  Purpose: To develop a general framework for Parallel Imaging (PI) with the
use of Maxwell regularization for the estimation of the sensitivity maps (SMs)
and constrained optimization for the parameter-free image reconstruction.
  Theory and Methods: Certain characteristics of both the SMs and the images
are routinely used to regularize the otherwise ill-posed optimization-based
joint reconstruction from highly accelerated PI data. In this paper we rely on
a fundamental property of SMs--they are solutions of Maxwell equations-- we
construct the subspace of all possible SM distributions supported in a given
field-of-view, and we promote solutions of SMs that belong in this subspace. In
addition, we propose a constrained optimization scheme for the image
reconstruction, as a second step, once an accurate estimation of the SMs is
available. The resulting method, dubbed Maxwell Parallel Imaging (MPI), works
seamlessly for arbitrary sequences (both 2D and 3D) with any trajectory and
minimal calibration signals.
  Results: The effectiveness of MPI is illustrated for a wide range of datasets
with various undersampling schemes, including radial, variable-density
Poisson-disc, and Cartesian, and is compared against the state-of-the-art PI
methods. Finally, we include some numerical experiments that demonstrate the
memory footprint reduction of the constructed Maxwell basis with the help of
tensor decomposition, thus allowing the use of MPI for full 3D image
reconstructions.
  Conclusions: The MPI framework provides a physics-inspired optimization
method for the accurate and efficient image reconstruction from arbitrary
accelerated scans.
