Tracking is the process by which objects are dynamically localized in the patient's coordinate system. Of particular interest to us is the tracking of medical instruments and the patient's head. Optical trackers use multiple cameras to triangulate the 3D location of flashing LEDs that may be mounted on any object to be tracked. Such devices are generally perceived as the most accurate, efficient, and reliable localization system [2,4]. Other methods, such as acoustic or magnetic field sensing are being explored as well, but tend to be more sensitive to environmental effects. We use optical tracking, the Flashpoint system by IGT Inc., Boulder, CO, USA, due to its accuracy and ease-of-use benefits.
Tracking patient head motion is often necessary since the head is not always clamped to the operating table, the head may move relative to the clamp, the operating table may be moved, or the hardware performing the tracking may be moved to rearrange lights or other equipment in the operating room. Although not all image-guided surgery systems account for patient motion, [1,2,5,15,18] solve this problem by attaching trackable markers to the head or clamp. We also currently utilize an optically trackable configuration of markers attached to a Mayfield clamp (Figure 3). We have also experimented with directly attaching trackable LEDs to the skin surface of the patient. Our experience is that while in most cases this worked well, it required that the surgeon carefully plan the location of the LEDs to ensure that they did not move between initial placement and opening of the skin flap.
We require direct line-of-sight from the Flashpoint cameras to the LEDs at times when the surgeon requires image-guidance. In order to maintain such line-of-sight we can re-locate the scanning bar such that it is both out of the way of the surgeon but maintains visibility of the LEDs. Such dynamic reconfiguration of the scanning bar is a benefit of the head tracking process.
Instrument tracking is performed by attaching two LEDs to a sterile pointer. The two LEDs allow us to track the 3D position of the tip of the pointer as well as its orientation, up to the twist angle which is not needed for this application. Figure 3 shows the surgeon using the trackable pointer in the opened craniotomy.
