Учебники / Computer-Aided Otorhinolaryngology-Head and Neck Surgery Citardi 2002

FIGURE 3.6 The InstaTrak system (Visualization Technology, Lawrence, MA) uses electromagnetic tracking technology with a radiofrequency transmitter mounted to the patient headset and a receiver attached to the surgical handpiece.

registration process where fiducials embedded in the patient’s headset, rather than anatomical landmarks, are used to align the CT data and patient anatomy.

The suction instrument is calibrated by holding its tip in a divot in the center of the headset, while the shaft is rotated 360° around the tip at 90° increments. Next, a verification point is established to ensure the system’s accuracy throughout surgery. This verification point is an adhesive strip with a central divot placed on the skin of the nasal dorsum. The tip of the suction is placed in the divot at 15-minute intervals to verify the sustained accuracy of the system. If the measurement is found to be greater than 2 mm (indicating drift), the registration process can be repeated.

Because the electromagnetic system uses radiofrequency transmission, metallic objects in the field will disrupt the operational signal. Distortion of the

FIGURE 3.10 This intraoperative photograph shows the relative positions of the headset and instruments during surgical navigation with an optically based system using infrared tracking.

eral practical differences in terms of clinical utility based upon differences in their hardware and software design.

3.5.1 Signal Distortion/Blocking

Because the optical-based system relies upon an infrared signal for correct operation, it is necessary to maintain a clear line of sight between the infrared camera and the DRFs mounted on the surgical instruments and patient headset. Therefore, the surgeon must hold the instrument with the DRF uncovered and pointed in the direction of the infrared camera whenever the system is activated. Furthermore, operating room personnel and equipment cannot be placed between the patient’s head and the camera array lenses, which are positioned at their focal length (generally 6 feet) above the head of the table (Figure 3.10).

Since the electromagnetic system utilizes a radiofrequency signal for localization, metallic objects in the surgical field cause signal distortion. To ensure proper operation of the electromagnetic system, patients have to be placed on

two surgical table mattress pads to increase the distance between the metal operating table and the electromagnetic emitter and sensor. This step may not be necessary in operating rooms where thicker mattresses are used. In addition, instrument tables, anesthesia equipment, and other sizable metallic devices must be positioned at an appropriate distance from the immediate surgical field.

3.5.2 Patient Headset

Automatic headset-based registration protocols require patients to wear a headset during the preoperative CT scan. Although patients occasionally report discomfort when wearing the headset, this issue rarely prevents completion of the scan. Use of the headset may contribute to patient and physician inconvenience, particularly if the patient forgets to bring the headset on the day of surgery.

Unlike the headset for manual registration, which is held in place by a band or rubber pads that encircle the head at the level of the brow, the headsets for automatic registration are secured at the ear canals and nasal bridge. This configuration necessitates intraoperative coverage of a portion of the medial orbit and frontal regions. For most sinus surgery, this design is not of clinical importance; however, it does preclude use of headsets with this design for procedures, which involve external incisions or manipulations in the frontal, medial canthal, or auricular regions.

All registration protocols for sinus surgery require a headset to be worn in the operating room. Because these devices must be secured in a fashion to minimize slippage during surgery, they may cause patient discomfort when surgery is performed under local anesthesia. In addition, temporary skin erythema at the headset contact points is commonly observed. In rare instances of prolonged surgery performed under general anesthesia, superficial skin necrosis has been observed at these contact points. The regular use of foam tape between the headset and skin appears to add to skin protection without increasing headset slippage.

3.5.3 Operating Room Time

The use of image-guided technology requires additional time for equipment setup and operation. Surgeons have reported 15–30 minutes per case of increased operating room time when first learning how to use the image guidance system [14]. Once the surgeon and nursing staff became familiar with the equipment, this time is reduced to 5–15 minutes. An automated registration process would actually be expected to save time as compared to a manual registration process, which requires the surgeon to preselect anatomic landmarks for fiducial points. In a study that compared the use of an optical system with manual registration and electromagnetic system with automatic registration at a single institution, total operating room time averaged 17 minutes longer for the electromagnetic

system. However, this difference was thought to reflect the fact that the electromagnetic system was the first image-guidance system used at the institution [10].

3.5.4 Expense

The use of image-guidance technology increases operating room expense. In one study, hospital charges were increased by approximately $500 per case when a surgical navigation system was used [12]. In addition to the capital outlay for equipment purchase, one must consider the cost of nonreusable supplies. If the system requires the use of disposable headsets and suction handpieces, greater operating costs may be anticipated.

3.5.5 Surgical Instrumentation

The electromagnetic system uses a specialized, nonmetallic suction, which was found to be helpful when brisk bleeding was present. However, difficulty may be encountered when trying to reach into the frontal sinus with the curved version of this suction because of its diameter and unusual tip configuration. The optical systems often use standard endoscopic sinus surgery instruments, which are mated to DRFs for localization. This feature may be especially advantageous when working in the region of the frontal recess. The narrow, curved, olive-tip suction can be readily inserted into the frontal sinus and can even verify the localization of different compartments within a large sinus.

Since the introduction of image-guidance technology, the number and variety of surgical instruments, which can be used with surgical navigation systems, has greatly increased. Many of the optical systems now offer universal instrument registration. With this process almost any standard surgical instrument can be digitized during surgery and used for anatomical localization.

Optical systems are also available that offer passive infrared technology. The camera array emits an infrared signal that is reflected by specialized spherical surfaces on the surgical handpiece. This technology allows for the use of wireless instrumentation and eliminates the problem of multiple cables, which may become tangled entwined.

3.5.6 Accuracy of Localization

Anatomical localization has been shown to be accurate to within 2 mm for both the electromagnetic and optical systems [10,12]. Although this degree of accuracy can be very reassuring to the surgeon, navigational systems have been found to be most useful to confirm the identity of large compartments within the sinus cavities, rather than to distinguish between millimeter increments. For example, the straight probe or suction is commonly used to confirm the identity of the

exposed ethmoid roof or to verify that the sphenoid sinus had been entered, rather than a large posterior ethmoid cell. The curved suction cannula is most useful to demonstrate that an ostium that has been opened in the frontal recess leads to the true frontal sinus, rather than an adjacent supraorbital ethmoid cell.

The positions of fiducial points may influence the accuracy of localization. For instance, the arrangement of fiducial markers in a planar surface may compromise accuracy at points distant from that plane. The selection of fiducial points in an array that surrounds the surgical site should theoretically increase system accuracy deep at points throughout the operative field. It is for this reason that the left and right tragus are typically used as fiducials during anatomical fiducial point registration. Similarly, the SAVANT headset for automatic registration incorporates fiducial markers that are separated in depth (i.e., out of plane).

3.5.7 Patient Selection

Intraoperative image-guidance systems have been found to be most useful for cases which present the surgeon with the greatest technical challenge due to abnormal anatomy or distorted landmarks [14]. Both types of systems have been widely utilized to facilitate sinus surgery in patients who have extensive disease or require revision surgery. Because the headsets used for automatic registration preclude access to the medial orbital and frontal regions, they cannot be used for surgeries, which require external access to these regions (e.g., frontal trephination, external ethmoidectomy, frontal sinus obliteration, or endoscopic dacryocystorhinostomy).

3.6 CONCLUSION

Navigational systems have demonstrated their clinical utility and relative ease of use for surgical procedures of the head and neck. Their greatest benefit appears to be for patients who have poor surgical landmarks from extensive disease or previous surgery. Both the optical and electromagnetic tracking systems provide the surgeon with accurate information regarding anatomical localization during sinus surgery. The different technologies utilized by these systems result in distinct differences in their design and function that can be used to formulate individual preferences for equipment selection.

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hardware components, provides a user interface, and displays image data. The tracking system can monitor the relative position of instruments in three-dimen- sional (3D) space.

Tracking technology has improved dramatically over the past 15 years. Early systems relied upon electromechanical arms. A series of acoustic emitters and receivers also can also serve as a mechanism for tracking. Most CAS surgical navigation platforms rely upon an optically based technology, that incorporates light-emitting diodes (LEDs) or highly reflective spheres into arrays (known as intraoperative localization devices, or ILDs) that are integrated into surgical instruments (Figure 4.1). Finally, an electromagnetic emitter and receiver can also provide localization data. Each of these specific technologies has its relative advantages and disadvantages, but it is important to realize that the principles of registration apply to all tracking systems and are not specific to any one tracking technology. Regardless of the specific hardware, the CAS surgical navigation system must have the capability to ‘‘learn’’ the location of the operative field.

Conceptually, all points in the operating field volume may be assigned a unique x, y, z coordinate value that defines the position of the point in relation to all other points in the operating field volume. Similarly, all points in the volume depicted by the preoperative imaging may be assigned a unique x, y, z coordinate value that defines the position of the point in relation to all other points in the preoperative imaging data set volume. During registration, the CAS system simply maps selected points (known as fiducial points) in the operating field volume with their corresponding points in the preoperative imaging data set volume. In this way, there is a one-to-one relationship between corresponding points in the real world and imaging data set.

The data generated by the registration process then serve as the basis for all surgical navigation. All further localizations are relative to the registration data. If the registration data are inaccurate or if the registration data become corrupted, then surgical navigation will suffer accordingly. Surgical navigation depends upon registration.

Inaccuracies in surgical navigation may be traced to a variety of problems, but often the biggest problem is poor registration. An active, optically based tracking system under laboratory conditions (not in the operating room) theoretically has submillimetric precision, while other tracking technologies are only slightly less precise. Clinically, the best surgical navigation accuracy is perhaps 1–2 mm and often 2–3 mm. This dramatic difference in accuracy illustrates the challenge of registration. The challenge is not tracking instrument position in a precise manner; the challenge lies in calculating instrument tip positions relative to the preoperative imaging. Registration protocols are answers to this challenge.