With advances in imaging technology, the development of fundus angiography from static to dynamic is an inevitable trend. However, one of remaining difficulties is that the entire record required a huge amount of memory. For instance, a traditional angiography device is adequate for recording short, one-minute videos. The size of a one-minute 1080P30 (1920*1080 image with progressive scan at 30 frames per second) high definition UWFA video could approach 100 GB. Time-lapse photography is a technique that meets the need for recording the complete angiography process. With the use of a time-lapse UWFA video, hundreds of GB of imaging data could be reduced to dozens of MB. In previous studies, it has been proven to be useful in visualizing abnormal vasculature leakage [14] and illustrating the progression of macular hole [15]. The present study used time-lapse technique to visualize the complete UWFA process in DR patients. Compared to static NPI, dynamic NPI generated using a time-lapse technique was significantly lower in the total retina (0.25 vs 0.28, p = 0.005), far-periphery (0.30 vs 0.34, adjusted p = 0.040) and superior quadrant (0.27 vs 0.34, adjusted p = 0.045) in eyes with severe NPDR and PDR. The far-periphery NPI was associated with posterior segment IRMA density in both groups, and posterior segment IRMA quantity in the static group.
NPI appeared to increase from the center to the periphery in both groups. The lower perfusion pressure at further distances from the posterior pole may cause a higher NPI in the peripheral retina [20]. We noticed that NPI could be quite variable. The NPI in Son et al’s study [21] was found to be highest in the temporal quadrant and lowest in the superior quadrant, with a global NPI of 0.59. In Silva et al’s [19] study, the NPI was observed to be the highest in the modified superotemporal quadrant. On one hand, variable NPI values may be caused by the different grading protocols and study characteristics. On the other hand, areas of hypofluorescence is a nonspecific hallmark on static images, which can be related to non-perfusion, subretinal hemorrhage, or vitreous opacity.
In previous study, patients with severe NPDR and PDR were reported to have a significantly greater vision-related functional burden than those without DR, which correlated well with degree of retinopathy [22]. It is important to precisely identify ischemic lesions in these DR patients. The present study showed significant differences between dynamic and static NPI among 16 eyes with severe NPDR and PDR in the far-periphery and in the superior quadrant. It demonstrated that low image contrast and eyelash artifact in the most peripheral region in UWFA imaging can contribute to a higher NPI. Some non-perfusion in peripheral retinal zones may actually be perfused. We considered that a 3.58% NPI may be an overestimation when calculated on a single UWFA image. In comparison with static pictures, videos are helpful for the differential diagnosis of hypofluorescence and for providing more information with regard to the peripheral retina.
We observed that the highest NPI was in the far-periphery, while IRMAs and NV occurred more frequently in the mid-periphery and posterior area. Analysis by Spearman’s rank correlation showed the severity of non-perfusion in the peripheral zone was associated with the prevalence of IRMAs in the posterior zone. Lange et al [23] found that far-peripheral NPI was significantly associated with mid-peripheral NV index (linear regression: Y = 0.103*X + 0.841, p = 0.007). These results indicate that the ischemia-induced vascular abnormalities usually occurs at the border between the perfusion and non-perfusion areas, in concordance with previous study [24]. Of note, static far-peripheral NPI was associated with posterior segment IRMA quantity (R = 0.517, p = 0.04). No association was found of dynamic far-peripheral NPI and posterior segment IRMA quantity (R = 0.471, p = 0.066). The results imply that the correlation between NPI and vascular abnormalities tend to be magnified in a single UWFA image. Moreover, the presence of vascular abnormalities might be impacted by multiple factors, besides ischemia. Posterior vitreous detachment has been a common occurrence in the vitreous, though this has not been mentioned in either previous or current studies. It has been shown that vitreoretinal attachment plays an important role in retinal vascular proliferation [25, 26]. Other risk factors of retinal vascular abnormalities include the long duration of diabetes, male gender, insulin use, hypertension, and PDR in the contralateral eye [27, 28].
The present study demonstrated that dynamic UWFA imaging allows for the accurate measurement of retinal non-perfusion. As a result, the precise application of panretinal photocoagulation and targeted retinal photocoagulation based on accurate non-perfusion delineation may be possible. An additional advantage is that a time-lapse video formed by a sequence of UWFA images is more likely helpful in differentiating retinal vascular lesions [see Additional file 2]. While leakage on fluorescein angiography conventionally helps to evaluate NV activity [8], a single image may be insufficient to distinguish NV from other lesions causing leakage, such as IRMA or other vascular abnormalities (Fig. 4). Therefore, with regard to clinical and scholarly presentations, active classic neovascularization on disc and neovascularization elsewhere could be easily detected in the videos (corresponding to the arrows in [Additional file 3]). Vascular leakage, non-perfusion, and granular background fluorescence at the far periphery, with increasing hyperfluorescence in the late phase, were clearly detected (corresponding to the arrows in [Additional file 4]). In addition, the UWFA time-lapse imaging allowed the posterior vitreous detachment with a Weiss ring as hypofluorescence at the posterior pole (corresponding to the arrow in [Additional file 5]).
The strength of this study lies in the fact that we assessed the value of dynamic UWFA imaging in precise identification of peripheral retinal non-perfusion and vascular abnormalities in DR. In addition, images were analyzed and retinas graded by two independent examiners. It should be noted that greater peripheral DR lesions in UWFA have been associated with more severe DR [29–31]. However, whether UWFA will attain a key position as a clinically relevant and irreplaceable tool remains unknown. Future studies, including the ongoing Intravitreal Aflibercept as Indicated by Real-Time Objective Imaging to Achieve Diabetic Retinopathy Improvement (registered at http://www.clinicaltrials.gov, with a registration number of NCT03531294) and Peripheral Diabetic Retinopathy Lesions on Ultrawide-field Fundus Images and Risk of DR Worsening over Time (DRCR.net, Protocol AA) [32], will help shed light on the potential role of UWFA findings in the evaluation and clinical management of DR eyes.
Limitations of this study were that we did not correct the peripheral warp present in UWFA. Since the most peripheral part of a UWFA image is magnified, we evaluated retinal non-perfusion using NPI instead of evaluating absolute areas of non-perfusion. Tan et al [33] used stereographic projection software to calculate precise NPA (in mm2) and compared corrected NPI to original NPI. They found that corrected NPI correlated with original NPI (Spearman correlation R = 0.978, p < 0.001), with no significant difference between the two NPI values (Wilcoxon signed-rank test, p = 0.239). Therefore, we believe that the discrepancy in NPI values may not significantly alter our conclusions. Besides, the significant differences between dynamic and static NPI in the far-periphery (adjusted p = 0.040) and superior quadrant (adjusted p = 0.045) are near minimal. The small sample size of this study increased the possibility of biased statistical significance (with a higher type 2 error). In addition, this technique requires patients to maintain good fixation long enough to attain high-quality images, which can be modified with the development of imaging technology.