April 2014
Volume 55, Issue 4
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Retina  |   April 2014
Quantification of the Image Obtained With a Wide-Field Scanning Ophthalmoscope
Author Affiliations & Notes
  • Akio Oishi
    Department of Ophthalmology and Visual Sciences, Kyoto University Graduate School of Medicine, Kyoto, Japan
  • Jiro Hidaka
    HOYA, Inc., Tokyo, Japan
  • Nagahisa Yoshimura
    Department of Ophthalmology and Visual Sciences, Kyoto University Graduate School of Medicine, Kyoto, Japan
  • Correspondence: Akio Oishi, Department of Ophthalmology and Visual Sciences, Kyoto University Graduate School of Medicine, 54 Kawahara, Shogoin, Sakyo, Kyoto 606-8507, Japan; aquio@kuhp.kyoto-u.ac.jp
Investigative Ophthalmology & Visual Science April 2014, Vol.55, 2424-2431. doi:10.1167/iovs.13-13738
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      Akio Oishi, Jiro Hidaka, Nagahisa Yoshimura; Quantification of the Image Obtained With a Wide-Field Scanning Ophthalmoscope. Invest. Ophthalmol. Vis. Sci. 2014;55(4):2424-2431. doi: 10.1167/iovs.13-13738.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

Purpose.: To quantify the angle of view, the magnification, and the quality of images acquired with an Optos 200Tx.

Methods.: We obtained fundus images of a model eye with the Optos 200Tx and recorded the maximal scale imaged in each direction. We measured the length of the scale bar and the interval of the scale bars at each angle and calculated the magnification. We also measured the contrast between scale bars and the intervals between scale bars.

Results.: The fundus image obtained with the Optos 200Tx covered 96, 101, 76, and 102° in the up, right, down, and left directions, respectively. Quantitative measurement showed that the overall image is stretched 1.12-fold in the horizontal direction with respect to the vertical. The magnification with respect to the posterior pole increased quadratically in all directions, most steeply in the vertical direction, reaching 2.0 × 1.5 in the most extreme case. The image quality was best in the left part and was worse in the inferior and superior parts.

Conclusions.: The Optos 200Tx can obtain an image duplicated, with an angular range of approximately 200° horizontally and 170° vertically, with greatest limitation in the inferior direction. It should be noted that the most peripheral part of the image is significantly and unequally magnified. In addition, the contrast is not the same at all positions. The present data would serve as a guide to correct the peripheral magnification in future studies.

Introduction
Fundus imaging is an essential part of ophthalmologic practice. 1 Images are taken with a fundus camera or a scanning laser ophthalmoscope, and the recorded fundus images enable objective evaluation or comparison of physiologic or pathologic findings. With these devices, however, it is not easy to obtain images of the peripheral part of the fundus. Rotating the eye allows the observation of the peripheral part of the fundus, but this results in decreased image quality due to decreasing the width of the pupil, induced astigmatism, and aberrations. 2 Several attempts have been made to overcome this limitation including scleral transillumination and use of special contact lenses. 3  
Recently, the ultra-wide–field scanning ophthalmoscope Optos 200Tx (Optos, Marlborough, MA, USA) has become commercially available. This device uses an ellipsoid mirror and creates a wide-field image of the fundus. According to the manufacturer's brochure, the device can image up to 200° of the retina in a single capture. The device has enabled fast and noninvasive observation of the peripheral retina and provides previously unavailable information. Researchers are now exploring the utility of the images. 3  
While the ability to evaluate the peripheral fundus objectively is indubitably a great advance, there are still some disadvantages of the device including the use of a false-color image created from the two laser wavelengths used in the device and the relatively low resolution of the image compared with the standard fundus camera or the confocal scanning laser ophthalmoscopes. 3 In addition, considering that the device creates planar images from the spheric fundus, there must be some magnification. 2 This magnification prevents quantitative measurement of the area and limits the value of the examination. An approximation that accounts for the magnification in each part of the image would be helpful for future studies. 
In the present study, we investigate the angle of view of the image and quantify the magnification at several settings. 
Methods
We used a model eye, which was provided by Makoto Inoue (Kyorin Eye Center, Kyorin University School of Medicine, Tokyo, Japan). 4,5 The draught, the appearance of the model eye—which was constructed based on Gullstrand's model of the human eye—and the jig to fix the model eye are shown in Figure 1. The axial length of the model eye is 24.0 mm. The cornea was made of polymethylmethacrylate, and the radius of curvature of the anterior surface is 7.70 mm, and that of the posterior surface is 7.46 mm. Two sizes of artificial pupil were prepared: 8.11 and 2.14 mm, as measured via the cornea (Wavefront Analyzer KR1W; Topcon Corp., Tokyo, Japan). We used a +20-diopter (D) intraocular lens AF-1 (HOYA, Tokyo, Japan) and a +20-D diffraction multifocal lens (AMO, Tokyo, Japan). The model eye was filled with distilled water or room air. The experiments were performed with an 8.11-mm pupil, a +20-D regular lens, and distilled water unless otherwise specified. Angular scales were attached to the inner surface of the model eye. 6 Please note that the angles in the present study or in the specifications of Optos are from the center of the eye, while in a conventional fundus camera, the angles are measured from the pupil. The model eye was mounted in the appropriate position with a jig that we created. We adjusted the position of the model eye to set the center of the fundus in the center of the image. The model eye was moved and rotated using the attachment. 
Figure 1
 
(A, B) The appearance of the model eye used to quantify the images obtained with the wide-field scanning funduscope Optos 200Tx. As shown in the draught, the axial length is 24 mm, and the refraction was comparable to that of the human eye. The body of the model eye was made of metal, and the cornea was made of polymethylmethacrylate. (C) We attached the angular scales to the inner surface of the model eye and quantified the image. (D, E) We created the jig, which fixes the model eye in the appropriate position and enables horizontal and vertical rotational movement of the eye.
Figure 1
 
(A, B) The appearance of the model eye used to quantify the images obtained with the wide-field scanning funduscope Optos 200Tx. As shown in the draught, the axial length is 24 mm, and the refraction was comparable to that of the human eye. The body of the model eye was made of metal, and the cornea was made of polymethylmethacrylate. (C) We attached the angular scales to the inner surface of the model eye and quantified the image. (D, E) We created the jig, which fixes the model eye in the appropriate position and enables horizontal and vertical rotational movement of the eye.
We obtained images in the straight direction, while moving the jig to different distances from the device, and in configurations where the jig is rotated to 15° with respect to the straight direction (where the rotation was in four directions: up, down, left, and right). The device turns on a green light when the laser appropriately irradiates the fundus. For the back-and-forth movement, we moved the eye in the range where the green light stayed on. The images were exported as 3900 × 3072 pixel Joint Photographic Experts Group (JPEG) files and analyzed using photo editing software (Photoshop CS5; Adobe Systems, San Jose, CA, USA). The most peripheral readable scale was recorded, and the lengths of the vertical and horizontal scale bars at the posterior pole were measured for each setting. In addition, we quantified the magnification throughout the standard image. The length of each scale bar and the interval between the scale bars were measured in the image obtained with the following settings: 8.1-mm pupil, +20-D spheric lens, fluid-filled, straight direction, and close position (Fig. 2). Since each interval was too small to measure precisely, we measured the distance between every fifth scale bar for each degree—for example, the positions between 3 and 7° are all rounded to correspond to the measurement at 5°. We analyzed the red-channel images because that channel had better contrast than both the green channel and the composed image. Each measurement was performed for five independent images and averaged. 
Figure 2
 
The fundus image obtained with the wide-field scanning funduscope Optos 200Tx. We measured the length of the scale bar and interval of scale bars to quantify the magnification in the peripheral part of the image. Thick red and blue bars indicate the length of the scale bar and the interval of five scale bars at 0, 30, 60, and 88°, respectively. Note that the length, as well as the interval of the scale bar, is larger in the periphery.
Figure 2
 
The fundus image obtained with the wide-field scanning funduscope Optos 200Tx. We measured the length of the scale bar and interval of scale bars to quantify the magnification in the peripheral part of the image. Thick red and blue bars indicate the length of the scale bar and the interval of five scale bars at 0, 30, 60, and 88°, respectively. Note that the length, as well as the interval of the scale bar, is larger in the periphery.
We also calculated the contrast in each part of the image, which consists of 256 gray steps. The mean intensity in the central four pixels of the black stripe was used to set the value of I B, and that of the white stripe (the interval of the scale) was used to determine I W. The contrast was calculated as (127 – I B + I W)/(127 + I BI W). 
Statistical analysis was performed using statistical software (SPSS version 19; IBM Japan, Tokyo, Japan). The descriptive analyses were reported as the mean ± standard deviation unless otherwise specified. Comparisons among groups were performed using the unpaired t-test. P values < 0.05 were considered to be statistically significant. 
Results
Images were successfully recorded for each setting (see Figs. 3 15525). The angle of view in each setting is shown in Table 1. At first, we checked what would happen when we moved the model eye back and forth. When the model eye with the 8.1-mm pupil was moved backward, the edge of the intraocular lens appeared in the image, as shown in Figure 3B. Although the intraocular lens disturbed the image at around 70°, the more peripheral part was recorded outside the intraocular lens, and the angle of view increased. In the 2.1-mm pupil, moving the model eye back made the angle of view quite small (approximately 15°), as shown in Figure 3D. Thus, the analysis thereafter for any setting was done with the model eye in the close position. Then we investigated to what angular extent we were able to record images when we rotated the eye. When the fluid-filled model eye was rotated by 15° in each direction, maxima of 106.6, 121.2, 91.0, and 120.4° in the up, right, down, and left directions were able to be recorded, respectively, as shown in Figure 4. The angle of view was larger in aphakic (Fig. 5C) or air-filled eyes (Fig. 5D). 
Figure 3
 
The angle of view in the images obtained with the wide-field scanning funduscope Optos 200Tx with different settings. (A) A fluid-filled eye with an 8.1-mm pupil and an intraocular lens of +20 D was used. (B) When the model eye was moved backward (away from the machine), the edge of the intraocular lens appeared in the image. (C) The peripheral fundus up to around 95° could be recorded even with a 2.1-mm pupil, but (D) moving the small pupil eye backward made the angle of view very small.
Figure 3
 
The angle of view in the images obtained with the wide-field scanning funduscope Optos 200Tx with different settings. (A) A fluid-filled eye with an 8.1-mm pupil and an intraocular lens of +20 D was used. (B) When the model eye was moved backward (away from the machine), the edge of the intraocular lens appeared in the image. (C) The peripheral fundus up to around 95° could be recorded even with a 2.1-mm pupil, but (D) moving the small pupil eye backward made the angle of view very small.
Figure 4
 
The fundus images of the model eye obtained with the wide-field scanning funduscope Optos 200Tx. The model eye was rotated by 15° in the (A) upward, (B) rightward, (C) downward, and (D) leftward directions. Rotating the eye resulted in increased visibility in each direction.
Figure 4
 
The fundus images of the model eye obtained with the wide-field scanning funduscope Optos 200Tx. The model eye was rotated by 15° in the (A) upward, (B) rightward, (C) downward, and (D) leftward directions. Rotating the eye resulted in increased visibility in each direction.
Figure 5
 
The fundus images of the model eye obtained with the wide-field scanning funduscope Optos 200Tx. We also investigated the effect of the media. (A) +20-D spherical lens, (B) +20-D diffraction multifocal lens, (C) aphakia, and (D) +20-D spherical lens; the vitreous cavity was filled with air. Note that the angle of view is larger in aphakic and gas-filled eyes.
Figure 5
 
The fundus images of the model eye obtained with the wide-field scanning funduscope Optos 200Tx. We also investigated the effect of the media. (A) +20-D spherical lens, (B) +20-D diffraction multifocal lens, (C) aphakia, and (D) +20-D spherical lens; the vitreous cavity was filled with air. Note that the angle of view is larger in aphakic and gas-filled eyes.
Table 1
 
Angle of View in Each Setting and Each Direction in the Image Obtained With the Wide-Field Fundus Ophthalmoscope Optos 200Tx
Table 1
 
Angle of View in Each Setting and Each Direction in the Image Obtained With the Wide-Field Fundus Ophthalmoscope Optos 200Tx
Superior, deg (P Value) Inferior, deg (P Value) Right, deg (P Value) Left, deg (P Value)
Standard position 96.2 76.4 100.6 101.8
Distant position 99.6 (<0.01) 81.0 (<0.01) 111.2 (<0.01) 106.4 (<0.01)
Small pupil 89.4 (<0.01) 75.0 (<0.01) 95.0 (<0.01) 91.6 (<0.01)
Small pupil, distant position 1.8 (<0.01) 21.3 (<0.01) 39.3 (<0.01) 1.8 (<0.01)
Up-gaze 106.6 (<0.01) 53.8 (<0.01) 104.8 (0.20) 102.4 (0.80)
Down-gaze 80.0 (<0.01) 91.0 (<0.01) 101.0 (0.72) 101.5 (0.43)
Right-gaze 90.5 (<0.01) 77.0 (0.07) 88.0 (<0.01) 120.4 (<0.01)
Left-gaze 90.0 (<0.01) 77.0 (0.07) 121.5 (<0.01) 87.0 (<0.01)
Multifocal lens 97.0 (0.08) 79.0 (<0.01) 102.5 (0.53) 105.0 (<0.01)
Aphakia 93.0 (<0.01) 79.0 (<0.01) 111.6 (<0.01) 110.2 (<0.01)
Air-filled 119.8 (<0.01) 118.8 (<0.01) 118.4 (<0.01) 121.2 (<0.01)
Next, we measured the length of the vertical and horizontal scale bars at the posterior pole for each setting. The result is shown in Table 2. In the model eye with a +20-D lens, 1 mm corresponded to 84.9 pixels in the horizontal direction and 75.5 pixels in the vertical direction, which means that the overall image is stretched in the horizontal direction by a factor of 1.12. When we moved the eye away from the device, the difference was minimal compared to the initial position, as shown in Figures 3A and 3C. When we rotated the eye, however, the scales were imaged in different positions in the picture, and the magnification was changed accordingly. The length of the vertical scale bar was smallest in the down-gaze position (73.5 pixels, P = 0.015) and largest in the up-gaze position (78.1 pixels, P = 0.006) The length of the horizontal scale bar was the smallest in the right-gaze position (83.5 pixels, P = 0.81) and the largest in the up-gaze position (87.9 pixels, P = 0.007), as shown in Figure 4. We also investigated the effect of media in several conditions. The multifocal lens did not show a significant difference compared with the regular lens (Fig. 5B). Under aphakic status, the length of the horizontal and vertical scale bars were 83.7 and 73.9 pixels, respectively, and the horizontal length was significantly smaller than under the phakic condition (P = 0.004, Fig. 5C). When the vitreous cavity was filled with air, the lengths were prominently minified (Fig. 5D). 
Table 2
 
The Length of the Vertical and Horizontal Scale Bars at the Posterior Pole Measured for Each Setting
Table 2
 
The Length of the Vertical and Horizontal Scale Bars at the Posterior Pole Measured for Each Setting
Vertical Scale, Pixel (P Value) Horizontal Scale, Pixel (P Value)
Standard position 75.5 84.9
Distant position 75.2 (0.65) 84.1 (0.41)
Small pupil 75.8 (0.50) 85.2 (0.74)
Up-gaze 78.1 (0.01) 87.9 (0.01)
Down-gaze 73.5 (0.02) 84.9 (0.94)
Right-gaze 76.0 (0.70) 83.5 (0.18)
Left-gaze 75.9 (0.69) 84.4 (0.50)
Multifocal lens 75.7 (0.80) 84.8 (0.97)
Aphakia 73.9 (<0.01) 83.7 (<0.01)
Air-filled 61.9 (<0.01) 66.6 (<0.01)
Then we compared the magnification at the posterior pole and in the peripheral fundus. The results demonstrated that the peripheral part of the fundus was magnified exponentially and unequally, as shown in Figure 6. For example, the tangential × radial magnifications at 60° in the inferior, superior, and horizontal directions, compared with that at the posterior pole, were respectively 1.6 × 1.3 times more, 1.4 × 1.1 times more, and 1.2 × 1.1 times more. The analysis showed that quadratic approximations fit the change of the magnification well; for an angle x, and a magnification rate of y, the tangential magnification in the vertical-direction fit is y = 0.0002x 2 – 0.0012x + 0.9969 (R 2 = 0.987), the tangential magnification in the horizontal-direction fit is y = 0.00005x 2 – 0.0001x + 1.016 (R 2 = 0.983); the radial magnification in the vertical-direction fit is y = 0.00006x 2 – 0.0015x + 0.9904 (R 2 = 0.980); and the radial magnification in the horizontal-direction fit is y = 0.00004x 2 – 0.0003x + 0.9474 (R 2 = 0.859). For the radial magnification in the horizontal direction, the quartic approximation showed a better fit (y = 6 × 10−9 x 4 + 2 × 10−7 x 3 – 2 × 10−6 x 2 – 0.0004x + 0.9909, R2 = 0.9722). 
Figure 6
 
Tangential and radial magnification at each position of the fundus. The x-axis indicates the angle from the posterior pole, and the y-axis indicates the magnification compared with the posterior pole. Note that the length of the scale bar is magnified exponentially toward the periphery. In addition, the rate was not equal in the vertical and horizontal directions or the tangential and radial directions. Quadratic approximations generally fit the change of the magnification well (thin lines). For tangential magnification in the horizontal direction, the quartic approximation fit better (thick line). In the most peripheral part, the scale bar was visible but was not clear enough to be quantified.
Figure 6
 
Tangential and radial magnification at each position of the fundus. The x-axis indicates the angle from the posterior pole, and the y-axis indicates the magnification compared with the posterior pole. Note that the length of the scale bar is magnified exponentially toward the periphery. In addition, the rate was not equal in the vertical and horizontal directions or the tangential and radial directions. Quadratic approximations generally fit the change of the magnification well (thin lines). For tangential magnification in the horizontal direction, the quartic approximation fit better (thick line). In the most peripheral part, the scale bar was visible but was not clear enough to be quantified.
Finally, we investigated the quality of the image by analyzing the contrast of the scale bars. The result showed variation in the image quality that depended on the position in the image, as shown in Figure 7. For example, the contrast at 60° degrees in the left direction was 0.25, and it was significantly better than that in the right direction (0.16, P < 0.001), in the superior direction (0.01, P < 0.001), or in the inferior direction (0.06, P < 0.001). While the angle of view was not significantly impaired in the 2.1-mm pupil, as shown above, the impairment of the contrast was observed in the small pupil (0.14 versus 0.06, at the posterior pole, P < 0.001). 
Figure 7
 
Contrast at each position of the fundus. The x-axis indicates the angle from the posterior pole, and the y-axis indicates the contrast. The contrast was best in the left part of the image. The upper and lower parts showed suboptimal contrast. In addition, the image obtained with a small pupil showed lower contrast than that obtained with a dilated pupil.
Figure 7
 
Contrast at each position of the fundus. The x-axis indicates the angle from the posterior pole, and the y-axis indicates the contrast. The contrast was best in the left part of the image. The upper and lower parts showed suboptimal contrast. In addition, the image obtained with a small pupil showed lower contrast than that obtained with a dilated pupil.
Discussion
The newly developed Optos 200Tx device enabled the observation of the peripheral part of the fundus and is attracting attention for enabling previously impossible investigations. The present study provides the basis for future analysis. The data shown herein including the angle of view and the exponential and unequal magnifications in the peripheral image should provide practical guidance for planning future studies. 
The evaluation of the peripheral retina is becoming a topic for screening 7 or evaluation of diseases including diabetic retinopathy, 8 retinal vein occlusion, 9 age-related macular degeneration, 10,11 retinopathy of prematurity, 12 uveitis, 1315 and retinal/choroidal dystrophy. 16,17 It was not clear, however, how far into the peripheral region the retina could be imaged or how magnified the obtained images would be. The present study confirmed that the device can image up to 120° (except for the inferior part) by rotating the eye. Even in eyes with a small pupil, up to 90° of the fundus can be imaged. This angle of view would be sufficient for clinical practice or most studies investigating peripheral lesions. 
The angle of view was smaller in the vertical direction than in the horizontal direction. Before we performed the experiment, we believed that the limited visibility of the upper and lower retina is due to eyelids and eyelashes. However, the present study showed that the angle of view of the upper and lower part of the image is limited even in the model eye, which does not have eyelids. In addition, the brightness of the image was lower especially in the upper part. These properties of the images should be noted when comparing the findings in different parts of the image. 
The image was significantly magnified and distorted from the original scale. The present data showed that the overall image is stretched by a factor of 1.12 horizontally and that the peripheral part of the image is magnified in an exponential manner, probably because the device uses an ellipsoid mirror rather than a spherical one. The most peripheral part was magnified to 2 × 1.5 times as large. This result confirmed that the actual size of the peripheral lesion is not what it appears to be and cannot be compared directly to that of the posterior lesion. The quantification of the peripheral image requires significant corrections. In addition, if an examined eye rotates and a lesion is recorded in a different part of the image, the size would change and affect the longitudinal or comparative analysis. When performing these analyses, care should be taken to record the image at the same angle as much as possible. 
This study also showed the differences in image quality between the individual parts of the image. The contrast was the best in the left part of the image and the worst in the upper part of the image. This result shows that the upper and lower parts of the image are not as clear as the right and left parts regardless of the presence of eyelid and eyelashes. This result would probably be due to the optical characteristics of the machine. Physicians should take care when performing a qualitative analysis because a pathologic change, which can be seen in the right or left part of the image, might be missed in the upper or lower part. 
The present experiment suggests some tips to obtain good images. Taking a picture with the eye positioned as close to the device as possible is generally recommended. That prevents artifacts from the intraocular lens in eyes with dilated pupils and prevents the decrease of the view angle in eyes with small pupils. In addition, we confirmed that moving the eye back and forth has little effect on the size of the image. If an eye is pseudophakic and the pupil is fully dilated, moving the eye position backward would sometimes enable recording of the most peripheral part of the retina. To record a specific direction, it is not necessary to move the patient's face or eye drastically. Only a small rotation of up to 15° is enough to record the most peripheral parts. Although the device can record the peripheral fundus through a small pupil, dilation of the pupil would enhance the image contrast. 
The angle of view was larger in aphakic or gas-filled eyes. Around 110 and 120° of the fundus could be imaged in aphakic and gas-filled eyes, respectively. The expanded visibility in these settings would be beneficial for monitoring patients who underwent surgery, especially for retinal detachment with peripheral retinal breaks. On the other hand, the magnification was lower in these settings, and the image of the gas-filled model eye contained prominent artifacts. Further investigation is required to address whether we can evaluate the post-operative patients' fundus in detail. 
A limitation of this study is the fixed size of the model eye. The axial length of the human eye is variable. For example, the axial length in a population-based study ranged from 18.55 to 32.66 mm. 18 The magnification in hyperopic or myopic eyes should be confirmed by further analysis. 
In summary, we presented quantitative data for the image capabilities, magnification, and distortions of the Optos 200Tx wide-field scanning laser ophthalmoscope. Algorithms that correct this magnification and these distortions automatically should be developed to facilitate future studies. 
Acknowledgments
The authors thank Makoto Inoue (Kyorin University, Tokyo, Japan) for providing the model eye. 
Supported in part by Grants-in-Aid for Scientific Research (No. 24791847) from the Japan Society for the Promotion of Science, Tokyo, Japan. The authors alone are responsible for the content and writing of the paper. 
Disclosure: A. Oishi, None; J. Hidaka, HOYA (E); N. Yoshimura, Topcon (F), Nidek (F, C), Canon (F) 
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Figure 1
 
(A, B) The appearance of the model eye used to quantify the images obtained with the wide-field scanning funduscope Optos 200Tx. As shown in the draught, the axial length is 24 mm, and the refraction was comparable to that of the human eye. The body of the model eye was made of metal, and the cornea was made of polymethylmethacrylate. (C) We attached the angular scales to the inner surface of the model eye and quantified the image. (D, E) We created the jig, which fixes the model eye in the appropriate position and enables horizontal and vertical rotational movement of the eye.
Figure 1
 
(A, B) The appearance of the model eye used to quantify the images obtained with the wide-field scanning funduscope Optos 200Tx. As shown in the draught, the axial length is 24 mm, and the refraction was comparable to that of the human eye. The body of the model eye was made of metal, and the cornea was made of polymethylmethacrylate. (C) We attached the angular scales to the inner surface of the model eye and quantified the image. (D, E) We created the jig, which fixes the model eye in the appropriate position and enables horizontal and vertical rotational movement of the eye.
Figure 2
 
The fundus image obtained with the wide-field scanning funduscope Optos 200Tx. We measured the length of the scale bar and interval of scale bars to quantify the magnification in the peripheral part of the image. Thick red and blue bars indicate the length of the scale bar and the interval of five scale bars at 0, 30, 60, and 88°, respectively. Note that the length, as well as the interval of the scale bar, is larger in the periphery.
Figure 2
 
The fundus image obtained with the wide-field scanning funduscope Optos 200Tx. We measured the length of the scale bar and interval of scale bars to quantify the magnification in the peripheral part of the image. Thick red and blue bars indicate the length of the scale bar and the interval of five scale bars at 0, 30, 60, and 88°, respectively. Note that the length, as well as the interval of the scale bar, is larger in the periphery.
Figure 3
 
The angle of view in the images obtained with the wide-field scanning funduscope Optos 200Tx with different settings. (A) A fluid-filled eye with an 8.1-mm pupil and an intraocular lens of +20 D was used. (B) When the model eye was moved backward (away from the machine), the edge of the intraocular lens appeared in the image. (C) The peripheral fundus up to around 95° could be recorded even with a 2.1-mm pupil, but (D) moving the small pupil eye backward made the angle of view very small.
Figure 3
 
The angle of view in the images obtained with the wide-field scanning funduscope Optos 200Tx with different settings. (A) A fluid-filled eye with an 8.1-mm pupil and an intraocular lens of +20 D was used. (B) When the model eye was moved backward (away from the machine), the edge of the intraocular lens appeared in the image. (C) The peripheral fundus up to around 95° could be recorded even with a 2.1-mm pupil, but (D) moving the small pupil eye backward made the angle of view very small.
Figure 4
 
The fundus images of the model eye obtained with the wide-field scanning funduscope Optos 200Tx. The model eye was rotated by 15° in the (A) upward, (B) rightward, (C) downward, and (D) leftward directions. Rotating the eye resulted in increased visibility in each direction.
Figure 4
 
The fundus images of the model eye obtained with the wide-field scanning funduscope Optos 200Tx. The model eye was rotated by 15° in the (A) upward, (B) rightward, (C) downward, and (D) leftward directions. Rotating the eye resulted in increased visibility in each direction.
Figure 5
 
The fundus images of the model eye obtained with the wide-field scanning funduscope Optos 200Tx. We also investigated the effect of the media. (A) +20-D spherical lens, (B) +20-D diffraction multifocal lens, (C) aphakia, and (D) +20-D spherical lens; the vitreous cavity was filled with air. Note that the angle of view is larger in aphakic and gas-filled eyes.
Figure 5
 
The fundus images of the model eye obtained with the wide-field scanning funduscope Optos 200Tx. We also investigated the effect of the media. (A) +20-D spherical lens, (B) +20-D diffraction multifocal lens, (C) aphakia, and (D) +20-D spherical lens; the vitreous cavity was filled with air. Note that the angle of view is larger in aphakic and gas-filled eyes.
Figure 6
 
Tangential and radial magnification at each position of the fundus. The x-axis indicates the angle from the posterior pole, and the y-axis indicates the magnification compared with the posterior pole. Note that the length of the scale bar is magnified exponentially toward the periphery. In addition, the rate was not equal in the vertical and horizontal directions or the tangential and radial directions. Quadratic approximations generally fit the change of the magnification well (thin lines). For tangential magnification in the horizontal direction, the quartic approximation fit better (thick line). In the most peripheral part, the scale bar was visible but was not clear enough to be quantified.
Figure 6
 
Tangential and radial magnification at each position of the fundus. The x-axis indicates the angle from the posterior pole, and the y-axis indicates the magnification compared with the posterior pole. Note that the length of the scale bar is magnified exponentially toward the periphery. In addition, the rate was not equal in the vertical and horizontal directions or the tangential and radial directions. Quadratic approximations generally fit the change of the magnification well (thin lines). For tangential magnification in the horizontal direction, the quartic approximation fit better (thick line). In the most peripheral part, the scale bar was visible but was not clear enough to be quantified.
Figure 7
 
Contrast at each position of the fundus. The x-axis indicates the angle from the posterior pole, and the y-axis indicates the contrast. The contrast was best in the left part of the image. The upper and lower parts showed suboptimal contrast. In addition, the image obtained with a small pupil showed lower contrast than that obtained with a dilated pupil.
Figure 7
 
Contrast at each position of the fundus. The x-axis indicates the angle from the posterior pole, and the y-axis indicates the contrast. The contrast was best in the left part of the image. The upper and lower parts showed suboptimal contrast. In addition, the image obtained with a small pupil showed lower contrast than that obtained with a dilated pupil.
Table 1
 
Angle of View in Each Setting and Each Direction in the Image Obtained With the Wide-Field Fundus Ophthalmoscope Optos 200Tx
Table 1
 
Angle of View in Each Setting and Each Direction in the Image Obtained With the Wide-Field Fundus Ophthalmoscope Optos 200Tx
Superior, deg (P Value) Inferior, deg (P Value) Right, deg (P Value) Left, deg (P Value)
Standard position 96.2 76.4 100.6 101.8
Distant position 99.6 (<0.01) 81.0 (<0.01) 111.2 (<0.01) 106.4 (<0.01)
Small pupil 89.4 (<0.01) 75.0 (<0.01) 95.0 (<0.01) 91.6 (<0.01)
Small pupil, distant position 1.8 (<0.01) 21.3 (<0.01) 39.3 (<0.01) 1.8 (<0.01)
Up-gaze 106.6 (<0.01) 53.8 (<0.01) 104.8 (0.20) 102.4 (0.80)
Down-gaze 80.0 (<0.01) 91.0 (<0.01) 101.0 (0.72) 101.5 (0.43)
Right-gaze 90.5 (<0.01) 77.0 (0.07) 88.0 (<0.01) 120.4 (<0.01)
Left-gaze 90.0 (<0.01) 77.0 (0.07) 121.5 (<0.01) 87.0 (<0.01)
Multifocal lens 97.0 (0.08) 79.0 (<0.01) 102.5 (0.53) 105.0 (<0.01)
Aphakia 93.0 (<0.01) 79.0 (<0.01) 111.6 (<0.01) 110.2 (<0.01)
Air-filled 119.8 (<0.01) 118.8 (<0.01) 118.4 (<0.01) 121.2 (<0.01)
Table 2
 
The Length of the Vertical and Horizontal Scale Bars at the Posterior Pole Measured for Each Setting
Table 2
 
The Length of the Vertical and Horizontal Scale Bars at the Posterior Pole Measured for Each Setting
Vertical Scale, Pixel (P Value) Horizontal Scale, Pixel (P Value)
Standard position 75.5 84.9
Distant position 75.2 (0.65) 84.1 (0.41)
Small pupil 75.8 (0.50) 85.2 (0.74)
Up-gaze 78.1 (0.01) 87.9 (0.01)
Down-gaze 73.5 (0.02) 84.9 (0.94)
Right-gaze 76.0 (0.70) 83.5 (0.18)
Left-gaze 75.9 (0.69) 84.4 (0.50)
Multifocal lens 75.7 (0.80) 84.8 (0.97)
Aphakia 73.9 (<0.01) 83.7 (<0.01)
Air-filled 61.9 (<0.01) 66.6 (<0.01)
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