As its name indicates, this space is associated with the surgical process, where the volume is probed using a digitizing device. At the MNI, a mechanical passive arm is used (see Chapter 1), which allows the three-dimensional coordinates of the location of the tip of the probe to be communicated to the computer. The coordinate system in surgical space is defined by this digitizing device, with the coordinates of a specific point on the patient depending on the relative position between the patient and a reference point associated with this device (for the FARO arm, this reference point is situated at its base). Many varieties of localizing devices have been described in the literature: mechanical [15,64,52,17,43], ultrasonic [49,3], optical [26,5] and magnetic [33].
Mechanical localizers are generally articulated passive arms that can be manipulated by the surgeon. This type of device typically uses potentiometers to determine the angle of flexion of each joint, and from this information, computes the position of the pointer situated at the end of the arm. This type of device can be subject to failure of the mechanical components to which it largely relies and can lose its accuracy after repeated use due to deterioration of these components.
Optical devices use a set of cameras to track the position of light-emitting diodes (LED) by triangulation. This type of digitizer requires that the path between the cameras and the LEDs not be obscured, a condition that can be sometimes difficult to satisfy in the OR. Optical tracking systems are however considered to be more accurate than mechanical arms.
The principle of ultrasonic localizing devices is to determine the position of an ultrasound emitter from the time delay between the emission and the reception of pulses by a set of receivers. The accuracy of type of system, relying on the constance of the speed of sound in air, will be affected by temperature variations. Echo can also constitute a significant accuracy-limiting factor for these systems.
Magnetic localizers use magnetic field gradients, surperimposed on the operating field, that can be detected by a probe. The various magnetic field perturbations coming from ferromagnetic surgical instruments and from radiation from electronic equipment seriously limits the accuracy of such devices in the OR.