and they are simultaneously digitized in the image data. When registration (spatial correlation) is com- plete, the computer can calculate the spatial position of the instrument in the 3D model and display it in relation to the preplanned trajectory and the target- ed surgical site. It must also be possible to continual- ly update changes in the patient’s position (“reregis- tration”), because movements during the operation cannot be prevented and are even necessary for some maneuvers. This is accomplished with an LED array that is securely attached to the patient. Generally a metal clamp with three steel prongs is mounted on the patient’s head for this purpose – a procedure that is done routinely in almost all neurosurgical opera- tions. When the setup is complete, the surgeon can move a tracking instrument during the operation while watching the corresponding movements of the instrument tip on the monitor. Any rigid surgical in- strument such as a drill, chisel, or even an endoscope or the focus of an operating microscope can be tracked in this way, providing a means of intraopera- tive navigation.
Intraoperative navigation is comparable to the navi- gation systems used in automobiles. While the posi- tion of an automobile is determined by satellite re- ceivers that track waves emitted from the vehicle, an optical-based intraoperative navigation system uses infrared cameras to detect the light waves emitted by LEDs mounted on the surgical instruments. Road maps are analogous to the CT or MRI data sets that are acquired from the patient prior to the operation.
To calibrate the system, it is necessary to define a starting position so that the virtual patient on the monitor corresponds anatomically to the real patient on the operating table. This is done by using refer- ence points that can be uniquely identified on the pa- tient and can be located in the data set. This process of correlating the patient’s images to the patient’s ac- tual anatomy is called registration. Usually it is based on three non-coplanar reference points that can be uniquely identified in the image data and on the skull of the patient. At the start of the operation, these ref- erence points are sequentially touched with a localiz- ing system, such as an instrument fitted with LEDs,
Intraoperative Navigation
Chapter 4 9
