Plain Film Radiography

Plain film radiography was the first imaging technique applied to spine surgery. The segmented bony anatomy of the spinal column lends itself readily to this form of imaging. Images are easily obtained, relatively inexpensive, and generally reliable. Differential attenuation of x-rays by the various tissues provides the image contrast seen in a radiograph. Bone attenuates x-rays the most, followed by muscle, fat, and air, which attenuate very few x-rays. Soft tissue structures such as the spinal cord, nerve roots, or intervertebral discs are not well visualized by plain film radiography.

In spine surgery, plain radiography is typically used to localize a specific spinal level. During the surgical exposure of the cervical, lumbar, or thoracic spine, a radiopaque marker (e.g., clamp, probe, spinal needle) is positioned in the surgical field. For the lumbar or cervical spine, a lateral radiograph is obtained with the lumbosacral or occipitocervical junction at one end and the inserted marker at the other end. The number of vertebral levels between the imaged junction and the marker can be determined and the appropriate spinal level easily localized. In general, the range of view from the occipitocervical junction extends caudally to the cervicothoracic junction, depending on the prominence of the patient’s neck and shoulders. The range of view from the lumbosacral junction extends rostrally to the lower thoracic region.

Spinal levels in the mid- and upper thoracic spine can be more difficult to precisely localize because of their distance from a reliable anatomic reference point. In this case, an alternative to the conventional method of radiographic localization is to obtain two adjacent lateral images. The first film is positioned to include the lumbosacral or occipitocervical junction at one end and an instrument marker attached to the spinal anatomy at the other end. A second film can then be positioned to include the first instrument marker and a second instrument marker centered in the operative field. The location of the first instrument marker can be determined on the first film by its relationship to the lumbosacral or occipitocervical junction. The location of the appropriate spinal level can then be determined on the second film by determining its relationship to the localized first instrument marker.

Radiographic imaging of the upper thoracic spine can be difficult because the patient’s shoulders may obstruct the view on a lateral radiograph. An alternative to the lateral radiograph is the anteroposterior (AP) view. This view can be obtained by positioning the patient on a radiolucent operating table. The film cassette can then be placed immediately below the table and an AP view obtained. With this view, either the T1 or T12 vertebra and its associated ribs serve as the reference point for spinal level identification.

When intraoperative localization involves exposure of the spinal column at the level of a collapsed or fractured vertebra, the relationship of the operative field to the lumbosacral or occipitocervical junction is not as critical. A film that centers over the operative field can approximate the appropriate level. The abnormal vertebrae can usually be identified on a lateral radiograph and the appropriate exposure confirmed.

The most common error related to the use of intraoperative plain radiography is incorrectly identifying the appropriate spinal level. This can be due to inadequate image quality that obscures the appropriate spinal anatomy. This can lead the surgeon to misinterpret the radiograph and incorrectly count the number of spinal levels from the lumbosacral or occipitocervical junction to the operative field. Suboptimal imaging should not be accepted. If satisfactory localization cannot be obtained, the radiograph should be repeated until correct localization is confirmed.

Even when the correct level is identified, localization errors may occur. For example, localization in the lumbar spine typically involves placing a marker on a spinous process and obtaining a lateral radiograph. Although the correct spinous process may be visualized on the film, continued surgical dissection down to the level of the lumbar canal may cause the surgeon to easily drift to an adjacent lumbar segment above or below the desired level. Placement of a marker at the level of the lumbar lamina in addition to the marker on the spinous process can help prevent this error.

In addition to localization of the spinal level, plain radiography can be used in isolated instances to assess the extent of neural decompression. This technique is particularly useful during transoral decompressive procedures. It involves placing a radiopaque contrast medium into the decompressed site and obtaining a lateral radiograph. The radiograph will show the extent of the decompression and can be compared with a preoperative study demonstrating the neural compression. If the configuration and location of the contained contrast medium does not approximate the configuration and location of the epidural compression on the preoperative studies, the surgical site can be modified until a satisfactory decompression of the neural elements is achieved (Fig. 175-1).

FIGURE 175-1 Lateral radiograph after a transoral odontoidectomy. Radiopaque contrast media has been placed into the decompressed site. The position and configuration of the contrast media can be compared to the preoperative studies to confirm a satisfactory decompression.

Plain radiography can also be used in conjunction with intraoperative myelography to determine the extent of canal decompression.¹ In the setting of a thoracolumbar burst fracture decompressed through a dorsolateral approach, radiopaque dye can be placed in the subarachnoid space by a small-gauge spinal needle to assess the adequacy of the surgical decompression. A lateral radiograph is taken and the relationship of the dye to the ventral spinal cord can be assessed (Fig. 175-2).

FIGURE 175-2 Intraoperative lateral myelography after lumbar decompression and fixation.

Fluoroscopy

Fluoroscopy is used when real-time imaging or multiple images of the spine are required during surgery. Fluoroscopic images are generated with a lower radiation dose than that for standard radiography. A camera records the image and displays it on a television monitor. The images can be recorded on x-ray film, cinefluoradiography film, or videotape.²

The primary use of fluoroscopy in spine surgery is to facilitate the optimal placement and positioning of spine fixation screws, interbody cages, or arthroplasty devices and to monitor the injection of methyl methacrylate into osteoporotic vertebral body fractures. Fluoroscopy can provide the surgeon with real-time imaging of the spinal column in several planes depending on the positioning of the fluoroscope’s C-arm. When used for the insertion of pedicle screws, the C-arm can be positioned to provide a lateral view of the spinal column. This shows the sagittal (rostrocaudal) orientation of a pedicle marker or screw (Fig. 175-3). By rotating the C-arm from the lateral position toward the AP position, an oblique view of the spinal column can be obtained. This view demonstrates the position of a pedicle marker or screw in relationship to a cross-sectional view of each pedicle. If the correct entry point and trajectory in both the sagittal and axial planes has been properly selected, the oblique fluoroscopic view will demonstrate these markers within the cortical margins of the individual pedicles (Fig. 175-4). When the correct entry point and trajectories have been confirmed, the markers can be removed and the pedicle screws inserted along the same path.

FIGURE 175-3 Lateral fluoroscopic view of the lower lumbar spine showing the sagittal orientation of Steinmann pins placed within the pedicles of L3, L4, and L5.

FIGURE 175-4 Oblique fluoroscopic view of the lumbar spine showing the pedicles in cross section. Three Steinmann pins have been placed within the pedicles and are noted to be lying within the cortical margins of the pedicles.

When used for cervical screw fixation (e.g., ventral odontoid screw fixation), an AP as well as a lateral view may be required. This can be obtained by alternating the position of a single C-arm unit from the lateral to the AP position or by using two separate fluoroscopy machines with arms positioned 90 degrees with respect to each other.³

Fluoroscopy has also been used for ventral cervical plate instrumentation procedures to assist in the positioning and placement of the fixation plates and screws.⁴ However, the lower cervical and upper thoracic regions can frequently be difficult to visualize because of image obstruction by the patient’s shoulders. This problem can be addressed by placing an interscapular roll beneath the patient and taping the shoulders down to the table or by using wrist slings to pull down on the patient’s arms during imaging.

A potential problem with fluoroscopic imaging is difficulty in obtaining a true lateral image of the spinal anatomy. This can produce a parallax effect that can result in the suboptimal positioning of fixation screws. In the cervical spine, this problem can be addressed by ensuring that the angle of one mandible viewed on the fluoroscopy screen overlaps the angle of the other mandible. In the thoracic and lumbosacral spine, the superior and inferior cortical margins of the vertebrae can be used to ensure a true lateral image. If two cortical margins are seen at one end of a vertebral body, a true lateral image has not been obtained and an adjustment of the C-arm is needed.

The recent development of minimally invasive techniques for spine surgery has led to a significantly increased use of intraoperative fluoroscopy. The limited exposure of soft tissue in these procedures reduces the ability of the surgeon to directly visualize the pertinent surgical anatomy. This makes fluoroscopy a critical component of any minimally invasive procedure.

The primary disadvantage of intraoperative fluoroscopy is the exposure of the surgical team to repeated amounts of low-dose ionizing radiation. Several studies have attempted to quantify the amount of occupational radiation exposure to health-care professionals under different and real clinical scenarios.^5–9 These studies have typically demonstrated that the radiation exposure to hands, eyes, head and neck, and body during fluoroscopically assisted procedures is well below the recommended values for annual allowable occupational radiation exposure as outlined by the International Commission on Radiological Protection.¹⁰ However, the health-related risks of this chronic exposure to radiation remains relatively unknown and may not be realized for years.

Unlike other fluoroscopically assisted musculoskeletal procedures (e.g., intramedullary femoral nailing), the techniques and instruments used for pedicle fixation bring the surgeon’s hands very close to the primary area of exposure.^11,12 The amount of radiation exposure needed to achieve adequate visualization of the lumbar spine is greater than that for other anatomic sites.¹³ Rampersaud et al. reported on the radiation exposure occurring during the placement of thoracolumbar pedicle screws.¹⁴ Pedicle screws were placed into six cadaver specimens using fluoroscopy. Dosimeter badges and rings were placed on the surgeon’s neck, torso, and dominant hand. Radiation exposure to these sites was quantified.

Ninety-six pedicle screws were placed with an average of 8.5 fluoroscopic images taken per screw. The average exposure time per screw was 9.3 seconds. The results of this study demonstrated that fluoroscopically assisted pedicle screw fixation exposes the spine surgeon to significantly greater radiation levels than other, nonspinal musculoskeletal procedures involving the use of fluoroscopy. The radiation exposure to the dominant hand was noted to be as high as 10 to 12 times the amount of hand exposure experienced in other procedures. The greatest level of exposure was noted on the side ipsilateral to the beam source due to backscatter radiation. This study emphasizes the need for appropriate lead shielding, including the use of a thyroid shield and leaded glasses. The use of radiation attenuation gloves resulted in a 33% decrease in dose rate to the hand. Using limited, pulsed-image acquisition and maintaining a safe distance (3–4 feet) from the radiation source also helped reduce the dosage to negligible levels.¹⁴ This study confirmed that spine surgeons who regularly use fluoroscopy should monitor their annual radiation dose.

Ultrasonography

The application of ultrasonography to spine surgery was first reported in 1982.¹⁵ Like fluoroscopy, ultrasonography provides continuous and real-time imaging. However, unlike fluoroscopy, which only provides images of the bony spinal column, ultrasonography can image the adjacent neural and soft tissue structures as well.

Diagnostic ultrasound scanners consist of a scan head containing transducers that convert electrical energy to mechanical or sound energy. To obtain a sonogram the scan head of the ultrasound probe is placed against a contact surface. A short ultrasonic pulse is generated by the transducers and transmitted into the contact surface. This pulse travels continuously into the medium until it strikes another surface or encounters a change in acoustic impedance. Acoustic impedance, the product of the density of a material and the velocity of the sound wave, depends not only on tissue density but also on the actual composition and internal structure of a tissue substance.¹⁶

When the ultrasound pulse strikes a surface, some of the energy is reflected back to the transducer and some of the energy continues forward. Structures that reflect most or all of the ultrasound pulses striking them, specifically air and bone, prevent the imaging of structures behind them. A reflected ultrasonic pulse returns to the scan head and hits the transducer. The shape of the crystal in the transducer is changed, inducing a voltage across it. This voltage is recorded—its amplitude dependent on the strength of the reflection that contacts the transducer face. The location within the tissue from which the reflection came can easily be calculated once the speed of sound in the tissue being scanned and the interval of time between the generation of the impulse and the reception of the reflected wave are known.

The spinal canal can be imaged with ultrasonography during surgery by filling the wound with saline after a laminectomy has been performed. The tip of the transducer is immersed in the saline and manipulated in several directions to give the surgeon an orientation to the specific anatomy. By rotating the scan head 90 degrees, the images can be changed from a longitudinal to a cross-sectional image of the structure.

Dense anatomic structures capable of reflecting ultrasonic pulses are termed echogenic and appear white on the ultrasound images. These structures include the spinal cord, the vertebral body, and any solid intramedullary or extramedullary lesion. The nonechogenic black layers consist of the saline immediately beneath the transducer tip, the subarachnoid space on either side of the spinal cord, and any intramedullary syrinx or cystic lesion.

Ultrasonography can be used to localize a variety of lesions within the spinal canal. It is effective for localizing and characterizing intramedullary spinal cord tumors as well as intradural and extramedullary tumors. Ultrasonography can be used before opening the dura to visualize the extent of the tumor and to localize specific areas of interest such as a cyst within the tumor. The images guide the opening of the dura and can often direct the surgeon to a particular location to start the myelotomy and exploration.¹⁷ When a cystic tumor such as a hemangioma is present, ultrasonography can help identify the specific mural nodule of the tumor.¹⁸

Edema proximal or distal to the tumor also causes widening of the cord seen on ultrasonography and can create confusion in determining the absolute limits of the tumor. However, in most cases, the tumor border is well defined and the edema is less echogenic than the tumor.¹⁹

In the setting of syringomyelia, ultrasonography can be used to localize the maximal extent of the syrinx as well as to identify any small septae present (Fig. 175-5).¹⁸ If a drainage catheter is placed into the syrinx, ultrasonography can be used to assess the degree of collapse of the cavity and determine whether additional catheters are needed.

FIGURE 175-5 A, Sagittal ultrasound image of the cervical spinal cord demonstrating a large syrinx (arrows). B, Axial ultrasound image of the same patient demonstrating the syrinx (arrow).

Ultrasonography has also been used in the setting of vertebral body fractures or dislocations to assess the spinal canal following a decompressive procedure. This is particularly useful in the thoracic and thoracolumbar areas when a dorsolateral approach is used to access ventral and ventrolateral epidural compression. Following removal of the lamina, the ultrasound probe can be placed over the dorsal dura and the extent of the ventral decompression assessed.^20,21

Image-Guided Spinal Navigation

Image-guided spinal navigation (IGSN) involves the use of an image-processing workstation coupled with a digitized localizer system to virtually link preoperative CT images of the spine to the corresponding surgical anatomy. By enabling the manipulation of multiplanar images through any surgical point, IGSN gives the surgeon a greater degree of orientation to the unexposed, and therefore nonvisualized, spinal anatomy.

The development of image-guided technology for spine surgery was influenced by the difficulties of intraoperative spatial orientation associated with surgery for complex spine disorders.^22,23 The 3D anatomy of the spinal column can present challenges for even the most experienced surgeon. Standard dorsal surgical approaches expose only a portion of the spinal column at a given level. Although this partial exposure is not problematic for most laminectomy or discectomy procedures, it can be limiting in the setting of complex spinal column disorders such as fractures, neoplasms, or deformities.

The placement of spine fixation devices such as pedicle screws requires an accurate orientation to the unexposed spinal anatomy. Although intraoperative fluoroscopy and serial radiography have proven useful, they are both limited because they provide only 2D imaging of a complex 3D structure. Consequently, the surgeon is required to extrapolate the third dimension based on an interpretation of the images and knowledge of the pertinent anatomy. This so called “dead reckoning” of the anatomy can result in varying degrees of inaccuracy when placing screws into the unexposed portions of the spinal column.

Several studies have shown the unreliability of routine radiography in assessing pedicle screw placement in the lumbosacral spine. The rate of penetration of the pedicle cortex by an inserted screw ranges from 21% to 31% in these studies.^24–26 However, Steinmann et al., using an image-based technique for pedicle screw placement that combined axial CT of cadaver spine specimens with fluoroscopy, was able to demonstrate a reduction of this screw insertion error rate to 5.5%.²⁷

The application of image-guided navigational technology to spine surgery provides 3D and multiplanar views of image data to improve the surgeon’s “visualization” of the spinal anatomy. This image data is provided in near-real time and can be manipulated to show every aspect of the intraoperative spinal anatomy through any selected point in the surgical field.^{22,23,28–30}

Navigational Technique

The primary components of an IGSN include an image-processing computer workstation interfaced with a two-camera optical localizer (Fig. 175-6). Customized navigational probes with three to four small reflective spheres attached in a known arrangement serve as the tools that link the surgeon to the navigational system (Fig. 175-7). The optical localizer camera system emits infrared energy toward the surgical field, where it is captured by the spheres on the probe and reflected back to the camera. The spatial orientation of the reflected light is passed on to the computer workstation, which can then use mathematical principles of localization by triangulation to localize and track not only the exact position of the probe’s tip but also the position of any anatomic structure in the surgical field on which the probe tip rests.

FIGURE 175-6 Image-guided navigation system. Image-processing computer workstation (right) and an infrared camera (left).

FIGURE 175-7 Navigational probe (with passive reflectors) and hand-held drill guide.

Bone landmarks on the exposed surface of the spinal column provide the frame of reference necessary for image-guided navigation. Specifically, any anatomic landmark that can be identified during surgery and in the preoperative image data set can be used as a reference point. Typically, for spine surgery, the reference points are the tips of the spinous process and transverse processes at each spinal level to be instrumented, although other bony landmarks such as facet joints or prominent osteophytes can also be used.^{22,23,28–30}

After a surgical exposure, each selected reference point in the image data set is mapped onto its corresponding point in the exposed surgical anatomy by a process termed registration. This process involves highlighting one of the selected reference points in the image data set while placing the navigational probe on the corresponding point in the surgical field. As the camera localizer system tracks the position of the probe, the spatial location of the probe tip is relayed to the computer workstation and the selected anatomic point in the surgical field is linked spatially to its corresponding image data point. This paired point registration step is then repeated for each of the three to five reference points required for accurate registration.

Alternatively, the surgeon can perform the registration process by creating a surface map of the exposed spinal anatomy. This technique involves placing the probe on multiple nondiscreet points on the exposed and debrided surface in the surgical field. The positional information of these points is transferred to the workstation, and a topographic map of the selected anatomy is created and “matched” to the patient’s CT data set. This surface mapping technique is typically more time-consuming and inherently less accurate than the paired point registration technique.³⁰

Registration can have the greatest effect on navigational accuracy. Registration accuracy depends on the surgeon carefully selecting the correct reference points or performing the contour mapping process. If properly performed, registration will allow for the display of reformatted, multiplanar CT or MRI images to assist the surgeon with orientation to the unexposed spinal anatomy.²³

The registration process establishes a precise spatial relationship between the image data and the spinal anatomy. If the patient is inadvertently moved after registration, the spatial relationship is distorted and the navigational information is inaccurate. This problem can be minimized by the optional use of a spinal tracking device, which consists of a separate set of passive reflectors attached to a spinous process in the surgical field. It alerts the system if the spine is inadvertently moved and provides for the necessary correction within the navigation data set to keep the registration process accurate and to eliminate the need to repeat registration.

Once the registration process is completed, the surgeon can place the navigational probe on any exposed point in the operative field and the workstation will automatically generate three reformatted images through the selected point. The three planar images are oriented perpendicular to each other in relationship to the long axis of the probe. For pedicle screw fixation, these three reformatted images will be the corresponding axial, coronal, and sagittal images through a selected point. On each reformatted image, a cursor or trajectory line marks the position of the navigational probe in the surgical field. The diameter of the cursor and the width of the trajectory line can be adjusted in proportion to the selected screw diameter. The length of the trajectory line relative to the imaged spine is displayed in millimeters, providing for accurate screw length selection (Fig. 175-8). As the surgeon moves the probe through the surgical field, each planar image updates immediately to show the probe’s orientation to the spinal column and provides the surgeon with an optimal orientation to the pertinent spinal anatomy and to the precise screw entry point and trajectory.

FIGURE 175-8 Screen from image-guided workstation demonstrating a satisfactory trajectory selection through the L4 pedicle. The upper left quadrant displays the coronal image with the computer cursor positioned within the cross section of the pedicle (arrow). The left and right lower quadrants show the selected trajectory in the axial and sagittal planes, respectively. The length of the trajectory line is set at 45 mm and its diameter set at 6 mm. The upper right quadrant displays a surface-rendered image of the spinal anatomy.

At each level to be instrumented, the surgeon identifies standard bony landmarks used for selection of a screw’s entry point and trajectory. The navigational probe is placed through a drill guide onto the selected entry point. The navigational images are generated, and the surgeon can then make subtle modifications to the selected entry point and trajectory as needed by simply changing the angle or location of the drill guide-probe assembly. When the appropriate entry point and trajectory have been determined, the navigational probe is removed from the drill guide and a 3-mm pilot hole is drilled along the selected trajectory. The process is then repeated for the contralateral side. Each level to be instrumented undergoes a separate registration process. This segmental registration eliminates the potential error that can occur with changes of vertebral body position between the preoperative scanned position and the intraoperative position. Because each vertebra is a rigid body, the spatial relationship of registration points at a single vertebral level to the pedicles of the same vertebrae remains unchanged regardless of changes in patient positioning.

At any point during the orientation procedure, the accuracy of the system can be tested by placing the probe on a known bony landmark and confirming its location on the workstation screen. If there is any discrepancy between the position of the probe and the corresponding position of the cursor on the workstation screen, the registration process can easily be repeated.

When applied to screw fixation of the spinal column, IGSN reduces or eliminates the need for intraoperative imaging.^{22,23,28–30} It can facilitate the optimal placement of screws within the spinal column. However, unlike intraoperative fluoroscopy, IGSN does not provide true real-time imaging. It does not show changes in the spinal anatomy as they occur. The system functions as a confirmation tool to assist the surgeon in identifying the pedicle and relating its position and orientation to the exposed spinal anatomy. It is an alternative method to the more conventional means of interpreting 2D images of the spine, relating them to the surgeon’s knowledge of the pertinent anatomy, and estimating the location of the pedicle. It is not intended to function as a substitute for an understanding of the appropriate spinal anatomy and the indications and techniques for insertion of pedicle screws.

Applications of this technology to spine surgery have proven IGSN to be a practical and extremely useful alternative and adjunct to conventional intraoperative imaging. It has been used routinely for pedicle screw fixation in the lumbosacral as well as the thoracic spine. It is very useful for placing transarticular screws at the C1-2 level (Fig. 175-9) and for providing optimal orientation to spinal anatomy during transoral surgery and during ventral thoracolumbar surgery.^{22,23,28–30} Additionally, it serves as a highly effective image manipulation system allowing the surgeon to scroll through CT images in any selected plane prior to and during surgery. This feature allows the surgeon to better conceptualize the complex 3D anatomy of the spinal column than is possible by the standard method of viewing multiple 2D image sheets.

FIGURE 175-9 Screen from image-guided workstation demonstrating screw entry point and trajectory for C1-2 transarticular screw fixation. The left upper quadrant displays the virtual position of a screw tip placed along the selected trajectory (arrow). The two lower quadrants display the selected screw trajectory in the sagittal plane (right lower quadrant) and in a plane that is perpendicular to the long axis of the navigational probe (left lower quadrant). This latter view represents an oblique plane that is midway between a coronal and an axial plane. The length of the trajectory line is set at 36 mm, and its diameter set to 4 mm. The right upper quadrant shows a surface-rendered image of the cervical anatomy.

Fluoroscopic Navigation

Fluoroscopic navigation is the combination of standard fluoroscopy with image-guided navigational technology. It was developed to address the difficulties of some earlier image-guided systems that typically took much longer to use than standard fluoroscopy.³¹ Although standard fluoroscopy is employed with this technique, the amount of fluoroscopic time is significantly reduced.

With the patient in position prior to surgery, an AP and lateral fluoroscopic view of the pertinent spinal anatomy is obtained. This is done with a customized reference frame attached to the C-arm. The frame serves to superimpose a specific reference grid on the two images obtained. The navigational workstation can then take the two images with the superimposed grid and relate the spatial position of the imaged anatomy to a navigational probe. As the navigational probe is placed on the patient’s anatomy, a corresponding trajectory line and cursor can then be superimposed on the lateral and AP images, respectively (Fig. 175-10).

FIGURE 175-10 Screen from fluoroscopic workstation demonstrating selected trajectory through the L2 pedicle in the coronal (arrow, left panel) and sagittal (arrow, right panel) planes. An image in the axial plane is not available with fluoroscopic navigation.

Despite the advantages of fluoroscopic navigation, it still presents some of the same difficulties experienced with standard fluoroscopy. Although the radiation dosage to the patient and surgical team is reduced, it is not eliminated. Positioning difficulties are the same in the upper thoracic region. Both the upper thoracic region as well as the lumbosacral region in obese individuals can be difficult to adequately visualize with fluoroscopic imaging.

The primary disadvantage of fluoroscopic navigation compared with CT-based navigation is the limitation of the image plane. As with conventional fluoroscopy, fluoroscopic navigation provides the surgeon with only AP and lateral planar images. Unlike CT-based navigation, it does not provide an axial image, which, in most spinal screw fixation procedures, is the critical plane for identifying intrusion into the spinal canal by a medially displaced screw.

A variation of conventional fluoroscopy, isocentric fluoroscopy, offers some improvements to the limitations of fluoroscopic navigation. This device acquires images intraoperatively by rotating the C-arm in a 180-degree arc around the patient. As with conventional CT imaging, the acquired images can then be reconstructed into multiplanar images, including images in the axial plane. Although the images are not of the same quality as standard CT scans, they are sufficient for navigational purposes. Image acquisition can also be repeated during surgery if needed to assess the adequacy of decompression or screw positioning.

The most recent advancement in intraoperative imaging involves the use of flat panel detector technology to improve intraoperative CT image acquisition and quality. A flat panel detector can be mounted onto a mobile imaging unit similar to a conventional C-arm fluoroscope. While this unit can be used to acquire standard anteroposterior and lateral images, its C-arm configuration can be “closed” to completely encircle the patient. This allows the flat panel detector to be swept in a 360-degree arc around the patient, thereby significantly improving the acquired image quality. The reformatted images are similar in quality to conventional CT imaging and superior to isocentric C-arm imaging. This ability to acquire high-quality planar images in addition to reformatted CT images makes this technology ideal for minimally invasive spine surgery. It is also easily adaptable to IGSN technology.

Summary

Intraoperative imaging of the spinal column is an important aspect of spine surgery. Although the primary role of intraoperative imaging is to facilitate correct spinal localization, recent technologic advancements in imaging and image processing allow for a greater degree of orientation to the unexposed spinal anatomy. The appropriate use and interpretation of intraoperative imaging enhance the accuracy, precision, and safety of spine surgery.