Abstract: This invention relates to the fusion of one data set acquired by either two-dimensional or three-dimensional imaging data acquisition means, such as MRI, onto another data set taken by another imaging means which has been mapped into stereotactic coordinates. As an example, an MRI data set which has intrinsic distortion in its dimensions or linearity can be fused with a CT data set which does not have image distortion, the CT data set being rendered in stereotactic coordinates by a stereotactic localizer means that has been attached to the patient's body. In this way, a distorted data set or a non-stereotactic data set such as the MRI image data can be mapped into a non-distorted or stereotactic data set so as to render the MRI data set in stereotactic coordinates and/or to render it with less distortions in its dimensions.