All participants underwent examination of visual acuity (logarithm of the minimal angle of resolution), intraocular pressure (IOP), slit-lamp biomicroscopy, average keratometry (Avek) (Topographic Modelling System, TMS-4; Tomey Corporation, Nagoya, Japan), AL and central corneal thickness (CCT) (Haag-Streit Diagnostics, LS 900; Haag-Streit AG, Koeniz, Switzerland), cycloplegic refraction for SE, ocular fundus, and retinal oximetry (ROSV-M18, Healthsun Vision, China). IOP was measured using a noncontact computerized tonometer (CT-800; Topcon Co., Ltd, Tokyo, Japan). For cycloplegic refraction, tropicamide phenylephrine eye drops (Mydrin-P; Santen, Osaka, Japan) were used three times at intervals of 15 minutes. SE was defined as the sum of the spherical diopters and half of the astigmatic diopters.
Retinal oximetry was performed using the Multi-wavelength Structure-function Coupled Retinal Imager (ROSV-M18; Healthsun Vision, Chengdu, China) (
Fig. 1). The retinal image acquisition subsystem can simultaneously obtain dual-wavelength retinal images (570 and 600 nm) simultaneously.
The retinal image acquisition subsystem was composed of a fundus camera, objective imaging lens, right-angle reflecting prism, two reflectors, two interference filters, objective imaging lens, and a three-charge-coupled device (CCD). At the retinal image output end of the fundus camera, the retinal image acquisition subsystem images the retinal images at infinity through the objective imaging lens, which forms the telecentric optical path of the images. The light beam was divided into two paths by the right-angle reflection prism. After being reflected by the first and second reflectors, respectively, the imaging light of the required wavelength was filtered through the first and second interference filters of a specific wavelength (570 nm and 600 nm), respectively. After imaging by the objective lens, the imaging lights with specific wavelengths were simultaneously imaged at different positions on the CCD target surface to achieve simultaneous dual-wavelength imaging of the retina.
There are two main systems for dual-wavelength retinal vascular oximetry based on fundus cameras. In 2008, Hammer et al.
14 used a double-bandpass filter specifically designed for oximetry to record fundus dual-wavelength images, which was inserted into the illumination path of a fundus camera. In 2012, another retinal oximetry system was developed, composed of two digital cameras. Using a custom-made image splitter, images at two different wavelengths of light, 570 and 600 nm, were recorded on two CCDs separately.
15
The disadvantages to using two cameras to acquire dual-wavelength retinal images are that it is costly and the difference in responses between the two cameras would impact retinal vascular oxygen measurements.
15 The overall optical path design of the retinal oximeter used in this study was simple in structure. The dual-wavelength retinal images were imaged at different locations on the same CCD, reducing hardware costs. The images of two specific wavelengths of light could be recorded simultaneously, avoiding the effect of eye movements on retinal oximetry compared to separate imaging.
Before the examination, the pupils were dilated to at least 6 mm using tropicamide phenylephrine drops, and the participants sat comfortably for at least 20 minutes. All fundus images were centered on the optic disc in a dark room. Performed by one experienced operator, the flash power was 50 W, and the acquisition view angle was 45°.
16 If satisfactory images were not acquired after three attempts, the measurement was not continued (
Fig. 2).
Healthsun retinal image quantification analysis (HIQA-1, V1) was used to analyze overall oxygen saturation. Images with a quality above 6.0/10.0 were analyzed. The calculation process is shown in
Figure 3. Two concentric circles were made with the center of the optic disc as the center and 1.5 to 3.0 optic disc diameters as the radius.
16 The retinal vessels in the area between the two concentric circles were selected in the analysis with a width of no less than 8 pixels and a length longer than 50 pixels.
13 The main blood vessel was selected as the vessel with branches, and branch vessels were selected if the length of the main blood vessel was insufficient. After detailed vessel segment selection, the mean arteriolar and venule oxygen saturation values and blood vessel width and length were analyzed.
In our study, we calculated the weighted mean of arterioles and venules of oxygen saturation in the retina. The difference in the retinal oxygen saturation of the weighted mean between the arterioles and venules was calculated as AVD. The weighted average is calculated by the sum of each vessel diameter in the fourth power,
S represents oxygen saturation, and
D represents the vessel's width. If five vessels are included in the analysis, the weighted mean is expressed as follows:
\begin{eqnarray*}S = \frac{{{S_1} \times D_1^4 + {S_2} \times D_2^4 + {S_3} \times D_3^4 + \ldots + {S_n} \times D_n^4}}{{D_1^4 + D_2^4 + D_3^4 + \ldots + D_n^4}}.\end{eqnarray*}