Rhinology, 40, 1–9, 2002
* Received for publication: February 6, 2002; accepted: February 10, 2002
We report on our experience with navigational tools in paranasal sinus and anterior skull
base surgery, especially with electromagnetic guidance systems. During the last five years we
operated over 80 selected cases with the InstaTrak®system from VTI (Lawrence, MS, USA).
Applicability and user friendliness were explored.
The InstaTrak®3500 employs a Sun®Workstation and is a frameless and free-arm and nav-
igation system. Two different suction devices, used as sensors (receivers), and one transmitter
are interconnected to this workstation. The position of the tip of the aspirator is displayed as
a pair of crosshairs on the screen in axial, coronal and sagittal planes of the patient’s CT-
scan on the computerscreen online.
Our results showed high accuracy-level, usually better than one millimeter and a setup-time
less than ten minutes, on average. No additional personnel is required in the OR. We believe
that the system enhances efficacy in selected cases like revision surgery, tumor surgery or dif-
ficult anterior skull base surgery. However, one should consider that medicolegal responsibil-
ity stays always with the surgeon and not with any navigation system.
Key words: computer assisted surgery, image guided surgery, functional endoscopic sinus
surgery, FESS, ESS
Image guided surgery of paranasal sinuses and
anterior skull base - Five years experience with
Wolfgang Koele1, Heinz Stammberger1, Andreas Lackner1, Pia Reittner2
1ENT Department, University Medical School, Graz, Austria
2Department of Radiology, University Medical School, Graz, Austria
Since the 1970s functional endoscopic sinus surgery (FESS) is
the method of choice for surgical treatment of sinus problems.
This method was developed at the University ENT-
Department at Graz, Austria and became widely spread and is
now one of the most frequently performed operations in ENT
worldwide (Stammberger, 1991). Unfortunately, this operation
also can be accounted responsible for several, partially serious
complications worldwide. Even though statistically the rate of
severe complications after FESS is much lower when com-
pared to external approaches, the absolute numbers climbed
due to the increased popularity of endoscopic sinus surgery. It
is true however, that there have been severe complications fol-
lowing FESS and surgeons have to be aware that they are
operating in an anatomically complex and delicate area
(Maniglia, 1991; Kainz et al., 1993; Kennedy et al., 1994).
In order to improve safety of this operation attempts have
been made in implementing computer aided image guidance
systems, as they are already in clinical use for neurosurgical
procedures (Mösges et al, 1993; Roth et al., 1995; Fried et al.,
1996). In recent years we collaborated with different companies
and became involved in development and testing of such
devices for applications in sinus and anterior skull base
(Luxenberger et al., 1999).
The basic idea of such systems is to provide a real time control
of orientation for the surgeon while operating in a complex
anatomical field - in this case the paranasal sinuses. These sys-
tems are able to calculate motion of a sensor in relation to the
patient in a three-dimensional field and display the current
position of this sensor on the patient’s CT scans on a monitor.
This allows surgeons to evaluate their assessment of the
patient’s individual surgical anatomy (Fernandez et al., 1997;
Javer et al., 2000).
Major drawbacks of such devices are considered their cost
intensity, complicated setup in the OR as well as circumstantial
scanning of the patient. Some systems also require fixation of
the patient’s head (Freysinger et al., 1997). Other problems
include lack of reliability and accuracy of these systems (Fried
et al., 1997; Cartellieri et al., 2001).
Koele et al.
MATERIAL AND METHODS
Since the mid-1980s the authors have used and tested most of
the early computer-assisted navigational devices during many
FESS courses and workshops worldwide. Despite significant
progress in design, handling and accuracy of several electrome-
chanical or optical systems the development of electromagnet-
ic technologies have finally allowed for a breakthrough of
intraoperative navigation in the paranasal sinus and anterior
skull base regions. Today however, the optical guidance sys-
tems are widely spread as well.
In January 1997 we started to test the InstaTrak® system of
VTI (Visual Technology, Inc., Lawrence, MS, USA). Eighty
patients have been operated by now using InstaTrak®. Though
in the beginning we were skeptical whether this technology
already was advanced enough for clinical routine, we have to
admit that our overall experience with the InstaTrak®system
was very positive. It was not anymore this indeed promising,
but still rather circumstantial and not always reliable technique
we were used to. Operating the system is easy and does not
require a software specialist or extra PhD in the OR. We were
surprised by the accuracy of the system, when we tested it in
our first 80 patients (Luxenberger et al., 1999).
The VTI InstaTrak®System is the first system that compen-
sates for or avoids several disadvantages of other systems. The
patient’s head can be moved during a surgical intervention
without the need for recalibration. The system is frameless, so
no rigid fixation in a frame or to the operating table is
required. Neither are fiducial markers, which might lead to
inaccuracy when reattachment becomes necessary. As rigid
head frame or fiducials are not required, one can operate on
the patient a few hours but as well weeks after the CT scan-
ning. Once trained, the operating room team is able to run the
system with high accuracy, and no additional technician is
required in the operating room.
The system consists of a headset that fits securely over the
nasion and into the patient’s external ear canals. An electro-
magnetic transmitter is connected to the headset. The electro-
magnetic receiver is integrated into the handle of a surgical
aspirator. Transmitter and receiver are interconnected via the
system’s computer, a Sun®Ultra10, employing an UltraSPARC
IIi processor at 440 MHz, 1024 MB RAM and a 21 GB hard-
disk. The resolution of the 17” LCD touch screen is 1280 x
1024 (Figure 1). First, an axial CT scan of the patient is per-
formed using the algorithms listed in Table 1. The patient
must wear the headset during the CT scan. The headset has
integrated fiducial markers, which must be included in the
scan. Their position remains constant in relation to the
patient’s anatomy within a margin of fractions of a millimeter,
despite the relative elasticity of the headset itself. The position
of the fiducials is automatically identified via an image-pro-
cessing algorithm. The CT data are transferred either via a
direct network (“Ethernet”) or magneto optical disks and
loaded into the InstaTrak®computer in the operating room.
Table 1. Parameters of the CT sanners we use.
512 x 512
GE LightSpeed QX/i
512 x 512
Algorithm / Filter:
on the anatomy;
has to be re-done
on the anatomy;
has to be re-done
Duration of Scan
Figure 1. OR-setup of the system.
Image Guided Surgery with the InstaTrak®-System
Here, the axial, coronal and sagittal sections are reconstructed
and a triplanar CT display is generated (Figure 2).
Movements of the electromagnetic receiver with attached VTI
aspirator in the electromagnetic field are registered and tracked
in relation to the position of the transmitter. The system’s soft-
ware calculates online the position of the tip of the aspirator-
receiver. This position is displayed on the monitor (represent-
ed by the center of a crosshair) in all three planes (sagittal,
axial and coronal). Thus the surgeon can identify the position
of the instrument in space intraoperatively on the CT scan in
all three planes, and correlate this with the direct endoscop-
ic/microscopic view. The InstaTrak®System and the
ConneCTstat workstation support the DICOM®3.0 (Digital
Imaging and Communications in Medicine), standard in most
modern CT scanners.
During the operation the patient has to wear again the very same
headset used for the CT scan. One of the first problems we ran
into at Graz resulted from not all seven fiducials of the headset
being included in the CT scan, which led to problems with auto
registration. Therefore, the radiologist must identify and include
all of the headset’s seven fiducials on their scan. For proper data
acquisition, therefore a very close relationship between the radi-
ology department and ENT-surgeons is mandatory.
During the surgical procedure, dense and heavy metallic
objects may interfere with the electromagnetic field and cause
distortion, if for example a metal operating table is used, a 4-
inch (10 cm) foam pad must be placed between the table and
the patient to avoid interference and distortion of the electro-
magnetic field. A metal detector integrated into the VTI
System informs the surgeon of potential magnetic distortion
by flashing a special warning on the monitor screen.
We were pleasantly surprised at how little time the system
needs for preoperative setup. A few minutes usually suffices.
On average in all our cases we stayed under 10 minutes of total
setup time in the operating room. This included adjusting, cali-
brating and verification of the system, positioning the headset
Advantages and Limitations of the InstaTrak®System
Until recently, some of the major disadvantages of most navi-
gational systems have been the almost unacceptable amounts
of expensive technology, bulky hardware, time consuming
setup and, most of all, the need for additional operating room
personnel (Freysinger et al., 1997; Cartellieri et al., 2001). We
believe this problem has been solved, by and large, with the
According to our experience, the major advantages of the
- High practicability and user-friendliness in clinical
- Setup time of less than 10 minutes.
- No specialized personnel (i.e. “expensive” additional techni-
- Days to weeks interval possible between scan and surgery.
- Full motility of the patient’s head intraoperatively, no need
for recalibration after or during movement, no fixation to a
frame or the surgical table required.
- No problem with covering LED’s (like in optical systems) by
surgeon’s hand, arms or body.
- Highly reproducible accuracy, in our cases better than one
millimeter on average (Fried et al., 1997; Luxenberger et al.,
Figure 2. System’s monitor: splitscreen in four quadrants, the original
axial view and the reconstructed coronal and sagittal views. In the right
inferior quadrant is the reconstructed 3-dimensional image. The
patient is an 11-year old boy with a bullet in his left orbit, stuck
between the medial and inferior rectus muscles and the optic nerve.
Both bullet fragments were safely removed via an endonasal route and
vision recovered completely.
Figure 3. Picture – in – Picture (PIP) on the screen: If the navigation
instrument is within the surgical volume, and a camera is attached to
the system, the right inferior quadrant of the splitscreen shows the
endoscopic image, in this case a recurrence of massive polyposis, lack-
ing of known anatomical landmarks.
Koele et al.
1999; Cartellieri et al., 2001).
- Availability of a “picture-in-picture” system. All relevant
information is presented on one monitor screen: the tripla-
nar CT display plus the corresponding actual endoscopic pic-
ture. All information required to obtain accuracy and ori-
entation can be seen on a single monitor.
(We are proud to have been involved in the development and
installation of this “PIP” system: The first video clips of surgical
procedures with synchronous display of triplanar VTI-CT-recon-
struction and endoscopic pictures world-wide originated from
our department at the Graz University Hospital (Figure 4).)
The “hardware” consists of one single tower and is easily trans-
portable. The entire device can be rolled from one operating
room into another, even “over the street” to other depart-
ments. There is no need for any devices to be mounted to the
ceiling or for a special type of operating table.
The aspirator is used as a localizer (Figure 4). This avoids the
disadvantage of using a pointer and saves frequent change of
instruments. Other instruments that can be used as localizers
are under construction.
Should the endoscopic picture become blurred due to blood or
debris topographical information does not get lost (Figure 5).
To fully appreciate the possibilities of the system and to avoid
navigational problems, surgeons must understand its working
principle and consider the following:
- Due to its flexibility, relatively minor pressure on the head-
set by direct or indirect force (leaning on it, pulling the cable,
pressure on the draping, etc.) may distort the headset slightly
and temporarily decrease accuracy.
- If the VTI aspirator is used as an instrument, i.e. if pressure
is put on it, distortion of several millimeters may occur due
to the flexibility of the steel shaft (Figure 6).
- As with all navigational systems relying on preoperatively
acquired imaging data, only the localization of the tip of the
navigational instrument is indicated in the triplanar display.
The system cannot give the surgeon information on the
amount of tissue, which was or is removed during the proce-
The headset can be placed in a wrong position: During scan-
ning or when reapplying the headset preoperatively, the tragus
of one or both sides may be “trapped”. This may lead to a
slight inaccuracy that would not be identified during verifica-
tion of the system. Only the direct correlation of the visual
information through the endoscope and identification of
known landmarks there and on the triplanar reconstruction
can precisely confirm the actual position.
In our department, in one case, the headset was misplaced a
full 180 degrees by a radiology assistant. This mistake, how-
ever, led to extended possibilities: by placing the headset in
this way, considerable areas of the infratemporal fossa adjacent
Figure 5. The aspirator tip is clearly in position inside the right frontal
sinus, despite not visible in the direct endoscopic image, which is
blurred by debris.
Figure 6. Example of deviation: The VTI aspirator is flexed by the sur-
geon (incorrect handling) resulting in erratic display of crosshairs intra-
orbitally. Endoscopically the instrument is clearly not inside the orbit
but in the posterior ethmoid.
This is the favorable one of two possible erratic situations: it is worse if
the cross-hairs indicate a safe position but if the instrument has actual-
ly gone astray already (for instance) intracranially, intraorbitally, or
dangerously close to the optic nerve or carotid artery.
This however, is not the system’s fault. Medico-legally the surgeon
must realize such a potentially dangerous situation and correct it.
Figure 4. We use two different types of aspirator-localizers: straight
Image Guided Surgery with the InstaTrak®-System
to the retro- and parapharyngeal space could be scanned and
navigation applied to this region. With this technique we were
able to place devices used for brachytherapy with extremely
high precision absolutely parallel into tumors of the skull base.
This enabled our radiotherapists to precisely deliver the best
radioisodoses possible for the individual case (Figure 7).
The distance between the front of the headset where the fidu-
cial markers are located and the forehead is rather large.
Therefore significant “empty space” has to be scanned
between the fiducials and the areas of interest, which adds to
the radiation dose.
The handle of the receiver (aspirator) is still rather bulky, not
all “corners” of the sinuses can be reached even with the
curved navigational aspirators currently available. This can be
of disadvantage especially in the frontal sinus region.
Like in all electromagnetic systems, interference may occur if
massive metallic instruments (like the shaft and handle of the
endoscope) are too close to the receiver in the handle of the
suction device. This, however, is clearly indicated as a warning
on the screen of the VTI System.
There is a relative contraindication to use electromagnetic sys-
tems in patients supported by implanted electronic devices
such as cochlear implants or cardiac pacemakers as the electro-
magnetic fields may cause interference.
A cranial pin for lateral skull base or cranial approaches is
available. The pin is screwed into the patient’s skull; the trans-
mitter is attached to the pin. This of course requires placement
of fiducial markers prior to the CT-scan. These markers cannot
be moved or replaced after the scan until the end of surgical
procedure. Cranial pin and fiducial markers are replacing the
Instead of the CT-scan a MRI can be used, if necessary. Image
fusion is not possible by now.
After the patient has been intubated for endoscopic surgery
and the face been prepared, the headset is put in place by the
surgeon, who is responsible for the correct positioning of the
device. The nurse then completes the draping and both nurse
and surgeon put in place the clear sterile drape to cover the
headset. Then, decongestant-soaked swabs are placed into the
nose and after this, the instruments are calibrated and verifica-
tion performed. While waiting for the decongestant to act (~ 5
- 8 minutes), the triplanar scans are studied again, special
anatomical features pointed out and danger areas highlighted.
We considered the possibility of scrolling through the triplanar
anatomy (and pathology) one of the most valuable features of
the system as it enables the surgeon to build a triplanar/3-
dimensional conception in his/her mind of the patient’s anato-
my and individual pathology. If necessary, magnification, con-
trast and brightness are modified to parameters considered
best for the individual case (depending on the CT scanning
Then we acquire the video signal from the endoscope’s digital
camera, which is connected with the VTI System. The VTI
System allows either a BNC or an Y/C (Super-VHS) input, the
latter providing the better quality.
After pledgets have been removed, the all-important “system
vs. reality-check” takes place. Under direct endoscopic vision,
anatomical landmarks are visually identified and touched with
the electromagnetic receiver with attached VTI aspirator.
“Real” anatomy and virtual triplanar display must match pre-
cisely (“optical cross-matching”). Various points – pathology
permitting – along each axis are cross-matched in this fashion,
all with clear bony or non-shiftable soft tissue substrate.
Commonly used landmarks are septal spurs, spines or crests,
anterior tip of middle turbinate, insertion of the middle
turbinate or any characteristic structure in revision surgery
Figure 7. Inverted position of the headset: Areas of the infratemporal
fossa adjacent to the retro- and parapharyngeal space could be scanned
and navigation applied to this region. With this technique we were
able to place devices used for brachytherapy with extremely high preci-
sion absolutely parallel into tumors of the skull base.
Figure 8. Optical cross-matching: Under direct endoscopic vision,
anatomical landmarks are identified and touched with the electromag-
netic receiver with attached VTI aspirator. “Real” anatomy and virtual
triplanar display must match precisely, like here at the anterior tip of
the middle turbinate.
Koele et al.
With RMS values (Root-Mean-Square deviation, a mathemati-
cal function indicating deviation between points in the CT-vol-
ume and the “real” patient) usually better than 1.0 mm, follow-
ing a comprehensive “cross-matching” procedure, a high
degree of certainty and accuracy are confirmed for the sur-
geon, thus enhancing the confidence in the system’s displays.
Clinical application examples
Patient 1: (Figure 9) A 70-year old female with an expansion
originating from the apex of the right pyramid.
This lesion turned out to be a dermoid cyst. The triplanar dis-
play demonstrates best accessibility from the choanal region
just behind the posterior end of the middle turbinate and
above the tubal lip. This information could not be generated
with the same accuracy from the standard CT Scan and the
endoscopic findings alone.
During surgery, the sphenopalatine artery (crosshair on axial
scan) and the pterygopalatine fossa can precisely be located
and bleeding thus avoided (see axial and coronal displays
The cyst contained cholesterol masses; hairs and other compo-
nents have been evacuated (Figure 11). The tip of the probe sits
on the horizontal course of the internal carotid artery, the wall
of which is slightly indented by the probe (see endoscopic
image). The artery was without any bony cover for two centime-
ters in the cavity during its course through the floor of the mid-
dle cranial fossa. A large communication was created towards
the nasopharynx and toward the maxillary sinus (not visible
here). There were no problems post-operatively. The patient is
free of symptoms without any recurrence after 4 years.
Case 2: (Figure 12) A 73-year old female with recurrence of
adenoid cystic carcinoma on the right side, 13 years after the
primary surgery on the left side.
Image Guided Surgery with the InstaTrak®-System
Endoscopic approach: crosshairs indicating arrival at lateral
Figure 13. Behind the pterygoid process, which has completely
resected endoscopically, and beyond the orbital apex.
Figure 14. Identification and exenteration of the cavernous
sinus on the right and arriving at middle cranial fossa at tem-
poral lobe dura.
Figure 15. Skeletonizing the trigeminal ganglion.
Figure 16. Approaching the infratemporal fossa lateral to the
Koele et al.
Despite our initial skepticism, our experiences in the first
selective difficult cases over the first few months have been
extremely positive. Today, after five years of VTI-use, we see
very good applications for special indications, for example in
very difficult anatomical relationships after previous opera-
tions, especially in revision surgery of the frontal recess/sinus,
in the vicinity of critical structures of the anterior skull base,
the medial wall and apex of the orbit, the sphenoid, and espe-
cially when operating on tumors. Here, the limits of possibili-
ties are clearly extended in the hands of experienced surgeons
and operating time can be reduced (Figure 17).
We have operated on more than 40 cases of tumors with com-
plete destruction of all anatomical landmarks, especially sur-
rounding the internal carotid arteries and the optic nerves,
with a radicality, precision and relative safety that would not
have been possible without the navigational system. The
InstaTrak® System enhances the degree of confidence of the
surgeon, once the accuracy has been verified in the individual
case. There are good applications for teaching and training, as
the system “forces” the resident to deal with and perfectly mas-
ter CT as well as endoscopic anatomy.
However, it remains to be seen whether and to which degree
navigational devices make sense in everyday routine cases (we
did not use the system for those), and whether lesser-experi-
enced surgeons will improve their quality of surgery when
using these devices. Under no circumstances must surgeons
forget that “opacifications” displayed in CT and/or MRI do not
automatically relate to pathological structures that should be
removed. Only in this way can routine radical operations and
surgical “overkill” be avoided.
There is a (slight) theoretical possibility that under the impres-
sion of additional security, surgeons might be encouraged to
approach cases upon which, based on their experience and
abilities, they probably should not attempt to operate.
The single most important criterion when using this intraoper-
ative navigational device is the direct visual control of the sur-
gical field. Only in this way can reality be correlated to the tri-
planar reconstructions with a high degree of certainty.
Misguidance can occur, but rarely is it the system’s fault.
Inexact placing of the headset leads to different positions of
the fiducials in the CT-scan and on the “real” patient. Only the
surgeon can detect this error by optical cross-matching.
Another possibility of misguidance not automatically detected
by the system is forced pressure either on the navigation aspi-
rator, so that it will bend and give wrong information about
the actual position, or pressure on the flexible headset by
assisting staff or wrong draping during the procedure.
From this it becomes evident that the final responsibility is
always with the surgeon. He or she must understand the work-
ing principles of their system and readily identify potential
interferences or deviations.
Based on our experience, the VTI InstaTrak®System is the
most user-friendly and reliable of all systems we tested for use
in very difficult cases in the paranasal sinus and anterior skull
base regions. Despite the high number of very complex cases
performed, no complications occurred in our series.
Intraoperative computer-assisted navigation is a very helpful
technology in selective cases, providing high accuracy and reli-
ability. The surgeon must be familiar with anatomical struc-
tures of the surgical field, however, no system can take away
that responsibility. Navigational systems provide intraoperative
help, but surgeons will remain responsible for all of their deci-
There is no answer yet as of today whether routine use of navi-
gational systems will decrease the number of complications of
endoscopic or other sinus surgery, nor whether such systems
will improve any surgeon’s dexterity, talent or results. We have
become convinced however, of the increased efficacy of the
surgical treatment in selected, difficult cases, when the confi-
dence of the surgeon is significantly enhanced when working
in topographically delicate areas.
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Wolfgang Koele, M.D.
University Medical School Graz
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