Structured Modality-Aware Token Interaction for Multimodal Medical Imaging

Multimodal medical imaging benefits from the global context modeling of transformers, yet most existing models fuse modalities implicitly by channel concatenation, leaving cross-modal interaction unstructured or relying on costly multi-stream cross-attention. We propose Modality-Aware Token Interaction (MATI), an architecturally lightweight and backbone-agnostic module that structures multimodal interaction within a single token stream by partitioning embedding channels into modality-aligned subspaces. MATI performs modality-preserving intra-subspace self-attention and gated global mixing for controlled inter-subspace exchange. We instantiate MATI in UNETR and introduce two architectures: ModaUNETR^S refines selected skip-token representations, and ModaUNETR^E injects MATI into the transformer encoder to progressively shape the token hierarchy. Experiments on the BraTS 2020 benchmark employ five-fold cross-validation and report segmentation and efficiency metrics on the official training and validation sets. Both models consistently improve over UNETR across all tumor subregions, with larger gains for tumor core and whole tumor, and ModaUNETR^E further improves upon ModaUNETR^S. An ablation study confirms monotonic gains from structured interaction. Compared with heavier transformers, the proposed models achieve a favorable accuracy-efficiency trade-off without modality-specific encoders or quadratic cross-attention, supporting structured multimodal interaction as a first-class architectural principle. Implementation of MATI and its instantiations is available at https://github.com/S3l11/MATI.