One Model to Unite Them All: Personalized Federated Learning of Multi-Contrast MRI Synthesis

Learning-based MRI translation involves a synthesis model that maps a source-contrast onto a target-contrast image. Multi-institutional collaborations are key to training synthesis models across broad datasets, yet centralized training involves privacy risks. Federated learning (FL) is a collaboration framework that instead adopts decentralized training to avoid sharing imaging data and mitigate privacy concerns. However, FL-trained models can be impaired by the inherent heterogeneity in the distribution of imaging data. On the one hand, implicit shifts in image distribution are evident across sites, even for a common translation task with fixed source-target configuration. Conversely, explicit shifts arise within and across sites when diverse translation tasks with varying source-target configurations are prescribed. To improve reliability against domain shifts, here we introduce the first personalized FL method for MRI Synthesis (pFLSynth). pFLSynth is based on an adversarial model equipped with a mapper that produces latents specific to individual sites and source-target contrasts. It leverages novel personalization blocks that adaptively tune the statistics and weighting of feature maps across the generator based on these latents. To further promote site-specificity, partial model aggregation is employed over downstream layers of the generator while upstream layers are retained locally. As such, pFLSynth enables training of a unified synthesis model that can reliably generalize across multiple sites and translation tasks. Comprehensive experiments on multi-site datasets clearly demonstrate the enhanced performance of pFLSynth against prior federated methods in multi-contrast MRI synthesis.