xensor provides a new approach to 3d electrode digitization for clinical and cognitive research. The electrode digitizer system uses a very accurate technique based on infrared light to track tools (like a digitizer pen or head tracker). The system takes care of the electrode digitization procedure and records, visualizes and stores the digitized electrode positions. The sequence of the digitization procedure is pre-defined by the user. Positions are acquired by locating the respective electrode at the scalp, placing the digitizer pen at that location and pressing a button. Markers (nasion, left and right pre-auricular points) are digitized as well. This way, the electrode positions are transformed to the nasion-ear coordinate frame.

The Xensor electrode digitizer system is based on the NDI VicraTM system. This position sensor hardware uses infrared light to track tools (like a digitizer pen or head tracker).

The position of the digitizer pen is update in real time. If the digitizer pen is located at the correct position and this electrode position is digitized, it will immediately become visible in the 3D view. Also, all previously digitized electrode positions will be shown.

Using only the three fiducial markers for the transformations between the tool-based coordinate system and the MRI coordinate system based on the subject's MRI and head model can result in high inaccuracies. In order to minimize these inaccuracies the following step is measuring additional points from the subject's head using the pointer.

A reference tool is either mounted on the EEG cap or a flexible head band that comes with the Xensor system. This allows us to track every head movement, so that head fixation isn't necessary.

Three fiducial positions are required in order to create a reference coordinate system. The points are the nasion, left and right ear point (preauricular points). The nasion-ear-coordinate system is defined such that x points to the nasion, y to the left ear and z to the top of the head.