A computer system and corresponding method are used to assist in planning a trajectory for a surgical insertion into a skull to a target representing an anatomical region. The computer system is configured to: provide the computer system with a three-dimensional image representation of the skull and brain which has been parcellated into anatomical regions, including an identification of critical objects comprising structures within the brain to be avoided during the surgical insertion; provide the computer system with a region of interest comprising an anatomical region within the brain representing the target of the trajectory for the surgical insertion; determine a metric for voxel locations within the anatomical region corresponding to the region of interest, the metric representing the suitability of each of the voxel locations to be a target location for the trajectory; select a set of one or more voxel locations having the greatest suitability according to the metric within the region of interest, each of the one or more selected voxel locations representing a potential target location for the trajectory; and identify a trajectory for the surgical insertion to a potential target location in the region of interest.