Computer-assisted image-guided intervention, also known as computer-assisted surgery, is the name given to the technology in which preoperative slice data from magnetic resonance imaging, computed tomography, ultrasonography, or other imaging modalities are used to guide interventional procedures. The images are acquired prior to or during the procedure and loaded onto a computer workstation. The workstation is connected to a position sensor, and the images are registered to the patient just before or during the procedure.
During the intervention, the physician uses instruments that are similar to those normally used to perform the procedure, except that they are specially modified to hold a tracking element or "tracker" that enables the position and orientation of the instrument tip to be determined. The instrument's position and orientation are then shown as an overlay graphic icon, such as crosshairs or a rendering of the device, on the registered images.
Tracking is one of the fundamental requirements of computer-assisted image-guided intervention. All image-guided intervention systems require a tracking device. This device is used to determine both the location and the angle of the tip of the instrument being used during the intervention. In addition, it may be used to track the location of tissue or tissue fragments to correct for motion of the tissue or simply to track gross patient motion, including respiration.
Several types of position-sensing devices have been used in computer-assisted surgery over the years, including mechanical spatial linkage devices, sound-based position sensors, optical trackers, and electromagnetic tracking systems. Most position sensors contain two parts: a tracking system and a tracker device. The tracking system is placed near the patient in the procedure room. Typically this takes the form of a camera or an electromagnetic field generator, which can be large. The tracker is smaller and is attached to an instrument or to the patient; it is the location of this local device that is tracked. Both passive (wireless) and active (wired) trackers exist.
Spatial Linkage Systems
Spatial linkages were one of the first technologies used in image-guided intervention. They were restricted mainly to cranial procedures such as neurosurgery and endoscopic sinus surgery1,2. Mechanical linkages consist of passive, multi-jointed mechanical arms in which encoders placed in the joints are used to determine the location and orientations of instruments placed at the end of the arm. Although accurate, these systems tend to be bulky and are difficult to adapt for monitoring multiple objects and tracking patient motion. Nonetheless, articulated arms were widely and successfully used in the first-generation image-guided intervention systems.
Ultrasonic Tracking Systems
Sound-based systems3 were also used commercially for a short time but tended to be less accurate than required; they were easily disturbed by ambient sounds and temperature fluctuations in the surrounding environment. Typically, they contained at least three microphones arranged on a rigid frame. Emitters (such as spark gaps) placed on instruments emitted short bursts of high-frequency sound that could be picked up by the microphones and used to triangulate the location of the instrument. With the advent of affordable optical systems, ultrasonic trackers were quickly replaced.
Optical Tracking Systems
Optical tracking technology for image-guided intervention originated with photogrammetry—calibrated camera systems that were used in topographic mapping, architecture, and manufacturing as early as the middle part of the 1800s. The technique was later applied to gait analysis and finally to image-guided intervention. There are two main technologies used in optical tracking: active and passive.
Active Optical Systems
Active optical systems make use of infrared light-emitting diodes that are viewed by cameras placed in the room (Fig. 1). The infrared light-emitting diodes are rigidly attached to the tracker at known locations, and the tracker is fixed to the instrument or to the patient. A cable feeding back to the camera system or to batteries in the tracker powers the infrared light-emitting diodes. The cameras then detect the light from the infrared light-emitting diodes and calculate the position of the tracker and the instrument or the patient.
Passive Optical Systems
Passive or freehand optical systems employ either special printed patterns (e.g., checkerboard patterns) or retroreflective markers (such as spheres or disks) placed on the trackers. The cameras of systems that make use of retroreflective markers employ arrays of infrared light-emitting diodes around each lens of the camera. Light generated by the illuminators is strongly reflected by each retroreflective target back to the camera lens.
Clusters of at least three reflectors are attached to the tracker (Fig. 2-A), allowing the orientation and location of the tracker to be determined. Systems that make use of retroreflective targets include the Polaris series of cameras (Northern Digital, Waterloo, Ontario, Canada), and these systems are widely used in computer-assisted orthopaedic surgery. They are almost as accurate as active optical systems and are cable free. Some hybrid camera models support both active and passive tracking.
Systems that make use of high-contrast printed patterns and that rely on ambient room light include the MicronTracker (Claron Technology, Toronto, Ontario, Canada). These systems make use of trackers comprised of checkerboard targets of unique geometries (Fig. 2-B).
While most commercial optical tracking systems for computer-assisted surgery applications are based on customized camera systems consisting of two or three cameras attached to a rigid support, it is also possible to construct such devices with use of off-the-shelf cameras. Moreover, most modern computer-assisted surgery systems employ passive-only trackers or a combination of passive and active trackers.
Optical systems are extremely accurate; individual markers can be located with an accuracy of <1 mm (typically 0.25 mm)4, with tool tips locatable in the 1 to 2-mm range. The biggest disadvantage of optical systems is the requirement that there be a direct line of sight from all of the cameras being used to all targets on all trackers being used. Surgical fields are often cluttered, and it is sometimes challenging to maintain line of sight. Optical tracking systems also rely on rigid instruments. Even small bends of the instrument between the tip and the tracker induce error since the tip is assumed to be in a fixed relationship to the tracker. Optical systems are not suitable for tracking needles or other flexible devices.
Electromagnetic Tracking Systems
Electromagnetic tracking systems represent another class of commonly used tracking devices. Although these systems are considered to be one of the older technologies, they have recently begun to be used more widely in image-guided intervention. Initially, electromagnetic tracking devices were highly susceptible to metal interference and were generally not as accurate as optical systems. These systems found use in motion capture for animation in the entertainment industry.
Recent improvements in the technology have enabled much smaller and more accurate sensors to be developed and have greatly reduced the effects of metal artifacts. Modern commercial systems suitable for medical applications are currently manufactured by Northern Digital, Ascension Technology (Burlington, Vermont), and Biosense Webster (Diamond Bar, California). All of these systems are "active" systems, requiring wires between the sensor in the instrument and the tracking system. Passive electromagnetic tracking systems rely on radiofrequency identification-type technology, which makes use of the position sensor to wirelessly power the tracking element with a pulse of electromagnetic energy. The position sensor then transmits a signal to a receiver to determine its location. Only the Calypso System (Calypso Medical, Seattle, Washington) currently makes use of wireless technology, which is currently limited to shorter ranges and larger trackers than are achievable with the active counterparts.
Electromagnetic systems generate a series of electromagnetic signals from the field generator or "transmitter." The field generator is the electromagnetic equivalent of the camera in the optical systems. These signals induce a voltage in the sensors that are attached to the instruments. Trackers for electromagnetic systems include small sensor coils that are attached to or embedded in the instrumentation (Fig. 3). These are cylindrical and in the range of 0.5 to 1 mm in diameter and 5 to 10 mm long. They are capable of resolving translation and orientation in two planes (i.e., pitch and yaw), but the cylindrical shape of these devices makes it difficult to resolve the roll motion around the coil's longitudinal axis. Two or more coils can be combined or dual-directional winding can be used to construct trackers that can measure instrument location in all six degrees of freedom. The accuracy of electromagnetic sensors has been reported to be approximately 0.9 mm, with tools typically achieving accuracy in the 1.5 to 2.5-mm range5.
The small size of the sensors makes them particularly attractive for embedding into devices such as needles, catheters, guidewires, and flexible endoscopes6,7. They can also be placed into Kirschner wires and screws and can thus track even small osseous fragments. As electromagnetic tracking systems do not require line of sight, the sensor can be embedded within the tip of the instrument and still be tracked (Fig. 4). When combined with tracked ultrasonography, these devices enable physicians to navigate needles with the aid of composite images consisting of live ultrasonography blended with preprocedural computed tomographic or magnetic resonance images reformatted to the same scan plane as the ultrasonographic image (Figs. 5-A and 5-B).
The main disadvantage of these systems is the influence of metal, which can distort measurements and influence accuracy. While modern systems are still affected by metal, considerable improvement has been achieved recently. Current systems are influenced only by large metal objects, and newer systems make it possible to detect when the system is being adversely affected by metal. With careful attention paid to the environment and with close monitoring of the amount of distortion, the systems can be used safely and effectively for many applications. Orthopaedic environments remain challenging, however, as it simply may not be possible to eliminate some of the potentially offending devices and equipment from many of the more invasive open procedures. Nonetheless, Medtronic Navigation (Boulder, Colorado) produces an electromagnetic system, named AXIEM, for total knee replacement and GE Healthcare (Milwaukee, Wisconsin) has an electromagnetic tracking system combined with a mobile c-arm called FluoroTrak.
Electromagnetic systems are mostly active systems, requiring the use of tethering cables connected to the position sensor. Wireless electromagnetic systems are not yet practical for tracking instrumentation. Electromagnetic systems are less accurate than optical systems, but, because the tip can be tracked directly, they are still accurate enough for many applications.
Optical tracking systems represent a mature and proven technology. Thousands of medical procedures—ranging from neurosurgical operations to spine surgery, trauma, and general orthopaedic procedures—have been performed with use of this equipment. The systems that make use of optical tracking technology are generally accurate and reliable. The systems also have a good tracking volume that is typically several cubic meters. Accuracy is in the range of 1 mm.
Two main disadvantages are present in all optical systems, however. Because the accuracy of optical systems is related to tracker size, the trackers themselves tend to be large. They contain at least three targets (e.g., retroreflective spheres or infrared light-emitting diodes) that must always be visualized by the camera system. The second major problem with optical systems is that they are line-of-sight devices, requiring a direct optical path between the position sensor system (comprised of two or more cameras) and the three or more targets that are being tracked. Cluttered environments, such as medical procedure rooms and operating rooms, sometimes make this difficult to achieve. In addition, line of sight places restrictions on instruments, since the tracker must always be visible. Normally it is attached to the proximal end of rigid instruments, with the assumption that the tip of the instrument does not deflect relative to the tracker. While this is readily achievable for many instruments, particularly those used in orthopaedic surgery, less invasive approaches that use fine-gauge needles or flexible endoscopes to deliver therapy are more of a problem; these instruments cannot be effectively tracked with use of optical technology because the visible portion of the tracker may deflect relative to the location of the tip of the instrument.
Electromagnetic systems have also been widely used, although not quite as widely as optical systems, in orthopaedics. Electromagnetic systems are finding more frequent use in soft-tissue interventions, where their smaller size makes them particularly appealing. The ability of electromagnetic systems to track without requiring a direct line of sight to the instrumentation is a great advantage of this technology; however, these systems have smaller tracking volumes, which can lead to problems in some image-guided procedures, such as total knee or hip arthroplasty, in which trackers are routinely attached to the hip, tibia, and femur to establish joint kinematics that are used in the alignment of the implant8,9. Also, when tracking mobile c-arms that are used in trauma or pedicle screw applications, electromagnetic field generators may have insufficient volume. Large metal devices such as c-arms may also adversely affect the accuracy of electromagnetic tracking systems10,11. Although the c-arm can normally be removed from the field for parts of the procedure, initial frame captures also require accurate tracking data from the electromagnetic tracker. Moving the c-arm in and out frequently can make the procedure awkward and time-consuming.
Currently, a plethora of optically tracked instruments is available, including probes, drills, drill guides, saw guides, reamers, biopsy guides, trial implant devices, anatomical trackers, and fluoroscopic trackers. Together these devices are used for a variety of procedures, including pedicle screw placement12, distal locking and alignment of long-bone fractures, alignment of knee and hip implants, screw placement, osteotomy, intracranial procedures, and navigated ear, nose, and throat procedures.