1. A medical system for determining information describing a probable position of a neural fibre in at least part of a patient's brain, the system comprising: at least one computer having at least one processor that executes instructions stored on a computer readable storage medium to determine information describing a probable position of the neural fibre in the at least one part of the patient's brain, the instructions operable to cause the computer to: acquire, at the at least one processor, patient-specific medical image data describing the at least part of the brain of the patient; acquire, at the at least one processor, atlas data defining an image-based model of at least part of a human brain; determine, by the at least one processor and based on the patient-specific medical image data and the atlas data, seed region data describing seed regions in the patient-specific medical image data in which the ends of neural fibres of the patient's brain may be located, wherein the seed region data is determined by determining a transformation between the atlas data and the patient-specific medical image data, and determining the position of white brain matter in the patient-specific medical image data based on the transformation, wherein the seed regions are at least two spatially disjunct and individually coherent seed regions and are constituted by white brain matter; determine, by the at least one processor and based on the patient-specific medical image data and the seed region data, neural fibre tract data describing a plurality of potential tracts which a specific neural fibre may take through the patient's brain; determine, by the at least one processor and based on the atlas data and the neural fibre tract data, a figure of merit for each one of a plurality of potential tracts; wherein the positions of a plurality of spatially disjunct and individually coherent regions of white brain matter are determined in the patient-specific medical image data based on the transformation, and wherein the coherent regions of white brain matter are sorted according to the geometric properties of model neural fibre tracts in the image-based model connecting regions of white brain matter and according to the probability of the respective model neural fibre tracts to contain fibre crossings, and wherein regions of white brain matter having a high probability for containing a fibre crossing are selected as the seed regions for determining the neural fibre tract data.