A method for defining image quality characteristics of X-ray based medical projection imaging devices is provided. A spatial frequency-dependent signal-to-noise ratio function includes image quality parameters of spatial resolution, object contrast and noise. The detectability of an object embedded into a defined background, such as a cardiac guide wire in a patient is determined. An X-ray system may be defined and set up for obtaining an optimized image quality to determine the best object detectability for a given patient dose.