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, the neural network being an artificial feedforward neural network having one hidden layer and an output layer consisting of one neuron and having sigmoid neurons' activation functions; wherein the histogram values are calculated using a bin width; 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.