1. A method for processing a 2D or 3D reconstruction image, recorded by a magnetic resonance device including a gradient coil to generate a gradient field, and distortion-corrected with regard to a given nonlinearity, leading to an image distortion of the gradient field by using an algorithm that processes the measurement signals at various measuring points lying in an imaging volume, the algorithm processing, with respect to each signal processed by the algorithm, a first input value describing a real gradient field given at a real measuring point of the signal, the method comprising: back-transforming the distortion-corrected reconstruction image into a distortion-uncorrected reconstruction image by using the first algorithm.