WaveJoint: Frequency-Guided Scale Transitions for OCT Angiography Segmentation

Optical coherence tomography angiography (OCTA) supports two complementary biomarkers: the retinal vessel (RV) tree and the foveal avascular zone (FAZ). The capillaries of the RV tree span only 1–3 pixels and their decorrelation signals are weak, whereas the FAZ is a compact region whose accurate delineation depends on stable contextual information. Jointly segmenting both structures from a single en-face projection is desirable for clinical quantification, but the two targets place conflicting demands on the network. We present WaveJoint, a compact (12.4M-parameter) dual-task Transformer and, to our knowledge, the first OCTA joint-segmentation architecture built around an explicit frequency decomposition. At each encoder scale transition, a wavelet-guided downsampling module (WaveDown) applies fixed orthogonal Haar filters, which are parameter-free and satisfy perfect reconstruction, to compute directional detail through a 2-D discrete wavelet transform and to reweight the downsampled path before aliasing suppresses high-frequency detail. Two task-specific heads then predict the RV and FAZ masks under a balanced Dice–BCE objective. A 2×2 factorial ablation with interaction terms attributes the model's fine-vessel advantage to this downsampling path: this path shows a 2.1 percentage-point main effect on capillary-scale recall and is the only module whose individual removal incurs a significant structural cost. The same analysis explains why single-module ablations judged on aggregate Dice alone can suggest, incorrectly, that no module is important. We also contribute a structural evaluation protocol comprising skeleton recall stratified by vessel caliber, HD95 and ASSD boundary distances, a fragmentation ratio, and Holm-corrected paired Wilcoxon tests. Under this protocol, WaveJoint exceeds a protocol-matched U-Net by 3.25 percentage points of capillary recall with 48% lower fragmentation; under Dice alone, the two models are statistically indistinguishable. On the official OCTA-500 3 mm split, WaveJoint attains an RV Dice of 0.9220±0.0002 and an FAZ Dice of 0.9820±0.0013 over three seeds, matching the best published joint-segmentation systems while using a single 2-D input and no pretraining. The structural advantage persists under shifts in field of view, device, and pathology (OCTA-500 6 mm; FAROS), with capillary-recall gains between 3.3 and 12.5 percentage points. These results indicate that OCTA segmentation should report structural metrics and that the contribution of an architectural mechanism should be localized rather than presumed.