Compensation for Reflectance Variation in Vessel Density Quantification by Optical Coherence Tomography Angiography

Abstract

PurposeTo compensate for reflectance variation when quantifying vessel density by optical coherence tomography angiography (OCTA).MethodsHealthy participants received 6×6-mm macular and 4.5×4.5-mm optic nerve head (ONH) angiography scans on a 70-kHz spectral-domain optical coherence tomography system. The split-spectrum amplitude-decorrelation angiography (SSADA) algorithm was used to compute the OCTA signal. Mean reflectance projection and maximum decorrelation projection were used to create en face OCT and OCTA images. Background OCTA noise in static tissue was evaluated in the foveal avascular zone (FAZ). Vessel density was calculated from en face retinal OCTA that was binarized according to a decorrelation threshold.ResultsThe average retinal decorrelation noise in the FAZ was linearly related to the average logarithmic-scale OCT reflectance signal. Based on this relationship, a reflectance-adjusted decorrelation threshold equation was developed to filter out 97.5% of background OCTA noise. A fixed threshold was also used for comparison. The superficial vascular complex vessel density in the macula and ONH were significantly correlated with reflectance signal strength index (SSI) using the fixed threshold. This correlation was removed by using the reflectance-adjusted threshold. Reflectance compensation reduced population variation in 25 healthy eyes from 8.5% to 4.8% (coefficient of variation) in the macula and from 6.7% to 5.4% in the peripapillary region. Within-visit repeatability also improved from 4.4% to 1.8% in the macula and from 3% to 1.7% in the peripapillary region.ConclusionsCompensating for reflectance variation resulted in more reliable vessel density quantification in OCTA.

Projection-Resolved Optical Coherence Tomography Angiography of Macular Retinal Circulation in Glaucoma

“…29 To demonstrate the importance of this reflectance compensation method, we also calculated the SVC VD using a fixed threshold that does not compensate for reflectance variation. In normal participants, the population variation of the reflectance-adjusted macular SVC VD was 6.6% CV, which was smaller (P=0.013, F test) than the variation without reflectance adjustment (9.7% CV).…”

“…We developed a method to adjust for the effect of reflectance on vessel density. 29 This algorithm effectively removes the dependence of VD on the signal strength index. However, it was not known a priori whether the reflectance-adjustment would aid or hinder glaucoma diagnostic accuracy.…”

“…due to media opacity or pupil vignetting). 29 The method is based on statistical analysis of the relationship between the flow noise in the FAZ with reflectance in retinal tissue, in which the reflectance was manipulated by simulated media opacity (optical filters). In the extrafoveal retina, the average reflectance in the INL, OPL, and ONL layers to adjust the threshold flow signal value used to classify vessel v. static tissue on en face OCTA.…”

“…Clinical validation showed that this algorithm was able to remove the dependence of retinal VD measurements on OCT signal strength index and reduce population variation. 29 …”

Quantitative optical coherence tomography angiography of the peripapillary circulation in glaucoma

“…An additional consideration when interpreting OCTA data is the effect of local OCT reflectance signal strength, which can be weakened by a number of factors including vitreous opacities and pupil edge vignetting resulting in decreased decorrelation signal. Adjustment for reflectance variation has been shown to increase vessel density measurement reliability in OCTA and should be incorporated into future studies (14). …”