1. A magnetic resonance imaging system comprising: a denoising unit configured to denoise a partially reconstructed image and provide a measure of a denoising effectivity, the measure including a matrix of values which represent the amount of noise removed for each partially reconstructed image location, the denoising effectivity comprising an estimate of a local amount of information relating to anatomical structure that has been removed, or damaged, or affected by the denoising unit; and a reconstruction unit configured to iteratively reconstruct received parallel magnetic resonance (MR) data with a parallel imaging reconstruction technique which generates partially reconstructed images and denoises the partially reconstructed images with the matrix during the iterative reconstructing to generate an output image.