Today, the use of neuronavigation is standard in most intracranial surgeries. What could be considered a clinically acceptable accuracy depends on the type of surgery. While some neurosurgical procedures, such as defining the borders of a bone flap, place lower demands on accuracy, other procedures such as tumor resection in eloquent areas, frameless biopsies or drilling of skull base structures require sub-millimeter accuracy. SM is the current gold standard for patient registration[3, 8, 19]. It relies on the matching of facial features of the patient to preoperative 3D-imaging. However, SM can be difficult to perform depending on patient positioning. When the head is turned, or the patient is prone, line-of-sight problems may occur relative to the camera of the navigation system. Facial features may also shift with gravity to create uncertainties in matching. In this study, we have compared the accuracy of a standard SM method for patient registration to the accuracy of AIR based on intraoperative CBCT using the Universal AIR registration matrix. In theory, AIR has the advantage of being indifferent to patient positioning as it relies on intraoperative imaging with the patient placed in the intended position for the surgical procedure. We found that accuracy of AIR was superior compared to SM.
In SM, an unexpected number of cases with a considerable TRE were noticed. Although not exclusively so, this was prominent in patients placed in prone or more laterally rotated positions. Several factors may contribute to this. As mentioned, extreme positions often lead to the obscuring of facial features from the navigation camera, precluding the use of a laser pointer (Z-touch, Brainlab) to map the face and provide positional data. Moreover, a pointer is also difficult to use in these positions. Extreme angulations of the pointer may be necessary to avoid the head obscuring the marker spheres from the view of the navigational camera. At these angles, it may not be feasible to accurately point towards the desired anatomical positions. Prone or rotated positions are also often used for posterior lesions, which are more distant to the face. As distance enhances angular errors, surface matching based on facial features is less reliable for these lesions[19, 21, 24].
In most navigated neurosurgical procedures, all accuracy hinges on the DRF. If it is dislodged, accuracy is lost. In one patient (ID 18) a 21 mm increase in TRE was found between the pre-drape and post-drape measurements, indicating a severe movement of the DRF in relation to the skull. In this case, the loss of accuracy was recognized intraoperatively, and the use of the navigation discontinued. A retrospective analysis of this case showed that the errors of all 4 screws were in the same rotational plane in the Z-axis compared to the DRF, indicating a shift of the DRF-position, most likely reflecting incorrect remounting after sterile draping. The overall finding that accuracy decreases after draping suggests that similar but less evident remounting errors may occur. In a clinical situation, AIR can be used after draping to avoid this error. AIR, contrary to SM, does not require access to the uncovered facial features.
Finally, using a heat map analysis, we showed that alterations of the facial features in relation to the preoperative MRI, may impact the registration accuracy of SM. Facial features may shift with gravity, pull from Mayfield pins or other factors that differ between the preoperative MRI and the intraoperative condition. TREs were generally larger when SM was used in rotated or prone positions. TREs of SM were lower when recalculated to match the CBCT performed intraoperatively, suggesting that errors were the result of differences in facial features rather than to the registration procedure itself. These potential sources of error could be circumvented by using AIR, as indicated by the observed decrease in TRE.
4.1 Strength and limitations
A major limitation of AIR is of course the availability of intraoperative CT or CBCT. In this study, surgery was performed in a hybrid OR with a ceiling mounted CBCT system[4, 12]. However, the registration method is independent of the imaging system, and could be performed using any CT or CBCT system able to export images in a standard DICOM format. Another concern would be the time dedicated to registration. Since the setup of this study required several extra steps of screw insertion and navigated point acquisition the specific time for the registration procedure could not easily be measured. However, it is our estimate that in a dedicated environment, the registration procedure by itself could be performed in a 10-minute timeframe, compared to the approximately 5 minutes for a SM registration[21]. Of note, the improved accuracy by AIR comes at the cost of additional radiation exposure to the patient and staff[4, 5, 12]. However, protective equipment can be used to protect OR staff and from the perspective of the patient, a slight increase in radiation exposure may be acceptable to ensure accuracy of a neurosurgical procedure. Moreover, the exposure from a single CBCT is lower than that of a routine CT and negligible compared to oncological adjuvant radiation.
The use of micro screws to assess accuracy was based on the assumption that their rigid fixation in the skull and their well-defined centers would help to provide the precise positional data needed for the analysis. The precision data of the point acquisition in the pair-wise distance test supports this assumption. However, the use of micro screws limited the target positions to the surface of the skull and to avoid unnecessary trauma, the screws were only placed within the intended surgical field.