Quantification of choriocapillaris with Phansalkar's local thresholding: pitfalls to avoid.

PURPOSE To demonstrate the proper use of the Phansalkar's local thresholding method (Phansalkar method) in choriocapillaris (CC) quantification with optical coherence tomography angiography (OCTA). DESIGN Retrospective, observational case series. METHODS Swept source OCTA imaging was performed using 3x3 mm and 6x6 mm scanning patterns. The CC slab was extracted following semi-automatic segmentation of the retinal pigment epithelium/Bruch's membrane complex. Retinal projection artifacts were removed before further analysis, and CC OCTA images from drusen eyes were compensated using a previously published strategy. CC flow deficits (FDs) were segmented with two previously published algorithms: fuzzy C-means approach (FCM method) and Phansalkar method. With the Phansalkar method, different parameters were tested and a local window radius of 1-15 pixels was used. FD density (FDD), mean FD size (MFDS) and FD number (FDN) were calculated for comparison. RESULTS Six normal eyes from six subjects and six eyes with drusen secondary to age-related macular degeneration from six subjects were analyzed. With both 3x3 mm and 6x6 mm scans from all eyes, the FD metrics were highly dependent on the selection of the local window radius when using the Phansalkar method. Larger window radii resulted in higher FDD values. FDN increased with the increase in the window radius but then decreased, with an inflection point at about 1 - 2 inter-capillary distances (ICDs). MFDS decreased then increased with increasing window radii. CONCLUSIONS Multiple parameters, especially the local window radius, should be optimized before using the Phansalkar method for the quantification of CC FDs with OCTA imaging. It is recommended that the proper use of the Phansalkar method should include the selection of the window radius that is related to the expected ICD in normal eyes.