1. A method of registering ultrasound image data of an object ( 3 ) and second image data (SD) of the object ( 3 ), wherein the second image data (SD) are three-dimensional (3-D) image data and wherein the method comprises the steps: obtaining sets (first sets) of the ultrasound image data, wherein each set of ultrasound image data consists of ultrasound image data of locations within a single plane ( 36 ) of a coordinate system (X-Y) of the ultrasound image data (ultrasound coordinate system); extracting sets (second sets) of 2-D image data from the second image data (SD), wherein each set of 2-D image data consists of image data of locations within a single cut plane ( 36 ) of a coordinate system (U-V-W) of the second image data; comparing one of the first sets with one of the second sets and automatically generating a measure of similarity between the first set and the second set; and optimising the similarity by varying the first set and/or the second set to be compared.