Inner-Loop-Free ADMM for Cryo-EM

Thanks to recent advances in signal processing, the interest for fast ℓ1-regularized reconstruction algorithms in cryo-electron microscopy (cryo-EM) has intensified. The approaches based on the alternating-direction of multipliers method (ADMM) are particularly well-suited due to the prime convergence speed and flexibility of use of this algorithm. Yet, the standard ADMM scheme still relies on a nested conjugate gradient (CG) to solve the linear step in its alternating-minimization procedure, which can be costly when handling large-scale problems. In this work, we present an inner-loop-free ADMM algorithm for 3D reconstruction in cryo-EM. By using an appropriate splitting scheme, we are able to avoid the use of CG for solving the linear step. This leads to a substantial increase in algorithmic speed, as demonstrated by our experiments.