Assessing the Accuracy of Foveal Avascular Zone Measurements Using Optical Coherence Tomography Angiography: Segmentation and Scaling

Abstract

PurposeThe foveal avascular zone (FAZ) is altered in numerous diseases. We assessed factors (axial length, segmentation method, age, sex) impacting FAZ measurements from optical coherence tomography (OCT) angiography images.MethodsWe recruited 116 Caucasian subjects without ocular disease, and acquired two 3 × 3 mm AngioVue scans per each right eye (232 total scans). In images of the superficial plexus, the FAZ was segmented using the AngioVue semiautomatic nonflow measurement tool and ImageJ manual segmentation. In images from the full retinal thickness, the FAZ was segmented using the AngioAnalytics automatic FAZ tool. Repeatability, reliability, and reproducibility were calculated for FAZ measurements (acircularity, area).ResultsFAZ area (mean ± SD) for manual segmentation was 0.257 ± 0.104 mm2, greater than both semiautomatic (0.231 ± 0.0939 mm2) and automatic (0.234 ± 0.0933 mm2) segmentation (P < 0.05). Not correcting for axial length introduced errors up to 31% in FAZ area. Manual area segmentation had better repeatability (0.022 mm2) than semiautomatic (0.046 mm2) or automatic (0.060 mm2). FAZ acircularity had better repeatability with automatic than manual segmentation (0.086 vs. 0.114). Reliability of all area measurements was excellent (intraclass correlation coefficient [ICC] = 0.994 manual, 0.969 semiautomatic, 0.948 automatic). Reliability of acircularity measurements was 0.879 for manual and 0.606 for automatic.ConclusionWe identified numerous factors affecting FAZ measurements. These errors confound comparisons across studies and studies examining factors that may correlate with FAZ measures.Translational RelevanceUsing FAZ measurements as biomarkers for disease progression requires assessing and controlling for different sources of error. Not correcting for ocular magnification can result in significant inaccuracy in FAZ measurements, while choice of segmentation method affects both repeatability and accuracy.

An Optical Coherence Tomography Angiography Study of the Relationship Between Foveal Avascular Zone Size and Retinal Vessel Density

“…34 In addition, intra-rater repeatability analysis was performed using repeatability coefficient and measurement error, as in previous studies. 34,[43][44][45] Repeatability coefficient was calculated as 2.77*average within-subject standard deviation (r w ), and measurement error was calculated as 1.96*r w . 34 The 95% confidence intervals of repeatability coefficient were calculated as 1:96Ãrw=…”

“…The OCTA system (Optovue, Inc.) has a default AL of 23.95 mm. 34,35 During the measurement of FAZ area and perimeter, Littmann's method was used to correct the ocular magnification by each AL. [34][35][36][37][38] Details of the formula are provided elsewhere.…”

“…34,35 During the measurement of FAZ area and perimeter, Littmann's method was used to correct the ocular magnification by each AL. [34][35][36][37][38] Details of the formula are provided elsewhere. 34,35,37,38 In brief, the relationship between the measured OCTA image diameter, Dm, and the true diameter, Dt, can be formulated as Dt ¼ p*q*Dm, where p and q are the image magnification factor of the imaging system and the eye, respectively.…”

“…[34][35][36][37][38] Details of the formula are provided elsewhere. 34,35,37,38 In brief, the relationship between the measured OCTA image diameter, Dm, and the true diameter, Dt, can be formulated as Dt ¼ p*q*Dm, where p and q are the image magnification factor of the imaging system and the eye, respectively. The p of the OCTA system (Optovue, Inc.) is 3.460, which can be calculated from 1/[0.01306(23.95 À 1.82)], and q can be determined from Bennett's formula, as q ¼ 0.01306*(AL À 1.82), where 1.82 is a constant related to the distance between the corneal apex and the second principal plane.…”

“…The p of the OCTA system (Optovue, Inc.) is 3.460, which can be calculated from 1/[0.01306(23.95 À 1.82)], and q can be determined from Bennett's formula, as q ¼ 0.01306*(AL À 1.82), where 1.82 is a constant related to the distance between the corneal apex and the second principal plane. 34,35,37,38 Thus, AL-related magnification factor can be corrected by Dt ¼ 3.460*0.01306*(AL À 1.82)*Dm. 34,35,37,38 Poor quality OCTA images were excluded from the analysis, including signal strength index (SSI) < 48 (minimum ¼ 1, maximum ¼ 100) 23,25,26,36,[39][40][41][42] ; poor-clarity images due to severe media opacities; motion artifacts or localized weak signal by vitreous floaters; residual motion artifacts visible as irregular vessel patterns or disc boundary on the en face angiogram; images not centered on the fovea for macular OCTA (fixation error); and segmentation failure.…”