A medical data processing method and system determines the position of an artifact in patient image data describing a set of tomographic slice images of an anatomical structure of a patient. The images are described by color Values. Color value difference data describing differences in color values for image elements in adjacent slice images is determined. At least one of positive or negative difference data, describing a subset of the differences and consisting of differences having a positive or negative value are determined. Smoothed difference data describing a smoothing of the differences contained in the positive or negative difference data are determined and, based on the positive or negative difference data and the smoothed difference data, artifact position data is determined describing the position of an artifact in the patient image data.