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

thinner slice thickness (often 1.0–1.5 mm); these scans depict more data. Furthermore, the CAS system reconstructs the axial data into coronal and sagittal images, and the user can scroll through any of these images. Finally, the CAS computer presents axial, coronal, and sagittal images for each orthogonal plane through each point in the scan volume; that is, the axial, coronal, and sagittal images are all arranged 90° to each other. The net result is a complete set of anatomical information that can be easily accessed. This notion of accessibility is also critical. Simply printing all of the 1 mm axial, coronal, and sagittal images to x-ray film cannot be equivalent, since that approach would create an overwhelming morass of visual information. The key point is that CAS provides more information in clinically useful ways.

Computer-enabled review of the preoperative CT scan can directly influence preoperative surgical planning, as shown in Figure 12.2. The patient, a 61- year-old woman who had undergone one previous osteoplastic frontal sinus procedure as well as three endoscopic sinus surgery procedures, presented with persistent frontal headache. Her nasal endoscopic examination showed healthy, relatively thin mucosa in each frontal recess; in fact, on each side, the frontal ostium was well seen and the internal frontal sinus mucosa also seemed healthy. However, the apparent volume of the frontal sinus on each side seemed small, since the endoscopic view did not extend as far superiorly as anticipated. A CT scan was obtained. Two coronal images are shown in Figure 12.2A and B. In Figure 12.2A, the image showed that the frontal sinus was opacified bilaterally, that the right frontal sinus floor was absent/eroded, and bone with a hazy appearance formed most of the frontal sinus floor. In another coronal image (Figure 12.2B), the midline frontal sinus seems well aerated. The CT scan was reviewed on the CAS workstation. After this review, it became apparent that the inferior aspect of each frontal sinus was aerated and draining into the frontal recess, but that the superior aspect of each sinus was opacified and separated from the inferior frontal sinus by reactive new bone formation (Figure 12.2C and D). This new bone formation occurred at the location of the bone cuts for the previous osteoplastic flap. Intraoperative findings confirmed these conclusions.

12.5SURGICAL NAVIGATION FOR SINUS SURGERY

Through intraoperative surgical navigation, CAS also provides precise localization relative to the preoperative imaging data set. The paranasal sinuses are truly an ideal location for surgical navigation, since they are contained inside a rigid bony space. In this environment, surgical navigation can be performed without concern about soft tissue deformations due to surgical manipulations. Furthermore, as FESS is performed, the relative positions of the adjacent critical softtissue structures (namely brain and orbit) are not disturbed unless an inadvertent violation of an adjacent cavity has occurred. As a result, for FESS the navigation

is confined to the space of the paranasal sinuses; the boundaries of this space are relatively inviolate. Therefore, localization is relatively straightforward, especially compared to the challenges of neurosurgery, where soft-tissue deformation (brain shift, etc.) is almost unavoidable.

Other endoscopic techniques (such as endoscopic optic nerve decompression and endoscopic closure of cerebrospinal fluid leaks) differ from standard FESS, since the surgical procedure for these so-called ‘‘advanced’’ techniques deliberately occurs outside the paranasal sinuses, which only serve as a route of access. Even in these circumstances, localization information relative to the known bony anatomy can be helpful. In fact, such information may be of even greater importance for these challenging cases.

Surgical navigation depends upon registration, which is basically a calibration step for surgical navigation. All registration strategies map corresponding points in the preoperative imaging data set and the operating room. Registration may be divided into two categories:

1.In automatic registration, the patient wears a special headset at the time of the preoperative CT scan and surgery (Figure 12.3). The headset contains special fiducial markers that the computer can recognize automatically in the CT data. Since the headset fits the patient in only one way and since the relationship between the tracking device (also known as the dynamic reference frame or DRF) and the headset is predefined, the registration process is complete as soon as the computer has determined the location of the fiducial points in the CT scan. All localizations are relative to these points.

2.If the preoperative CT scan does not contain any information about the position of the DRF relative to the surgical volume; manual registration must be performed. In point-mapping registration, the surgeon must manually correlate corresponding points on the preoperative imaging data set and the surgical field. Anatomic fiducial points (such as the tragus and/or the medial canthus) may serve this purpose. Alternatively, external fiducial markers can be applied prior to the preoperative CT scan; in this case, the markers cannot be moved before surgery. Early CAS systems used bone-anchored fiducial points. Although such markers provide the best registration, they are cumbersome and impractical, and the other alternatives provide satisfactory registration accuracy. Surface contour maps also can support registration. In this approach, the surgeon maps approximately 30–40 points on relatively immobile surface contours, and the computer computes a registration by fitting the curve defined by these points to the surface contour of a three-dimensional model that has been reconstructed from the thincut axial CT data. A new system that automates the surface matching process has been developed (Figure 12.4).

FIGURE 12.3 The CBYON SAVANT ENT headset (Cbyon, Palo Alto, CA) is typical of CAS headsets that support automatic registration for sinus surgery. These headsets rest on the patient’s nose and are secured in place at each external auditory canal (inset photo). The headset components include (A) dynamic reference frame (DRF) platform for the tracking system, (B) posts for reflective spheres, which can be recognized by the CAS camera array, (C) virtual mouse pad, and (D) fiducial markers. In the SAVANT headset, a plastic frame houses the metal fiducial markers. The headset for the InstaTrak (Visualization Technology, Inc., Lawrence, MA), which is also designed for automatic registration, is similar; the major difference is that VTI system uses electromagnetic tracking rather than optical tracking. (Original headset photo courtesy of CBYON, Palo Alto, CA.)

During registration, the CAS computer calculates a registration accuracy value, which is actually an arithmetic expression of the average standard deviation of each fiducial point relative to the whole registration. If the registration accuracy is less than a critical value, then the registration is deemed potentially acceptable. The CAS user must manually accept the registration in order to proceed to surgical navigation.

In general, lower registration accuracy values are associated with greater surgical navigation accuracy; however, this may not be so in each specific case. As a result, the surgeon must estimate surgical navigation accuracy manually by localizing to known points in the surgical volume and making a visual estimate of localization accuracy. It is important that the estimates of surgical navigation

FIGURE 12.4 The Xomed FAZER (Medtronic Xomed, Jacksonville, FL) automates surface merge registration by performing a surface laser scan; during this scan, the reflected light contains the information about the surface contour. Then registration is calculated through a surface matching algorithm, which relates the surface anatomy information with the CT images that have been reconstructed from the preoperative CT scan. During registration, the CAS system tracks the locations of the headset DRF and the FAZER. (Photo courtesy of Medtronic Xomed, Jacksonville, FL.)

accuracy be completed at several points randomly distributed in the surgical volume, since localization accuracy may vary among points in different parts of the surgical volume. In addition, surgical navigation accuracy should be verified throughout the duration of the procedure, since inadvertent drift may alter accuracy during the case.

Registration accuracy (registration error calculated by the CAS system) and surgical navigation accuracy are not equivalent. Registration accuracy is the error calculated by the CAS system; to a large extent, it reflects the position of the fiducial points on the CT scan. Surgical navigation accuracy is the real-life value that is much more important. Numerous factors—including tracking system precision, registration accuracy, and instrument calibration (i.e., bent vs. ‘‘true’’ instruments)—all influence surgical navigation accuracy. Although surgical navigation accuracy is just a visual estimate of localization accuracy, it is the only way to confirm the precision of surgical navigation and monitor the continued performance of the system during surgery.

Available CAS systems routinely provide a surgical navigation accuracy of 2 mm or better, and in many cases an accuracy of 1 mm or better can be achieved. This level of accuracy must be considered relative to the anticipated dimensions of relevant structures (such as the lamina papyracea, which has a thickness of less than 1 mm). Since the critical dimensions of these structures are less than 1 mm, one may assume that the intraoperative margin of error for surgical navigation should also be less than 1 mm; however, this assumption is inaccurate. The net intraoperative margin of error must be smaller than that critical dimension, but the net margin of error includes surgical navigation accuracy as well as the judgment of the operating surgeon. Information from CAS localization is not used in isolation; rather, CT scan images, the endoscopic view, and surgical navigation are all complementary. The net surgical precision is greater than the estimated accuracy of surgical navigation alone; the net surgical precision with CAS is also greater than endoscopy and CT scans together (in the absence of CAS). The bottom line is that the effective intraoperative accuracy of CAS surgical navigation is greater than the estimated accuracy of isolated localizations provided by CAS surgical navigation.

12.6THE SPECIFIC UTILITY OF CAS IN FESS

The practical implications of IG-FESS are best explained through a series of examples, which illustrate the impact of CAS on surgical decision making and execution [16]. In this series of examples, the focus is not upon the novelty of CAS. Similarly, the focus is not simply point localization. Rather, the emphasis is on how CAS positively influences surgical planning and the actual procedure.

Case 1: Frontal Sinus

R.V., a 46-year-old man with a long history of sinonasal polyposis, chronic rhinosinusitis, and asthma, presented with complaints of facial pressure, nasal congestion/obstruction, and purulent rhinorrhea. He also described periodic asthma exacerbations that he subjectively related to rhinosinusitis exacerbations. He had undergone surgery at another facility in the recent past. Since his medical treatment had not been maximized, he was first treated with systemic steroids and culture-directed antibiotics, but his symptoms persisted. He then was brought to the operating room for IG-FESS. Initial review of his preoperative CT seemed to reveal a septated frontal sinus; however, careful examination of the images on the CAS workstation showed the presence of a right supraorbital ethmoid cell and a left frontal sinus that had pneumatized across the midline into the right frontal bone (Figure 12.5). Intraoperatively, a right frontal ostium could not be identified.

CAS surgical navigation was used to confirm localization relative to the preoperative CT scan; in doing so, the CAS-enabled assessment of the frontal