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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 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)
i “SIREMOBIL Iso-C3D:Breathtaking Views in the OR,” Siemens Aktiengesellschaft Medical Solutions (2001): 1.
ii D. Ritter, et al. “Markless Navigation with the Intra-Operative Imaging Modality SIREMOBIL Iso-C3D,” Electromedica 70, no.1 (2002): 32.
iii D.G. Karahalios, et al. “Image-Guided Spinal Surgery,” Textbook of Neurological Surgery. Philadelphia: Lippincott Williams and Wilkins, 2.
iv Ibid, 3.
v Ibid, 3.
vi “StealthStation® Siemens SIREMOBIL® Iso-C3D C-arm imaging system interface,” <http://www.stealthstation.com/physician/spine/library/siremobil.jsp> (April 20, 2005).
vii E. Euler, et al. “3D-Imaging with an Isocentric Mobile C-Arm,” Electromedica 68, no. 2 (2000): 126.
viii J. S. Hott, et al. “Intraoperative Iso-C C-arm navigation in cervical spinal surgery: review of the first 52 cases,” Spine 29, no. 24 (2004): 2856-2860.
ix D. Ritter, et al. “Markless Navigation with the Intra-Operative Imaging Modality SIREMOBIL Iso-C3D,” Electromedica 70, no.1 (2002): 33.
x Ibid, 31.
xi Ibid, 33.
xii Ibid, 32.
xiii Ibid, 33.
xiv D.G. Karahalios, et al. “Image-Guided Spinal Surgery,” Textbook of Neurological Surgery. Philadelphia: Lippincott Williams and Wilkins, 4.
xv M. Y. Wang, et al. “Reliability of three-dimensional fluoroscopy for detecting pedicle screw violations in the thoracic and lumbar spine,” Neurosurgery 54, no. 5 (2004): 1138-1142.
xvi E. Euler, et al. “3D-Imaging with an Isocentric Mobile C-Arm,” Electromedica 68, no. 2 (2000): 122.
xvii D.G. Karahalios, et al. “Image-Guided Spinal Surgery,” Textbook of Neurological Surgery. Philadelphia: Lippincott Williams and Wilkins, 15.
xviii Ibid, 4.
xix Ibid, 16.
xx Ibid, 18.
xxi Ibid.
xxii D.G. Karahalios, et al. “Image-Guided Spinal Surgery,” Textbook of Neurological Surgery. Philadelphia: Lippincott Williams and Wilkins, 20.
xxiii Ibid, 20-21.
xxiv Ibid, 21.
xxv Ibid.
xxvi Ibid, 22.
xxvii Jonathan S. Hott, et al. “Intraoperative Iso-C C-Arm Navigation in Craniospinal Surgey: The First 60 Cases,” Neurosurgery 54, no. 5 (2004): 1131-1137.
xxviii D.G. Karahalios, et al. “Image-Guided Spinal Surgery,” Textbook of Neurological Surgery. Philadelphia: Lippincott Williams and Wilkins, 22.