Учебники / Computer-Aided Otorhinolaryngology-Head and Neck Surgery Citardi 2002
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3
Surgical Navigation Technology:
Optical and Electromagnetic
Tracking
Ralph B. Metson, M.D.
Harvard Medical School and Massachusetts Eye and Ear Infirmary, Boston, Massachusetts
3.1 INTRODUCTION
The performance of surgery in the head and neck demands a high degree of precision and allows little room for misjudgments regarding anatomical relationships. These demands can be particularly challenging when operating on a patient with poor anatomical landmarks, as is often the case from extensive disease or previous surgery. In an attempt to enhance surgical safety and efficacy of otolaryngologic procedures, navigational systems have been developed that can track the movement of a surgical instrument in the operative field. This information is used to provide the surgeon with real-time localization of the instrument on a three-dimensional (3D) video display of the patient’s preoperative computed tomography (CT) or magnetic resonance (MR) scan.
The earliest intraoperative guidance systems were developed in the 1980s for neurosurgical procedures. They utilized a stereotactic frame that required fixation of the patient’s head during surgery with cranial screws [1]. An articulated mechanical arm or ‘‘wand’’ held a probe, which could be localized in the surgical field by means of electromechanical sensors. A frameless system was eventually
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developed [2], which was used for a variety of sinonasal procedures [3–5]. However, many surgeons found this system to be awkward to use and time-consuming [6,7]. In addition, the patient’s head still had to be immobilized during surgery. The introduction of wandless and frameless systems in the mid-1990s greatly facilitated the utility of image-guided technology for otolaryngologic surgery [8– 14]. These devices required neither articulated arms nor head immobilization and could be readily adapted for use during commonly performed endoscopic sinus procedures.
Commercially available systems are currently based upon two different tracking technologies—optical and electromagnetic. The optically based systems utilize an infrared camera array to track light-emitting diodes (LEDs), which are attached to the surgical instrument and a headset worn by the patient. Some optical tracking systems are ‘‘passive,’’ i.e., the infrared camera array tracks the positions of highly reflective spheres, which are substituted for the standard LEDs. In this approach, the camera array also houses an infrared emitter, which illuminates the reflective spheres so that the camera array can recognize their positions. Obviously, the passive systems eliminate the need for wires for the tracking system in the operative field. The electromagnetically based systems track the position of the surgical instrument relative to the patient by means of a radiofrequency transmitter mounted to the headset and a radiofrequency receiver incorporated in the surgical handpiece. These two different navigational systems have both demonstrated their ability to provide the surgeon with accurate information regarding anatomical localization during surgery [10]. However, the different technologies utilized by these systems result in distinct differences in design and function, which can be used to formulate individual surgeon’s preferences for equipment selection.
3.2 EQUIPMENT OPERATION
Certain operational features—calibration, registration, and accuracy verifica- tion—are common to all navigational systems. These steps are necessary to ensure the integrity of the equipment at the start of surgery and the accuracy of localization throughout the procedure. For the purposes of this discussion, the mechanics of surgical navigation for endoscopic sinus surgery will be emphasized, since this specific application of surgical navigation is the most well-devel- oped application of this technology within the domain of otorhinolaryngology– head and neck surgery.
3.2.1 Calibration
Prior to utilization of the navigational system, the surgeon must perform calibration. It is typically done after the patient is draped and the cables from the headset
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and handpiece have been attached to the computer. This step usually requires the surgeon to hold the tip of the surgical instrument (usually a straight probe or suction) in a divot on the patient headset while stepping on a pedal. The system verifies the proper geometric relationships between the handpiece and headset to ensure that the equipment is in proper working order and that the surgical instrument has not been bent or damaged.
3.2.2 Registration
Registration is the process whereby the patient’s digitized preoperative CT or MR scan is aligned with the patient’s actual anatomy in the operating room. The reference points used to perform this function are known as fiducial points. Most surgical navigation systems that use optical tracking generally rely upon anatomical fiducial points, which are selected by the surgeon prior to surgery from surface landmarks on the 3D video model. Registration is completed when the surgeon touches these same points on the patient while activating the footpedal.
The electromagnetic system uses fiducial points, which have been embedded into the patient headset. Since the patient wears this headset during both the preoperative CT scan and surgery, the computer can align the CT scan with the patient’s anatomy. The surgeon performs the registration process by holding the tip of the suction handpiece in a divot in the center of the headset and rotating the instrument around the tip at 90° increments.
It should be noted that registration protocols do not reflect the limitations of either optical or electromagnetic tracking. In theory, both electromagnetic and optical tracking systems can support anatomical fiducial point registration and automatic headset-based registration. The engineers of each system have developed approaches to registration independently of the design for tracking technology.
3.2.3 Sustained Accuracy
Slippage of the patient headset or equipment malfunction can result in a loss of system accuracy during surgery. In order to assure that anatomical drift has not occurred, navigational systems incorporate timed reminders for the surgeon to verify system accuracy. This process requires the surgeon to select a precise anatomical point at the start of surgery, usually at the nasion. This point can be identified with an ink spot or adhesive strip with a central divot. The tip of the probe or suction is touched to this point at the start of surgery and at regular intervals throughout the procedure. If anatomical drift of more than 2 mm has occurred, the system should be reregistered prior to continuing its use.
It is strongly recommended that the surgeon develop a habit of checking the image-guidance system against known anatomical landmarks prior to its use for surgical navigation. For example, before using the probe to identify the level
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of the ethmoid roof, it should be touched to the tip of the middle turbinate or the posterior wall of the opened maxillary sinus to verify that the information the system is providing is accurate and correct.
3.3 OPTICAL TRACKING SYSTEMS
Optical image-guidance systems (Figure 3.1) for otolaryngologic surgery include the LandmarX (Medtronic Xomed, Jacksonville, FL), StealthStation (Medtronic
FIGURE 3.1 The LandmarX surgical navigation system (Medtronic Xomed, Jacksonville, FL) utilizes optical tracking technology incorporates an infrared camera array (arrows) to monitor the position of light emitting diodes (LEDs) mounted to the surgical instrument and patient headset.
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Surgical Navigation Technologies, Louisville, CO), Surgical Navigator (Zeiss, Montauk, NY), Optical Tracking System (Radionics, Burlington, MA), SAVANT (CBYON, Palo Alto, CA) and VectorVision (BrainLab, Tuttlingen, Germany). Most of these systems have software and hardware packages available that upgrade existing neurosurgical and orthopedic systems for use during otolaryngologic surgery. Most manufacturers also offer dedicated, less expensive systems, which are equipped only for ENT procedures.
These optical systems employ an infrared tracking system to locate the real-time position of surgical instruments on the patient’s preoperative CT scan. The tip of the instrument is depicted by crosshairs on a video display of a reformatted 3D image, as well as axial, coronal, and sagittal CT views (Figure 3.2). The optical sensor is an array of two or three infrared cameras mounted in a mobile, horizontal bar. Positioning of the cameras approximately at their focal length (typically 6 feet) from the head of the operating table provides optimal
FIGURE 3.2 During surgical navigation, the instrument tip localization is depicted relative to the preoperative CT scan. A representative screen capture from the LandmarX system (Medtronic Xomed, Jacksonville, FL) is shown.
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resolution. The cameras monitor the coordinate position of LEDs that are attached to standard sinus instruments or a straight probe (Figure 3.3). A separate set of LEDs is mounted in a dynamic reference frame (DRF), which is connected to the headset worn by the patient during surgery to monitor head movement (Figures 3.4 and 3.5). (In passive optical systems, reflective spheres serve the same functions as the LEDs, as described above.) This information is processed by an optical digitizer and computer workstation.
In most optical systems, patients do not wear a headset during the preoperative CT scan. Since the SAVANT and VectorVision support automatic headsetbased registration as well as anatomical fiducial registration, the patient may wear the SAVANT or VectorVision headset at the time of the scan, if the surgeon wishes to use the headset-based registration protocol. Scanning protocols vary among manufacturers but generally utilize standard spiral CT scan techniques. Nonoverlapping axial slices of 1–3 mm thickness can also be used. Images are transferred to the guidance system through the hospital’s computer network or by means of optical disk or storage tape.
The surgical handpiece is calibrated by placing the tip of the instrument in a divot on the headset DRF array while the foot pedal is depressed. The registration process involves correlating points in the operative field with the corre-
FIGURE 3.3 A standard surgical pointer for surgical navigation incorporates a LED array, which can be tracked by the overhead camera array. (LandmarX, Medtronic Xomed, Jacksonville, FL).
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FIGURE 3.4 This headset from the StealthStation ENT module (Medtronic Surgical Navigation Technologies, Louisville, CO) supports anatomical fiducial point registration. The headset contains LEDs (arrows) that are tracked by an infrared camera. This headset compensates for any intraoperative head movement and/or repositioning.
sponding points on the CT images. If an anatomical fiducial point registration protocol is used, the surgeon, resident, or nurse selects five to seven anatomical fiducial points on the 3D reconstructed model at the computer workstation with a standard computer mouse. This selection process can be done anytime prior to surgery, but it is often convenient to perform this step just prior to the patient’s arrival in the operating room. The most commonly used anatomical fiducial points are the nasal dorsum at the level of the medial canthi, the tip of the nose ( at the point of greatest anterior projection), the base of the nose (where the columella meets the upper lip), the right and left lateral orbital rims at the level of the lateral canthi (the bony rims can be palpated), and the right and left ear tragus (at the points of greatest lateral projection). Although any anatomical sites on the head can be used for fiducial points, it is preferable for the points to surround the surgical field (i.e., the paranasal sinuses) in order to optimize navigational accuracy. Sites at which skin is in close proximity to underlying bone have been found to provide the most reliable reproducibility and result in the least amount of distortion during surgery.
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FIGURE 3.5 This intraoperative photograph shows the relative positions of the headset and instruments during surgical navigation with an optically based system using anatomic fiducial point registration.
The location of the selected surface fiducials on the patient’s face are then touched with the tip of a calibrated probe or surgical instrument while depressing the foot pedal. For the remainder of the surgery, the location of the instrument tip is displayed on the CT images whenever the foot pedal is depressed. When the foot pedal is released, the current location of the instrument is frozen on the computer display. The system can also be set with the foot pedal in a ‘‘locked on’’ position, so that localization of the surgical instrument remains activated throughout the surgery, even when the foot pedal is not depressed.
For automatic headset-based registration for the SAVANT, the patient wears a special headset, which houses fiducial points, at the time of the preoperative CT scan. The patient wears the same headset at the time of surgery. Since the headset can fit each patient in only one way, the spatial relationships between the headset’s fiducial points and the patient’s anatomy are maintained. Furthermore, the relationship between the headset DRF and the fiducial points is fixed. During registration, the SAVANT computer automatically recognizes the fiducial
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points on the preoperative CT scan and calculates the registration. Surgical navigation may then proceed. The VectorVision automatic registration protocol is similar.
At approximately 15-minute intervals, many systems display a screen icon, which notifies the surgeon that navigational accuracy needs to be verified. For this step, the surgeon places the tip of a calibrated instrument on the previously selected point (typically a point on the nasal dorsum), while depressing the foot pedal. This step ensures that anatomical drift, which could result in a significant decrement in system accuracy, has not occurred. Such drift is usually the result of inadvertent movement of the headset during surgery. If the system demonstrated a decrement in accuracy of more than 2 mm, the registration process should be repeated before continuing with surgery.
3.4 ELECTOMAGNETIC TRACKING SYSTEMS
The first navigational system (Figure 3.6) introduced for sinus surgery was the InstaTrak (Visualization Technology, Inc., Lawrence, MA), which utilizes electromagnetic tracking technology. This system employs a radiofrequency transmitter mounted to a specialized headset that is worn by the patient during the operative procedure (Figure 3.7). A radiofrequency receiver is incorporated into the handpiece of a nonmagnetic suction (Figure 3.8). Cables connect both the transmitter and receiver to the computer workstation. The tip of the suction cannula is depicted by crosshairs on axial, coronal, sagittal images of the patient’s preoperative CT displayed on a video monitor (Figure 3.9).
The InstaTrak imaging protocol requires the patient to wear the exact same headset during both the preoperative CT scan and the operative procedure. When placing the headset, care must be taken not to allow objects that can cause distortion to push against the headset during the scan. Using helical mode, the scans are obtained with 3 mm slice thickness by 3 mm table movement and reconstructed to 1 mm table increments in the bone algorithm. If direct coronals are needed, the headset must be removed before scanning. The patient is scanned from the bottom of the maxilla to approximately 2.5 cm above the most superior of the seven metal spheres incorporated in the headset. Multiplanar CT views are obtained by reformatting the axial images. The digitized CT data is loaded into the InstaTrak directly by transporting the unit to the radiology department, via the hospital’s computer network, or from an optical storage disk.
The patient must bring the same headset worn during the CT scan to the hospital to wear during the surgery. The headsets are neither interchangeable nor reusable. The headset serves two functions—it compensates for head movement during the procedure and it serves as a method for automating registration. Registration is the means by which digitized CT data in the computer workstation is correlated with the patient’s actual anatomy. This system uses an automatic