Image-Guided Surgery

Image-guided surgery (IGS) is the use of a real-time correlation of the operative field to a preoperative imaging data set that reflects the precise location of a selected surgical instrument to the surrounding anatomic structures. Although first developed for neurosurgery, endoscopic sinus surgery (ESS) rapidly became one of the leading indications for this technology. ^[1]

Image-guided surgery is one of the most significant advances in endoscopic sinus surgery since the inception of the endoscopic approach in the mid-1980s. This technology enables the surgeon to follow the anatomic dissection of the sinuses on a computer monitor in the operating room in real time. Difficult anatomic relationships can more easily be understood and treated with the assurance that the critical landmarks are secured. Although the initial expense is substantial, these procedures have minimal per-case costs. The decision whether to use an optical or an electromagnetic system is less critical than the decision to use computer technology. Both systems are widely accepted and provide excellent anatomic information.

IGS should not be considered as a way to palliate lack of experience or understanding of sinonasal surgical anatomy but rather as an adjunctive tool designed for otolaryngologists properly trained in ESS.

In neurosurgery, the primary use of image-guided surgery is to locate an intracranial lesion for resection or biopsy. In endoscopic sinus surgery, the main advantage is to avoid disrupting hazardous areas such as the brain and orbit. The development and rapidly growing popularity of image-guided surgery in sinus surgery are directly attributable to the risks of such disruptions. ^{[2, 3, 4]}

IGS begins with obtaining a CT scan. The CT scan acquisition protocol used for the authors' needs consists of a helical, 2-mm–thickness axial CT scan with the use of a specially designed headset incorporating built-in metallic fiducial land marking. The specially designed headset allows automatic registration of the imaging to the patient's anatomy in the operating room. The imaging data set is transferred via optical disk, CD-ROM, or computer network to the operating room, where it is loaded into the workstation. The images are brought up on the IGS system prior to the procedure and checked for image quality and accuracy.

An image depicting image-guided surgery is shown below.

Today, most CT scanners provide data sets compatible with commonly used guidance systems. Radiology also has the ability to provide the required data on a CD-ROM, or to transfer the images directly from the radiology station to the IGS system through a secured broadband network. These advances, as well as the decreasing cost of the technology, have allowed IGS to be available to a sizable number of otolaryngologists.

Image-guided endoscopic sinus surgery is also reaching increasing acceptance in pediatric otolaryngology, although fewer reports have been published compared with the adult literature. ^[5]

Initially, because of the cumbersome nature of the systems, increased operating time, and expertise required of operating room personnel, image-guided surgery was selected only by tertiary referral centers for revision or unusual sinus cases in which the anatomy was expected to be distorted. ^[6] As experience has been gained, image guidance has become an integral part of surgery for many otolaryngologists performing endoscopic sinus surgery, particularly in the following situations:

• Sinus surgery in the absence of normal landmarks

• Revision sinus surgery ^[7]

• Disease that abuts the skull base ^[8]

• Disease that extends into the frontal or sphenoid sinus

• Dehiscent lamina papyracea

• Orbital pathology

To date, in the authors' institution, more than 1000 computer-assisted ESS procedures have been performed over a period of 10 years. ^[9] Currently, the time from anesthesia induction until completion of registration is less than 5 minutes.

Computer-assisted endoscopic sinus surgery (ESS) has no absolute contraindications except for lack of experience and training. Physicians must be aware that the technique is an adjunct to surgery and does not replace surgical skills and knowledge. ^[10]

The relevant anatomy is that of the paranasal sinuses, orbits, and cranial base and is comprehensively treated in many textbooks such as Stammberger's Functional Endoscopic Sinus Surgery (1991). ^[11]

Anesthesia is induced on a normal operating table. However, when electromagnetic systems are used, a thick foam mattress is needed to keep the patient off of the metal table in order to prevent interference. With systems using optical technology, the image-guided surgery (IGS) unit must be in direct line to the operating room table with no line-of-sight obstruction, ^[12] although recent developments alleviated the latter limitation. ^[13]

Registration

Registration generates a correlation between the position of the instrument in the surgical field and the corresponding location on the CT images. The instruments are registered to show their position with respect to the orthogonal CT images of the patient. The location is materialized by a set of cross hairs on the screen that moves through the CT image data in concordance with the movement of the pointer.

Image-guided surgery, using either of the most recent optical or electromagnetic systems, accommodates for head movement. This has enormous implications for surgeons who prefer local or intravenous sedation. Prior to the advent of the new systems, general anesthesia was necessary to ensure absolute fixation of the head relative to the tracking system (see the first image below). With the newer systems (see the second image below), the headset moves along with the head, so registration is maintained throughout the procedure, although frameless registration can also be performed. Therefore, any anesthetic technique may be used.

With an electromagnetic system, the headset that was worn by the patient during the preoperative CT scan acquisition is again applied on the patient's head in the operating room. This correlates head position with the tracking system.

Testing of accuracy

Accuracy is verified by testing various known landmarks on the patient's face and intranasally to the images on the computer monitor (see the image below). These locations' coordinates are stored and used throughout the procedure to monitor any changes in the accuracy of the device. With most systems, these preliminary steps take less than 2 minutes with the collaboration of trained operating room staff.

Once registered and verified, the system allows the surgeon to verify surgical position on the monitor depicting the preoperative CT scan in 3 dimensions, along with an additional frame displaying the endoscopic view of the procedure (see the image below).

Although technical advances have allowed navigation devices to be attached to virtually any sinus surgery instrument, the true value of the technology is that it maps out difficult anatomy at critical points in the surgery. Clearing the operative field of blood and debris and then using either a tracking pointer or a tracking curved suction to elucidate anatomic questions is most prudent for the surgeon, who may then proceed safely with the next step of surgery. The following are examples of critical points when computer guidance is of greatest assistance during surgery:

• Localizing a difficult frontal sinus

• Localizing a small sphenoid sinus

• Delineating a skull base contour during a revision procedure

• Distinguishing smooth-walled peripheral cells from surrounding landmarks

These localization maneuvers should be performed with diagnostic instruments, not surgical instruments. The use of a microdebrider with tracking is of exceptional benefit while removing nasal and sinus polyp disease.

The following observations of IGS have been consistent since the earliest cases in the authors' institution :

• Image-guided surgery assists the experienced surgeon in delineating ambiguous or distorted landmarks. It is not a replacement for thorough anatomical training

• Accuracy within 2 mm is the norm

• Based on experience with both of the currently used systems, registration can be accomplished with minimal additional operating room time

• Inconveniences related to the logistical setups of either system are minimal and do not affect the value of the technology

• Image-guided surgery allows the surgeon to routinely perform a more complete exploration of the paranasal sinuses, particularly when it comes to smaller cells occupying the crevices of the sinus cavities

• Difficult sphenoid sinus and ethmoid sinus anatomy can be approached with more surgical confidence using computer-guided dissection

• Frontal sinus anatomy can be approached with greater confidence, particularly in the presence of a false lateral terminal cell

The authors' experience has been with an electromagnetic system. ^[14] Metson et al prospectively compared an optical system and an electromagnetic system. ^[15] Despite a significant decrease in operating time with the optical system, this difference was not readily explained. Furthermore, no significant difference was noted in other parameters (eg, blood loss, complications).

Recent advances have allowed magnetic resonance images (MRI) to be fused directly to CT images. This technology is beneficial in cases that involve both otolaryngologists and neurosurgeons, as one image-guidance system can be used for navigation. CT images are indeed able to better delineate the bony anatomy of sinonasal cavities for the nasal approach performed by the otolaryngologist, whereas the neurosurgeon will require the assistance of MRI guidance with enhanced intracranial soft tissue definition to pursue the combined procedure. This is particularly relevant in cases that involve the pituitary and anterior skull base. ^[16] The image-guidance system can create a hybrid image that has both excellent soft tissue and bony detail.

Newer methods are available for automated registration; for instance, small cranial pin transmitters designed to replace the more bulky headsets currently in use may have exceptional benefit in open skull base or orbital cases.

Some studies in image guidance have focused on real-time updates, which allow the preoperative images used for navigation to be updated throughout the surgery. Although the senior author has experimented with magnetic resonance for this purpose, this imaging modality was deemed inadequate for routine use because of extremely high costs and implementation constraints. ^[2]  We have also studied the use of fluoroscopy for this purpose. However, the images produced by this modality remain inadequate for near–real-time navigation. ^[17]

Future developments in surgical simulation will also likely contribute to broaden the applications of image-guided surgery. Patient-specific preprocedural rehearsal devices will allow surgeons to add critical annotations and observations to the imaging data set preoperatively. These annotations will then be available in real-time on the IGS system during surgery.