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ORIGINAL ARTICLE
Year : 2014  |  Volume : 62  |  Issue : 5  |  Page : 503-509

Accuracy and precision of targeting using frameless stereotactic system in deep brain stimulator implantation surgery


1 Department of Neurosurgery, Center of Neuromodulation, Wexner Medical Center, The Ohio State University, Columbus, USA
2 Department of Neurosurgery, University Hospitals, Cleveland, Ohio, USA
3 Department of Neurosurgery, Emory Hospital, Atlanta, Georgia, USA

Date of Submission17-Jul-2014
Date of Decision24-Aug-2014
Date of Acceptance03-Oct-2014
Date of Web Publication12-Nov-2014

Correspondence Address:
Milind Deogaonkar
Department of Neurosurgery, Center of Neuromodulation, Wexner Medical Center, The Ohio State University, 480 Medical Center Drive, Columbus, Ohio-43210
USA
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0028-3886.144442

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 » Abstract 

Objectives: To assess the accuracy of targeting using NexFrame frameless targeting system during deep brain stimulation (DBS) surgery. Materials and Methods: Fifty DBS leads were implanted in 33 patients using the NexFrame (Medtronic, Minneapolis, MN) targeting system. Postoperative thin cut CT scans were used for lead localization. X, Y, Z coordinates of the tip of the lead were calculated and compared with the intended target coordinates to assess the targeting error. Comparative frame-based data set was obtained from randomly selected 33 patients during the same period that underwent 65 lead placements using Leksell stereotactic frame. Euclidean vector was calculated for directional error. Multivariate analysis of variance was used to compare the accuracy between two systems. Results: The mean error of targeting using frameless system in medio-lateral plane was 1.4 mm (SD ± 1.3), in antero-posterior plane was 0.9 mm (SD ± 1.0) and in supero-inferior plane Z was 1.0 mm (SD ± 0.9). The mean error of targeting using frame-based system in medio-lateral plane was 1.0 mm (SD ± 0.7), in antero-posterior plane was 0.9 mm (SD ± 0.5) and in supero-inferior plane Z was 0.7 mm (SD ± 0.6). The error in targeting was significantly more (P = 0.03) in the medio-lateral plane using the frameless system as compared to the frame-based system. Mean targeting error in the Euclidean directional vector using frameless system was 2.2 (SD ± 1.6) and using frame-based system was 1.7 (SD ± 0.6) (P = 0.07). There was significantly more error in the first 25 leads placed using the frameless system than the second 25 leads (P = 0.0015). Conclusion: The targeting accuracy of the frameless system was lower as compared to frame-based system in the medio-lateral direction. Standard deviations (SDs) were higher using frameless system as compared to the frame-based system indicating lower accuracy of this system. Error in targeting should be considered while using frameless stereotactic system for DBS implantation surgery.


Keywords: Accuracy, deep brain stimulation, frame-based stereotactic system, frameless, frameless stereotactic system, stereotactic, surgery, targeting system


How to cite this article:
Sharma M, Rhiew R, Deogaonkar M, Rezai A, Boulis N. Accuracy and precision of targeting using frameless stereotactic system in deep brain stimulator implantation surgery. Neurol India 2014;62:503-9

How to cite this URL:
Sharma M, Rhiew R, Deogaonkar M, Rezai A, Boulis N. Accuracy and precision of targeting using frameless stereotactic system in deep brain stimulator implantation surgery. Neurol India [serial online] 2014 [cited 2018 Aug 21];62:503-9. Available from: http://www.neurologyindia.com/text.asp?2014/62/5/503/144442



 » Introduction Top


Since the introduction of Cartesian stereotactic system by Spiegel and Wycis in 1947, this technique has been the gold standard for a variety of stereotactic and functional procedures. [1] Stereotactic targeting in functional neurosurgery is traditionally done using a stereotactic frame. However, the advent of computer-assisted, image-guided navigation made it possible to steer from the conventional frame-based techniques to frameless system for stereotactic targeting in the last decade. [2],[3],[4],[5],[6],[7],[8] Though image-guided navigation system provides real time feedback and proved to be more useful in tumor resection surgeries, the utility of this technique in functional neurosurgical procedures needs to be assessed against the time-tested stereotactic frames. Moreover, rigid stereotactic frame provides a stable framework for multiple trajectories, microelectrode recordings and macro stimulations over a prolonged operative duration without compromising the accuracy of the system. Targeting accuracy of frameless stereotactic system has been previously evaluated in laboratory and clinical settings with no significant differences as compared to frame-based systems. [4],[5],[7] There are several advantages of frameless system as compared to frame-based system for functional neurosurgical procedures such as improved patient's comfort, independent of head/neck size or configuration, dissociation of imaging and surgical procedure, reduced operative time, avoidance of technical difficulties associated with imaging patients with stereotactic frame, real time microelectrode recording as well as integration of multiple information and increased accuracy. [2],[5],[9],[10] The aim of the study was to investigate the precision and accuracy of targeting of NexFrame (Medtronic, Minneapolis, MN) frameless system using bone fiducials as compared to the frame-based targeting system during deep brain stimulation (DBS) surgery. We also compared the accuracy of targeting using frameless system between the first and second half of our series.


 » Materials and Methods Top


Clinical data

This retrospective study was approved by the Institutional review board at the Cleveland clinic and carried out in accordance with the Health Insurance Portability and Accountability act (HIPAA) over a period of 3 years. Patients were informed and consented prior to enrollment in the study. A total of 50 DBS leads were implanted in 33 patients using the NexFrame (Medtronic, Minneapolis, MN, USA) targeting system during the study period by one of the senior authors (N.B). Anxious and claustrophobic patients were enrolled for frameless guided implantation of DBS. We retrospectively reviewed the inpatient case records, imaging details (preoperative and postoperative Computed automated tomography (CAT) scans, Magnetic resonance imaging [MRI] scans), outpatient data and operation notes of all the selected patients. All charts were studied for the patient demographics and intraoperative details with the techniques used. The target coordinates were calculated using the Medtronic stealth station (Medtronic, Minneapolis, MN, USA) in reference to the coordinates of anterior commissure (AC), posterior commissure (PC) and Intercommissural line (IC). Trajectory was planned from the vertex to the target through an appropriate gyrus avoiding the ventricular walls and intracranial vessels. Postoperative thin cut (1 mm) CT scan slices were used for lead localization in all the selected patients. X, Y, Z coordinates of the tip of the lead in reference to AC-PC coordinates were calculated and compared with the intended target coordinates to assess the targeting error. Target, age and diagnosis matched, comparative 33 patients who underwent 65 lead placements using Leksell stereotactic frame during the same period were retrospectively selected from the database. Error was calculated using the same protocol. Frame-basedprocedures were performed by senior surgeons (N.B, A.R and M.D), who alternated in different surgical procedures.

Surgical techniques

NexFrame System (Medtronic, Minneapolis, MN, USA):

NexFrame is a frameless system with a skull-mounted device which is designed to achieve a high degree of targeting accuracy. Three-dimensional volumetric T-1 weighted MRI of the entire head with 1-mm slices, 1-mm T-2 weighted MRI slices in the axial plane and T-2 weighted 2 mm slices in the coronal plane through the subthalamic nucleus were obtained within 30 days prior to the procedure. One to four days prior to surgery, patient's head was shaved and five to six bone fiducial markers (Stryker-Leibinger, Kalamazoo, MI) were screwed into the skull bone under local anesthesia. Serial fine cut (1-mm thick) continuous CT scan slices were obtained after the application of bone fiducial markers. These CT scan images were then exported to the stealth station in Digital Imaging and Communications in Medicine (DICOM) format for planning the trajectories and target nucleus. The MRI images were fused to the reference CT scan in the usual fashion to target the appropriate nucleus. The patient's head was immobilized in a head holder device with a cervical collar. Following adequate head immobilization, the registration of the fiducial markers was obtained using a nonsterile reference arc and registration probe. The surgical field was then prepared and draped in a usual fashion. Burr hole was placed at the marked site and Stimloc system was screwed to the skull bone. Then the NexFrame/tower system was attached to the skull bone. A sterile reference arc was then attached to the base of the tower and re-registration of the fiducial markers was carried out. The alignment of the system was checked and microelectrode recordings were performed. The quadripolar DBS lead (model 3389/3387, Medtronic, Minneapolis, MN, USA) was passed to the target based on anatomical and physiological mapping including intraoperative macro stimulation. A stealth CT scan (1 mm cuts) was performed immediately after surgery to assess the lead position as well as to rule out postoperative complications. These postoperative images were exported to the stealth station and fused with the preoperative MRI and CT images to assess the differences between actual and expected lead locations. The final lead location coordinate was identified as the center of the beam hardening artifact in the postoperative CT scans representing the deepest electrode contact. [4] These differences were calculated in x, y and z directions in reference to AC-PC coordinate [Figure 1] and [Figure 2].
Figure 1: The patient's head immobilized in a head holder device with a cervical collar during the frameless navigation. Note the bone fiducials screwed firmly to the skull bone

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Figure 2: (a) The reference arc attached firmly to the bone fiducial into the skull bone. (b-d) The NexFrame with Microdrive and reference system attached firmly to the tower and skull using bone screws during DBS surgery

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Leksell frame system

Leksell frame (Elekta AB, Stockholm, Sweden) was used for frame-based implantation ofDBS electrodes (model 3389/3387, Medtronic, Minneapolis, MN, USA). Stealth MRI scans were obtained using the protocol as previously described, within 30 days prior to the procedure. The patient's head was shaved and stealth CT scan was performed after the application of Leksell frame under local anesthesia and sedation on the day of surgery and images were then exported to the stealth station for planning. The MRI images were fused to the reference CT scan for localizing the targeted nucleus. The surgical site was prepared and draped in the usual manner. Burr hole was placed at the marked site and microelectrode recordings were performed in all patients. The DBS lead was then placed based on anatomical and physiological mapping including intraoperative macrostimulation. After lead placement, lateral X-ray was obtained to confirm the exact position of the electrode. The indicator box with fiducials on all sides which projects on the X-ray film helps in avoiding the tilts and parallax, thus confirming the final location of the tip of the electrode. The stereotactic frame system consists of a side cross-bar which is always parallel to the frame with an X-ray film. A stealth CT scan was performed immediately after surgery and fused with the preoperative images to assess the differences between actual and expected lead locations as described in the above section.

Statistical analysis

Statistical analysis was carried out using SPSS v20 (IBM, Inc., Chicago, IL), Microsoft Excel (Microsoft, Redmond, Washington, USA) and the OpenEpi online calculators (provided by the Centers for Disease Control, Atlanta, Georgia, USA). Multivariate analysis of variance was used to compare the accuracy between the two systems. Error of placement was calculated in each of the X (medio-lateral), Y (antero-posterior), Z (superior-inferior) planes and the combined 3D vector and documented as means ± SD. Euclidean Distance was measured between the intended and actual point of placement to calculate the directional error and was calculated using the formula (x 2 + y 2 + z 2 )½ [Figure 3]. This 3D vector represents the shortest distance to the center of the intended target. The mean lengths of the vectors in the frame-based and frameless groups were compared using a t-test. Chi-square and Pearson's correlation tests were used to evaluate the association between the variables. Comparisons were considered significant only if the two-tailed P < 0.05.
Figure 3: The Euclidean distance (d) between the intended and actual point of placement of DBS leads. (x, y and z) are medio-lateral, antero-posterior and superior-inferior planes, respectively)

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 » Results Top


A total of 50 (n = 33) and 65 DBS leads (n = 33) were implanted using the frameless (NexFrame system) and Leksell stereotactic frame respectively during the study period. In the frameless cohort, Subthalamic nucleus (no. of leads = 36, 72%) was the most common nucleus targeted in 21 patients, followed by Ventralisintermedius nucleus of thalamus in 10 patients (no. of leads = 11, 22%) and Globus pallidusinternus in 2 patients (no. of leads = 3, 6%), [Table 1]. All the procedures were uneventful without adverse events. The electrophysiological mapping was done in both frame and frameless cohorts, however final comparison was made between the target as defined after intraoperative electrophysiological findings (intended target) and that depicted on postoperative imaging studies (actual target).
Table 1: The distribution of DBS leads across different targets using the frameless navigation system (NexFrame) during the study period

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Comparison between frame-based and frameless DBS lead implantation: [Table 2]

The mean error of targeting using frameless system was greater than frame-based system inmedio-lateral (X) [1.4 ± 1.3 mm vs. 1.0 ± 0.7 mm], antero-posterior (Y) [0.9 ± 1.1 mm vs. 0.9 ± 0.5 mm] and in superior-inferior (Z) planes [1.0 ± 0.9 mm vs. 0.7 ± 0.6 mm.] However, this error in targeting reach significance only in the medio-lateral plane (P = 0.03) using the frameless system as compared to the frame-based system. Mean targeting error in the Euclidean directional vector using frameless system was larger [2.2 ± 1.6 mm (mean ± SD)] as compared to using frame-based system [1.7 ± 0.6 mm]. However, this difference in Euclidean directional vector using frame and frameless-based system did not reach statistical significance (P = 0.07). Moreover, standard deviations (SDs) were higher in X, Y, Z planes and 3D vector using frameless system as compared to frame-based system indicating lower accuracy of this system. There was no difference in error according to different targets in the frameless group(P = 0.44).
Table 2: Table comparing the mean error of targeting using frame based and frameless (NexFrame) systems

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Comparison between first 25 and second25 patients who underwent DBS lead implantation using frameless system: [Table 3] and [Figure 4]

The mean error of targeting using frameless system in the first half (25 patients) of the series in medio-lateral (X) plane was 1.9 ± 1.7 mm (mean ± SD), antero-posterior (Y) plane was 1.2 ± 1.3 mm and in superior-inferior (Z) plane was 1.3 ± 0.9 mm. Similarly, the mean error of targeting in the second half of series in medio-lateral (X) plane was 1.0 ± 0.6 mm, antero-posterior (Y) plane was 0.6 ± 0.6 mm and in superior-inferior (Z) plane was 0.7 ± 0.7 mm. This error of targeting was significantly more in the first half of the series only in the medio-lateral and antero-posterior planes as compared to second half using the frameless system (P = 0.02, 0.03, respectively). Mean targeting error in the Euclidean directional vector using frameless system in the first half of series was 2.9 ± 1.9 mm (mean ± SD) and in the second half was 1.5 ± 0.8 mm. There was significantly more error in the Euclidean directional vector in the first 25 leads placed using the frameless system as compared to the second 25 leads. (P = 0.0015) [Figure 4] Moreover, SDs were higher in X, Y, Z planes and 3D vector in the first 25 patients using frameless system as compared to second half of the series which indicates that the accuracy of the system improved with the experience of the surgical team.
Figure 4: Increased dispersion of 3D vector error during the initial half of our series (first 25 leads) as compared to the latter half (second 25 leads) using frameless technique

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Table 3: Table comparing the mean error of targeting in the first and second half of our series using frameless (NexFrame) system

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There was no significant difference in error between the first and second half of the series in frame-based groups. (P > 0.05) [Figure 5].
Figure 5: Dispersion of 3D vector error during the DBS implantation using stereotactic frame-based technique

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 » Discussion Top


Though stereotactic frame system is a well-established technique for targeting deep brain structures with submillimetric accuracy, recent studies comparing the accuracy of frame and frameless systems in trajectory-based neurosurgical procedures such as deep brain stimulator placement reported conflicting results. [4],[5],[7],[9],[11] In this study, the aim was to investigate the accuracy of targeting using frame-based and frameless systems during DBS surgery.

This study showed that the difference between intended and actual lead locations were greater in the frameless cohort as compared to the frame-based group, though this difference reached significance only in the medio-lateral (X) plane, P = 0.03. Moreover in our study, SDs were larger in all planes using frameless system as compared to frame-based system indicating larger dispersion of the final electrode contacts and lower accuracy of this system. Previous studies reported the accuracy of Leksell and CRW frames to be 1.8 ± 0.11 mm and 1.7 ± 0.1 mm on phantom models without weight bearing. [4] Findings of this study reiterate the results of Bjartmarz et al., who compared the accuracy of targeting using frame and frameless system for DBS implantation in 14 patients with essential tremors and reported that conventional frame-based stereotaxy has higher accuracy than frameless technique. [9] In this study the error of targeting using frameless system was significantly greater in the medio-lateral (X, P < 0.001)) and antero-posterior (Y, P < 0.05) planes as compared to the frame-based system. In contrast, Henderson et al. studied the accuracy of a skull mounted trajectory guide system using a plastic skull phantom for functional neurosurgical procedures and concluded that image guided localization system can achieve accuracy similar to or even higher than the conventional stereotactic frame system. [11] Disconcordant to our findings, Holloway et al., reported that there was no statistical difference in the accuracy of the frame-based and frameless systems (P = 0.22). [4] In this study, the mean error of targeting using frame-based system was higher than frameless system only in the antero-posterior plane (Y, 1.6 mm vs. 1.3 mm respectively), whereas this difference was reversed in other two planes (X,1.4 mm frame-based vs. 1.6 mm frameless; Z, 1.7 mm frame-based and 2mm frameless) and none of these differences reached statistical significance. In our study, the mean vector difference between intended and actual lead location was higher in the frameless group (2.2 mm) as compared to frame-based system (1.7 mm), however this difference did not reach statistical significance (P = 0.07). In contrast, another study reported mean vector error of 3.15 mm using either of the techniques (frameless or frame-based). [4] This difference in findings can be attributed to the fact that Holloway et al. compared their phase 3 frameless data with the frame-based raw DBS data from another study by Starr et al., which could potentially impairs the uniformity of this study. [4],[12] Starr et al., reported a mean deviation of 3.15 mm from the expected lead location following 76 STN DBS implantation using a frame-based technique. [12] To overcome this discrepancy, we compared the frameless and frame-based data from a single institution operated by a same group of neurosurgeons. Another study reported mean vector error of 2.3mm for STN, 2.9 mm for Vim and 2.2 mm for Gpi targets in a total of 217 DBS implantations. [4] Fukaya et al., in their study of 10 planned targets using NexFrame system reported mean errors of 1.3 mm, 1.0 mm and 0.5 mm in medio-lateral (X), antero-posterior (Y) and supero-inferior (Z) planes whereas the mean targeting errors using frame-based system in their institute were 1.5mm, 1.1mm and 0.8 mm in the respective planes. [13] This difference in the mean errors of targeting did not reach statistical significance in either of X, Y or Z planes in their study. They concluded that although there is no difference in the accuracy of two systems (frameless vs. frame-based), the frameless system is associated with narrow surgical field for multiple electrode insertion and multitract recordings. [13] In our study, the mean error of targeting and range of dispersion decreased in the second half of the series compared to the first half which can be attributed to the fact that increased experience with this technique tend to increase the accuracy of frameless system by reducing subjective errors. Therefore, the widespread use of frameless technique instead of conventional frame-based stereotaxy in trajectory-based functional neurosurgical procedures is a matter of contentious debate.

In Frameless system, the whole brain is registered using 5-6 bone fiducial markers and these fiducials need to be registered and verified manually by the surgeon in the navigation system. In contrast, in the frame-based system, the brain is registered in reference to the skull-mounted frame with nine different fiducials on the frame. Moreover CT scan slice of 1mm register all fiducial points visible on each slice in all planes on the planning station. Therefore, frameless system is associated with increased number of interfaces and sharing of information across these interfaces might increase the likelihood of inaccuracies. [9],[14] Also, frameless technique uses fusion of CT and MR images to localize the target and guide the implantation, which increase the accuracy and precision of planning. [9],[15] Steinmeier et al. and Holloway et al. reported that registration accuracy does not correlate with and underestimate the error of localization in phantom and clinical studies, respectively. [4],[14] In our study, we used fusion of CT and MRI images in both frame-based and frameless systems to improve the accuracy. In addition, we verified the actual position of the lead using postoperative CT scans and fused them to preoperative MRI so at to minimize distortions. In frame-based systems, the target always lies in the center of a sphere and the trajectory with its axis of movement in the center, whereas in frameless systems the axis of movement lie at or above the skull bone which can increase the deviations at the level of the target. [9] This indicates that frameless systems are more susceptible to error of targeting with even minor deviations above the axis of movement. Clinical scenarios such as CSF loss during surgery with brain shift, discrepancies in selecting the tip of the lead and the AC-PC coordinates on imaging, weight bearing by the frame, deviation of leads prior to anchoring and deviation of the electrode as it traverses through the brain tissue might increase the error of targeting with either the frame-based or frameless systems. [4],[5],[10],[16],[17] Discrepancies in type of imaging scanners, thickness of imaging slices and placement of fiducial markers also tend to influence the mean error of targeting. [10],[14] Another study reported the effect of a change in patient's head position on the registration error using a Brown-Roberts-Wells frame. [17] In this study they concluded that to minimize the localization error the patient's head position should be identical during imaging and surgical intervention. The mean error of localization tends to be higher in clinical settings as compared to phantom laboratory settings using either frame-based or frameless techniques. [4]

A major limitation in our and similar studies include difficulty in localizing the exact center and depth of the electrode in postoperative imaging and its relation to the intended target. [4],[12] Both postoperative CT and MRI scans are associated with beam hardening and magnetic susceptibility artifacts, respectively and therefore the exact location of the electrode is more of estimation in such scans. Moreover, the distal contact on the DBS lead is 1.5 mm above the tip of the electrode which further makes it difficult to localize the distal contact on postoperative scans. Another study reported no significant differences in localizing the postoperative lead positions using either of the imaging modalities (CT or MRI). [18] Furthermore, fusion of these postoperative scans with the preoperative images and transferring of coordinates to calculate the mean error of targeting tend to increase the inaccuracies associated with the system.

There are several advantages of frameless system as compared to frame-based systems. Henderson et al., reported that frameless system offers an advantage of real time positional feedback of the target and therefore confers more safety and confidence in targeting deep brain structures. [5] This technique also offer advantages in patient comfort, ease of neurological examination during surgical procedure, dissociation of imaging and surgical procedure as well as decreased operating time. [4],[5] In addition, patients can tolerate prolonged surgical procedures due to greater mobility and less rigid fixation associated with the frameless techniques.

In this study, the stereotactic frame-based system has greater accuracy and fewer deviations as compared to frameless navigation system for targeting deep brain structures. The targeting accuracy of the frameless system using bone fiducials was not significantly different as compared to stereotactic frame-based system except in medio-lateral direction where frameless system had a significantly more error. The mean error in targeting using frameless system improved significantly over a period of time. The error was not specific to any deep brain target. Frameless system can be an alternative to stereotactic frame-based system in anxious and claustrophobic patients.

 
 » References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]

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Online since 20th March '04
Published by Wolters Kluwer - Medknow