Frameless System Research Articles

SU‐E‐T‐280: DSA as the Primary Image Modality in Frameless SRS for Treatment of Arteriovenous Malformations

Purpose: Stereotactic radiosurgery (SRS) is an effective alternative to microsurgical resection or embolization for arteriorvenous malformations (AVMs). Digital Subtraction Angiography (DSA) is the gold standard for diagnosis and characterization of vascular anatomy, but it requires a frame attached to the skull. Proton and the new image‐guided LINAC SRS systems are frameless. These frameless systems represent a major progress in radiotherapy — however, they prevent the usage of DSA. Without DSA as the primary image, the outcome may be less‐than‐optimal and sometimes even impossible to treat. This study intends to solve the problem. Methods: The solution is based on a conceptual breakthrough whereby, in contrast to the previous practice where a localizer was fixed to the skull during angiography, the patient can now be angiographed under standard clinical conditions without frame and localizer, while a newly‐designed localizer box is radiographed separately. Based on several fiducials implanted into the skull, a target volume on DSA can be transferred to the localizer system in 3D, then further transferred to contours in multi‐CT slices. Combined with other imaging modalities, a treatment can be planned and executed by a routine frameless SRS. Results: Phantom tests demonstrated that the uncertainty in transferring a point target to the localizer is less than 0.3 mm. The maximum transformation errors from the localizer to CT system are ∼0.3 mm. The overall uncertainty of placing a point target to CT is 0.4 mm. For volume targets the volume determined is accurate within < 6%. The algorithm and software are robust and accurate. The method has been successfully applied clinically. Conclusions: A new reliable method for frameless Stereotactic Radiosurgery has been developed, which allows using DSA as the primary imaging in AVM treatment. Implanted fiducials are required for the time being, but using non‐implanted fiducials is feasible.This work is partially supported by the JCRT Foundation.

Setup Accuracy of the Novalis ExacTrac 6DOF System for Frameless Radiosurgery

Stereotactic radiosurgery using frame-based positioning is a well-established technique for the treatment of benign and malignant lesions. By contrast, a new trend toward frameless systems using image-guided positioning techniques is gaining mainstream acceptance. This study was designed to measure the detection and positioning accuracy of the ExacTrac/Novalis Body (ET/NB) for rotations and to compare the accuracy of the frameless with the frame-based radiosurgery technique. A program was developed in house to rotate reference computed tomography images. The angles measured by the system were compared with the known rotations. The accuracy of ET/NB was evaluated with a head phantom with seven lead beads inserted, mounted on a treatment couch equipped with a robotic tilt module, and was measured with a digital water level and portal films. Multiple hidden target tests (HTT) were performed to measure the overall accuracy of the different positioning techniques for radiosurgery (i.e., frameless and frame-based with relocatable mask or invasive ring, respectively). The ET/NB system can detect rotational setup errors with an average accuracy of 0.09° (standard deviation [SD] 0.06°), 0.02° (SD 0.07°), and 0.06° (SD 0.14°) for longitudinal, lateral, and vertical rotations, respectively. The average positioning accuracy was 0.06° (SD 0.04°), 0.08° (SD 0.06°), and 0.08° (SD 0.07°) for longitudinal, lateral and vertical rotations, respectively. The results of the HTT showed an overall three-dimensional accuracy of 0.76 mm (SD 0.46 mm) for the frameless technique, 0.87 mm (SD 0.44 mm) for the relocatable mask, and 1.19 mm (SD 0.45 mm) for the frame-based technique. The study showed high detection accuracy and a subdegree positioning accuracy. On the basis of phantom studies, the frameless technique showed comparable accuracy to the frame-based approach.

Intra-operative Imaging Techniques During Surgical Management of Gliomas

The goal of glioma surgery is to maximize tumor removal while preserving existing function. Intra-operative imaging techniques play an important part in achieving this goal. This article surveys those techniques and discusses the indications, advantages, and drawbacks of each. Structural techniques such as intra-operative magnetic resonance imaging (MRI), ultrasound, diffusion tensor imaging, and 5-aminolevulinic acid staining offer anatomic information. Functional techniques such as functional MRI, magnetoencephalography, and transcranial magnetic stimulation provide information about the functionality of brain regions. When incorporated into a frameless stereotactical neuronavigation system, these modalities increase both the efficacy and safety of glioma surgery by allowing the surgeon to achieve the most extensive and safe resection possible.

Perturbation Analysis of Relationship of Conebeam CT and Stereoscopic Optical Localization System in Frameless Stereotactic Radiosurgery

Purpose/Objective(s): Treatment targets under frameless stereotactic radiosurgery (SRS) can be localized by optical guidance platform (OGP) using a stereoscopic camera. The registration of the target to isocenter is by IR markers on a patient bite block. The rigidity of the bite block and treatment target will determine the accuracy of the treatment. However, the offset between the biteblock and target cannot be visually detected; therefore conebeam CT(CBCT) is used to verify the accuracy of target localization based on information of anatomy volume. In this study, the relationship between these two localization methods was investigated by sequential perturbations of the setup.

Analysis of Stereotactic Accuracy of the Cosman-Robert-Wells Frame and Nexframe Frameless Systems in Deep Brain Stimulation Surgery

Introduction: The primary goal of stereotactic systems in deep brain stimulation (DBS) surgery is accurate delivery of a DBS lead to a target identified on imaging. Thus, it is critical to understand the accuracy of the stereotactic systems and the factors which may be associated with a decrease in accuracy. Methods: Ninety patients underwent microelectrode recording-guided placement of 139 DBS leads by a single surgeon using the Cosman-Roberts-Wells (CRW) frame (n = 70) or a frameless skull-mounted trajectory guide (Nexframe; n = 69). The final DBS location was identified on a postoperative CT fused to the preoperative CT and MRI scans. The difference between this final location and the expected location was calculated. Results: The vector error was 2.65 mm (standard error, 0.22) for the frame and 2.78 mm (standard error, 0.25) for the frameless methods (p = 0.69). There was a gradual decline in error for both systems over time, as the vector error of the last 20 implants was 1.99 for the CRW frame and 2.04 for the Nexframe (p = 0.86). Conclusions: This study shows that the CRW frame and Nexframe frameless systems have equivalent accuracy. Furthermore, the accuracy of both techniques improved over time, from 3 mm initially to 2 mm with current techniques.

WE-A-BRB-01: Single and Hypo-Fractionated Stereotactic Irradiation of Brain & Body Lesions

Stereotactic irradiation has been in clinical use for decades. It was initially performed almost exclusively for brain lesions with Gamma Knife radiosurgery. In the 1990s, it was expanded to the rest of the body. Over the years the emphasis has shifted from benign lesions like AVMs to malignancies such as early‐stage lungcarcinoma in medically inoperable patients. Stereotactic body radiation therapy(SBRT) now is available at approximately 700 institutions in the United States (American Hospital Association guide 2009). Almost 1500 articles have been published on SBRT, and there are several ongoing prospective randomized trials comparing SBRT with standard radiation therapy. Major considerations in single and hypo‐fractionated radiotherapy are the therapeutic gain and technical challenges for precise imaging, planning, and dose delivery. In this educational session, we will first briefly review the results of recently completed studies for intracranial and extracranial stereotactic irradiation with an emphasis on SBRT; and then discuss the technical aspects in image registration, stereotactic localization, target delineation uncertainties particularly using FDG‐PET for the lungcancer; and finally address the physics QA procedures to ensure optimal planning that differentiate the doses between tumor and normal tissue without detracting of target coverage and to deliver the desired dose under image‐guidance using stereotactic frame or frameless refixation systems. Learning Objectives: 1. Understand the therapeutic gain in stereotactic radiosurgery(SRS) or hypo‐fractionated SBRT and recognize that the success of SBRT relies on both the dose escalation to the tumor and the margin reduction and tumor shrinkage 2. Discuss the rationale and technical details about how to use fine CT or MRI scans for the delineation of small targets and critical structures, how to eliminate motion or other image artifacts using stereotactic‐type image localization, and how to combine anatomic information from CT or MRI with physiological information from PET or functional MRI for optimal treatment planning 3. Learn the physics QA procedures for plan evaluation with empirical rules on the conformity index, target coverage, and periphery dose gradient, and QA for dose delivery under image‐guided target refixation and patient tracking 4. Develop mock training for SBRT in procedural design and implementation for annual training of the SBRT team to prevent the occurrence of systematic and human errors

Nexframe Frameless Stereotaxy with Multitract Microrecording: Accuracy Evaluated by Frame-Based Stereotactic X-Ray

Objective: The development of image-guided systems rendered it possible to perform frameless stereotactic surgery for deep brain stimulation (DBS). As well as stereotactic targeting, neurophysiological identification of the target is important. Multitract microrecording is an effective technique to identify the best placement of an electrode. This is a report of our experience of using the Nexframe frameless stereotaxy with Ben’s Gun multitract microrecording drive and our study of the accuracy, usefulness and disadvantages of the system. Methods: Five patients scheduled to undergo bilateral subthalamic nucleus (STN) DBS were examined. The Nexframe device was adjusted to the planned target, and electrodes were introduced using a microdrive for multitract microrecording. In addition to the Nexframe frameless system, we adopted the Leksell G frame to the same patients simultaneously to use a stereotactic X-ray system. This system consisted of a movable X-ray camera with a crossbar and was adopted to be always parallel to the frame with the X-ray film cassette. The distance between the expected and actual DBS electrode placements was measured on such a stereotactic X-ray system. In addition, the distance measured with this system was compared with that measured by conventional frame-based stereotaxy in 20 patients (40 sides). Results: The mean deviations from 10 planned targets were 1.3 ± 0.3 mm in the mediolateral (x) direction, 1.0 ± 0.9 mm in the anteroposterior (y) direction and 0.5 ± 0.6 mm in the superoposterior (z) direction. The data from the frame-based stereotaxy in our institute were 1.5 ± 0.9 mm in the mediolateral (x) direction, 1.1 ± 0.7 mm in the anteroposterior (y) direction and 0.8 ± 0.6 mm in the superoposterior (z) direction. Then, differences were not statistically significant in any direction (p > 0.05). The multitract microrecording procedure associated with the Nexframe was performed without any problems in all of the patients. None of these electrodes migrated during and/or after the surgery. However, the disadvantage of the system is the narrow surgical field for multiple electrode insertion. Coagulating the cortex and inserting multiple electrodes under such a narrow visual field were complicated. Conclusion: The Nexframe with multitract microrecording for STN DBS still has some problems that need to be resolved. Thus far, we do not consider that this technology in its present state can replace conventional frame-based stereotactic surgery. The accuracy of the system is similar to that of frame-based stereotaxy. However, the narrow surgical field is a disadvantage for multiple electrode insertion. Improvement on this point will enhance the usefulness of the system.

Evaluation of patient setup uncertainty of optical guided frameless system for intracranial stereotactic radiosurgery

The optically‐guided frameless system (OFLS) has been used in our clinic for intracranial stereotactic radiosurgery (SRS) since 2006, as it is especially effective in IMRT‐based radiosurgery (IMRS), which allows treating multiple brain lesions simultaneously using single isocenter approach. This study reports our retrospective analysis of patient setup accuracy using this system. The OFLS consists of a bite block with fiducial markers and an infra‐red camera system. To test reproducibility, patients are taken for reseat verification after bite block construction. Upon the completion of radiosurgery planning, the isocenter position(s) and images are sent to the optical guidance computer where fiducials are manually registered from the CT scan. During treatment, patient setup is monitored and guided by the camera readings on the fiducials. In addition, two orthogonal kV images are acquired and used as an isocenter verification tool. In addition, we have analyzed the reseat and fiducial digitization data of 56 patients. Retrospective comparison of kV images with reference images has been carried out for all the patients to evaluate actual patient setup accuracy at the time of treatment. The histogram of the findings shows that 82.2% of patients had 3D isodisplacement (E≤1mm; 5.2% had 1<E≤2mm). Hence, for 87.5 % of the patients in the study, treatments were finished under the optical guidance with a maximum setup error of 2 mm and the median setup error of 0 mm. For the remaining 12.5% of patients in the study, the isodisplacements were greater than 2 mm and the treatment records showed that those patients were repositioned, guided by the orthogonal kV‐images. It is found that the OFLS in the SRS treatment has acceptable accuracy when used in conjunction with orthogonal kV images, and the use of orthogonal kV images as a verification tool ensures the efficacy of frameless localization in the radiosurgery treatment.PACS numbers: 87.53.Ly, 87.61.Tg, 87.55.Qr, 87.56.‐v, 29.20.Ej

A modified stereotactic frame as an instrument holder for frameless stereotaxis: Technical note

Background:In order to improve the targeting capability and trajectory planning and provide a more secure probe-holding system, a simple method to use a stereotactic frame as an instrument holder for the frameless stereotactic system was devised.Methods:A modified stereotactic frame and BrainLab vector vision neuronavigation sys¬tem were used together. The patient was placed in the stereotactic head-holder to which a reference array of the neuronavigation system was attached. The pointer of the frameless system was placed in the probe-holder of the frame. An offset in distances was kept between the radius of the arch of the frame and the tip of the pointer so that the pointer was always outside the head during navigation. The offset correction was made on the BrainLab monitor so that the center of the arc of the frame was at the tip of the probe line on the monitor. Then, using the frame’s coordinate adjuster system, the center of the arc was positioned on the target. This method was used to insert depth electrodes (seven procedures) and gain access to the temporal horn (three procedures).Results:Post-operative scans showed that the accuracy was within 2.5 mm in all three planes for depth electrode placement, and easy access to the temporal horn was obtained in two other patients.Conclusion:This is a simple method to use a stereotactic frame to improve coordinate and trajectory adjustments and provides a better method to stabilize the pointer and the probe-holder during frameless stereotactic procedures.

Rapid fabrication of custom patient biopsy guides

Image‐guided surgery is currently performed using frame‐based as well as frameless approaches. In order to reduce the invasive nature of stereotactic guidance and the cost in both equipment and time required within the operating room, we investigated the use of rapid prototyping (RP) technology. In our approach, we fabricated custom patient‐specific face masks and guides that can be applied to the patient during stereotactic surgery. While the use of RP machines has previously been shown to be satisfactory from an accuracy standpoint, one of our design criteria – completing the entire build and introduction into the sterile field in less than two hours – was unobtainable. (1) Our primary problems were the fabrication time and the nonresistance of the built material to high‐temperature sterilization. In the current study, we have investigated the use of subtractive rapid prototyping (SRP) machines to perform the same quality of surgical guidance, while improving the fabrication time and allowing for choosing materials suitable for sterilization. Because SRP technology does not offer the same flexibility as RP in terms of prototype shape and complexity, our software program was adapted to provide new guide designs suitable for SRP fabrication. The biopsy guide was subdivided for a more efficient build with the parts being uniquely assembled to form the final guide. The accuracy of the assembly was then assessed using a modified Brown‐Roberts‐Wells phantom base by which the position of a biopsy needle introduced into the guide can be measured and compared with the actual planned target. These tests showed that: 1) SRP machines provide an average technical accuracy of 0.77 mm with a standard deviation of the mean of 0.07 mm, and 2) SRP allows for fabrication and sterilization within three‐and‐a‐half hours after diagnostic image acquisition. We are confident that technology is capable of reducing this time to less than one hour. Further tests are being conducted to determine the registration accuracy of the face mask on the patient's head under IRB‐approved trials. The accuracy of this new guidance technology will be verified by judging it against current frame‐based or frameless systems.PACS number: 87.57.Gg

SU-FF-T-528: Retrospective Analysis On Patient Localization Accuracy for Linac-Based Intracranial Stereotactic Radiosurgery Using Frameless System

Purpose: The frameless system has been used in our clinic for intracranial stereotactic radiosurgery. This study reports our retrospective analysis and findings on patient setup and isocenter localization of radiosurgery treatment. Method and Materials: The frameless immobilization consists of a bite-block with fiducial markers and an infra-red camera system. To test bite-block seating, patient was taken for reseat verification before imaging. The SRS-plan and images were sent to the camera computer for fiducials digitization. Prior to treatment, patient setup was guided by the camera readings on fiducials for isocenter localization. Orthogonal kV-images were taken as verification only. In this study, we have analyzed reseat test and fiducial digitization data of 53 patients. Retrospective comparison of KV-images with planning DRRs was carried out. Results: The patient reseat test is found of no correlation with the actual 3D treatment isocenter displacements. The sample mean of predicted error at isocenter from fiducial digitization is 0.46 mm. The histogram of 3D iso-displacement shows that 86.8% of patients have localization error E⩽1 mm, 5.7% of 1<E⩽2 mm, 3.8% of 2<E⩽3 mm, and 3.8% of 3<E⩽5.1 mm. As a result, for 92.5% of patient sample, treatments were proceeded through optical guidance, where iso-displacement errors range from 0 to 2 mm with a median of 0.0 mm, and sample mean of 0.4 mm. For the rest 7.5% of patient population, where 3D iso-displacements were greater than 2 mm as compared with KV images, isocenter shifts were applied and patient positions were realigned using OBI, and treatments proceeded without the optical guidance. Conclusion: Our study concludes that the frameless localization for radiosurgery treatment has acceptable accuracy and using KV-imaging as a verification tool is a necessity to ensure its efficacy. Also, adding 2 mm margin to CTV to form PTV in the radiosurgery planning is strongly recommended.

VARIOGUIDE

VarioGuide (BrainLAB AG, Feldkirchen, Germany) is a new system for frameless image-guided stereotaxy. In the present study, we aimed to assess target point accuracy in a laboratory setting and the clinical feasibility of the system. Using the phantom of our frame-based stereotactic system (Riechert-Mundinger; Inomed Medizintechnik GmbH, Teningen, Germany), target points were approached from different angles with the frameless system. Target point deviation in the x, y, and z planes was assessed. Furthermore, patients harboring intracranial lesions were diagnostically biopsied using VarioGuide. Phantom-based accuracy measurements yielded a mean target point deviation of 0.7 mm. Between February 2007 and April 2008, 27 patients were diagnostically biopsied. Lesion volumes ranged from 0.2 to 117.6 cm3, trajectory length ranged from 25.3 to 64.1 mm, and the diagnostic yield was 93%. Concluding from the phantom measurements with ideal image-object registration, assumed spherical lesions with a volume of 0.524 cm can be biopsied with 100% target localization. Early clinical data revealed VarioGuide to be safe and accurate for lesions of 0.2 cm3 and larger. Thereby, the system seems feasible for the biopsy of most intracranial lesions.

SU‐GG‐T‐451: Clinical Evaluation of New BrainLab Image Guided Frameless SRS System

Purpose: To evaluate the results and accuracy of Image Guided Frameless SRS. Method and Materials: A new image guided frameless system for Novalis has been installed at our institution for Stereotactic Radiosurgery (SRS) treatment. The main features of the system are its utilization of an Infrared camera, a pair of kV x‐ray, and a 6D couch for patient setup. The system works at any couch angle. The tracking computer will generate DRRs for the corresponding couch angle and compares them with OBI kV images to determine the shift corrections in 6 Dimensions (3 Translational and 3 Rotational). The shift corrections are carried out by the 6D robotic couch, The same process is repeated to confirm the correction was successful, and that further correction is no longer needed. The same process is required for every couch angle during the SRS treatment. The SRS fields are shaped by employing either BrainLab micro MLC or BrainLab cones. We have treated various types of brain tumors including trigeminnal nerves. Results: The system is very intutive, and in general only one OBI guided frameless correction procedure is needed for each couch angle. The OBI kV images obtained for confirmation of correction on average show that the mean deviation by automatic imaging fusion computer are 0.5±1, 0.2±1 and 0.3±1 degree for pitch, roll and yaw, 0.3±0.3, 0.3±0.4, 0.2±0.8 mm for longitudinal, lateral and vertical translation movement. Included are the pre and post trigeminal nerve MRI scan that shows clearly that we hit the target of planned treatment. Conclusion: The new type of BrainLab' Image Guided Framless SRS system coupled with an 6D robotic couch, imaging at any couch angles, is accurate, and easy to use. Initial experience shows that clinical results are equivalent to the Frame based SRS system.

Quality Assurance Procedures for Stereotactic Body Radiation Therapy

Cranial stereotactic radiosurgery (SRS) and radiotherapy (SRT) are established treatment modalities. Initial implementations of these techniques rigidly attached frames to the patient's head for single-fraction treatments. The head frame accommodates an external fiducial marker system that is a reliable reference for targets within the cranium and accurately links the imaging equipment used for treatment planning to the treatment device. Fractionated SRT treatments use noninvasive "relocatable"-type head immobilization that fixes to the patient's head and face features. Clearly defined quality assurance (QA) procedures exist for both cranial SRS and SRT but are not as well developed for extracranial SRT. Procedures for demonstrating the geometric relationship between the planning imaging and treatment have to some degree copied the techniques used for intracranial stereotactic irradiation. However, there are some unique QA issues that are specific to extracranial irradiation. One major consideration is the large number of methodologies available for stereotactic body radiation therapy. In addition to the variety of integrated image-guided frameless systems, there are immobilization devices (called body frame systems) that use a fiducial reference system similar to the cranial devices. This article describes generic QA approaches that can be adapted to the various stereotactic body radiation therapy methodologies.

Validation of the PathFinder™ neurosurgical robot using a phantom

Minimally invasive surgery was born out of recent advances in neuro-imaging and stereotactic technology. As a result, the scale of neurosurgical procedures will soon be so small that it will not be within the ability of the most gifted and skilled neurosurgeons of today. Hence, neurosurgical robotics is the natural evolution in this field. The aim of this study was to evaluate the performance of a new robotic system in a neurosurgical phantom, comparing it to standard frame-based and frameless technology of today. In total, 19 different targets were approached by two standard stereotactic frames, the Stealth Station frameless system and the robot. The CRW and the ZD stereotactic frames were used. The frameless system was the Stealth Station image guidance system. The phantom used was a replica of the human skull fitted with 10 surface and nine deep targets. The robotic system outperformed both frame-based and frameless systems in all experiments in this study. The application accuracies were: robot, 0.5 mm; stereotactic frames, 0.98 mm; and frameless system, 1.96 mm. The robotic system was as accurate as the stereotactic frame, but without technical restrictions and cumbersome manual adjustments. Furthermore, the robotic system had near-absolute geometric accuracy, was reliable to perform the same procedure over and over without tiresomeness, variation or boredom, and would be impervious to biohazards and hostile environments.

Navigation-assisted transsphenoidal deflation of a detachable balloon in the cavernous sinus after embolization of a direct carotid-cavernous fistula

We describe a case of transsphenoidal deflation of a detachable balloon after embolization of a carotid-cavernous fistula (CCF). The patient developed complete third and sixth nerve palsies immediately after detachable balloon embolization of the CCF, which was considered to be caused by cavernous sinus (CS) compression by the over-inflated balloon. We performed direct puncture of the balloon via the transsphenoidal route using a frameless neuronavigation system. Navigation-assisted transsphenoidal approach (TSA) is technically feasible for balloon deflation in cases of severe cranial nerve palsies due to an over-inflated balloon.

Application of neuronavigation in resection of intracranial deep tumor

目的 评价神经导航系统在脑深部肿瘤显微手术中的应用价值.方法应用StealthNavigation神经导航系统对46例脑深部肿瘤手术治疗,术前采用神经影像MRI导航扫描,对病灶进行三维重建,制定显微手术治疗计划,术中实时导航指导手术切除范围,分析治疗效果.结果肿瘤及其周围结构定位准确,手术全切除39例(84.7%),次全切除3例(6.6%),大部分切除4例(8.7%);术后症状改善或不变41例(89.1%),症状加重或出现新症状5例(10.9%),无死亡病例.结论应用神经导航系统可以精确定位脑深部肿瘤,设计最佳手术入路,有助于提高显微手术疗效,防止手术副损伤,降低手术并发症。

フレームレス医用断層画像オーバーレイ表示システムの開発

フレームレス医用断層画像オーバーレイ表示システムの開発

Frameless stereotaxy using bone fiducial markers for deep brain stimulation

Functional neurosurgical interventions such as deep brain stimulation (DBS) are traditionally performed with the aid of a stereotactic frame. Although frameless techniques have been perceived as less accurate, data from a recent phantom study of a modified frameless approach demonstrated a laboratory accuracy exceeding that obtained using a common frame system. The present study was conducted to evaluate the accuracy of a frameless system in routine clinical use. Deep brain stimulation leads were implanted in 38 patients by using a skull-mounted trajectory guide and an image-guided workstation. Registration was accomplished with bone fiducial markers. Final lead positions were measured on postoperative computerized tomography scans and compared with the planned lead positions. The accuracy of the Leksell frame within the clinical situation has been reported on in a recent study; these raw data served as a comparison data set. The difference between expected and actual lead locations in the x plane was 1.4 mm in the frame-based procedure and 1.6 mm in the frameless procedure. Similarly, the difference in the y plane was 1.6 mm in the frame-based system and 1.3 mm in the frameless one. The error was greatest in the z plane, that is, 1.7 mm in the frame-based method and 2 mm in the frameless system. Multivariate analysis of variance demonstrated no statistically significant difference in the accuracy of the two methods. The accuracy of the frame-based and frameless systems was not statistically significantly different (p = 0.22). Note, however, that frameless techniques offer advantages in patient comfort, separation of imaging from surgery, and decreased operating time.

Frameless stereotactic placement of depth electrodes in epilepsy surgery

Depth electrodes are useful in the identification of deep epileptogenic foci. Computerized tomography-magnetic resonance (CT/MR)- and angiography-guided frame-based techniques are safe and accurate but require four-point skull fixation that limits cranial access for the placement of additional grids and strips. The authors investigated the viability and accuracy of placing depth electrodes by using a commercially available frameless system. A slotted, custom-designed adapter was built to interface with the StealthStation Guide Frame-DT and 960-525 StealthFighter. The Cranial Navigation software was used to plan the trajectory and entry site based on preoperative spoiled gradient MR imaging studies. Forty-one depth electrodes were placed in 51 targets in 20 patients. Thirty-one of these electrodes were inserted through the temporal neocortex following craniotomy and placement of subdural grids, whereas 10 were placed through burr holes. All electrodes had contact either within (71%) or touching (29%) the target, 50 of which (98%) provided adequate recordings. Although the mean distance of the distal electrode contact from the intended target was 3.1 +/- 0.5 mm, the mean distance to the edge of the anatomical structure was 0.4 +/- 0.9 mm. Placement via the laterotemporal approach was significantly (p < 0.001) more accurate than that via the occipitotemporal approach. No complication occurred. Depth electrodes can be placed safely and accurately by using a commercially available frameless stereotactic navigation system and a custom-made adapter. Depth electrode placement to record ictal onsets during epilepsy surgery only requires the contacts to touch rather than to reside within the intended structure. The laterotemporal approach is a more accurate method of placing electrodes than is the occipitotemporal one, likely due to the increased distance from the entry point to the target.