Apparatus, comprising: a magnetic sensor; a magnetic tracking system configured to measure locations of the magnetic sensor; a probe comprising the magnetic sensor that is configured to measure magnetic-system-positions of the probe in the system; and a processor, configured to: receive a computerized tomography image comprising voxels of a head of a subject; generate a surface image of the head of the subject; find the highest point of the surface image that is above the coronal plane of the subject, wherein the highest point of the surface image corresponds to the nose tip of the subject; analyze the CT image to identify respective locations of a left eye and a right eye of the subject in the CT image, so as to define a first line segment joining the respective locations; identify a voxel subset, comprising voxels, overlaying bony sections of the head, lying on: a second line segment parallel to the first line segment, by selecting voxels of the surface image that are a preset distance, in a direction away from the highest point of the surface image, from the first line segment, and a third line segment orthogonal to the first line segment, by looking for local maxima in the surface image between the highest point of the surface image and the second line segment, wherein the third line segment overlays a nose tip of the subject; activate the magnetic tracking system; receive magnetic-system-positions from the probe of a surface of the head overlaying the bony sections; form a correspondence between the magnetic-system-positions from the probe and the voxel subset; and generate a registration between the CT image and the magnetic tracking system in response to the correspondence. a magnetic sensor; a magnetic tracking system configured to measure locations of the magnetic sensor; a probe comprising the magnetic sensor that is configured to measure magnetic-system-positions of the probe in the system; and a processor, configured to: receive a computerized tomography image comprising voxels of a head of a subject; generate a surface image of the head of the subject; find the highest point of the surface image that is above the coronal plane of the subject, wherein the highest point of the surface image corresponds to the nose tip of the subject; analyze the CT image to identify respective locations of a left eye and a right eye of the subject in the CT image, so as to define a first line segment joining the respective locations; identify a voxel subset, comprising voxels, overlaying bony sections of the head, lying on: a second line segment parallel to the first line segment, by selecting voxels of the surface image that are a preset distance, in a direction away from the highest point of the surface image, from the first line segment, and a third line segment orthogonal to the first line segment, by looking for local maxima in the surface image between the highest point of the surface image and the second line segment, wherein the third line segment overlays a nose tip of the subject; activate the magnetic tracking system; receive magnetic-system-positions from the probe of a surface of the head overlaying the bony sections; form a correspondence between the magnetic-system-positions from the probe and the voxel subset; and generate a registration between the CT image and the magnetic tracking system in response to the correspondence. receive a computerized tomography image comprising voxels of a head of a subject; generate a surface image of the head of the subject; find the highest point of the surface image that is above the coronal plane of the subject, wherein the highest point of the surface image corresponds to the nose tip of the subject; analyze the CT image to identify respective locations of a left eye and a right eye of the subject in the CT image, so as to define a first line segment joining the respective locations; identify a voxel subset, comprising voxels, overlaying bony sections of the head, lying on: a second line segment parallel to the first line segment, by selecting voxels of the surface image that are a preset distance, in a direction away from the highest point of the surface image, from the first line segment, and a third line segment orthogonal to the first line segment, by looking for local maxima in the surface image between the highest point of the surface image and the second line segment, wherein the third line segment overlays a nose tip of the subject; a second line segment parallel to the first line segment, by selecting voxels of the surface image that are a preset distance, in a direction away from the highest point of the surface image, from the first line segment, and a third line segment orthogonal to the first line segment, by looking for local maxima in the surface image between the highest point of the surface image and the second line segment, wherein the third line segment overlays a nose tip of the subject; activate the magnetic tracking system; receive magnetic-system-positions from the probe of a surface of the head overlaying the bony sections; form a correspondence between the magnetic-system-positions from the probe and the voxel subset; and generate a registration between the CT image and the magnetic tracking system in response to the correspondence.