Учебники / Computer-Aided Otorhinolaryngology-Head and Neck Surgery Citardi 2002
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FIGURE 4.1 (A) An array of light emitting diodes (LEDs) can provide information about the relative positions of instruments in the operating field volume; an overhead camera array senses the light emitted by the LEDs and triangulates their position. This example shows the typical characteristics of an optical ILD. (B) Optical tracking can also occur passively; i.e., the ILD does not emit an active signal. Instead, an array of highly reflective spheres is used. The overhead camera array emits an infrared signal that is reflected by the spheres. Two representative passive ILDs are shown.
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The ideal registration protocol would yield a precise correlation between corresponding points in the operating field volume and the image space. For practical reasons, it is also desirable that surgeons can easily and simply implement the registration without significant disruption of the typical operating room activities. Unfortunately, combining the features of precision and simplicity is not a simple task; in fact, skeptics would propose that precision and simplicity in registration are inversely related to each other. As a practical matter, the easiest registration protocols tend to offer greater potential for inaccuracies, and the most precise registration protocols are the most cumbersome. The registration protocols that are commercially available are compromise solutions that seek to balance the need for precision and the need for usability.
Almost all registration protocols support dynamic registration, which compensates for patient movement during surgical navigation. Dynamic registration can be achieved by simply attaching an ILD to the volume of interest patient. This ILD, which may be more precisely termed a dynamic reference frame (DRF), then functions as a frame of reference for the coordinate system of the operating field volume, since all localizations are relative to the DRF (Figure 4.2). (Colloquially, ILD and DRF are used interchangeably, since for all practical purposes they serve the same function, i.e., they permit tracking by CAS tracking systems.)
FIGURE 4.2 An ILD that is attached to the patient monitors the position of the operating field volume. This ILD, known as a DRF, may be a standard ILD, or a special ILD may be adapted for this purpose. The CBYON ENT headset (CBYON, Palo Alto, CA) with its DRF is shown.
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Of course, maintaining a constant 3D relationship between the DRF and the operating field volume is critical. In this arrangement the CAS tracking system is tracking the position of the patient. After dynamic registration is achieved, the CAS system must track both the DRF and a second ILD, which is attached to a surgical instrument. This arrangement involves tracking two ILDs (the instrument ILD and the DRF, which is the ILD attached to the patient). Tracking two ILDs is inherently less precise than tracking a single ILD; however, this tradeoff in accuracy is small and not clinically significant. Since dynamic registration greatly enhances the usability of CAS in the real work, it is felt that dynamic registration is an essential feature for almost all registration protocols.
Obviously it is critically important that the DRF maintain a constant geometric relationship to the target area of interest in the operative field volume if dynamic registration is used. For this reason, dynamic registration works best for most sinus surgery and neurosurgery procedures. In these cases the DRF can be attached directly to the patient’s head or skull through a variety of means. In contrast, dynamic registration is not feasible for any surgery involving soft tissues of the neck, since the tissue will inevitably deform before, during, and after registration. Similarly, registration for the bony spine is also problematic, since the vertebra to which the DRF is attached may move in relation to adjacent vertebrae. Consequently, robust registration strategies for spinal surgery is a very active area of technological development.
At the conclusion of the registration protocol, the CAS software calculates an error value, which summarizes the average calculated error of each fiducial point relative to all of the fiducial points. This calculation, known as the root mean square (RMS), expresses the standard deviation of each fiducial point of the registration data set compared to the entire set. Clinical experiences indicate that lower RMS values are associated with better surgical navigation accuracy, but this is not necessarily true in every case.
RMS data may be presented graphically as regions in which the mathematical calculations indicate the greatest statistical likelihood of precise registration (Figure 4.3). Although a graphical representation portrays more information than a single value, it should not be considered a substitute for direct estimation of surgical navigation accuracy.
Of course, smaller RMS error values are desirable; however, a registration with a small error value does not guarantee that the surgical navigation will provide accurate localizations. For this reason, it is critically important that surgeons verify the accuracy of surgical navigation by localizing against known landmarks so that practical limitations of the system for each case are fully known. Surgeons should also recheck accuracy by localizing against known landmarks throughout the entire case so that inadvertent drift caused by shifting ILDs can be recognized early. Similarly, surgeons should monitor surgical navigation accuracy at multiple landmarks widely separated in 3D space in the operating field volume. It is con-
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FIGURE 4.3 Registration error may be calculated mathematically; this information can then be presented in a graphical format. The CBYON Suite (CBYON, Palo Alto) displays zones of anticipated accuracy by displaying yellow or red masks on the preoperative axial, coronal, and sagittal CTs; the central zone—where the unaltered imaging is shown— shows the area of maximum anticipated accuracy. In this screen capture, the two left images convey information about the relative positions of the CAS pointer and DRF, and the central image on the upper row shows the current set of fiducial points.
ceivable that registration may yield good surgical navigation accuracy in one part of the operating field volume while the surgical navigation accuracy is quite poor in an adjacent region.
After registration is complete, CAS surgical navigation systems provide a means for monitoring the continued performance of the system; i.e., the systems will permit calculations of so-called sustained accuracy or verification points. (CAS vendors have applied multiple names to this function; in general, they all serve the same purpose.) In order to use this feature, the surgeon must localize to an arbitrary point, whose coordinates are stored by the CAS software. Throughout the case, the surgeon may localize the same target, and the system will indicate the drift (distance between the initial point and subsequent localizations). Any drift can be ascribed to DRF movement, CAS pointer damage, tracking failure, and other factors.
It should be emphasized that registration and calibration are not synonymous. Calibration confirms the geometry between the instrument tip and the
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FIGURE 4.4 For instrument calibration, the desired CAS pointer (in this case, a modified suction with an attached DRF) is placed in a calibration divot/tube that is incorporated into the DRF. Both the CAS pointer and calibration point must have attached ILDs. (From CBYON, Palo Alto, CA)
attached ILD (Figure 4.4). In some systems, this geometry is predefined, and calibration only verifies the programed parameters that describe the relative positions of the ILD and instrument tip. If the instrument cannot be calibrated, then it is probably bent damaged. Obviously, this damaged pointer cannot be used. In other systems, an ILD can be clamped to any instrument at the discretion of the surgeon. Since CAS computer does not have a profile that describes the relationship between the instrument tip and this ILD, manual calibration must be performed. For manual calibration, the surgical instrument with the ILD must be touched to a predefined point while the surgeon clicks a specific button that initiates the calibration routine. Of course, if the position of the ILD is altered, the calibration step must be repeated. It also should be noted that some systems require periodic recalibration or at least verification of calibration so the usage of a damaged pointer can be minimized.
4.3 MANUAL REGISTRATION PARADIGMS
As described above, all registration paradigms deliver a system for aligning corresponding fiducial points in the operating field volume and the preoperative imaging data set volume. In manual registration, the surgeon must select points the fiducial points in the preoperative imaging data set and then localize with the
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surgical navigation pointer to the corresponding points in the operating field volume (Figure 4.5).
The steps for manual registration are as follows:
1.The surgeon reviews the preoperative images on the computer workstation and selects the desired fiducial data points. This step can be performed at almost any time after the preoperative images have been transferred to the computer workstation.
2.At the beginning of surgery, the CAS equipment must be set up as necessary. This step includes DRF placement. For sinus surgery, the DRF is typically attached to the patient’s head with a band or visetype device (Figure 4.6). For neurosurgery, the patient’s head is placed in a Mayfield head holder, and the DRF is bolted to the Mayfield head holder.
3.Next, the CAS pointer (namely the operative instrument with an attached ILD) must be calibrated by localizing to a fixed point on the DRF.
4.Subsequently, the surgeon then localizes against the selected fiducial points.
5.The CAS software calculates the registration by mapping the coordinates of the points from the operating field volume with the corresponding coordinates from the preoperative imaging data set volume.
6.If the registration error is below a critical value, the system may automatically accept it, or the surgeon may need indicate that the registration is acceptable.
7.Of course, the surgeon must confirm accuracy of surgical navigation by localizing against known anatomical landmarks.
A variety of fiducial markers may be used. Bone-anchored fiducial markers are rigidly fixed to bone, while external fiducial markers are merely taped to the patient’s skin (Figure 4.7). Sometimes, simple skin staples can serve as fiducial markers. Of course, all of these fiducial markers must be placed before the preoperative imaging study.
Bone-anchored fiducial markers are cumbersome, but they are clearly the gold standard since they provide excellent data for registration. The obvious practical issues limit their widespread use. External fiducial markers are often acceptable, but there are limitations. The external soft tissues exhibit a surprising amount of varying deformation so that the positions of the taped-on fiducial markers demonstrated a surprising amount of movement between the preoperative imaging and the actual surgery. The taped-on markers also cannot be moved until the manual registration is complete; therefore, the preoperative imaging must be done shortly before the planned surgical procedure.
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FIGURE 4.5 (A) In manual registration, fiducial points must be selected. In this example, five anatomical points were chosen. (B) For manual point mapping, the user must localize to the corresponding fiducial points in the operating field volume. In this example, the indicated fiducial point corresponds to point 3 in (A). (From CBYON, Palo Alto, CA)
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FIGURE 4.6 In manual registration, the ILD must be attached to the patient securely. This example shows the ENT headset for LandmarX (Medtronic Xomed, Jacksonville, FL).
FIGURE 4.7 External fiducial markers may be simply taped to the patient’s skin. In this instance, seven other markers were attached to other areas of the patient’s head; these other markers are not seen in this image.
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Anatomical fiducial landmarks can also support registration. Other limitations apply here. In some patients, the landmarks may be difficult to reproducibly identify. For instance, identification of facial landmarks on very round faces of obese patients can be problematic. Also, surgeons must learn to select these points reliably, and there are no rigid rules to guide this anatomical fiducial landmark selection.
4.4 AUTOMATIC REGISTRATION PARADIGMS
All automatic registration paradigms share similar features; i.e., they require the use of a relocatable frame that may be reproducibly placed on a patient in a noninvasive fashion. The frame contains a series of fiducial points that the computer software can automatically recognize. Intraoperatively, a DRF is attached to the relocatable frame so that the relationship between the DRF and the fiducial points in the relocatable frame is fixed. Intraoperative surgical navigation occurs by CAS computer projection of the tip of the CAS instrument onto the preoperative imaging study relative to the fiducial points in the relocatable frame. Since the relocatable frame fits the patient in only one way, this serves to show localization information relative to the patient’s unique anatomy.
Automatic registration is uniquely suited to sinus surgery procedures. The CBYON ENT headset (CBYON, Palo Alto, CA) and the InstaTrak headset (Visualization Technology, Laurence, MA) both achieve automatic registration for most sinus surgery procedures (Figure 4.8). The fiducial system in the CBYON ENT headset is contained in a series of bars that are arranged in a cage-like shape. The InstaTrak headset houses a series of small, metallic spheres that act as fiducial points.
The steps for automatic registration are as follows:
1.The preoperative imaging is performed with the patient wearing the relocatable frame.
2.After the preoperative imaging data set is loaded into the computer, the CAS software must process the data so that the fiducial points are recognized. This step may occur automatically after loading the data, or the software may require an input from the user to proceed (Figure 4.9).
3.If the calculated error is within acceptable limits, the registration is complete. The registration process does not truly require that the relocatable frame be on the patient, although the user interface for the software may suggest that this is necessary.
4.The relocatable headset is placed on the patient (Figure 4.10), and function of the tracking system is confirmed.
5.The CAS probe is calibrated.
6.The surgeon must confirm the accuracy of surgical navigation by localizing against known anatomical landmarks.
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FIGURE 4.8 (A) The CBYON ENT headset (CBYON, Palo Alto, CA) incorporates the standard features of relocatable headsets for registration. The headset is designed so that it may be reproducibly placed on the patient’s nose and ears. Built-in fiducial bars support registration. (B) The InstaTrak headset (Visualization Technology, Laurence, MA) is similar to the CBYON headset (shown in (A)). Built-in metallic spheres support registration.