We present a software tool for the simulation of an X-ray imaging systems. It consists of three virtual objects: the X-ray source, the human body and the detector. The X-ray source is modeled as a radiological tube for which the user can modify the tube potential, the anode material, the tube load, the filtration and some geometric parameters, such as source-skin distance, orientation and field size. The virtual body consists of a 3D voxel matrix in which CT numbers for each point of the body are stored, obtained from tomographic slices. The interactions of X-rays passing through the body are evaluated using pencil beam technique. The image is obtained computing the dose absorbed by the detector and converting it into optical density by the use of a proper response function. The dose absorbed in each point of the body is also computed and can be visualized both in 2D and 3D representations. The influence of each parameter on the beam spectrum, on the image quality and on the dose to the patient can be observed interactively.