The localization system is used to translate the position of an instrument in the surgical field into coordinates superimposed on the preloaded preoperative CT images displayed on the computer monitor. The critical component of this localization system is the digitizing sensor or tracker. Four different types of tracking technology have been used: electromagnetic, optical, electromechanical, and sonic. At present the optical and electromagnetic systems are the only two in widespread use.
Low-frequency magnetic fields recognize the position of an instrument via ferromagnetic probes that detect gradients in the magnetic field. The influences of these probes on the magnetic field can be calculated to work out their position. The equipment comprises a head-frame with an electromagnetic transmitter and a suction device with an electromagnetic receiver in its handle. Be aware that any ferromagnetic or paramagnetic object in the surgical field, such as aluminum, can distort the magnetic field. This is rarely a problem, however. The identical headframe needs to be worn for the preoperative CT scan and the operative procedure. This system is easy to use and is widely accepted.
There are two types of optical tracking systems: one active, the other passive. In the active system, the headframe and the instrument have infrared-emitting diodes that are detected by the cameras. These utilize a number of infrared imaging diodes attached to the operating probe or instrument in a distinctive geometric pattern. This system needs the hand-held instrument to be connected by a wire. A passive system does not need a wire as it depends on markers attached to the instrument that are detected by the infrared cameras. A three-camera array system positioned 2 meters from the headset detects the position of these diodes and detects the position of the probe (Klimek et al., 1998; Anon et al., 1997). The headset, which does not need to be worn during the preopera-tive scan, contains a sterile keyboard that is used in-traoperatively. The headset also contains diodes and is therefore crucial for registration of each instrument.
The disadvantage with this system is that a clear line of sight must be maintained between the instrument sensor, the headset, and the camera array. It is vital that the headframe does not slip during the procedure. Instruments available at present do not have infrared imaging diodes placed in a variety of positions that enable them to be used and detected by a camera in one position. This particularly applies to working in the frontal recess. This means that the camera and stand have to be moved so that the signal can be picked up.
Electromechanical systems rely on detectors located within the joints of a table-mounted, position-sensitive, articulated multijoint robotic arm. The position of the probe tip that is connected to the arm is calculated from the arm geometry and information from the joint detectors (Klimek et al., 1998). Problems with this system are that movement of the patient's head affects registration and therefore the head has to be immobilized. The device is also bulky and takes up considerable space in the operating room (Fried and Morrison, 1998).
Sonic systems are based on measuring the time for the sound emitted from several locations to be detected by several microphones. However, temperature differences and humidity affect the speed of sound, and echoes, airflow, and convection currents may diminish the reliability of the system (Anon et al., 1997; Klimek et al., 1998).
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