Surgical navigation:

For intra-operative navigation, a real-time, Light Emitting Diode-based (LED), frameless, stereotactic device developed in the laboratory was used. Prior to surgery, a dynamic reference frame carrying three LED's (Image Guided Technologies, Inc.) is fixed next to the patient's head. A series of points from the patient's skin are recorded using an LED probe (Image Guided Technologies, Inc.). The information is tracked using an optical digitizer (Flashpoint 5000; Image Guided Technologies, Inc.). The points recorded in real space are then matched to the 3D model in two stages. An initial alignment is obtained by recording the real-space location of three points, then manually matching those points on the MRI model. This initial registration is refined by finding the optimal transformation that aligns all of the points onto the skin surface of the model [6,14]. After registration the surgeon points to a known area on the patient using the LED probe, to confirm correspondence between the patient and the 3D model and MRI slices. During surgery, the surgeon uses a sterile LED probe to select any point on or inside the patient's brain. Any point may be recorded and displayed on the 3D model using an arrow or simple dot (see Figure 2).

  

Figure: 3D reconstruction showing the position of the subdural electrodes. 96 electrodes were applied directly to the cortical surface, including a posterior and an anterior interhemispheric strip. The parietal and frontal electrodes were localized using the trackable probe. The interhemispheric electrodes were segmented from an internal scan.