SpectFusion: Cross-modal Spectrum-aware Attention Network for Unsupervised Multimodal Medical Image Fusion

Medical image fusion aims to synthesize relevant and complementary information from different modalities, thereby enhancing clinical diagnosis. Current deep learning-based fusion approaches, particularly Transformer-based architectures, have achieved remarkable results due to their strong capacity for modeling long-range dependencies. However, there are still limitations in capturing sufficient global information because of the window-based local attention mechanism. Moreover, existing fusion schemes predominantly focus on spatial features while rarely considering spectral features, thus affecting the fusion performance. To address these challenges, we propose a new unsupervised cross-modal spectrum-aware fusion framework, named SpectFusion, for medical image fusion. Specifically, we devise a spatial-spectrum hybrid block, which effectively extracts fine-grained local features via a gradient retention strategy in the spatial domain, and captures global features with an image-wide receptive field through Fourier convolution in the frequency domain. Furthermore, we develop a novel cross-modal spectrum-aware attention to facilitate spatial-spectrum information interactions during fusion. It dynamically guides the retention of relevant spectral components while integrating multimodal spatial features. Additionally, to achieve more precise alignment image pairs, we incorporate a refined registration module to correct minor local deviations. We also define corresponding frequency and spatial domain losses to jointly constrain the proposed SpectFusion. By leveraging spatial-spectrum information interactions, fine-grained fusion can be adaptively realized. Extensive experiments, including clinical brain tumor image fusion, demonstrate that SpectFusion outperforms other state-of-the-art methods both qualitatively and quantitatively. We show that SpectFusion can boost performance in downstream tasks such as multimodal medical image segmentation. The code is available at https://github.com/PlumW/SpectFusion.