Polar-domain Segmentation of Peripapillary Atrophy and Optic Disc

Abstract

Automatic segmentation of pepripapillary (PPA) and the optic disc (OD) from fundus photographs enables quantitative morphological analysis in myopic eyes. However, variations in OD-PPA geometry and the indistinct appearance of PPA complicate the delineation of their shared and outer boundaries. Motivated by the spatial arrangement of PPA around the OD, we reformulate their delineation by indexing the OD and PPA contours as two radial profiles in polar coordinates. Built upon this representation, we propose UPolarSeg. To accommodate the polar geometric representation under non-uniform spatial sampling, UPolarSeg equips U-shape skip connections with Radial-Angular Decoupling Module (RADM) to capture radial transitions and circumferential continuity respectively. A training-only auxiliary boundary decoder further promotes contour fidelity, supporting direct morphological measurements from the predicted segmentation masks. On internal and external cohorts, UPolarSeg achieves PPA Dice scores of 89.59% and 77.46%, and OD Dice scores of 98.36% and 97.63%. For morphological quantification, the mean absolute errors are 2.4 and 6.7 pixels for maximum PPA width, and 0.017 and 0.036 for its ratio to the vertical OD diameter.