Back-Projection Diffusion: Solving the Wideband Inverse Scattering Problem with Diffusion Models

We present Wideband Back-Projection Diffusion, an end-to-end probabilistic framework for approximating the posterior distribution of the refractive index using the wideband scattering data through the inverse scattering map. This framework produces highly accurate reconstructions, leveraging conditional diffusion models to draw samples, and also honors the symmetries of the underlying physics of wave-propagation. The procedure is factored into two steps, with the first, inspired by the filtered back-propagation formula, transforms data into a physics-based latent representation, while the second learns a conditional score function conditioned on this latent representation. These two steps individually obey their associated symmetries and are amenable to compression by imposing the rank structure found in the filtered back-projection formula. Empirically, our framework has both low sample and computational complexity, with its number of parameters scaling only sub-linearly with the target resolution, and has stable training dynamics. It provides sharp reconstructions effortlessly and is capable of recovering even sub-Nyquist features in the multiple-scattering regime.