Introduction
Fluoroscopic navigation systems allow localization of the position of a surgical instrument relative to the anatomy of the patient [1, 2]. By superimpos- ing the instrument's geometry onto fluoroscopic images, the surgeon can follow live the progres- sion of the intervention.
Navigation Technology
The most common navigation technology used is based on infra-red tracking. A camera is used, which is equipped with two infra-red image sen- sors, where each one produces a two-dimensional image. By synchronizing the two sensors (stereo- graphic principle), the three-dimensional position of a tool can be localized.
Tools to be tracked emit infra-red light, which is detected by the camera. In order to track posi- tion and orientation, a tool is equipped with at least three infra-red sources. One distinguishes between active systems and passive systems.
Active systems consist of tools with light-emit- ting diodes. They actively send infra-red light, which is tracked by the camera. Passive systems consist of tools with infra-red reflecting balls. The camera emits infra-red light, which is reflected by the tool and again tracked by the camera.
The first generation of navigation systems had active tools, which were powered by cables, result- ing in handling limitations. Passive tools were in- troduced with the second generation of navigation systems. They do not need cables but suffer from ambiguity by partially covered or overlapping re- flecting objects and visibility problems when the reflecting surfaces are soiled or partially ob- scured. The latest generation of trackers is based on active and wireless tools, which are battery powered. They combine the advantages of both technologies. In addition, these trackers are equipped with on-board electronics and bidirec-
tional communication with the navigation system, which allows for automatic instrument detection and software remote control.
Principle of Fluoro-Navigation
The basic components of a fluoroscopic naviga- tion system are: infra-red camera, computer with monitor, patient tracker, instrument tracker, and C-arm tracker (Fig. 6.1.1).
Prior to the procedure, a C-arm tracker is at- tached to the image intensifier (Fig. 6.1.2). Beside tracking the position of the image intensifier, the C-arm tracker also compensates for image distor- tions and X-ray source shifts due to mechanical flex. Small metal balls are integrated into a plate (phantom), which is positioned in front of the imaging plane. In the acquired images the soft- ware detects the position of these balls, compares them with the measured positions and corrects for any image distortions.
During the procedure a patient tracker is ri- gidly attached to the bone that is going to be treated (Fig. 6.1.3). Different anchoring systems are available based on single or multiple screw fixations (Fig. 6.1.4).
As soon as a fluoroscopic image is taken, it is automatically transferred to the monitor of the navigation system via a standard video signal cable. At the same time, the camera localizes the position of both the image intensifier and the pa- tient tracker. This allows the image reference to be transferred from the image intensifier to the patient tracker. This step is called registration.
The uniqueness of fluoroscopic navigation is that registration is done automatically. The C-arm can now be removed from the surgical field without losing the image reference. Usually, two images from two perpendicular views are taken (e.g., anteroposterior and lateral), which allows for a three-dimensional determination of the instru- ment's position.
CHAPTER 6.1
Basic Principles of Fluoro-Navigation
A. Sarvestani
Fig. 6.1.1. Components and setup for a fluoroscopic navigation surgery (infra-red camera, computer with monitor and C-arm tracker)
Fig. 6.1.2.Example of a C-arm tracker
A tracker is attached to the surgical instrument followed by calibration of the assembly (Fig.
6.1.5). Calibration is necessary to determine the instrument's geometry relative to the tracker. The instrument tip is inserted into the calibration de- vice. At the same time the navigation system tracks the position of both devices, determines the position of the instrument tip and stores it either in the instrument itself or in the software.
After calibration the position and orientation of the instrument are superimposed simultaneously on all acquired images. The surgical intervention can now be monitored live on the screen (Fig.
6.1.6).
A very important feature of all fluoroscopic navigation systems is the virtual tip elongation.
The displayed instrument is extended by a user- defined value. This allows performing on-the-fly planning and measurements.
Fluoroscopic navigation systems also allow tracking and displaying the current position of the image intensifier relative to the acquired images (Fig. 6.1.7). By this the C-arm can be quickly maneuvered to the targeted field of inter- est without the need for taking extra shots.
Chapter 6.1 Basic Principles of Fluoro-Navigation 245
Fig. 6.1.3.Example of a pa- tient tracker attached to the iliac crest
Fig. 6.1.4.Example of an anchoring system based on a rota- tion safe single screw
Fig. 6.1.5.Calibration of a navigated power drill
Fig. 6.1.6.Example of hip screw navigation on a frontal and a lateral view. The blue element represents the actual position of the instrument while the yellow element represents the virtual tip extension
Fig. 6.1.7.C-arm guidance: the yellow circle represents the actual position of the image intensifier
Future Challenges
One of the main challenges of fluoroscopic navi- gation is to reduce the invasiveness of bone an- choring devices and the time to prepare and at- tach them.
To bring current fluoroscopic navigation sys- tems to the next level, dedicated application work- flows are needed, which guide the surgeon step by step through the application.
Current navigation systems focus on instru- ment navigation only. The ability also to navigate bone fragments will close an important gap in trauma surgery. In addition implant databases are needed optimally to select and position implants.
Smart instruments will eliminate the need for instrument calibration and will reduce user inter- action with the navigation software. Those instru- ments will contain integrated RF-ID tags, which store information about their geometry and func- tionality. The moment the surgeon starts using a
navigated instrument, the software automatically progresses to the corresponding screen and the instrument's geometry is loaded.
Recently, new navigation systems have been de- veloped to be coupled to three-dimensional C- arms to fully explore the advantages of intraoper- ative three-dimensional imaging.
All future development must improve the inte- gration of navigation into the operating room en- vironment to achieve a quick setup, an intuitive usage and a high robustness.
References
1. Hofstetter R, Slomczykowski M, Sati M, Nolte LP (1999) Fluoroscopy as an imaging means for computer-assisted surgical navigation. Computer Aided Surgery 4:65±76 2. Suhm N, Jacob AL, Nolte LP, Regazzoni P, Messmer P
(2000) Surgical navigation based on fluoroscopy ± clini- cal application for computer-assisted distal locking of intramedullary implants. Computer Aided Surgery 5:
391±400
Chapter 6.1 Basic Principles of Fluoro-Navigation 247
