Freqformer: Frequency-Domain Transformer for 3-D Visualization and Quantification of Human Retinal Circulation

We introduce Freqformer, a novel Transformer-based architecture designed for 3-D, high-definition visualization of human retinal circulation from a single scan in commercial optical coherence tomography angiography (OCTA). Freqformer addresses the challenge of limited signal-to-noise ratio in OCTA volume by utilizing a complex-valued frequency-domain module (CFDM) and a simplified multi-head attention (Sim-MHA) mechanism. Using merged volumes as ground truth, Freqformer enables accurate reconstruction of retinal vasculature across the depth planes, allowing for 3-D quantification of capillary segments (count, density, and length). Our method outperforms state-of-the-art convolutional neural networks (CNNs) and several Transformer-based models, with superior performance in peak signal-to-noise ratio (PSNR), structural similarity index measure (SSIM), and learned perceptual image patch similarity (LPIPS). Furthermore, Freqformer demonstrates excellent generalizability across lower scanning density, effectively enhancing OCTA scans with larger fields of view (from 3$\times$3 $mm^{2}$ to 6$\times$6 $mm^{2}$ and 12$\times$12 $mm^{2}$). These results suggest that Freqformer can significantly improve the understanding and characterization of retinal circulation, offering potential clinical applications in diagnosing and managing retinal vascular diseases.