Intraoperative Imaging Protocols

General Considerations

Several elements must be considered when selecting an intraoperative navigation modality. First is consideration of the operating room table. The vast majority of modern operating room tables are radiolucent; therefore, they are unlikely to produce significant imaging artifacts in either the anteroposterior (AP) or lateral views. More important considerations when selecting a table are the surgical approach and spinal region undergoing treatment. Anterior approaches remain the standard for many cervical pathologies, including spondylotic myelopathy, disc herniations, and ossified posterior longitudinal ligament. In these procedures, a conventional operating room table can be used and allows for effective maneuvering of the C-arm or portable radiograph for acquisition of images. Although less frequently used, the O-arm (Medtronic) can potentially also be used for cervical operations because it fits around the head of the surgical table. However, care must be taken when using a skull clamp because this may push the table–patient cross-sectional area to the point where it cannot be accommodated by the O-arm. Additionally, for low cervical pathologies or pathologies of the cervicothoracic junction, the central support of the conventional operating room table may impose a physical constraint that prevents effective imaging of the entire cervical spine with the C-arm or O-arm.

Many of the same considerations exist for posterior cervical operations. However, in these cases, where the patient is positioned prone, we favor the use of a radiolucent frame, such as the Jackson Frame or Wilson Frame (Mizuho OSI). These frames not only relieve intraabdominal pressure and potentially reduce blood loss, but they also allow for easier maneuvering of the C-arm and O-arm, and thereby may facilitate the use of intraoperative imaging. Similarly, the posterior approach is generally preferred for thoracolumbar operations, and so the aforementioned frames are favored over the conventional surgical table. Where a lateral approach (e.g., the transpsoas approach, extreme lateral, or oblique lateral approach) or direct anterior approach is pursued, either a frame or a conventional surgical table may be used. To avoid the physical constraints mentioned above, the patient may have to be positioned lower (i.e., more caudal) on the table so that the C-arm or O-arm can be positioned over the surgical site.

The second consideration we make when selecting an intraoperative imaging modality is the indication for intraoperative imaging. Aside from level localization, which is performed at the beginning of every surgical case, the most common indications for intraoperative imaging are instrumentation placement and tumor resection. These two indications have unique demands; instrumentation placement relies on imaging to confirm accurate instrumentation and to facilitate placement of said instrumentation. In contrast, tumor resection uses imaging to help define tumor margins, to dictate osteotomy planes (for resection of primary vertebral column lesions), and to confirm neural element decompression. For primary lesions of the spinal cord, roots, and meninges, ultrasound may also be used to define locoregional blood flow and evaluate extent of lesion resection.

The last consideration we make when selecting an imaging modality is radiation exposure. It is well known that, with the exception of ultrasound, intraoperative imaging is associated with radiation exposure to both the patient and the surgical staff. Although exposure to both parties deserves consideration, the chronic or repeated exposure seen in surgeons and operating room staff across the thousands of cases that comprise a surgical career means that minimization of radiation exposure is of greatest concern for the operating room staff. A recent metaanalysis by Pennington et al. investigated the radiation exposure experienced by patients and surgeons during navigated procedures and revealed that surgeon radiation exposure is minimized by using intraoperative navigation that relies upon preoperatively acquired images. This includes both hand-based navigation, such as the StealthStation (Medtronic) or BrainLab Spinal Navigation systems (BrainLab), and robotic systems, such as the ExcelsiusGPS system (Globus) and the Mazor family of systems (Medtronic). Unfortunately, these same technologies that limit surgeon radiation exposure are also associated with the highest radiation exposure to the patient when they require a pre- or intraoperative CT that would not otherwise be acquired. However, because the workflows of many surgeons involve the acquisition of preoperative CT to evaluate bone quality (and the need for cement augmentation), it is possible that these systems may actually decrease patient radiation exposure by eliminating the intraoperative component. Even in the case where use of preoperative image-based navigation systems requires acquisition of a new CT volume, we feel that the net benefit to all stakeholders favors the use of intraoperative navigation systems.

Plain Film Radiography

Plain film radiography is the oldest and most widely used imaging technology in spine surgery. Identical in function to diagnostic films, plain film radiography acquires images by positioning a radiation source and detection plate on opposite sides of the patient. X-rays are then projected through the patient, whose tissues absorb the rays in proportion to the tissue radiodensity. Rays passing through the patient are then captured on the plate, with areas of high radiodensity appearing white and those of low radiodensity appearing dark on conventional projections. Radiodense materials include instrumentation, bone (700–3000 Hounsfield units [HU]), and highly cellular tissues such as liver (45–65 HU).

In the context of spine surgery, plain-film radiographs are most commonly used in two contexts: (1) when verifying surgical level before incision, and (2) when confirming instrumentation positioning before closure at the end of the procedure. Additionally, in the absence of other technologies, plain films may be acquired intraoperatively to confirm the surgical level between completion of the soft tissue dissection and initiation of bony work (e.g., laminectomy, facetectomy, etc.).

When plain-film radiographs are used for level localization, a radiopaque marker (e.g., a towel clamp or spinal needle) is positioned in the surgical field. Lateral-view radiographs are then captured, and the level identified on the radiograph is correlated to the in situ clamp position. To ensure proper level counting and to avoid the possibility of wrong-level surgery caused by abnormal anatomy (e.g., a sacralized lumbar vertebra or the presence of cervical ribs), it is recommended that the film view include the lumbosacral junction for lumbar cases and the occipitocervical junction for cervical cases. These reference points can then be used to determine the marked level. Localization in the thoracic spine can be performed using similar methods; however, the larger amount of soft tissue (including the shoulders at the cervicothoracic junction) and greater distance from invariant anatomical landmarks makes accurate localization more challenging. For midthoracic imaging, serial images may be required for initial localization; a “reference frame” image can be acquired to localize the anatomic landmark (e.g., occipitocervical junction) and place an initial marker. A second, adjacent image can then be acquired that includes the first marker as well as the operative field. Alternatively, AP images can be acquired that include the twelfth pair of ribs (T12 level or thoracolumbar junction) or the vertebra prominens (spinous process of C7 or cervicothoracic junction), which eliminates the need to link together serial localization films. The AP view is also less affected by a patient’s body habitus, and may provide clearer images for localizing the cervicothoracic junction. The major limitation of this approach is that it increases the risk of field contamination when it is used after draping.

An alternative method for level localization uses a computerized algorithm to translate lateral film views into labeled levels based upon preoperatively acquired CT scans. ^, At present, this system is neither widely available nor designed to function as a standalone system. Regardless of the method used for localization, it is imperative that the correct working level is verified on imaging before the operation begins.

Historically, plain films have been described for the evaluation of neural element decompression. Oftentimes this was performed with radiopaque contrast injected into the decompression site, or into the thecal sac (i.e., intraoperative myelography). These techniques are no longer used at most centers, however, because of the advent of other modalities (e.g., intraoperative CT and ultrasound) that can be used to evaluate decompression. Additionally, in the case of ultrasound, Doppler can be used to confirm the restoration of normal cerebrospinal fluid flow—a sign of adequate decompression.

At present, the second main indication for intraoperative plain film acquisition is confirmation of instrumentation placement. With the increased availability of intraoperative CT, this is becoming less useful, as the latter provides much better localization of the pedicle screws that have been placed. However, we find that it is still useful for confirming placement of interbody devices and corpectomy cages/vertebral body replacement devices, which have the potential to be displaced anteriorly upon intraoperative reduction.