L2SR: Learning to Sample and Reconstruct for Accelerated MRI

Accelerated MRI aims to find a pair of samplers and reconstructors to reduce acquisition time while maintaining the reconstruction quality. Most of the existing works focus on finding either sparse samplers with a fixed reconstructor or finding reconstructors with a fixed sampler. Recently, people have begun to consider learning samplers and reconstructors jointly. In this paper, we propose an alternating training framework for finding a good pair of samplers and reconstructors via deep reinforcement learning (RL). In particular, we propose a novel sparse-reward Partially Observed Markov Decision Process (POMDP) to formulate the MRI sampling trajectory. Compared to the existing works that utilize dense-reward POMDPs, the proposed sparse-reward POMDP is more computationally efficient and has a provable advantage over dense-reward POMDPs. We evaluate our method on fastMRI, a public benchmark MRI dataset, and it achieves state-of-the-art reconstruction performances.