Image-Guided Surgery in the Paranasal Sinuses

Siow Jin Keat and Martin J. Citardi

In 1996, the International Society for Computer-Aided Surgery proposed: “The scope of computer-aided surgery (CAS) encompasses all fields within surgery, as well as biomedical imaging and instrumentation, and digital technology employed as adjunct to imaging in diagnosis, therapeutics and surgery…”. The term image-guided surgery (IGS) has been used to describe the specific CAS application of intraoperative surgical navigation.

• A computer-based system to track the position of an instrument tip that is projected as a virtual point in crosshairs on to a preoperative computed tomography (CT) scan rendered in triple orthogonal projections (coronal, axial, sagittal) simultaneously.

• With IGS, the preoperative CT scans in the coronal, axial, and sagittal planes are projected on a computer monitor during surgery, and the position of the instrument tip, which is tracked dynamically, is shown relative to preoperative CT scans.
  
• IGS facilitates preoperative surgical planning by providing a better CT scan “road map” as it provides three simultaneous orthogonal planes for interpretation instead of the conventional single coronal CT scan view. The surgeon may directly relate the surgical anatomy of the preoperative sinus CT with the intraoperative anatomy of the surgical field.

• The IGS computer tracks the movement of a mathematically designated virtual point in space relative to a frame of reference via electromagnetic sensors or a camera array that detects infrared light. The tip of the instrument is not directly tracked, but its constant geometric position relative to the tracking instrumentation attached to the instrument is used to project the virtual point on the computer monitor.
  
• The computer system displays both the preoperative sinus CT and the virtual point relative to the same reference frame, thus allowing the surgeon to interpret where the virtual point is in relation to the patient’s sinuses.
  
• It should be noted that this reference frame may be a rigid physical structure (as in traditional framed stereotaxy) or a virtual frame composed of fiducial points (as in most sinus surgery applications).

1. Computer workstation
   
2. Display system
   
3. Tracking system
   
4. Surgical instruments that can be tracked
   
5. Data transfer hardware
   
6. Integrating software

The computer workstation interprets the mathematical spatial model of tracking that is projected onto a display system (liquid crystal display [LCD] or cathoderay tube [CRT] monitor). The tracking system feeds information of movement of surgical instruments that are tracked to the computer workstation. In most systems, the tracking system also monitors the position of the surgical field. Data transfer hardware is required to transfer the preoperative CT scans into the computer workstation in the operating room. Such hardware may be a network system connection as well as magnetic or optical disk drives. Most commonly, data transfer of preoperative CT scan image data utilizes the Digital Imaging and Communications in Medicine (DICOM) standard. Integrating software allows for manipulation of CT images (e.g., window width and level adjustments for bone and soft tissue windows). Software may also allow coloring of a specific anatomical region such as the orbit and projecting the colored orbit in the display system during intraoperative surgical navigation for better identification. On some systems, virtual endoscopy, much like virtual colonoscopy, may also be performed by software manipulation pre- and intraoperatively.

Currently available IGS systems for sinus surgery include:

Work by detection of movement of a sensor within an electromagnetic field

• Advantage
  
• No line-of-sight problems
  
• Disadvantage
  
• Magnetic objects (including most surgical instruments and devices) disturb the accuracy of tracking.
  
• Present System Available
  
• General Electric Medical Systems Navigation and Visualization: InstaTrak and ENTrak

Work by detection of movement of an infrared emitter or reflector by an infrared camera array

• Advantages
  
• Tracking not influenced by magnetic objects
  
• When system is actively tracking, tracking error is submillimetric.
  
• Disadvantage
  
• Line-of sight-problems (the surgeon must maintain line of sight between the camera array and infrared emitter/reflector on the surgical instrument)
  
• Present Systems Available
  
• BrainLab (Heimstetten, Germany): Vector Vision and Kolibri
  
• Medtronic Xomed (Jacksonville, Florida): LandMarX and StealthStation
  
• Stryker Corporation (Kalamazoo, Michigan): Navigation System

Calibration Process of establishing or confirming the relationship between a tracker and the tip of an instrument.

Dynamic reference frame A tracker attached to the patient that serves as the reference for all localizations.

Operative field volume