Abstract
Purpose:
This article evaluates the clinical usefulness of an attachment involving a +20-D lens to gain extended field images on wide-angle optical coherence tomography angiography (OCTA).
Methods:
An attachment with a +20-D lens was developed to take OCTA images of anterior segments, and it was used to obtain extended field images of the posterior segment in this study. Ten eyes of 5 individuals who did not have a history of ocular or systemic disease underwent wide-angle OCTA with a 12 × 12-mm center field using the PLEX Elite 9000 with and without the attachment. The ratio of the area of the center field to the area of the extended field with the attachment was calculated.
Results:
The mean area of the center image was 125 disc areas and that of the field extended by the attachment was 210 disc areas. The mean ratio between the center field and the extended field was 1.67.
Conclusions:
The attachment involving the +20-D lens seems to be clinically useful to gain extended field images on wide-angle OCTA.
Keywords: attachment, extended field image, ocular coherence tomography angiography (OCTA), wide angle
Introduction
Optical coherence tomography angiography (OCTA) has been widely applied in the assessment of various retinal vascular diseases. 1 -3 OCTA can be used to detect blood flow and construct 2- and 3-dimensional images of flow within the eye based on decorrelation between resampled images. 3 A standard retinal and choroidal vascular examination, fluorescein angiography, sometimes induces nausea and, rarely but critically, anaphylaxis, even in healthy individuals, 4 whereas OCTA is a noninvasive, speedy, and reproducible examination to present retinal and choroidal vasculature.
OCTA is a novel imaging technique but is limited by its small field of view, which is significantly smaller than the standard photographic field. One of the latest OCTA instruments can obtain 12 × 12-mm fields, but the field of view on the wide-angle OCTA is still smaller than the field obtained with ultra-widefield fluorescein angiography. 5
Recently, Uji and Yoshimura reported on an extended field imaging (EFI) technique to obtain widefield OCT images. The EFI method includes imaging the posterior pole through trial frames fitted with a +20-D lens. 6 An auxiliary instrument, an attachment involving a +20-D lens (Carl Zeiss Meditec; not commercially available), can be attached to the wide-angle OCTA, PLEX Elite 9000 (Carl Zeiss Meditec) (Figure 1). This device can operate over wider (12 × 12 mm) OCTA scan protocols. The combination of the attachment and the wide-angle OCTA was expected to achieve a wider field of OCTA imaging. This study appraised the preliminary clinical usefulness of the attachment to gain widefield OCTA images.
Figure 1.
Attachment including a +20-D lens. (A) Front side of the attachment. (B) Back side of the attachment. (C) Eye piece of the Carl Zeiss Meditec PLEX Elite 9000. (D) The PLEX Elite 9000 with the attachment.
Methods
Study Participants
Participants were aged 20 years or older, did not have a history of ocular or systemic disease, and underwent a comprehensive ophthalmic examination. Individuals provided written informed consent in accordance with procedures approved by the institutional review board of Shiga University of Medical Science and the tenets of the Declaration of Helsinki.
This study, which estimates imaging instrument performance, was carried out at Shiga University of Medical School Hospital from March 2018 to May 2018. The protocol was approved by the institutional review board of Shiga University of Medical Science.
Wide-Angle Optical Coherence Tomography Angiography With or Without Attachment
Three-dimensional OCTA scans of 12 × 12-mm regions were acquired using the swept-source OCT PLEX Elite 9000 instrument. A center OCTA scan was obtained, and an OCTA scan was acquired from the same angle with the attachment including a +20-D lens to gain the EFI. All scans consisted of 500 A-scans per B-scan, repeated twice at each of the 500 B-scan positions.
Comparison of Field Area Between the Center Image and Extended Field Image
The optic disc area and field of the OCTA images were measured using Image J software (http://imagej.nih.gov/ij/) on each image. The area of the OCTA field was divided by that of the optic disc area to present the OCTA area in units of disc area in each eye (OCTA area [disc area units] = area of OCTA image / optic disc area). The ratio between the OCTA areas with and without the attachment was then calculated in each eye (ratio = area of OCTA image with attachment [disc area units] / area of OCTA image without attachment [disc area units]).
Results
Ten eyes of 5 participants (average age, 27.8 ± 5.4 years; range, 22-35 years) were imaged for the study. The measurement time without the attachment was the same as that with the attachment (Figure 2).
Figure 2.
Representative images. At left is a 12 × 12-mm optical coherence tomography angiography (OCTA) image gained without the attachment including the +20-D lens. At right is a 12 × 12-mm OCTA image gained with the attachment including the +20-D lens.
The mean OCTA field area without the attachment was 125 ± 34 disc areas (DA) (range, 93-210 DA). The mean OCTA field area with the attachment was 210 ± 66 DA (range, 129-371 DA). The mean ratio between the areas without and with the attachment was 1.67 ± 0.20 times (range, 1.38-2.01 times).
Discussion
This report presents the clinical usefulness of a new auxiliary instrument, an attachment including a +20-D lens, to quickly gain extended field images in widefield OCTA images. The EFI effect produced by this attachment was able to enlarge a 12 × 12-mm OCTA image by a mean of 1.67 times. The original EFI technique involved imaging the posterior pole through trial frames fitted with a +20-D lens. 6 Hirano and colleagues showed the application of the same original EFI technique for the currently larger 12 × 12-mm OCTA images, reporting an average extension rate by EFI of 1.80 times. 7 The present study resulted in an average extension rate of 1.67 times, a value comparable to the previous report.
In the original EFI, the distance from the eye to the trial frame lens was not the same among all participants when the attachment fitted with the +20-D lens was used. 6 The distance from the eye to the +20-D lens had to be adjusted to gain an extended OCTA image. In the present study, the distance from the eye to the surface of the attachment also had to be changed to gain an extended OCTA image. It might be easier to control the distance from the eye to the attachment by comparing the distance from the eye to the trial frame lens.
Previously, we reported that we obtained 12 × 12-mm OCTA images of 5 directions (center, nasal lower, nasal upper, temporal lower, and temporal upper) and composed panoramic images that were 2.4-times enlargements of 12 × 12-mm center fields. 5 The present method using the attachment resulted in smaller images than the composed panoramic images. The EFI results in our study have a limitation regarding image resolution of OCTA images. The PLEX Elite 9000 can obtain a 12 × 12-mm OCTA image consisting of 500 A-scans per B-scan, repeated twice at each of the 500 B-scan positions. The resolution of an extended widefield is therefore less than that of a default field because the scanning density (500 × 500 pixels) does not change. Because EFI expands the image field without increasing the scanning density, it results in decreased image resolution.
However, it took many more than 5 tries to obtain OCTA images of the participants in all 5 directions, and the method was difficult for individuals who cannot hold fixation. The present method using the attachment required only a short time, the same as that to obtain one 12 × 12-mm OCTA image, and might enable imaging in patients with poor visual fixation.
The limitation of this attachment is that it attaches only to the PLEX Elite 9000. However, the OCTA instrument images are the widest OCTA images among commercially available OCTA devices, and the extended images are also the largest that could be captured by the PLEX Elite 9000.
Conclusions
The EFI effect with the attachment including the +20-D lens was able to enlarge a 12 × 12-mm OCTA image by a mean of 1.67 times. The attachment seems to be a useful auxiliary instrument to gain extended field images on wide-angle OCTA.
Footnotes
Ethical Approval: This study was approved by the Institutional Review Board of Shiga University of Medical Science and adhered to the tenets of the Declaration of Helsinki.
Statement of Informed Consent: Written informed consent was obtained from all study participants.
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding: The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The attachment including the +20-D lens was provided by Carl Zeiss Medical Technology. The company had no role in the design, conduct, or decision to publish the research, nor did it have any role in the review or approval of the manuscript. The authors have no other financial or conflicting interests to disclose.
ORCID iD: Osamu Sawada, MD 
https://orcid.org/0000-0001-9752-0499
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