Principles and concepts of surgical navigation

Electromechanical systems

Electromechanical systems were the first to be used, being easy to construct. A jib supports an arm comprising several segments articulated to provide 5 or 6 degrees of freedom ( Figure 2.2 ). The articulations usually include a precision potentiometer encoding the angle between each segment. Accuracy was to within a few micrometers, but they required the patient to be totally immobile. Other reasons for their being abandoned were bulk and poor ergonomy.

Ultrasound-beam systems

Ultrasound-beam systems comprise a piezoelectric ultrasonic transmitter fixed on the instrument, generating a signal picked up by a set of at least three microphones remote from the operative field. If sound propagation time in air is known, the relative position of the instrument can be calculated by quadratic triangulation. Unfortunately, sound propagation time is highly dependent on temperature and local hygrometry, leading to cumulative error of as much as several centimeters, even after meticulous calibration. Moreover, interposition of the surgeon’s hands and instruments and any sudden displacement of air induce errors that are hard to detect. These systems have therefore remained at the prototype stage.

Opto-electronic systems

Opto-electronic systems were developed a little later. They are based on a set of two or three high-resolution charge-coupled cameras remote from the operative field. They analyze the spatial projection disparity of an identifiable object such as a set of near-infrared LEDs fixed on the instrument (active system), or microprism-covered spheres reflecting infrared emission from a projector in the camera bay (passive system), so that no cable is needed to feed the LEDs ( Figure 2.3 ). Accuracy is satisfactory, at around half a millimeter for electroluminescent diode systems, and slightly poorer for passive systems.

Some systems use a geometric or colored marker on the tracked instrument. The signal from one or several cameras is processed by a pattern-feature recognition algorithm to search for the marker structure and extract it within the image flow. After extracting the center of each marker, an algorithm determines the spatial position of the target and thus of the instrument.

All optical systems have the drawback of being interrupted whenever an object gets in the line of sight between the camera and what is supposed to be tracked. This makes ergonomics difficult to manage, as the surgeon is not free in positioning aids, instruments and hands. This is particularly critical in operating in deep narrow areas, as in transnasal pituitary adenoma resection. The camera bay is bulky and, above all, fragile, as it contains a system of cylindrical lenses liable to lose focus under shock, impairing accuracy. If these systems are finely suited to the more-or-less static procedures typically encountered in neurosurgery (for which they were originally designed), they are much less suited to dynamic surgery such as ENT craniofacial procedures.

Electromagnetic systems

Electromagnetic systems are a more recent development. They are based on generating an alternating magnetic field on three perpendicular axes. A sensor, comprising coils that are also perpendicular to one another, detects its position and orientation relative to the transmitter according to the voltages induced in each coil ( Figure 2.4 ). As magnetic field strength decreases with the cube of the distance from the source, sensitivity and resolution are excellent, to within a 10 ^th of a millimeter. The technology is very robust, and can be miniaturized to the point where the surgeon can just forget about its presence. Another advantage is that they function without restriction, whatever the position of the surgeon, instruments and aids, as they do not depend upon a line of sight.

The transmitter, measuring 1 or 2 cm ³ , can be fitted directly on the patient, without the kind of reference sensor needed by infrared systems, thus halving the risk of measurement error. The early versions had the major inconvenience of being sensitive to metallic objects, inevitably present in the operative field; this has now been minimized by sophisticated signal generation and processing technology eliminating most metal-induced distortion. This electronic technique can be further combined to algorithms able to predict onset of distortion to a high degree of probability and to correct any such error using a predefined mathematical model such as the finite elements method [ ]. Associating an extra control sensor further increases reliability, warning the user of onset of uncorrected error. This technology now ensures reliability at least as good as that of infrared systems, without their drawbacks.