N3 Bias Field Correction Explained as a Bayesian Modeling Method

Abstract. Although N3 is perhaps the most widely used method for
MRI bias field correction, its underlying mechanism is in fact not well
understood. Specifically, the method relies on a relatively heuristic recipe
of alternating iterative steps that does not optimize any particular objective
function. In this paper we explain the successful bias field correction
properties of N3 by showing that it implicitly uses the same generative
models and computational strategies as expectation maximization (EM)
based bias field correction methods.We demonstrate experimentally that
purely EM-based methods are capable of producing bias field correction
results comparable to those of N3 in less computation time.