Otolaryngol Clin N Am

38 (2005) 439–452

Image-Guided Sinus Surgery: CurrentConcepts and Technology

Martin J. Citardi, MD*, Pete S. Batra, MDSection of Nasal and Sinus Disorders, Head & Neck Institute,

Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland, OH 44195, USA

Since its introduction in the 1980s, image-guided surgery (IGS)technology has gained considerable acceptance for its applications infunctional endoscopic sinus surgery (FESS). Appropriate use of IGS duringsurgical procedures requires that the rhinologic surgeon understand thefundamental principles that guide the technique. Such knowledge will alsoserve to elucidate potential IGS limitations and minimize secondarycomplications.

Domain of computer-aided surgery

In 1996, the International Society for Computer-Aided Surgery (ISCAS,www.iscas.net) was established as a multidisciplinary forum for thedevelopment of semiconductor-based technologies for surgical applications.Since its founding, ISCAS has promoted a broad definition of computer-aided surgery (CAS): ‘‘The scope of Computer-Aided Surgery encompassesall fields within surgery, as well as biomedical imaging and instrumentation,and digital technology employed as an adjunct to imaging in diagnosis,therapeutics, and surgery’’ [1]. The domain of CAS includes surgical navi-gation, computer-aided image review, stereotactic surgery, robotic surgery,telemedicine, electronic medical records, and many other specific applica-tions of semiconductor-based technologies [2].

IGS falls within this broad definition of CAS. IGS is another term forintraoperative surgical navigation, which provides three-dimensional (3D)localization data about specific points in the operating field volume relative

Disclosure: Dr. Citardi was a consult for CBYON (Mountain View, CA) from 1999 to

2003. He has been a consultant for GE Healthcare Navigation and Visualization (Waukesha,

WI) since 2003.

* Corresponding author.

0030-6665/05/$ - see front matter � 2005 Elsevier Inc. All rights reserved.

doi:10.1016/j.otc.2004.10.019 oto.theclinics.com

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to a preoperative imaging study (Figs. 1 and 2). IGS has also come toinclude preoperative imaging review at the computer workstation andsoftware-enabled surgical planning. Rhinologic surgeons, spine surgeons,and neurosurgeons have all come to recognize the potential advantages ofIGS within their respective surgical disciplines.

Registration

Definition of registration

Registration is a process during which an IGS system calculates the one-to-one relationships between corresponding points, termed fiducial points, in

Fig. 1. This intraoperative still image capture, obtained from the InstaTrak 3500 Plus (GE

Healthcare, Waukesha, Wisconsin) during endoscopic resection of an ethmoid fibro-osseous

lesion, illustrates the typical screen layout on the computer monitor during surgical navigation.

The lower right panel is the analog endoscopic video image that is imported as a picture-in-picture

display with the three orthogonal CT views. The IGS system can track the position of the

instrument tip (in this case, the tip of the drill) and projects its calculated position on the

preoperative imaging as indicated by the cross hairs on axial, coronal, and sagittal CT images.

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two locations. For surgical navigation, these locations consist of theoperating field volume (ie, the patient) and the imaging data set volume (ie,the CT or MRI scan). Each point in each volume has a unique xyzcoordinate; registration simply aligns corresponding xyz points. Duringsurgical navigation, the IGS system can display the position of a probe inthe operating field volume in the preoperative imaging data set volume byextrapolating from this mapping of corresponding points.

The concept of registration also applies to image fusion, where twoimaging data sets are merged. In image-to-image registration, fiducial pointswithin each data set are aligned and the data from one imaging data set issuperimposed on the other imaging data set.

The term ‘‘registration’’ should not be used interchangeably with the term‘‘calibration.’’ Calibration is the process of defining (or confirming) theposition of an instrument tip relative to a tracking device.

Fig. 2. This intraoperative still image capture, obtained from the InstaTrak 3500 Plus (GE

Healthcare, Waukesha, Wisconsin) during revision endoscopic frontal sinusotomy, demon-

strates the usual appearance of intraoperative surgical navigation. In this instance, the position

of the aspirator tip at the sphenoid face (seen in the endoscopic image) is projected on the

preoperative axial, coronal, and sagittal CT images (cross hairs).

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Registration paradigms

Each IGS vendor has developed specific protocols for registration.Although these protocols differ in their details, all registration protocolsmay be classified as paired-point registration (ie, manual registration),automatic registration, or contour-based registration [2].

Paired-point registrationPaired-point registration is a three-step process that sequentially

identifies a series of fiducial points in the operating field volume. Thesefiducial points may be bone-anchored markers or taped-on skin markersthat are applied before scanning and left untouched until surgery.Alternatively, appropriate anatomic points (eg, the tragus, the lateralcanthus, and so forth) may be used. In the first step, the user must designatethe locations of the fiducial points in the preoperative imaging at thecomputer workstation. Some systems can automatically locate the fiducialpoints, but typically this step must be performed manually. Next, the usermanually maps each point by localizing it in the operating field witha tracked probe. Finally, the computer aligns corresponding fiducial pointsin the operating field volume and the imaging data set volume, andcalculates the registration.

Automatic registrationAutomatic registration depends on the placement of a headset that is

designed so that its positioning on a specific patient is reproducible (Fig. 3).The headset contains fiducial markers whose positions relative to the patient

Fig. 3. The InstaTrak (GE Healthcare, Waukesha, Wisconsin) headset for automatic

registration features metallic spheres (arrow) whose configuration can be recognized by the

IGS computer during registration. The patient must wear an identical or functionally equivalent

headset during CT acquisition and during surgery. The headset is designed so that the position

of the fiducial array relative to the patient’s head is maintained whenever the headset is placed

on the patient.

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are identical at the time of CT scan and at the time of surgery, because theheadset fits the patient in only one way. The patient wears this headsetduring image acquisition, and wears a functionally identical headset duringsurgery. The IGS software automatically recognizes the fiducial markers atthe time of surgery and calculates the registration. Intraoperative surgicalnavigation is performed relative to the headset. Because the implicit as-sumption is that headset position remains unchanged between image acqui-sition and surgery, intraoperative localizations provide useful anatomicinformation.

Contour-based registrationLike paired-point registration, contour-based registration is also a three-

step process. First, the IGS software builds a 3D model based on the axialimaging data and defines the surface contours. Next, the user must localizethose contours in the operating field volume. This may require an initialapproximate localization of three or more points in a fashion similar topaired-point registration. A standard probe may be used, although somesystems require a handheld laser whose reflected light defines the contour.This step defines a large number (40–500 discrete points) on the surfacecontour. Finally, the IGS system calculates the registration by aligning thecontours of the 3D model and operating field volume through an iterativesoftware algorithm.

Registration error concepts

Although it is not well described in the otorhinolaryngology literature,error is intrinsic to the registration process. Three principle sources ofregistration error include fiducial localization error (FLE), fiducialregistration error (FRE), and target registration error (TRE) [3].

FLE is the error of localizing the fiducial, and represents the error definedby the difference between the true position of a fiducial and its measuredposition. FRE is the difference after registration between the true position ofa fiducial point in the operating field and the point’s indicated position in theimaging data set volume. Root mean square (RMS) values are measures ofFRE. TRE is the difference after registration between the true position of ananatomic feature of surgical interest (ie, a surgical target) and the indicatedposition relative to the preoperative imaging data set. TRE answers thequestion ‘‘How close to target?’’ and thus defines the error of surgicalnavigation. Clinically, TRE is determined by a surgeon’s judgment withregard to the apparent robustness of surgical navigation accuracy.

The concepts of FLE, FRE, and TRE are strictly applicable only forpaired-point registration protocols. However, FLE, FRE, and TRE mayprovide insight into registration error associated with automatic registrationand contour-based registration. Discussion of the intricacies of FLE, FRE,and TRE are beyond the scope of this article.

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Clinical experiences and extrapolations relating to registration errortheory provide useful guides for the selection of fiducial points:

� The fiducial points should be relatively fixed and therefore reproducible.Placement on mobile areas such as loose facial tissues is suboptimal.

� In general, greater numbers of fiducial points are desirable, althoughdiminishing returns as a result of increasing the number of markers mustbe considered. For instance, six fiducial points are clearly preferableover three, but the difference between 10 and 11 fiducial points may notbe clinically significant.

� The 3D distribution of the fiducial points is critical. The ideal placementseparates the fiducial points in a 3D array around the area of surgicalinterest. Adequate separation among the points enhances accuracy.Arrangements that place the points in a linear or nearly linear alignmentare suboptimal. Experimental data corroborates this observation [4].

Similar principles are also relevant in considering headsets for automaticregistration. The theoretical TRE for a headset with a 3D box of fiducialpoints is less than the theoretical TRE for a headset with a planararrangement of fiducial points. Therefore the 3D arrangement shouldtheoretically yield greater intraoperative surgical navigation accuracy (ie,lower real-world TRE) [5]. The magnitude of these theoretical differences ismuch less than clinically observed TRE, suggesting that repositioning is themajor cause for error as a result of differences in the placement of theheadset at the time of imaging and at the time of surgery.

For contour-base registration, themajor consideration is the deformabilityof the soft-tissue contour. Clinically, differences in tissue hydration andmuscle tension while awake at the time of image acquisition and while underanesthesia at the time of surgery may produce dramatic changes (�2 mm) inthe position of surface contours. Contour-based registration protocolscompensate for this limitation by incorporating a large number of pointsover contours that are less likely to exhibit these variations. In addition,contour-based registration protocols seem to yield the greatest degree ofsurgical navigation accuracy close to the contour, rather than deep into it [6].

Regardless of the registration protocol, the quality of the preoperativeimaging study is also a major determinate of intraoperative surgicalnavigation accuracy. The optimal CT scan for image-guided sinus surgery isa direct 1-mm axial CT with a 512 � 512 pixel matrix. Furthermore, thescan’s field of view should be set so that the patient’s head (and headset, ifapplicable) fills the entire image. It must be remembered that the imagingdata is granular (ie, each slice is composed of discrete pixels of information).Because of this, there is no real information between slices (or betweenpixels). Thus, the slice thickness sets the lower limit for clinical TRE (upperlimit for surgical navigation accuracy). For instance, if the CT scan slicethickness is 2 mm, then the best possible TRE will be 2 mm, assumingperfect registration (which never happens in the real world). The quality of

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the preoperative imaging also influences the quality of the reconstructedsagittal and coronal images and any 3D models. Therefore, software andcomputer display features are a factor. Some software includes more robustroutines for image reconstruction, and some IGS systems include bettermonitors that provide crisper images.

Finally, the concept of precision must be discussed. Precision is a measureof reproducibility of a specific localization. A specific localization may beconsidered highly precise if repeated localizations at the same specific pointproduce similar results. Ideally, intraoperative navigation yields highlyprecise localizations so that the range of TRE values is small. Clinicalexperiences often vary considerably from this idealized situation becauseseemingly random errors often appear to degrade TRE. Using a laboratorymodel, Cartellieri et al [7] confirmed a wide range of precision (ie, largerange of TRE values) for various IGS systems and observed that thecalculated TRE seemed to vary depending on the angle between the pointerand tracking system. Therefore, changes in the trajectory of the trackedinstrument pointer may influence the apparent TRE. Clinically, theapproach to a surgical area may limit instrument positioning and thusincrease TRE. Regardless, simple changes in pointer trajectory may increaseor decrease TRE. During surgery, these alterations will appear to be randomerrors that may render surgical navigation useless.

System components

IGS vendors always try to distinguish their products from theircompetitor’s products. Although that may be a useful sales technique, itobscures the fact that all of these systems share similar components [8].

Hardware

Surgical navigation systems share a number of similar key components:

� Computer workstation. The computer workstation is the central com-ponent of all IGS systems. Originally, the operating system for manyIGS systems was UNIX. More recently, other operating systems, in-cluding Windows 2000, Windows XP, and LINUX, have become morecommon. A standard computer mouse and keyboard serve as inputdevices. A radiofrequency mouse may be substituted for a traditionalmouse.

� Display system. IGS systems include a standard computer monitor for theoutput of visual information. In some systems, a touch screen providesdirect access to software functions. High resolution, flat panel, liquidcrystal display screens have supplanted cathode-ray tube monitors.

� Tracking system. Surgical navigation must include specific hardwarethat monitors the relative position of the surgical instruments. Earlier

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systems in the late 1980s and early 1990s relied on an electromechanicalarm for instrument tracking. Although this technique was accurate, itwas also quite cumbersome. Current commercially available IGSsystems rely on electromagnetic or optical tracking technology. Forelectromagnetic tracking, the system senses positional information froman electromagnetic receiver (Fig. 4) in an electromagnetic field generatedby a specific emitter attached to the patient. For optical tracking, anoverhead camera array (ie, a digitizer) tracks the position of arrays oflight emitting diodes (LEDs or highly reflective spheres in a passivesystem) (Figs. 5 and 6).

� Specific surgical instrumentation. Early IGS systems used primitiveintraoperative pointers. Currently, numerous instruments, includingstraight and curved suctions, cutting forceps, drills, and microdebriders,have since been adapted for intraoperative surgical navigation and arecurrently available (Fig. 7). In theory, almost any surgical instrumentcan be tracked by the attachment of an intraoperative localizationdevice (ILD). In an optical system, the ILD is an array of LEDs, and inan electromagnetic system, the ILD is an electromagnetic sensor.

� Data transfer hardware. The preoperative data set must be transferred tothe IGS computer. This may be achieved through standard computernetworks that link the imaging equipment throughout the radiologydepartment to the IGS workstation. Alternatively, the data sets may betransferred on commonly available digital media, such as a CD-ROM orother portable media.

Software

A complete IGS system includes user-friendly software that integrates thevarious hardware components into a robust, functional unit that supportssurgical navigation in addition to other applications. Generally, discussionsabout IGS focus only on its utility for intraoperative surgical navigation.

Fig. 4. For electromagnetic tracking, the IGS computer determines instrument position

through an electromagnetic sensor (arrow) that is attached to the instrument tip. This sensor

functions as an intraoperative localization device. The InstaTrak system (GE Healthcare,

Waukesha, Wisconsin), with its curved aspirator, relies on this method for tracking.

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However, this overlooks other powerful software capabilities these systemsprovide. Most IGS systems include a variety of software tools that allow forcomplex manipulation of the preoperative imaging data set. The softwareallows the following tasks: (1) simultaneous viewing of axial, coronal, andsagittal images in the three orthogonal planes through a given point; (2)coronal and sagittal image reconstruction; (3) 3D model reconstruction; (4)3D cut-view reconstruction; (5) distance measurement; and (6) CT windowand level adjustment.

These software tools allow review of preoperative images in a mannerthat facilitates an understanding of the critical 3D anatomic relationshipsthat simply cannot be achieved through standard radiographic light-boxreview of the images. Thus, IGS is a technology that enables formulation ofa more detailed and accurate surgical plan.

Fig. 5. Intraoperative navigation may also rely on optical tracking. In this approach, an

overhead camera senses the position of arrays of light-emitting diodes (arrow). An array of this

type serves as an intraoperative localization device when it is attached to a surgical instrument.

Fig. 6. Optical tracking may also sense the position of arrays of reflective spheres (arrow). The

overhead camera emits infrared illumination which the spheres reflect back to the camera. This

passive system eliminates the need for wires. In this example, passive intraoperative localization

devices are attached to a straight and curved suction.

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Image-guided functional endoscopic sinus surgery

The integration of CAS technology into FESS produces the newparadigm of image-guided functional endoscopic sinus surgery (IG-FESS)[9]. Several tenets are central to the IG-FESS concept:

� The goal of sinus surgery is to restore normal mucociliary clearance formedically refractory chronic rhinosinusitis. Delicate anatomic dissectionand meticulous mucosal preservation are paramount in achieving thisgoal.

� The localization capability of the IGS system should not be used merelyin a point-and-hunt manner, as this trivializes the sophistication of thispowerful technology and violates the core principles of FESS.

� IGS platforms enable thorough review of the preoperative data set,which promotes an accurate understanding of the complex 3Drelationships. Thus, the IGS systems facilitate preoperative surgicalplanning.

IGS complements the standard techniques for FESS. It must beremembered that although the nasal telescopes provide brilliant illuminationand bright images, these images are only two-dimensional representations ofa complex 3D space. Furthermore, because the telescopes provide wide-angle views, there is a certain amount of intrinsic fish-eye distortion to theimages. The angled 30�, 45�, and 70� telescopes may also introduce further

Fig. 7. The attachment of an intraoperative localization device, followed by instrument

calibration, permits the IGS system to track the position of almost any surgical instrument. In

this example, the surgeon is attaching a passive intraoperative localization device for an optical

tracking system to a microdebrider. After calibration, the IGS system will display the calculated

position of the instrument tip relative to the preoperative imaging data set.

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perceptual error. By providing 3D anatomic information through directcomparisons to the preoperative imaging, IGS provides depth to thestandard endoscopic views and minimizes the surgeon’s perceptualdistortion.

Ideally, the surgical navigation platform must be used throughout theentire sinus procedure. To achieve this objective, various surgical instru-ments, including aspirators, forceps, pointers, microdebriders, and evendrills, should be modified to track their position throughout the procedure.

The indications for using IGS in sinus surgery have been the subject ofsome debate. The emerging consensus emphasizes the following indicationsfor IGS: (1) revision sinus surgery, (2) frontal sinus surgery, (3) posteriorethmoid and sphenoid sinus surgery, (4) previous maxillofacial trauma, (5)congential maxillofacial anomalies, (6) extensive inflammatory disease (eg,sinonasal polyposis, allergic fungal rhinosinusitis), and (7) extended skullbase procedures (eg, cerebrospinal fluid leak, orbital surgery, sinonasalneoplasia).

The efficacy and safety of intraoperative surgical navigation in FESShave been corroborated by data in the literature. Metson et al [10] noted anapparent increase in operative time and costs associated with IGS use, butno differences in blood loss and complication rates. More recently, Friedet al [11] reported that when comparing cases of FESS with and withoutIGS, intraoperative surgical navigation may result in reduction ofcomplications and may allow for a more complete operation, but operativetime and blood loss may be increased. Although Reardon [12], ina comparison of cases performed with and without IGS, did not find anydifference in the median operative time and frequency of major and minorcomplications, he did note that the patients whose surgery included IGS hadsignificantly more sinuses entered. Thus, application of IGS in routine FESSmay be associated with more complete surgery and lower morbidity andmortality. On the other hand, costs and operative time may be greater whenIGS is used. Anecdotal reports suggest that these negative aspects of IGS arefar outweighed by the positive impacts of IGS.

Assessment of surgical navigation accuracy

Although the potential benefits of IGS are well-recognized, relativelylittle consideration has been given to the issue of IGS failure. These losses ofintraoperative navigational accuracy may range from a catastrophiccomplete loss of accuracy (eg, obvious inaccuracy or total loss of navi-gation) to more subtle errors that the casual rhinologic surgeon may notobserve. Of course, the first scenario is easy to recognize, but fortunately itsoccurrence is uncommon. The second scenario is much more common andinsidious. Therefore, a strategy for real-time monitoring of surgical navi-gation accuracy is warranted.

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The surgeon must have continuous sense of the system’s TRE, or surgicalnavigation accuracy. In the clinical realm, TRE is an estimate based ona surgeon’s judgment of the system’s surgical navigation accuracy. Unlessbone anchored fiducial markers are employed, the IGS system cannot provideinformation about TRE. FRE values (ie, RMS) may suggest an acceptablelevel of TRE, but the real test for TRE is the surgeon’s estimate of TRE.

Clinical experiences have shown that some basic steps may facilitaterecognition of acceptable TRE (ie, good surgical navigation accuracy) andunacceptable TRE (ie, unreliable navigation accuracy). First, TRE shouldbe monitored throughout the entire procedure. This will ensure that thesurgeon can recognize specific patterns of errors. For instance, the positionof headsets may shift, or instruments may lose calibration at any time duringa procedure. Second, TRE should be assessed in the anatomic region wherethe surgery is being performed. Confirmation of surgical navigationaccuracy at the tip of the nose does not give reliable information aboutaccuracy in the ethmoid cavity. A small rotational error in the registrationcalculation may yield an acceptable clinical TRE in one region, but acompletely unacceptable clinical TRE in the region of most interest.

Intrinsic limitations

It is imperative that surgeons be cognizant of the limitations of IGS:

� Accurate surgical navigation depends on robust registration. If theregistration tightly maps the imaging data set volume to the surgicalfield, the intraoperative surgical navigation will be optimal. In reality, allregistration methods are less than optimal. Bone-anchored fiducialmarkers provide the best FLE, FRE, and TRE in paired-pointregistration. However, this strategy is impractical in almost all patients.Alternative registration paradigms, such as automatic registration,paired-point registration with anatomic landmarks, and contour-basedregistration, increase the usability of the technology, although all ofthese methods will degrade TRE to varying extents. In light of theimportance of registration, all surgeons should be able to troubleshootregistration errors and problems.

� Although IGS can serve as an important surgical tool, it is imperativethat surgeons understand that this technology is not a substitute forsurgical expertise and intimate knowledge of the paranasal sinusanatomy. CAS systems will not improve the capabilities of a poorlyskilled surgeon. Therefore, a FESS procedure that would not beattempted because of lack of proper training and instrumentationshould not be attempted simply because IGS is available. On the otherhand, IGS may facilitate the application of standard FESS surgicalprinciples to specific scenarios by providing greater opportunity forpreoperative assessment and surgical planning.

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� IGS is an enabling technology. It can help facilitate complex proceduresby providing detailed information about the 3D relationships. IGStechnology does not change the nature of the surgical procedure, itallows for completion of the procedure within the confines of currentlyaccepted surgical principles.

� All IGS platforms rely on preoperative imaging data sets. Therefore,intraoperative surgical navigation cannot accurately reflect the anatom-ical changes created by the surgical manipulations during the procedure.The success of this technology for sinus surgery relies on the fact thatstandard sinus surgery is performed in a ‘‘bony box,’’ which is the idealsetting for surgical navigation that relies on preoperative imaging dataset.

� As with all computer technology, hardware and software failures dooccur. Software diagnostic tools programs may help pinpoint the sourceof the system failure. Surgeons should become familiar with theseaspects of IGS software.

Conclusion

IGS technology offers numerous advantages to the rhinologic surgeonbecause it offers a platform for preoperative software-enabled image reviewand surgical planning, and intraoperative surgical navigation. When IGS isincorporated into FESS, it can enhance surgical outcomes by providinggreater anatomic information to the rhinologic surgeon. To truly realize allof the potential benefits offered by IGS, the rhinologic surgeon must fullyunderstand the principles on which it is based. Thus, the concepts ofregistration paradigms and theory, in addition to surgical navigationaccuracy and error, all become important. With such knowledge, therhinologic surgeon may also mitigate the impact of technical failures andmisapplications of IGS.

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