Head Mesh Fiducial Points

In order to coregister the digitizer coordinate system to the MRI coordinate system, three corresponding points on the head mesh of the subject have to be specified. While these three points can be chosen almost arbitrarily, a standard set of points is typically used: the Nasion, the Left Preauricular Point and the Right Preauricular Point. These points are easy to identify on a head mesh as well as on a persons real head and form the basis for coregistration. To specify the fiducials on a head mesh, load the respective surface into the OpenGL window. Open the Mesh Fiducials dialog by clicking Set Fiducials... in the TMS Neuronavigation menu. Then locate the Nasion on the mesh and CTRL-click (CMD-click on Mac OS X) the identified point (see figure below).

You will see a little white sphere appearing close to the mouse tip visualizing the specified point. Note that the coordinates for the nasion are automatically filled in the Mesh Fiducials dialog and that the corresponding Set option has been checked. If you define the three fiducials in the indicated order, you do not have to go to the dialog to manually click the Set option.

After having specified the Nasion, locate the Left Preauricular Point, i.e. by rotating and zooming the head mesh appropriately. Specify the fiducial by CTRL-clicking it. Another white sphere appears indicating the position of this fiducial. The coordinates for the Left Preauricular Point will be again automatically set in the Mesh Fiducials dialog. Continue in the same way for the Right Preauricular Point.

After having specified all three fiducials, you can save them for later use by clicking the Save... button. Provide a fiducial point (.FDP) file name in the appearing Save As dialog,  If you use the same subject later again, you can simply reload the respective .FDP file by clicking the Load... button. You may close the Mesh Fiducials dialog by clicking the Close button.

Note:  The fiducials might not be defined exactly at the point you have clicked. The reason for this behavior is that the program locates the vertex (mesh point), which is closest to the clicked point in order to define a fiducial point. You can fine-adjust the position of a fiducial by changing the values in the coordinates field. Alternatively, you can deselect the Set option of a fiducial, CTRL-click a slightly different location (and probably a different closest vertex) and then check the Set option to use the new coordinates.