1. A method for determining brightness in a region of interest of a medical digital X-ray image comprising: acquiring the image; calculating a histogram of the image; converting the histogram values into input arguments for a neural network; and calculating the brightness using the neural network; wherein the histogram values are calculated using a class interval; further comprising normalizing the histogram values to unity before using the histogram values as input arguments for the neural network; wherein the brightness is calculated as a linear function of an output value of the neural network; further comprising neural network learning using a training set calculated using an image database; further comprising using, as a set of target values, a plurality of brightness values calculated for a region of interest of each image and scaled to a range of a neuron activation function on an output layer of the neural network.