Images coming from digital subtraction angiography (DSA) are affected by a perspective distortion due to the use of image intensifiers. As a result, DSA cannot be used for the accurate definition of stereotactic coordinates. A correction method has been developed to enable the use of DSA for the radiosurgery of arteriovnous malformations. A software program and a special phantom tool were employed. The phantom is made by a computer-controlled drilling machine which makes holes in a Plexiglas plate. It has 865 calibration steel spheres with coordinates determined with a precision of 0.01 mm. A calibration image is acquired by a personal computer, and the software calculates the transformation algorithm to superimpose the image on the known positions of the phantom. This algorithm is saved and then recalled to transform the diagnostic images.