The present study aimed to describe FD metrics of the retinal vascular network using SS-OCTCA in healthy participants and to assess its repeatability and reproducibility in clinical practice. We used an SS-OCTA device with automatic layer segmentation to achieve 12×12-mm images and then quantified the retinal vascular network FD of the SCP and DCP using independent software. The mean retinal vascular network FD was 1.693 in the superficial and 1.694 in the deep vascular layer in our healthy cohort. These results are in agreement with previous studies based on fundus photographs, which found a normal retinal vessel FD of 1.7 [9, 12]. Although we did not use the same imaging device, our results were consistent since we acquired wide-field 12×12-mm images with a field of view of 48°, which allowed us to work on the fundus area close to that obtained with fundus photographs. Previous studies have investigated retinal vessel FD using SS-OCTA. Corvi et al. evaluated the retinal vascular network FD with seven different OCTA devices including the Cirrus Plex Elite 9000 (Carl Zeiss Meditec, Jena, Germany) using a 3×3-mm scan, and found a mean value of 1.68 and 1.69 in the superficial and deep vascular plexuses, respectively . Although the acquisition size in our study was different, our results are in agreement. Hirano et al. quantified the retinal vascular network FD with the same SS-OCTA device and image size in healthy participants and in patients with diabetes . They found a mean retinal vascular network FD of 1.627 and 1.612 in the superficial and deep capillary network in healthy eyes, respectively. However, to our knowledge, none have been interested in assessing the repeatability and reproducibility of this metrics. Furthermore, it should be noted that retinal vascular network FDs in the SCP and DCP were fairly similar. Few studies have compared the retinal vascular network FD in these two networks. It was reported that the patterns of these two networks are different, and that the central avascular zone is larger in the deep network compared with the superficial one . FD evaluates the retinal vascular network as a whole, which may explain why there were only a few differences in the mean FD metrics between these two networks.
The retinal vascular network FD measurements showed good repeatability and reproducibility in our study. The measurements obtained by the same operator and those obtained by two operators were not statistically significantly different when the same protocol was used. Intra-observer repeatability was good in the SCP and excellent in the DCP. However, we found that the measurements of the retinal vascular network FD in the DCP did not have very good inter-observer reproducibility. This could be explained by the difficulty of removing projection artefacts of vessels from the superficial vascular network to the deep vascular network. It has already been shown that analysis of the superficial retinal plexus is more accurate than the deeper plexus because of projection artefacts . Another explanation could be motion artefacts during OCTA acquisition. However, this did not occur between two acquisitions made by the same observer and therefore it could be related to changes in the head position between the two observers. Even small head movements or eye misposition can produce dramatic changes from one B-scan to another, although the SS-OCTA device eye-tracker tried to control for these artefacts.
The results regarding reproducibility in this study were only applicable to the SS-OCTA Cirrus Plex Elite 9000 (Carl Zeiss Meditec, Jena, Germany). Corvi et al. compared retinal vascular network FD measurements between seven different OCTA devices and found that the mean retinal vascular network FD in both the SCP and DCP were different between these instruments . Differences were even found between devices using the same algorithm and segmentation limits. Thus, measurements between different OCTA devices were not interchangeable. Moreover, we only included healthy eyes in our study and, therefore, the automated segmentation was correct for all images. We should expect that in eyes with retinal disorders, automatic segmentation might compromise the repeatability of these measurements.
In previous studies, the retinal vascular network FD was considered an interesting biomarker for the study of retinal and cardiovascular diseases. These studies used methods based on the measurement of FD on fundus photographs [12, 27]. With fundus photographs, it is impossible to distinguish the two vascular networks. Current advances in retinal imaging have allowed us to describe the retinal vascular network thoroughly. Furthermore, the swept-source technique has the advantage of being able to acquire larger images with better quality and contrast thus allowing us to study the vascular network as a whole . Moreover, the benefit of using a 12×12-mm wide-field acquisition zone is to simultaneously visualise the macula region and the optic nerve head and to analyse both the macular network and radial peripapillary capillaries on the superficial and deep vascular plexus.
Fractal analysis can provide greater insight into the development of retinal vascular diseases. Recent studies have demonstrated the value of the retinal vascular network FD in clinical practice. A reduced retinal vascular network FD was found in all stages of diabetic retinopathy with SS-OCTA [32, 34]. Nevertheless, retinal vascular network FD analysis of the retinal network is a global measure of the blood vessel pattern; as such, it is not sensitive to minor alterations in a small region of the total pattern . An earlier study explored the relationship between the retinal vascular network FD and neovascular age-related macular degeneration (nAMD) . The relationship between quantitative OCTA parameters in patients with active nAMD under treatment and those with remission nAMD was investigated. The retinal vascular network FD in the active nAMD group was significantly lower than that in the remission group (1.44 vs 1.50, P < 0.001) . This was explained by increased branching after arteriogenesis in the remission group. The retinal vascular network FD therefore seemed to be of interest for the study of retinal vascular diseases but also macular disorders. It would also be interesting to evaluate its contribution to the diagnosis of cardiovascular disorders. Combined with other quantitative parameters such as vascular density, vascular perfusion and the area of nonperfusion, the retinal vascular network FD offers the possibility of additional parameters to monitor retinal disease and to refine risk stratification.
The potential limitations of this study should be mentioned. First, it was a small-sample single-centre study, which may limit its external validity. Second, it was a self-report study; we did not carry out a medical assessment or blood test to eliminate vascular disease such as diabetes. Third, the age range was limited to individuals aged 24–52 years. Fourth, we had artefacts during OCTA image acquisition, including motion artefacts, as well as image treatment such as automated segmentation failure and projection artefacts .
In conclusion, the retinal vascular network FD provided new repeatable and reproducible quantitative data using SS-OCTA with healthy participants. The clinical relevance of these findings warrants further studies.