A medical data processing method of determining a virtual image of a part of a patient's body, the method being constituted to be executed by a computer and comprising the following steps: a) acquiring patient image data describing at least two two-dimensional patient slice images of anatomical body parts of the patient's body; b) acquiring atlas data describing a general structure of at least part of the patient's body comprising correspondence parts corresponding to the anatomical body parts; c) acquiring correspondence part position data describing the positions of the correspondence parts in the general structure; d) determining (S4.1, S4.2, S4.3), based on the patient image data and the atlas data and the correspondence part position data, virtual image data describing at least two two-dimensional virtual images being supplement slice images of a part (21, 22) of the general structure having a position in the general structure which lies between the positions of the correspondence parts, the method being characterised by e) comparing the virtual images with the patient images; and f) determining, based on comparing the virtual images with the patient images, which one of the virtual images matches the patient images best, wherein, if it is determined that none of the virtual images matches the patient images within a predetermined degree of accuracy, an imaging transformation comprising at least one of a rotational and a translational positional transformation is applied to the virtual images such that at least one of the virtual images (21, 22) matches the patient images within the predetermined degree of accuracy, followed by g) updating (S7.2) the description of the general structure by the atlas data based on the virtual image data. a) acquiring patient image data describing at least two two-dimensional patient slice images of anatomical body parts of the patient's body; b) acquiring atlas data describing a general structure of at least part of the patient's body comprising correspondence parts corresponding to the anatomical body parts; c) acquiring correspondence part position data describing the positions of the correspondence parts in the general structure; d) determining (S4.1, S4.2, S4.3), based on the patient image data and the atlas data and the correspondence part position data, virtual image data describing at least two two-dimensional virtual images being supplement slice images of a part (21, 22) of the general structure having a position in the general structure which lies between the positions of the correspondence parts, e) comparing the virtual images with the patient images; and f) determining, based on comparing the virtual images with the patient images, which one of the virtual images matches the patient images best, wherein, if it is determined that none of the virtual images matches the patient images within a predetermined degree of accuracy, an imaging transformation comprising at least one of a rotational and a translational positional transformation is applied to the virtual images such that at least one of the virtual images (21, 22) matches the patient images within the predetermined degree of accuracy, g) updating (S7.2) the description of the general structure by the atlas data based on the virtual image data.