The image quality, of an image reconstructed by a computed tomography (CT) system, is presented as a factor depending on the standard deviation of the reconstructed image. The main factors affecting the image quality are classified to three main groups depending on the CT system (hardware), the CT algorithm (software), and the scanning parameters. Each of these groups is studied, explained, and applied on a real scan. The images reconstructed of an object (model of a solid propellant rocket motor) by varying the type of detector, the collimator, the CT algorithm, the image resolution, and the scanning parameter are presented and compared by calculating the image quality factor.