1. A method for registering two-dimensional image data, comprising the steps of: providing a registered three-dimensional image data set of an object under examination; providing a two-dimensional image data set of the object under examination which is to be registered; generating synthetic two-dimensional image data sets of the object under examination from the three-dimensional image data set, wherein the synthetic two-dimensional image data sets to be generated are parameterized with regard to parameters which describe the two-dimensional image data set to be registered; comparing the parameterized synthetic two-dimensional image data sets with the two-dimensional image data set to be registered, and finding the parameterized synthetic two-dimensional image data set having the greatest similarity to the two-dimensional image data set to be registered; on the basis of this, ascertaining the spatial relationship between the two-dimensional image data set to be registered and the registered three-dimensional image data set, and thus registering the two-dimensional image data set.