1. A method for determining brightness of a medical digital X-ray image comprising: creating an image database; training a feedforward neural network with one hidden layer and sigmoid functions of neurons; using transformed frequencies of image histograms as input arguments for the neural network; using brightness values calculated over a region of interest and scaled to an output value range of the neuron network as target values for the neural network training; acquisition of a current medical digital X-ray image; calculation of a histogram of the current image; transformation of histogram frequencies into input arguments for the neuron network; and calculation of the brightness as a linear function of an output value of the neural network.