Image Guidance in Endoscopic Sinus Surgery
M. Bradley Evans, M.D.
August 4, 2005

Our case presentation today is that of a 56-year-old female who presented with a several-month history of bilateral nasal obstruction, as well as occasional sinus pressure and pain. She had been treated previously by her primary care physician with a short course of antibiotics and nasal steroid spray without relief of her symptoms and was subsequently referred to us. She had a history of sinus surgery twenty years in the past for what she said were nasal polyps. She had no history of asthma or aspirin sensitivity. Other than well-controlled hypertension, she had no other known medical problems. Upon physical examination, she was noted to have polyps which were filling her nasal cavities bilaterally. A CT scan was obtained which confirmed the presence of sinonasal polyposis, as can be seen in this series of scans. She was counseled extensively regarding risks and benefits of revision endoscopic surgery, which she agreed to proceed with. A preoperative steroid taper was given to her. She was then taken to the operating room for revision endoscopic sinus surgery with nasal polypectomy using current image guidance technology. She was noted to have sinonasal polyposis with significant alteration of her anatomy secondary to the previous surgery, and it was with the assistance of the image guidance system that we were able to confirm critical anatomic structures because of her alteration in anatomy, such as the lamina papyracea, and confirm that we were in the frontal recess as well. She went through her surgery without complication and has done well postoperatively with resolution of her preoperative symptoms and no recurrence of her polyps.

I present this patient this morning just as a typical patient that benefits from current technology in image guidance sinus surgery, and I would like to review this subject with you today. The objective of today's talk is to present a definition of what image-guided surgery actually is, go through a little bit of the historical perspective, talk about the current technology, and then talk about how accurate these systems are, as well as a brief cost analysis, and some of the indications and applications of this technology, as well as the future of image guidance.

What, exactly, is image-guided surgery? The International Society for Computer-Aided Surgery, which was established in 1996 as a multidisciplinary forum for the development of semiconductor based technologies for surgical applications, put forth this broad definition of computer-aided surgery, which is also their mission statement. It states that the scope of computer-aided surgery encompasses all fields within surgery as well as biomedical imaging and instrumentation and digital technology employed as an adjunct to imaging and the diagnosis, therapeutics, and surgery.

The broad domain of computer-aided surgery includes several things, including surgical navigation, computer-aided imaging review, stereotactic surgery, robotic surgery, telemedicine, electronic medical records, as well as other specific applications as semiconductor-based technologies. Image-guided surgery, or intraoperative surgical navigation, falls under this domain. Specifically, this allows us to do several things. First, it provides three-dimensional localization data about specific points in an operating field relative to a preoperative imaging study, such as CT or MRI. But not only this, it also allows us as surgeons for preoperative imaging review at the computer workstation as well as software-enabled surgical planning, similar to a PAC system in the Radiology Department. This is also utilized by several surgical specialties such as rhinologic surgeons, spine surgeons, and neurosurgeons today.

To go to the roots of stereotactic surgery takes us back to the 1940s when Spiegel and Wycis described the use of a framed stereotactic surgical system, which used a variety of instrumentation for calibration; however, it was mainly driven by attaining plain radiographs those images along with standard anatomical landmarks on the patient, as well as standard anatomic atlases of the brain to provide a stereotactic type of surgery. It had a limited role in neurosurgical procedures because of its obvious lack of precision and accuracy and was limited mainly to the drainage of abscesses and creating lesions in the cerebrum to alter motor function in patients with epilepsy.

In 1976, Bergstrom and Greitz described the application of this frame to stereotactic surgery to CT scan in which they applied a fixation device to the patient's head which held a metal trajectory ring in place. The patient was then scanned with this, and the rigid headset worn during the surgery allowed at least the trajectory of the instrument to be known; however, the depth of penetration was unable to be seen at the tip of the instrument.

In the mid-1980s, with the improvement in CT scan images and the advent of personal computers, this enabled computers to provide intraoperative information on the placement of the instrument tip without the need for a frame. However, widespread acceptance was not gained in routine sinus surgery, mainly because these systems were difficult to use and ungainly. They were much too expensive and had unacceptable inaccuracies, and they required head fixation. As most of the sinus surgery was performed in the 1980s under local anesthesia, you can imagine that a patient would not tolerate being put in a Mayfield head fixator for a routine sinus surgery.

It was not until 1994 that Anon described the first intraoperative guidance system using the ISG Viewing Wand in the United States for endoscopic sinus surgery. This system worked on a probe, which can be seen in the top photograph here, which was used to localize the instrument in space. However, it was attached to a bulky arm with several joints to the computer system. This was used for multiple sinus procedures in the late 1990’s, as documented in the literature. As more popularity was gained with this technology, and with the rapid advances of images and computer technology, more and more companies entered the endoscopic sinus surgery market.

This takes us to the present day and the current technology. Basically, there are four components which are necessary for an image guidance system:

Second is the computer systems which are available today. They consist of three components which are mainly computer work stations, display systems, and the data transfer hardware. In talking about the computer work stations, this is the central component of the image guidance system and works on a variety of operating systems, such as Windows 2000 or XP as well as Linux. The display systems nowadays are high resolution, flat panel LCD monitors; and the data transfer hardware, or the ways to get the preoperative imaging to the computer system, can work in a variety of ways as well, either by networks which link the radiology directly to the image guidance system and are able to be uploaded onto that system, or transferred by any portable digital media such as Flash drives or CD ROMs.

There have also been vast advancements in the software programs, which are used to run these image-guided systems. They are now more user friendly and are able to integrate the hardware components into a functional unit. Not only are they important for the surgical navigation itself but they also allow for detailed review of preoperative images, such as the simultaneous viewing of axial, coronal, and sagittal images in the three planes to a given point, quality coronal and sagittal image reconstruction, as I previously mentioned, 3-D model reconstruction, as well as 3-D cut-view reconstruction. This allows us to perform much more detailed preoperative analysis of these scans and to provide the patient with a better operation. Also, distance measurements and CT window and level adjustments can be performed with these software systems.

Registration is essentially the process during which the image guidance system calculates a 1:1 relationship between corresponding points, which are termed fiducial points, in two different locations. The two locations are the operating field volume (the patient), and the imaging data set volume (the preoperative CT or MRI scan). Each one of these points in each volume has a unique X, Y, and Z coordinate. Registration simply aligns these corresponding X, Y, and Z points. Each IGS vendor has a specific protocol for registration for their own system. While the details differ in the registration, they can be classified in one of three general categories; paired point registration or manual registration, automatic registration, and contour-based registration.

Paired point registration, as mentioned, is a completely manual registration process. It is based on three steps which start with identifying a series of fiducial points on a patient. This can range from anything from bone-anchored markers to taped-on skin markers, which are applied before the CT scan and left untouched until the time of surgery, or standard anatomic points such as the tragus or the lateral canthus. The user must designate the locations of the fiducial points manually, and the preoperative imaging is in relation to the patient and the computer work station. While some systems automatically locate these fiducial points, it typically must be performed manually. The user then manually maps each point by localizing it in the patient with a tracked probe by the system, and then the computer aligns these corresponding fiducial points and calculates registration.

This is in contrast to the automatic registration, which most of us are more familiar with, such as employed by the InstaTrak system. It depends on a headset which is worn by the patient. This headset contains its own fiducial markers inside the headset, as noted by the arrow here. Because the headset only fits one way, the position is identically relative to the patient at the time of the CT scan and at the time of the surgery. The patient wears his headset during the CT scan and at the time of surgery, and then the image guidance system software automatically recognizes these fiducial markers at the time of surgery and calculates the registration based off the fiducial markers in the headset.

The third registration mechanism is that of contour-based registration. It is also a three-step process similar to paired point registration. However, it is more complicated. The IGS software first builds a three-dimensional model based on the axial imaging data and defines surface contours on the patient's face, as can be seen in this picture here. The user must then localize the contours in the patient with either a standard probe or a hand-held laser. Using a probe, it is traced over the surface contours of the face around the orbits and all the way down the nose to the maxillary spine. Similarly, the BrainLAB system uses a handheld laser which performs this from a distance away from the patient and is recognized by the system itself. This defines a large number (up to 500) of discrete points on the surface contour. At this point the IGS system then calculates the registration by aligning the contours of the three-dimensional model and that of the patient.

Once the patient is registered to the system, there is a mechanism to calibrate the instruments. In our specialty, there are two techniques in use: optical tracking devices and electromagnetic tracking devices. Examples of optical tracking devices on the market include the BrainLAB system, the Landmarx system, and the Stryker Navigation system. I think most people in this room are familiar with the electromagnetic tracking devices, or InstaTrak. Both technologies have been thought to be equivalent in accuracy, ease of use, and setup time. Which is used is really just a matter of surgeon preference.

The way the optical systems work is by infrared light-emitting diodes which localize the patient's head and track the movement of the instrument through space. This is performed by a bar, which is located above the patient and emits infrared beams. These infrared beams are then reflected up by sensors, which are called glions. The glions are attached to the reference headset on the patient as well as to a variety of instruments. As long as this marker can be placed on the instrument, pretty much any instrument can be used in image guidance, including microdebriders as well as a variety of suctions. The main disadvantage of this system is that you must have an unobstructed view between the overhead bar and the sensors, which can prove challenging at times in the operating room.

The electromagnetic systems, in contrast, work from a transmitter from the computer which is placed on the headset itself. The sensor is then attached to a probe or any of a variety of instruments, and then an electromagnetic field is generated to sense the position of the instrument tip after it has been calibrated to the headset. While the position of the system does not matter in the electromagnetic systems, there are several wires which are connected from the patient's headset to the computer work station, which can be cumbersome at times.

So, how accurate are these systems?Multiple studies have confirmed the accuracy of the image guidance systems to within 2 mm. It has been shown that there is really no difference in the accuracy between the optical systems and the electromagnetic systems. However, it is important to realize that there are potential sources of error or inaccuracy, and this must always be remembered when using this type of technology. Some of these potential sources of error include CT imaging variables, fiducial shifts after registration, soft tissue effects during surface registration, as well as operator-dependent errors during the registration process. I think the most important point I can make when talking about the potential sources of error here is that image guidance never serves as a substitute for a thorough knowledge of the surgical anatomy.

A brief cost analysis shows that the initial cost of purchasing the system is the most significant expense associated with using image guidance technology. The cost varies depending on the system capabilities and the software packages which are purchased. The Landmarx system from published data ranges anywhere from $86-$159 thousand; and the InstaTrak system ranges anywhere from $99-$190 thousand with OR disposable costs of approximately $69 per procedure. This wide range is because of the different levels of application which can be purchased from the system (spine surgery, neurosurgery, etc.) This is not the only thing to take into account when performing a cost analysis though. It has been shown in a study in 1999 by Madsen that image-guidance use increases the operating room time by an average of 17.4 minutes per case. As this system is used, that time probably goes down as the user and the operating room become more familiar with the use of the system. However, he calculated, based on cost of anesthesia for 15 minutes and operating costs at the Massachusetts Eye and Ear Infirmary, that this equaled about a $500 increase in cost per case. There are also several indirect costs which are related to the use of image guidance, including those that are incurred by the operating room, the Radiology Department, as well as the Division and the patient. Some of these costs may be reimbursed by insurance; however, they can be broadly viewed as general costs to the health care system. When doing a cost analysis, you also have to realize that there can be intangible savings by using image guidance technology, such as potentially lower revision rates of surgery, as well as potentially lower complication rates. This is obviously impossible to assign a specific value to. As far as reimbursement goes, these days you can bill for using image guidance; however, the payment is markedly variable in different geographic regions and depends on the insurance carrier. There is a CPT code which is available, 61795, which is defined in the 2005 book as Stereotactic Computer-Assisted Surgical Navigation.

As far as the indications and applications of this technology, I would first like to review that the use of this technology has greatly expanded in the last decade. In 2000, Madsen documented this and thought that this was a function of increased surgeon confidence secondary to the technology itself. There was about a 71% increase in the number of the procedures using image guidance in the first two years that this technology was available, and there was an almost 93% increase in the number of surgeons who used this technology during the same time period. Some of the indications for using this technology, which have been cited in the literature, include those patients with extensive sinonasal polyposis, revision sinus surgery, distorted sinus anatomy of development, postoperative or traumatic origin; pathology involving the frontal, posterior ethmoid and sphenoid sinuses; disease abutting the skull base, orbit, optic nerve, or carotid artery; and skull base procedures such as CSF rhinorrhea or conditions in which there is a skull base defect; as well as a variety of benign and malignant sinonasal neoplasms. As far as chronic sinusitis goes, there is a question whether this is really needed in routine endoscopic sinus surgery. Those patients who have limited sinus disease, as well as well-defined surgical landmarks, probably do not need the use of this technology in these situations. However, it can be helpful in complicated cases, such as those patients with extensive polyposis, dehiscence of the lamina papyracea, optic nerve, or carotid artery, as well as extensive frontal or sphenoid sinus disease, especially when a drill-out procedure is being considered for the frontal sinus and frontal recess. As far as revision endoscopic sinus surgery goes, because of the significantly altered anatomy in some of these cases as a function of the previous surgery that the patient may have had, as well as subsequent scar tissue formation, as well as barriers between the sinuses and the surrounding structures being thinned or even completely absent in cases, the use of this technology, as I said, has increased surgeon confidence and is felt by many to be a safer operation for the patient by identifying altered anatomy and critical structures intraoperatively.

There has been a trend toward more endoscopic resections of a variety of benign and even malignant tumors endoscopically. This includes anything from bony tumors to inverted papillomas and mucoceles, as can be seen in these pictures here of an inverted papilloma, which contains squamous cell carcinoma extending all the way back to the skull base and sphenoid sinus. It is amazing what people are doing nowadays endoscopically using image guidance. This is a picture of a clival chordoma, which is being taken out endoscopically through the nose. It has also been described in frontal sinus obliteration surgery to be able to outline the osteoplastic flap using the image guidance. It has been shown in a recent study that this is the equivalent in accuracy to the standard 6-foot Caldwell for this. You can also define your limits of resection of certain bony tumors, such as osteomas of the frontal sinus.

This technology is also being applied to orbital surgery, and endoscopic DCR’s. People are citing image guidance being helpful in these cases to identify precisely the lacrimal sac and lacrimal apparatus endoscopically, as well as the use in orbital decompression and optic nerve decompression to precisely locate where you need to do your optic nerve decompression and then, in this case, which is a severe trauma with panfacial fractures and optic nerve compression, it was useful to remove bony fragments from the optic canal. Also, the removal of a variety of orbital tumors has been described in the literature. The indications and applications toward skull base surgery include those patients that have skull base defects, such as a CSF leak shown in this scan, as well as encephaloceles from the anterior ethmoid with 3-D reconstruction of the anterior skull base and being able to see exactly where the defect lies.

It is also gaining widespread use in pituitary surgery and cited in several articles. Some authors feel that this is replacing the traditional transseptal-transsphenoidal approach to the pituitary and sella. These lesions are approached either through a nasal or a septal approach, and MRI/CT fusion technology is being applied to these cases as can be seen in this picture with the coronal CT scan showing the bony detail of the anatomy and the sagittal and axial MRI showing the detail of the soft tissue. Some authors also cite that you have a greater visibility than the microscope provides when performing this type of surgery.

With all this information, what are some practical considerations in when to use image guidance in sinus surgery? From an article in 2005 recently by Dr. Madsen, he stratified patients in one of three groups to try to simplify this. He stratified them based on the extent of their sinus disease, as well as the potential for image-guided technology to have a positive impact on their sinus surgery. Group I were those patients with localized disease, which he defined as image guidance not being necessary. Group II was intermediate disease where IGS is helpful but not necessary. Group III was advanced disease or when IGS is necessary. The Group I patients were those who have limited sinus disease and well-defined surgical landmarks, patients with disease limited to blocked ostiomeatal complexes or disease limited to the ethmoid or maxillary sinuses. Group II patients are those with advanced sinus disease who have surgical landmarks which are obscured by extensive disease or by previous surgery. Group II patients may be those with disease involving the frontal or sphenoid sinuses, as well as those with diffuse nasal polyposis, and patients needing revision sinus surgery. Group III, where he thinks that image guidance is necessary, is a very small, select number of patients with unusual sinus pathology who have severely altered anatomy from their disease process and require extended endoscopic applications to treat them. Examples of this include sinonasal fibrous dysplasia, and encephaloceles—some people are even draining petrous apex cysts endoscopically—as well as a variety of the nonmalignant tumors.

So, what does the future hold for image-guided surgery? I think the perfect image guidance system, which hopefully will be available in the future, will have improved sub millimeter accuracy and precision, as well as improved usability and improved affordability. These systems will probably be able to be more easily integrated into the operating room environment and allow seamless intranasal and external navigation with a variety of different equipment. They will also be able to incorporate any existing images for navigational purposes, which would avoid the need for repeat imaging. Also, to permit multimodal navigation, which is actually being used nowadays but promises to be advanced with diffusion technology, not only with CT and MRI but also with intraoperative ultrasound as well as PET scanning possibly. Also, to allow intraoperative updates of preoperative image data sets, which would allow the surgeon to continually be updated on the altered anatomy, as well as the extent of disease removal by performing intraoperative CT or MRI scanning. Also, to permit extended applications in robotic integration, which would open the doors for telemedicine and possibly even robotic surgery from remote locations. This is a vision of BrainLAB of the operating room of the future for image guidance, as you can see huge plasma screen displays that everyone can see, continually updated CT scans right next to that, with an intraoperative CT or MRI scanner, as well as the system being very incorporated into the operating room environment.

In summary, I would like to review one more article by Dr. Madsen, which was published in 2003, titled "Image Guided Sinus Surgery, Lessons Learned from the First 1,000 Cases." This is a review of the first 1,000 image guided sinus operations which were performed by 42 different surgeons at the Massachusetts Eye and Ear Infirmary. It provides several lessons which they learned based on these cases.

Again, I would like to emphasize that image guidance never serves as a substitute for thorough knowledge of the surgical anatomy. As with any technology which is new to any surgeon operating on anyone, it must be used with care.

Case Presentation:

L.B is a 56-year-old female who originally presented with severe bilateral nasal obstruction, occasional sinus pain, and pressure for several months. She had previously undergone “sinus surgery” at an outside institution approximately twenty years ago for nasal polyps with relief of her symptoms for several years.

She had been placed on a nasal steroid spray and a short course of antibiotics by her primary care physician, with no relief of her symptoms. She had no history of asthma or aspirin sensitivity. Other than well controlled hypertension, she had no other known medical problems.

Upon physical examination, the patient’s nasal exam was noted to be significant for the presence of extensive polyps which were filling the nasal cavities bilaterally (left > right). There was noted to be the presence of an inferior meatus nasoantral window on the right. The middle meatus was unable to be evaluated with nasal endoscopy secondary to the extensive nature of the polyps. A CT scan of the sinuses was obtained and the presence of extensive sinonasal polyposis was confirmed. The patient was counseled regarding the risks and benefits of revision endoscopic sinus surgery with image guidance, and she was taken to the operating room after an adequate steroid taper preoperatively.

The patient underwent nasal polypectomy, bilateral maxillary antrostomies, revision ethmoidectomies and right sphenoid sinusotomy using image guidance technology without complication. She was again noted to have extensive sinonasal polyposis with altered anatomical landmarks secondary to her previous surgery.

Postoperatively, the patient was maintained on a nasal steroid spray along with nasal saline irrigations, and is currently doing well with relief of her preoperative symptoms and without evidence of recurrence of her nasal polyps.

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