The invention relates to a method of registering ultrasound image data of an object and second image data of the object, wherein the second image data are three-dimensional (3-D) image data. Sets (first sets IUS) of the ultrasound image data are obtained, wherein each set of ultrasound image data consists of ultrasound image data of locations within a single plane of a coordinate system of the ultrasound image data. Sets (second sets IEX) of 2-D image data from the second image data are extracted, wherein each set of 2-D image data consists of image data of locations within a single cut plane of a coordinate system of the second image data (unit 41). One of the first sets (IUS) is compared with one of the second sets (IEX) and a measure of similarity between the first set (IUS) and the second set (IEX) is automatically generated (unit 42). The similarity is optimised by varying the first set (IUS) and/or the second set (IEX) to be compared (unit 43).