1. A method for determining a position of a region of interest serving as a start condition for conducting diffusion image-based tracking of nerve fibers, the method executing on a processor of a computer, and comprising: a) acquiring, at the processor, medical image data describing a medical image of an anatomical body part of a patient's body comprising the nerve fibers, the medical image generated by applying a diffusion-based medical imaging method to the anatomical body part; b) acquiring, at the processor, actual region size data describing the actual size of the region of interest; c) acquiring, at the processor, predetermined region size data describing a predetermined minimum size of the region of interest; d) determining, by the processor and based on the actual region size data and predetermined region size data, whether the region of interest has at least the predetermined size, and e) if step d) results in a determination that the region of interest has at least the predetermined size, e1) acquiring, at the processor, region position data describing the position of the region of interest in the anatomical body part described by a position of a virtual tool in the medical image; e2) determining, by the processor and based on the medical image data and the region position data, fiber tracking data describing a result of tracking a nerve fiber running through the region of interest; e3) determining, by the processor and based on the fiber tracking data and the actual region size data, whether the actual size of the region of interest is smaller or larger than a desired size of the region of interest required to achieve a desired result of the tracking, and if it is determined that the actual size is smaller than the desired size, e3a) acquiring, at the processor, changed region position data describing a new position of the region of interest different from the position described by the region position data; e3b) determining, by the processor and based on the medical image data and the changed region position data, changed region tracking data describing a result of tracking a nerve fiber running through the region of interest at the position described by the changed region position data; e3c) determining, by the processor and based on the changed region tracking data and the changed region position data, whether the position of the region of interest described by the changed region position data shall be added to the position of the region of interest described by the region position data, and if it is determined that the position of the region of interest described by the changed region position data shall be added to the position of the region of interest described by the region position data, e3c1) adding, by the processor, the position of the region of interest described by the changed region position data to the position of the region of interest described by the region position data, and continuing, by the processor, execution of the method with step d); if it is determined that the position of the region of interest described by the changed region position data shall not be added to the position of the region of interest described by the region position, e3c2) continuing, by the processor, execution of the method with step d); if it is determined that the actual size is not smaller than the desired size, e3d) determining, by the processor, whether the actual size is larger than the desired size, and if it is determined that the actual size is larger than the desired size, e3d1) acquiring, at the processor, changed region position data describing a new position of the region of interest which at least partly overlaps the position described by the region position data; e3d2) determining, by the processor and based on the medical image data and the changed region position data, changed region tracking data describing a result of tracking a nerve fiber running through the region of interest at the position described by the changed region position data; e3d3) determining, by the processor and based on the changed region tracking data and the changed region position data, whether the position of the region of interest described by the changed region position data shall be removed from the position of the region of interest described by the region position data, and if it is determined that the position of the region of interest described by the changed region position data shall be removed from the position of the region of interest described by the region position data, e3d3i) removing, by the processor, the position of the region of interest described by the changed region position data from the position of the region of interest described by the region position data, and e3d3ii) continuing execution of the method with step d); if it is determined that the position of the region of interest described by the changed region position data shall not be removed from the position of the region of interest described by the region position data, e3d3iii) continuing, by the processor, execution of the method with step d); and if it is determined that the actual size is not larger than the desired size, e3d4) ending, by the processor, execution of the method; and f) if step d) results in a determination that the region of interest does not have at least the predetermined size, continuing, by the processor, execution of the method with step e3a).