Advanced Intraoperative Imaging and Navigation Promise
to Make Spine Surgery Safer
The development of computer-aided procedures over the
last several years has led to significant qualitative
improvements in surgical interventions of the spine.
Advancements in intraoperative imaging and navigation
systems have been exceptional. They’ve also been
somewhat interdependent. The emergence of highly sophisticated,
three-dimensional intraoperative imaging systems has
broken down barriers for navigation systems, making
it safer to conduct spine surgery.
“The Iso-C3D C-arm imaging system is revolutionizing
the way minimally invasive spine surgery is performed,”
says Chicago Institute of Neurosurgery and Neuroresearch
(CINN) neurosurgeon Dean
G. Karahalios, M.D. “This system, which provides
images in real-time, allows us to see what is changing
as we move forward with the procedure.” A spine
surgeon’s reliance on preoperative anatomical imaging
is fading quickly. A higher level of efficiency is now
obtainable through Iso-C3D technology and complementary
image guidance software and hardware. Precision surgical
techniques, including minimally invasive procedures,
utilizing these new technologies are potentially safer
for the patient by minimizing risk, and limiting the
need for postoperative surgical revision.
Marriage of Technologies (intraoperative imaging +
navigation = image-guidance)
Intraoperative imaging has undergone a remarkable evolution
over the past several years. In its most primitive form,
intraoperative imaging consists of plain radiographs
taken during an operation to localize a specific target.
For spinal procedures, the fluoroscope has become indispensable
in providing two-dimensional imaging in real-time during
surgery. Intraoperative CT and MRI scanners have been
developed, but have significant limitations and for
most centers are prohibitively expensive. A new type
of fluoroscope, Iso-C3D C-arm imaging system, is now
able to provide three-dimensional (3-D) CT-like images
during surgery. The imaging data obtained with this
device can then be transferred to an intraoperative
navigation system providing the surgeon unprecedented
capabilities.
Intraoperative image-guided navigation has also been
evolving rapidly. A state-of-the-art intraoperative
navigation system consists of a computer workstation
with a powerful graphics processor. The computer analyzes
imaging data that it receives either prior to or during
an operation. In the past, only pre-operative images
were usable, but now we can uses images obtained during
surgery. The imaging data is then displayed on a monitor,
which the surgeon observes during surgery. The imaging
data can be manipulated to demonstrate the spine from
various perspectives. Until recently, these perspectives
have been limited to 2-D planar views. The Iso-C3D C-arm
imaging system not only provides real-time intraoperative
images, but also provides imaging data that the navigation
computer can reconstruct and display from any and all
perspectives. An infrared camera linked to the computer
is positioned in the corner of the operating room. It
acts as the “eye” of the computer, tracking
the position and movement of the patient’s spine
and the surgeon’s instruments during the procedure.
It accomplishes this by emitting an invisible infrared
light signal that bounces off of reflective spheres
mounted onto the end of a reference frame (which is
placed on the spine) and surgical instruments. A direct
line-of-sight is necessary between the spheres and the
camera. They also must be mounted on rigid instruments
so that there is no variability between the tip of the
instrument on the patient’s anatomy and the reference
spheres on the distal end. The precise position of the
tip of the surgeon’s instrument relative to the
spine is represented on the monitor. The surgeon can
then navigate in and around the spine, accurately targeting
lesions and avoiding injury to critical structures.
“It works like a car’s GPS navigation system”,
says Dr. Karahalios. “The infrared camera acts
like a satellite which tracks the patient and surgical
instruments. The computer then displays a roadmap for
the surgeon. However, unlike conventional GPS navigation
systems which rely on archived maps, the current intraoperative
navigation system use real-time data. Not only do you
know where the road is, but also if the there is traffic
ahead or if the bridge is out.”
The Path to Precision
Three-dimensional intraoperative imaging provides more
information and higher certainty during OR procedures.i
Such image data acquisition—notably, that conducted
with the SIREMOBIL Iso-C3D system (Siemens Medical Solutions)—provides
the best means of ensuring precise correlation between
image data, real-time patient anatomy and surgical instruments.(ii)
The frameless stereotactic image-guided technology,
with which the Iso-C3D C-arm is paired, was initially
pioneered for use in cranial neurosurgical procedures.
The joint application of long-standing, fundamental
stereotactic principles and high-speed computer processing
resulted in the ability to rapidly compare stereotactic
data with preoperative imaging data. A practical instrument
emerged, providing the surgeon with simultaneous multiplanar
images depicting the precise location of a target intraoperatively.
The utility of this technology has been evident for
some time now in cranial procedures; however its efficacy
in spinal procedures has been demonstrated only recently.
In spinal surgery, as in cranial surgery, frameless
stereotactic guidance can be used to localize anatomic
structures and pathologic lesions, and to implant instrumentation
with a greater degree of precision and safety.(iii)
Prior to the Iso-C3D system’s development, there
were basically two options for acquiring images of anatomical
structures for use during surgery. First, there was
preoperative radiographic image guidance technology.
Pre-operative radiographic imaging yielded axial, three-dimensional
and trajectory views, which markedly enhanced the surgeon’s
ability to appreciate and navigate through the anatomy
conceptually. However, pre-operative radiographic imaging
could not produce real-time views, and therefore the
images did not necessarily reflect the state of the
patient’s anatomy during surgery. The introduction
of a second option, intraoperative fluoroscopy, had
a substantial advantage in this respect. Real-time feedback
was especially valuable if the relevant information
required related to a change that a surgeon expected
to occur intraoperatively (e.g., gauging the extent
of a decompression, the advancement of an implant, or
the reduction of a fracture or subluxation).(iv)
“Fluoroscopic navigation was leading edge for
awhile,” recalls Dr. Karahalios. “It allowed
us to see limited two-dimensional images displayed on
the computer and to navigate the surgical field, but
it didn’t provide a complete view.” In direct
contrast to preoperative radiographic image guidance
technology, the images obtained through intraoperative
fluoroscopy and plain radiography were limited to 2-D
sagittal, coronal or oblique planes.v Iso-C3D technology
would adopt the benefits of both and the shortcomings
of neither. “What intraoperative MRI did for brain
surgery, the Iso-C3D imaging system will do for spine
surgery,” predicts Dr. Karahalios. “The technology
took the latest breakthroughs in imaging for the brain
and translated it to spine.”
Iso-C3D
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| The
Iso-C3D C-arm imaging system has enabled
neurosurgeons to view the spine three
dimensionally at any angle in real
time during surgery. |
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The SIREMOBIL Iso-C3D C-arm imaging system marks a
true evolution in imaging technology. “With the
Iso-C3D C-arm imaging system, which rotates around the
patient creating and combining multiple planar images,
we get a real-time view of the spine in ways we could
not before,” summarizes Dr. Karahalios. The motorized
C-arm captures a 3D data set—much like a CT scan—and
transfers it to a navigational system for processing.
Through the navigational system, the surgeon can utilize
real-time imaging data to guide his instrumentation.
The Iso-C3D system in place at NOIC operates in tandem
with the StealthStation® navigational system by
Medtronic Sofamor Danek. Other companies also offer
a navigational interface that is compatible with the
SIREMOBIL Iso-C3D C-arm imaging system, but approximately
90 percent of the market is using the Siemens/Medtronic
Sofamor Danek configuration. Together the two advanced
systems have taken intraoperative fluoroscopic navigation
to a higher level in cervical, thoracic and lumbar spine
surgery.(vi)
In comparison with other potential 3D modalities that
may be used in the OR for spine care, SIREMOBIL Iso-C3D,
as a mobile C-arm, allows virtually unlimited patient
access without increasing preparation time and procedure
complexity. It facilitates work in sterile conditions
and saves precious space in the OR.vii Intraoperative
CT and MRI provide real-time information but they restrict
access to the patient, preclude the use of traditional
operating room equipment, and are time consuming. By
contrast, a recent study of the Iso-C3D thoroughly demonstrated
its ability to provide quick, CT-like quality, real-time
acquisition of data, which can be transferred to navigational
systems like StealthStation to facilitate spine procedures.(viii)
During the C-arm’s continuous 190-degree, motorized
orbital rotation, the system records a set of 50-100
fluoro projection images at equidistant angles. Following
the rotation’s completion, a reconstruction phase
of approximately 120 seconds yields a high-resolution,
3D image data cube [2563 isotropic pixels of a volume
of approximately (120 mm)3]. At this point, physicians
at the OR table or monitor trolley can direct the unit
to execute Multiplanar Reconstruction (MPR) of desired
images in real time. This 3D rendering process may be
repeated throughout the surgical procedure to update
the registration or provide multi-slice images in combination
with images from other modalities (e.g., soft tissue
imaging using ultrasound).(ix)
Upon automatic registration, image-guided surgical
instruments can be used and displayed over the previously
reconstructed images immediately following acquisition
of patient image data. This accuracy and simplicity
is in sharp contrast to methods of conventional registration
using preoperative MRI or CT images used by many centers.
Conventional registration involves the spatial correlation
between the preoperative image data set, and the patient’s
true anatomy in the OR, as determined either by anatomical
reference points or artificial markers that have been
attached to the patient. If fiducial markers are used,
the markers may have to be attached to the patient a
day in advance of surgery. Furthermore, the marker points
have to be visible and clearly identifiable in the preoperative
images, so that at least three corresponding pairs of
points can be used to coordinate the exchange of information
between the image data set and the patient’s anatomy
on the operating table.(x)
Because this entire process must be repeated each time
that navigation calls for intraoperative readjustment,xi
this conventional, preoperative-image-based registration
process can be laborious and time consuming. But more
importantly, because the anatomy of a patient prior
to surgery can vary significantly from his or her anatomy
during surgery, it also can be misleading and potentially
dangerous to the patient. Patient repositioning or effects
caused by the surgical intervention itself may impede
or even prevent the function of a conventional navigation
system.xii Iso-C3D technology linked with the Stealth
system and its capacity for “markerless” registration
safeguards the patient from any such complications.
As long as the position of the C-arm can be determined
at the time of the 3D measurement, the position of the
navigation devices may be superimposed at any time within
the 3D data set. An offline calibration procedure determines
a registration matrix once, and the system utilizes
it to guide the surgeon throughout the procedure.(xiii)
The superior efficiency demonstrated by the Iso-C3D
system hasn’t been limited to the registration
process. Despite its high power output, the Iso-C3D
system also keeps radiation doses at an absolute minimum,
protecting the patient from unnecessary radiation exposure.
Each pass of the C-arm is equivalent to just 20 seconds
of radiation. Dose savings are achieved with Sirematic
image curves, pulsed fluoroscopy and adjustable dose
rates, along with digital image rotation and a laser
targeting.
Applications
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| Screenshot from the monitor of the Stealth system displaying reconstructed images of the spine in multiple planes and in real-time. Views of this type were previously unavailable and can be extremely helpful to the surgeon to optimize the safety of a spinal operation. |
The most common use of Iso-C3D technology in the spine
is for the placement of pedicle screws.(xiv) Using pedicle
screws as a means of spinal fixation, especially in
thoracic and lumbar applications, can be very effective.
However, its efficacy depends on the proper placement
of the screws. The malposition of screws can cause serious
and potentially life threatening complications. The
real-time intraoperative imaging of the Iso-C3D system
is proving to be a more reliable means of operating
with such instrumentation.(xv)
“Using the Iso-C3D system, we have extreme precision
when placing pedicle screws in single vertebrae,”
reports Dr. Karahalios. “Because of the ability
to utilize a wide array of reconstructed views such
as in the axial plane, we can minimize mediolateral
misses. It also allows us to check the positioning of
the instrumentation before we close. Studies show that
the break out rate of pedicle screws can be as high
as 10 percent, but according to unpublished data from
a live patient study currently underway at NOIC, our
break out rate using the Iso-C3D system may be far lower.”
These results are especially impressive when one considers
that in the early nineties, as many as 40 percent of
all pedicle screws were suboptimally placed.(xvi) Instrumentation
placement may be an increasingly difficult task in the
thoracic and cervical regions of the spine, where the
anatomy is smaller and more variable in shape, orientation
and angle. The power and utility of this technology
is even more evident in these regions, which require
more exacting precision.
“In some cases, you are dealing with pathologic
instability,” adds Dr. Karahalios. “We must
transfix two segments of the spine with one screw.”
Information gleaned from axial and trajectory views
with the Iso-C3D system (which is inherently difficult
to obtain with a conventional fluoroscope) become critical
for safe placement of the screws.(xvii) “Preoperative
imaging won’t give you the relative position of
individual mobile spinal elements in relation to each
other during surgery,” explains Dr. Karahalios.
“Before the Iso-C3D system, there was a lot of
guesswork. If you guessed wrong you could hit the spinal
cord or vertebral artery. With the Iso-C3D system we
can get real-time axial and trajectory views, which
wasn’t possible with 2-D data sets.” In this
regard, a procedure for which Iso-C3D technology is
quite useful is the placement of posterior transarticular
screws to fixate the atlas and the axis.(xviii) Because
the fusion procedure places a number of anatomical structures
at risk (i.e. vertebral artery, spinal cord and C2 nerve
root) the screw must be precisely placed through the
pars interarticularis, which in many cases can be quite
narrow. The vertebral artery’s path through the
axis is extremely variable and if it is found to run
too high and medial in a patient, the safe passage of
transarticular screws may be impossible. Algorithms
based on preoperative CT analysis of C2 have been used
to conclude that up to 18% of pars may be unsuitable
for screw placement. However, this yield can be significantly
increased by 75% with the help of image-guidance technology.(xix)
The Iso-C3D system can also be used to facilitate spinal
decompression such as in corpectomy or vertebrectomy
for degenerative and neoplastic processes.(xx) “The
Iso-C3D system is also valuable in revision surgery,”
says Dr. Karahalios. “Without the Iso-C3D technology
we could have off-center or incomplete decompression.
With it, however, we can effect a perfect decompression.”
Resection of spinal tumors is an excellent application
for the new technology as well. “The Iso-C3D system
allows us to ensure we have proper margins when dealing
with radioresistant tumors,” says Dr. Karahalios.
Using the Iso-C3D system, surgeons can precisely outline
the margins of a tumor to be resected preoperatively,
and then follow these highlighted margins intraoperatively
to achieve a gross total resection safely, and with
confidence.(xxi)
In addition to facilitating spine surgery, the SIREMOBIL
Iso-C3D system has been designed specifically for intraoperative
use with bones and joints of the lower and upper extremities.
The system is an ideal aid for precisely reconstructing
many types of joint surfaces and complex fractures,
as well as positioning screws and implants.
Future Trends
As frameless stereotactic image guidance technology
continues to evolve at a rapid pace, improvements in
workstation hardware and software will increase the
speed and functionality of these systems. Preoperative
planning will become even more sophisticated, eventually
reaching the point at which “virtual surgery”
can be performed prior to the actual procedure. Through
this rehearsal, residents-in-training and surgeons alike
will learn new techniques faster and apply them more
safely.(xxii)
The line-of-sight interference commonly experienced
with the infrared-based tracking system, which is the
current industry standard, will likely be overcome by
emerging tracking technologies such as magnetic field-based
systems. Instruments with small antennae at their tips
can be tracked by “sensing” their relative
positions in a magnetic field that surrounds the spine.
Both tracking and localization can occur transcutaneously,
including that of multiple independent vertebral motion
segments simultaneously. Tracking at the tips also lends
itself to the use of flexible guidance and localization
instruments such as endoscopes. Early limitations of
the technology include potential interference due to
adjacent metal instruments or implants, but once resolved,
magnetic field-based systems may become the new standard.(xxiii)
For all the impressive advancements that imaging has
made in recent years, viewing the monitor itself still
presents a problem. Currently, the surgeon’s eyes
must deviate away from the surgical field to view image-guidance
data but this, too, may soon be a forgotten frustration.
In cranial surgery, the use of the microscope has alleviated
the problem by projecting the images through oculars.
For spine surgery, systems now in development project
images onto a prism attached to the surgeon’s surgical
telescopic glasses. Experimental systems provide the
superimposition of images over the exposed anatomy or
onto a live video image of the anatomy. Future systems
may utilize a technology similar to that used by fighter
pilots for weapons targeting, and project semitransparent
2D and/or 3D images onto a visor worn by the surgeon.(xxiv)
Surgical robotic devices are also in development, which
will work in tandem with the advanced intraoperative
imaging and navigation technologies. Operated remotely
from a workstation, these devices can dampen normal
excessive physiologic movements or tremor, allowing
surgeons to perform intricate procedures more quickly
and with greater precision.(xxv)
Before long, a surgeon may be able to learn a technique,
perfect it and successfully perform it without ever
leaving the computer workstation. As these emerging
technologies develop, their integration will likely
lead to faster operating times, enhanced precision and
decreased patient morbidity.(xxvi)
Conclusion
When coupled with intraoperative navigation, real-time
3-D imaging technology promises to facilitate complex
spinal procedures by improving accuracy, precision,
and safety.(xxvii) A prime example of this technology
at work is the joint application of the SIREMOBIL Iso-C3D
system (Siemens Medical Solutions) and the StealthStation
(Medtronic Sofamor Danek) as a navigational aid in spine
surgery.
Despite the revolutionary prospects of the Iso-C3D
system, this new technology is not without controversy.
“Image-guided technology in general is controversial
and not widely embraced,” notes Dr. Karahalios.
“The learning curve on Iso-C3D and Stealth systems
is high. It requires a large investment in time from
the surgeon and the OR staff. Many surgeons feel that
they “don’t need this” to place instrumentation
safely. However, why would you operate with one hand
tied behind your back when you could use both hands
and all your faculties? This technology is just one
more aid in helping us perform safer operations.”
Again using the automobile analogy Dr. Karahalios adds,
“It’s like driving your car on an unfamiliar
road at night, then on the same road during the day.
No one would argue that the latter is inherently safer.”
Lastly, this technology is also costly. The Iso-C3D
C-arm unit alone costs around $300,000; the StealthStation
is an additional $200,000. These amounts are not easily
recouped by hospitals through billing.
As further advances in intraoperative imaging are made,
the utility of image guidance may expand to a level
that permits most or all spinal procedures to be performed
on patients using safer techniques.(xxviii)
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