Spatial resolution of x-ray imaging systems is crucial in small animal studies because of the small size of the subjects. A digital x-ray imaging system for small animal studies was specially designed to achieve such required resolution. The system features a small x-ray focal spot of 20 micrometers , the use of magnification radiography to improve the resolution, and a fiber-optics taper-coupled CCD detector with a 24-micrometers pixel size. The aim of this study is to characterize the system in terms of its spatial resolution and the effect of the focal spot size on spatial resolution. The spatial resolution of the system was determined from observer-based measurements of x-ray phantom images and from modulation transfer function calculation. The measured spatial resolution values were close to the theoretical values at low magnification factors as expected. At higher magnification factors, the measured spatial resolution values showed noticeable deviations from the theoretical values due to the Penumbra effect created by the use of magnification radiography. The Penumbra effect was characterized in our study. The advantages of this system such as small focal spot size and high resolution make it a suitable system for small animal studies.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.