1. A method for providing an estimation of a mean signal to noise ratio value for an output image comprising a number of image pixels, each image pixel of the output image having a pixel value, the method comprising: providing an image histogram divided into bins, the image histogram comprising information regarding the distribution of pixel values among the image pixels of the output image, each bin comprising a set of image pixels having pixels values within a predetermined range; attributing a signal to noise ratio value for each bin of the image histogram; weighting, for each bin of the image histogram, the signal to noise ratio value of that bin with the number of image pixels in the set of image pixels of that bin; summing the weighted signal to noise ratio values of the bins; and dividing the sum of weighted signal to noise ratio values of the bins by the total number of image pixels of the output image or the total number of image pixels in the bins of the image histogram.