Mitigating the effects of choroidal hyper- and hypo-transmission defects on choroidal vascularity index assessments using optical coherence tomography.

Background: Changes in choroidal vascularity index (CVI) are associated with multiple choroid-related ocular diseases. CVI is calculated as the area/volume ratio of vessels in the choroid, which could be affected by alterations in regional signal intensities due to hypo-transmission defects (hypoTDs) caused by drusen and retinal pigment epithelium (RPE) detachments, and hyper-transmission defects (hyperTDs) caused by the absence of RPE. To develop a simulation model to verify the CVI assessments in eyes with hyper/hypoTDs and demonstrate that accurate CVIs can be achieved after attenuation correction on swept-source optical coherence tomography (SS-OCT). Methods: A simulation model was developed on 6×6 mm macular scans from normal subjects. Signal intensity in a cylindrical region below RPE was altered to mimic hyper/hypoTDs. CVIs were compared inside and outside the simulated regions before and after attenuation correction. CVI assessments of OCT scans from patients with hyperTDs due to geographic atrophy (GA) and from patients with hypoTDs due to drusen that subsequently resolved with the disappearance of the hypoTDs were compared with and without attenuation correction. Results: Ten normal eyes were recruited to generate the hyper/hypoTD simulation model. In eyes with hypoTDs, CVIs were overestimated, and in eyes with hyperTDs, the CVIs were underestimated (P<0.001). After attenuation correction, the uneven distribution of signal intensity was eliminated and the resulting CVI showed no significant difference compared with the 'ground truth', which is measured from the original scans. Attenuation correction successfully eliminated the influence of hyperTDs caused by GA on CVI measurements (n=38). Quantitatively, no significant difference was found in the CVIs of eyes before and after drusen collapse with attenuation correction (n=8). Conclusions: The simulation model could reveal the impact of hypo/hyperTDs on CVI quantification in eyes with choroid-involved ocular diseases. The importance of attenuation correction to ensure accuracy in choroidal vessel segmentation was demonstrated by analyzing eyes with GA or drusen.