Intraoperative Imaging of the Spine

Summary of Key Points

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Spine surgery is highly dependent on intraoperative imaging for both localization and assessment purposes.
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Plain film radiography and fluoroscopy are the most common imaging modalities used for spine surgery.
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Exposure of the patient and the surgical team to radiation from the use of intraoperative fluoroscopy is a growing concern.
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Spine surgeons who use fluoroscopy have a greater degree of radiation exposure compared to nonspine surgeons using fluoroscopy.
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Image-guided navigation is an evolving technology that improves the accuracy of spinal screw placement and reduces or eliminates the need for fluoroscopy.

Few surgical specialties are as dependent on intraoperative imaging as is the field of spine surgery. Whether it involves obtaining a lateral radiograph to confirm the level of a lumbar disc herniation or using intraoperative computed tomography (CT) imaging for the navigation of pedicle screws, intraoperative imaging provides information that can significantly affect the course of the surgery.

The most commonly used intraoperative imaging techniques are plain film radiography, fluoroscopy, and, to a lesser degree, ultrasonography. These techniques each have their own advantages and disadvantages, but when used appropriately, each can provide valuable information. In addition to these standard intraoperative imaging techniques, image-guided spinal navigation (IGSN) and intraoperative CT have evolved into proven and versatile tools for orienting the spine surgeon to the complex three-dimensional anatomy of the spinal column. This chapter reviews each imaging technique as well as the indications for their use.

Plain Film Radiography

Plain film radiography is the most frequently used imaging technique applied to spine surgery. It is typically used to localize a specific spinal level prior to surgical manipulation and to assess positioning of spinal fixation devices. During the surgical exposure of the spinal column, a radiopaque marker (i.e., surgical clamp, spinal needle, etc.) is positioned in the surgical field. For the lumbar or cervical spine, a lateral radiograph is obtained that includes either the lumbosacral junction or the occipitocervical junction as a point of reference at one end of the film and the inserted marker at the other end of the film. The number of vertebral levels between the anatomic reference point and the marker can be determined and the appropriate spinal level 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 the position of the second instrument marker relative to the first instrument marker. This technique can also be used immediately prior to the incision by placing two spinal needles percutaneously in the upper lumbar and midthoracic regions ( Fig. 193-1A and 1B ).

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. A film cassette can be positioned below the patient’s chest and an AP view obtained. This view should include either the T1 or T12 vertebrae and their associated ribs, which then serve as the reference points 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 level confirmed.

Image quality can significantly affect intraoperative localization. This can occur due to poor imaging technique or due to difficulty penetrating soft tissue in obese patients. Failure to adequately image the selected reference point or the target vertebrae can lead to wrong level surgery. Imaging should be repeated until adequate visualization of the spinal anatomy is obtained.

Even when the correct level is identified, localization errors may occur. This typically happens when the localization film is obtained early in the surgical dissection. Although the correct spinous level may be visualized on the film, continued surgical dissection down to the level of the lumbar canal may cause the surgeon to migrate to an adjacent lumbar segment above or below the desired level. Obtaining the localization film after the dissection has been completed and then placing a permanent identifier (suture, surgical pen mark, etc.) at the identified level can help prevent this error.

Plain radiography can also be used to assess the extent of neural decompression particularly following transoral decompression or anterior corpectomy surgery. Prior to obtaining the image, radiopaque contrast material can be placed into the decompressed site. The lateral radiograph can then show the extent of the decompression and can be compared to a preoperative image. 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, additional decompression can be performed.

Another method of determining the adequacy of decompression is to perform a lateral with intraoperative myelography. Radiopaque dye is placed into the subarachnoid space through a small-gauge spinal needle. A lateral radiograph can then demonstrate the dye column and its relationship to the neural elements.

Fluoroscopy

Fluoroscopy is used when real-time imaging or multiple images of the spine are required during the surgical procedure. It is most commonly used during minimally invasive surgery to monitor positioning of pedicle screws and interbody cages inserted through small incisions. It is also commonly used to monitor the injection of methylmethacrylate into osteoporotic vertebral body fractures.

Fluoroscopy can provide the surgeon with imaging of the spinal column in several planes depending on the positioning of the fluoroscope’s C-arm. The arm can be moved into different positions to provide a lateral, oblique, or AP views of the surgical field. Alternatively, two C-arms can be positioned perpendicular to each other to better facilitate obtaining two different planar images. This is particularly helpful during anterior odontoid screw fixation surgery or during a vertebroplasty procedure.

A disadvantage of intraoperative fluoroscopy that is of growing concern is the exposure of the surgical team to repeated amounts of low-dose ionizing radiation. This is particularly concerning during minimally invasive spine surgery because the limited exposure through small incisions creates a greater need for fluoroscopy and a subsequent increase in radiation exposure during these procedures. Several studies have attempted to quantify the amount of occupational radiation exposure to health care professionals under different and real clinical scenarios. These studies have typically demonstrated that 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. 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 (i.e., intramedullary femoral nailing), the techniques and instruments used for pedicle fixation bring the surgeons’ hands very close to the primary area of exposure. The amount of radiation exposure needed to achieve adequate visualization of the lumbar spine is greater than for other anatomic sites due to the increased soft tissue mass that must be penetrated.

Rampersaud and associates reported the radiation exposure occurring during the placement of thoracolumbar pedicle screws. 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 and the amount of radiation exposure was measured. 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 nonspine procedures. The greatest level of exposure was noted on the side ipsilateral to the beam source due to backscatter radiation. 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 to 4 feet) from the radiation source also helped reduce the dosage to negligible levels. The study concluded 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. It emphasized the need for appropriate lead shielding including the use of a thyroid shield and leaded glasses as well as the need for spine surgeons who use fluoroscopy regularly to monitor their annual radiation dosage.

Image-Guided Spinal Navigation

Image-guided spinal navigation, or computer-assisted spine surgery, is a computer-based surgical technology that links spinal image data with the corresponding spine surgical anatomy. The technology provides the spine surgeon with the ability to manipulate multiplanar CT or fluoroscopic images during the procedure in order to gain a greater degree of orientation to the nonvisualized spinal anatomy. By improving anatomic orientation it helps to enhance the accuracy of spine surgery, particularly fixation screw insertion. It also minimizes or, in many cases, eliminates the need for conventional fluoroscopy reducing the radiation exposure to the patient and the surgical team.

Historical Perspective

Image-guided spinal navigation evolved from the principles of stereotaxy, which is defined as the localization of a specific point in space using three-dimensional coordinates. Stereotactic principles have been used for many decades for intracranial procedures. Early applications of stereotaxy required the use of an external frame attached to the patient’s head. This requirement made applying stereotaxy to other noncranial surgical procedures impractical. However, as computer-based technologies and techniques continued to evolve, the need for a physical reference frame attached to the patient was eliminated. This frameless navigation technology created the ability to apply stereotactic navigation to other surgical procedures, in particular, spine surgery.

The initial application of image-guided technology to spine surgery was reported by Kalfas and colleagues in 1994 and 1995. These studies demonstrated the feasibility of using navigational technology to improve the accuracy and efficiency of lumbar pedicle screw insertion without the need for intraoperative imaging. Other groups soon followed with their experiences using the technology for broader applications in spine surgery. The development of optical localizers, faster computing speeds, intraoperative CT imaging and surgical robotics has allowed the technology to evolve rapidly into a well-accepted and commonly used surgical option.

Principles of Image-Guided Spinal Navigation

The components of a navigation system include an image-processing computer workstation interfaced with a two-camera optical localizer ( Fig. 193-2 ). When positioned during surgery, the optical localizer emits infrared light toward the operative field. A handheld navigational tool mounted with a fixed array of passive reflective spheres serves as the link between the surgeon and the computer workstation ( Fig. 193-3 ). Passive reflectors can also be attached to standard surgical instruments such as a drill guide, a tap, or a pedicle screwdriver. The spacing and positioning of the passive reflectors on each navigational probe or customized trackable surgical instrument is known by the computer workstation. The infrared light that is transmitted toward the operative field is reflected back to the optical localizer by the passive reflectors. This information is relayed to the computer workstation, 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. This information can then be presented in real time on the workstation screen.