure 2.

Example of a healthy eye imaged consecutively using Cirrus spectral-domain optical coherence tomography (SD-OCT) and swept-source OCT (SS-OCT). The choroido-scleral interface is hardly visible on Cirrus non-enhanced depth imaging (non-EDI) except at the far temporal and nasal locations (yellow arrows) (A). The Cirrus image with EDI (B) results in a subtle enhancement of the visibility of the choroido-scleral interface (yellow arrows). The SS-OCT images show a complete visualization of the choroido-scleral interface (yellow arrows). Note that the SS-OCT image in (C) employs image averaging of 20 frames (B-scans) and that in (D) employs image averaging of 75 frames (B-scans).

Discussion

This study demonstrates an enhanced visualization of the choroido-scleral interface using SS-OCT when compared with non-EDI and EDI SD-OCT. While EDI SD-OCT improves visualization of the choroido-scleral interface somewhat (by 5%) (Figures 1 and 2 ), it appears that the prototype long-wavelength SS-OCT system with higher acquisition speed and deeper tissue penetration results in a significantly better identification of the choroido-scleral interface in healthy eyes.

It may be argued that the enhanced visualization of the choroido-scleral interface with the SS-OCT prototype in the present study is only due to the increased image averaging that is possible with the higher scanning speed of the SS-OCT system within the same image acquisition time when compared with the Cirrus SD-OCT system. Considering this possibility, all images obtained on the SS-OCT system were reprocessed to average 20 frames (B-scans) instead of 75 frames (B-scans) and reassessed for visualization of the choroido-scleral interface. The choroido-scleral interface was visualized in all eyes (19 of 19, 100%) examined using an averaging of 20 frames (B-scans), a result identical to that obtained by averaging 75 frames on SS-OCT (Figure 2 ). Recent studies suggest good reproducibility and agreement with regard to visualizing the choroido-scleral interface and measurement of choroidal thickness in healthy eyes among the different commercially available SD-OCT systems using comparable wavelengths (800 nm to 870 nm) that average from 16 to a 100 frames (B-scans) with or without EDI. 4,10 This suggests that after reaching a certain threshold, image averaging and EDI may have a limited benefit in further enhancing the visualization of the choroido-scleral interface and that higher acquisition speed and deeper penetration of the long-wavelength SS-OCT may allow for a much improved visualization of the choroido-scleral interface for choroidal analysis. Studies that look at the minimum number of averaged frames necessary for adequate visualization of the choroido-scleral interface are warranted.

In conclusion, this study shows an enhanced visualization of the choroido-scleral interface in healthy eyes using a prototype long-wavelength, high-speed SS-OCT system. Future studies in healthy and diseased eyes involving quantitative analysis of the choroid using SS-OCT are expected to help recognize the clinical utility of SS-OCT in various chorioretinal diseases.

References

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10.3928/23258160-20131101-08

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