Principles Of Image-Guided Spinal Navigation

The use of an image-guided navigational system for localizing intracranial lesions has been previously described.^7,8 Image-guided navigation establishes a spatial relationship between a preoperative CT image and its corresponding intraoperative anatomy. Both the CT image and the anatomy can each be viewed as a three-dimensional coordinate system with each point in that system having a specific x, y, and z Cartesian coordinate. Using defined mathematical algorithms, a specific point in the image data set can be matched to its corresponding point in the surgical field. This process is called registration and represents the critical step of image-guided navigation. At least three points need to be matched, or registered, to allow for accurate navigation.

A variety of navigational systems have evolved during the past decade. The common components of most of these systems include an image-processing computer workstation interfaced with a two-camera optical localizer (Fig. 305-1). When positioned during surgery, the optical localizer emits infrared light toward the operative field. A handheld navigational probe mounted with a fixed array of passive reflective spheres serves as the link between the surgeon and the computer workstation (Fig. 305-2). Alternatively, passive reflectors may be attached to standard surgical instruments. The spacing and positioning of the passive reflectors on each navigational probe or customized trackable surgical instrument is known by the computer work-station. The infrared light that is transmitted toward the operative field is reflected back to the optical localizer by the passive reflectors. This information is then relayed to the computer work-station, which can then calculate the precise location of the instrument tip in the surgical field as well as the location of the anatomic point on which the instrument tip is resting.

FIGURE 305-1 Image-guided navigational workstation with infrared camera localizer system.

FIGURE 305-2 Navigation probe and drill guide for spinal surgery.

The initial application of navigational principles to spinal surgery was not intuitive. Early navigational technology applied to intracranial surgery used an external frame mounted to the patient’s head to provide a point of reference to link preoperative image data to intracranial anatomy. This was not practical for spinal surgery. The current generation of intracranial navigational technology uses reference markers or fiducials that are attached to the patient’s scalp before imaging. However, the use of these surface-mounted fiducials for spinal navigation is not practical because of accuracy issues related to a greater degree of skin movement over the spinal column.^9,10 This is less of a problem with intracranial applications because of the relatively fixed position of the overlying scalp to the underlying anatomy.

The application of navigational technology to spinal surgery involves using the rigid spinal anatomy as a frame of reference. Bone landmarks on the exposed surface of the spinal column provide the points of reference necessary for image-guided navigation. Specifically, any anatomic landmark that can be identified intraoperatively as well as in the preoperative image data set can be used as a reference point. The tip of a spinous or transverse process, a facet joint, or a prominent osteophyte can serve as a potential reference point (Fig. 305-3). Because each vertebra is a fixed, rigid body, the spatial relationship of the selected registration points to the vertebral anatomy at a single spinal level and is not affected by changes in body position.

FIGURE 305-3 Navigational workstation screen demonstrating a paired point registration plan for the insertion of T12 pedicle screws. Three discreet bony landmarks are selected at the T12 level. In this case, the lateral margins of the two T12 transverse processes and the tip of the T12 spinous process have been selected.

The purpose of the registration process is to establish a precise spatial relationship of the image data with the physical space of the patient’s corresponding surgical anatomy. If the patient is moved after registration, this spatial relationship is distorted, making the navigational information inaccurate. This problem can be minimized by the optional use of a spinal tracking device consisting of a separate set of four passive reflectors mounted in a known configuration on a small frame. This reference frame can be attached to the exposed spinal anatomy and its position in space tracked by the infrared camera system (Fig. 305-4). Movement of the spinal anatomy and the attached frame alerts the navigational system, which can then make the appropriate correctional calculations to maintain accuracy and eliminate the need to repeat the registration process. The disadvantage of using a tracking device is the added time needed for its attachment to the spine, the need to maintain a line of sight between it and the camera, and the inconvenience of having to perform the procedure with the device placed in the surgical field. It is particularly cumbersome when image-guided navigation is used during cervical procedures. Alternatively, image-guided spinal navigation can be performed without a tracking device.^1,12 This involves acknowledging the effect of patient movement on the accuracy of image-guided navigation and maintaining reasonably stable patient position during the relatively short amount of time needed (i.e., 10 to 20 seconds) for the selection of each appropriate screw trajectory. Patient movement can potentially occur with respiration, from the surgical team leaning on the table, or from a change of table position. Movement associated with patient respiration is negligible and does not require any tracking, even in the thoracic spine. Although movement associated with leaning on the table or repositioning the table or the patient will affect registration accuracy, it can be easily avoided during the short navigational procedure. If inadvertent patient movement does occur, the registration process can be repeated.

FIGURE 305-4 Reference frame attached to a spinous process in the surgical field. The reference frame monitors inadvertent movement of the spinal anatomy that may affect navigational accuracy.

Three different registration techniques can be used for spinal navigation: paired point registration, surface matching, and automated registration. Paired point registration involves selecting a series of corresponding points in a computed tomography (CT) or magnetic resonance imaging (MRI) data set and in the exposed spinal anatomy. The registration technique is performed immediately after surgical exposure and before any planned decompressive procedure. This allows for the use of the spinous processes as registration points.

A specific registration point in the CT image data set is selected by highlighting it with the computer cursor. The tip of the probe is then placed on the corresponding point in the surgical field, and the reflective spheres on the probe handle are aimed toward the camera. Infrared light from the camera is reflected back, allowing the spatial position of the probe’s tip to be identified. This initial step of the registration process effectively links the point selected in the image data with the point selected in the surgical field. When at least three such points are registered, the probe can be placed on any other point in the surgical field, and the corresponding point in the image data set will be identified on the computer workstation.

Alternatively, a second registration technique called surface matching can be used. This technique involves selecting multiple, random (nondiscrete) points on the exposed surface of the spine in the surgical field. This technique does not require prior selection of points in the image set, although several discrete points in both the image data set and the surgical field are frequently required to improve the accuracy of surface mapping. The positional information of these points is transferred to the work-station, and a topographic map of the selected anatomy is created and matched to the patient’s image set.¹¹

Typically, paired point registration can be done more quickly than surface mapping. The average time needed for paired point registration is 10 to 15 seconds. The time needed for surface mapping is much longer, with difficult cases requiring as much as 10 to 15 minutes. With the need to perform several registration processes during each surgery, this time difference can significantly affect the length of the navigational procedure and the surgery.¹²

More recently, the development of intraoperative CT systems that incorporate navigational technology has allowed for the use of an automated registration technique. This approach involves attachment of a reference frame on the exposed spinal anatomy. The intraoperative imaging system is then positioned, and a CT image of the pertinent spinal anatomy obtained. A second reference frame attached to the CT scanner allows its position relative to the spine reference frame and, therefore, the spinal anatomy to be tracked as well. With these spatial relationships established, the navigational system can then perform an automated registration of the spinal anatomy eliminating the manual efforts required of the surgeon with the paired point and surface matching techniques. The scanner is removed, and real-time navigation of up to five separate spinal levels can be performed.

After accurate registration, a trackable probe with attached reflective spheres can be positioned on any surface point in the surgical field. As the probe is tracked by the camera system, three separate reformatted CT images centered on the corresponding point in the image data set are immediately presented on the workstation monitor. Each reformatted image is referenced to the long axis of the probe. If the probe is placed on the spinal anatomy directly perpendicular to its long axis, the three images will be in the sagittal, coronal, and axial planes. A trajectory line representing the orientation of the long axis of the probe will overlay the sagittal and axial planes. A cursor representing a cross section through the selected trajectory will overlay the coronal plane. The insertional depth of the trajectory can be adjusted to correspond to selected screw lengths. As the depth is adjusted, the specific coronal plane will also adjust accordingly, with the position of the cursor demonstrating the final position of the tip of a screw placed at that depth along the selected trajectory. As the probe is moved to another point in the surgical field, the reformatted images, as well as the position of the cursor and trajectory line, will also change. The planar orientation of the three reformatted images will also change as the probe’s angle relative to the spinal axis changes. When the probe’s orientation is not perpendicular to the long axis of the spine, the images are displayed in an oblique, or orthogonal, plane. Regardless of the probe’s orientation, the navigational workstation will provide the surgeon with a greater degree of anatomic information than can be provided by any intraoperative imaging technique.

Clinical Applications

Image-guided spinal navigation was initially evaluated for the insertion of pedicle screws in the thoracic and lumbosacral spines of cadaver specimens. The accuracy of screw insertion was documented by plain film radiography and thin-section CT imaging of the instrumented levels. Satisfactory screw placement was noted for 149 of 150 inserted screws.² The initial clinical application of image-guided spinal navigation was its use for lumbosacral pedicle fixation.^1,13,14 Other spinal applications gradually evolved including transoral decompression, cervical screw fixation, thoracic pedicle fixation, decompression of spinal metastasis, and anterior thoracolumbar decompression and fixation procedures.^12,15–20

The application of image-guided navigation to spinal surgery is directed by the complexity of the procedure and, specifically, by the need to visualize the unexposed spinal anatomy. Image-guided navigation can be used with or without standard intraoperative imaging techniques (i.e., fluoroscopy). In either case, image-guided navigation provides the surgeon with an improved orientation to the pertinent spinal anatomy, which subsequently facilitates the accuracy and effectiveness of the procedure.