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Terrance M. Darcey, Erik J. Kobylarz, Michael A. Pearl, Patricia J. Krauss, Stephanie A. Ferri, David W. Roberts and David F. Bauer

OBJECTIVE

The purpose of this study was to develop safe, site-specific procedures for placing and leaving subdermal needle leads for intraoperative monitoring (IOM) during intraoperative MRI procedures.

METHODS

The authors tested a variety of standard subdermal needle electrodes designed and FDA-approved for IOM in the conventional operating room. Testing was used to determine the conditions necessary to avoid thermal injury and significant image artifacts with minimal disruption of IOM and MRI procedures. Phantom testing was performed with a fiber optic (lead) temperature monitoring system and was followed by testing of leads placed in a healthy volunteer. The volunteer testing used electrode placements typical of standard IOM cases, together with radiofrequency (RF) coil placement and imaging sequences routinely employed for these case types. Lead length was investigated to assess heating effects for electrodes placed within the RF coil.

RESULTS

The authors found that conventional stainless steel (SS) and platinum/iridium (Pt/Ir) subdermal needles can be used safely without significant heating when placed outside the RF coil, and this accounts for the majority or entirety of electrode placements. When placed within the RF coil, Pt/Ir leads produced minimal image artifacts, while SS leads produced potentially significant artifacts. In phantom testing, significant heating was demonstrated in both SS and Pt/Ir leads placed within the RF coil, but only during high-resolution T2-weighted scanning. This problem was largely, but not completely, eliminated when leads were shortened to 25 cm. Human testing was unremarkable except for nonpainful heating detected in a few electrodes during thin-slice (1.5 mm) FLAIR scanning. Transient irritation (skin reddening along the needle tract) was noted at 2 of the electrodes with detectable heating.

CONCLUSIONS

The authors were satisfied with the safety of their site-specific procedures and have begun with off-label use (following institutional review board approval and obtaining patient informed consent) of tested monitoring leads in cases that combine IOM and MRI. The authors recommend that all facilities perform their own site-specific testing of monitoring leads before proceeding with their routine use.

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Kimon Bekelis, Atman Desai, Alex Kotlyar, Vijay Thadani, Barbara C. Jobst, Krzysztof Bujarski, Terrance M. Darcey and David W. Roberts

Object

Intracranial monitoring for epilepsy has been proven to enhance diagnostic accuracy and provide localizing information for surgical treatment of intractable seizures. The authors investigated the usefulness of hippocampal depth electrodes in the era of more advanced imaging techniques.

Methods

Between 1988 and 2010, 100 patients underwent occipitotemporal hippocampal depth electrode (OHDE) implantation as part of invasive seizure monitoring, and their charts were retrospectively reviewed. The authors' technique involved the stereotactically guided (using the Leksell model G frame) implantation of a 12-contact depth electrode directed along the long axis of the hippocampus, through an occipital twist drill hole.

Results

Of the 100 patients (mean age 35.0 years [range 13–58 years], 51% male) who underwent intracranial investigation, 84 underwent resection of the seizure focus. Magnetic resonance imaging revealed mesial temporal sclerosis (MTS) in 27% of patients, showed abnormal findings without MTS in 55% of patients, and showed normal findings in 18% of patients. One patient developed a small asymptomatic occipital hemorrhage around the electrode tract. The use of OHDEs enabled epilepsy resection in 45.7% of patients who eventually underwent standard or selective temporal lobe resection. The hippocampal formation was spared during surgery because data obtained from the depth electrodes showed no or only secondary involvement in 14% of patients with preoperative temporal localization. The use of OHDEs prevented resections in 12% of patients with radiographic evidence of MTS. Eighty-three percent of patients who underwent resection had Engel Class I (68%) or II (15%) outcome at 2 years of follow-up.

Conclusions

The use of OHDEs for intracranial epilepsy monitoring has a favorable risk profile, and in the authors' experience it proved to be a valuable component of intracranial investigation. The use of OHDEs can provide the sole evidence for resection of some epileptogenic foci and can also result in hippocampal sparing or prevent likely unsuccessful resection in other patients.

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Kimon Bekelis, Tarek A. Radwan, Atman Desai, Ziev B. Moses, Vijay M. Thadani, Barbara C. Jobst, Krzysztof A. Bujarski, Terrance M. Darcey and David W. Roberts

Object

Intracranial monitoring for epilepsy has been proven to enhance diagnostic accuracy and provide localizing information for surgical treatment of intractable seizures. The authors investigated their experience with interhemispheric grid electrodes (IHGEs) to assess the hypothesis that they are feasible, safe, and useful.

Methods

Between 1992 and 2010, 50 patients underwent IHGE implantation (curvilinear double-sided 2 × 8 or 3 × 8 grids) as part of arrays for invasive seizure monitoring, and their charts were retrospectively reviewed.

Results

Of the 50 patients who underwent intracranial investigation with IHGEs, 38 eventually underwent resection of the seizure focus. These 38 patients had a mean age of 30.7 years (range 11–58 years), and 63% were males. Complications as a result of IHGE implantation consisted of transient leg weakness in 1 patient. Of all the patients who underwent resective surgery, 21 (55.3%) had medial frontal resections, 9 of whom (43%) had normal MRI results. Localization in all of these cases was possible only because of data from IHGEs, and the extent of resection was tailored based on these data. Of the 17 patients (44.7%) who underwent other cortical resections, IHGEs were helpful in excluding medial seizure onset. Twelve patients did not undergo resection because of nonlocalizable or multifocal disease; in 2 patients localization to the motor cortex precluded resection. Seventy-one percent of patients who underwent resection had Engel Class I outcome at the 2-year follow-up.

Conclusions

The use of IHGEs in intracranial epilepsy monitoring has a favorable risk profile and in the authors' experience proved to be a valuable component of intracranial investigation, providing the sole evidence for resection of some epileptogenic foci.

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Atman Desai, Kimon Bekelis, Terrance M. Darcey and David W. Roberts

Intracranial electroencephalography monitoring of the insula is an important tool in the investigation of the insula in medically intractable epilepsy and has been shown to be safe and reliable. Several methods of placing electrodes for insular coverage have been reported and include open craniotomy as well as stereotactic orthogonal and stereotactic anterior and posterior oblique trajectories. The authors review each of these techniques with respect to current concepts in insular epilepsy.

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Atman Desai, Barbara C. Jobst, Vijay M. Thadani, Krzysztof A. Bujarski, Karen Gilbert, Terrance M. Darcey and David W. Roberts

Object

The authors describe their experience with stereotactic implantation of insular depth electrodes in patients with medically intractable epilepsy.

Methods

Between 2001 and 2009, 20 patients with epilepsy and suspected insular involvement during seizures underwent intracranial electrode array implantation at the authors' institution. All patients had either 1 or 2 insular depth electrodes placed as part of an intracranial array.

Results

A total of 29 insular depth electrodes were placed using a frontal oblique trajectory. Eleven patients had a single insular electrode placed and 8 patients had 2 insular electrodes placed unilaterally. One patient had bilateral insular electrodes implanted. Postoperative imaging demonstrated satisfactory placement in all but 1 instance, and there was no associated morbidity or mortality. Fourteen patients underwent a subsequent resection, involving the frontal lobe (9 patients), temporal lobe (4), or frontotemporal lobes (1), and of these, 11 currently have Engel Class I outcome. Two patients (10%) had seizures originating within the insula and another 5 patients (25%) demonstrated early specific insular involvement. Neither patient with an insular seizure focus went on to resection. All 5 of the patients with early specific insular involvement underwent an insula-sparing resective procedure with Engel Class I outcome in all cases.

Conclusions

Stereotactic placement of insular electrodes via a frontal oblique approach is a safe and efficient technique for investigating insular involvement in medically intractable epilepsy. The information obtained from insular recording can be valuable for appreciating the degree of insular contribution to seizures, allowing localization to the insula or clearer implication of other sites.

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Terrance M. Darcey and David W. Roberts

Object

The anatomical localization of electrodes in the human brain is important for the interpretation of pathophysiological (epileptifom spikes, seizures) and functional data (stimulation mapping, evoked potentials). Electroencephalography and evoked potentials are volume-conducted field effects that are most easily interpreted with knowledge of the location and topology of adjacent structures, and brain stimulation techniques produce current fields whose effects are highly dependent on the geometry of electrode assemblies in relation to adjacent structures. In this paper, the authors describe a straightforward method for implanted electrode localization, and detail their experience to date with the technique.

Methods

The described method is based on the coregistration of preoperative MR imaging studies with postimplant CT scans by using standard mutual information optimization of rigid body transformation of the CT to the MR image. Fused images of the MR and thresholded CT images are derived, and electrodes are visualized using various standard computer projections, renderings, and measurement tools.

Results

The authors have successfully used the described method over an extended period to localize electrode contacts in intracranial implants for seizure localization, and in long-term implants for movement disorders and seizure control. The accuracy of localization is very good, although it is dependent on image quality and possible brain shift between acquisition of the CT and MR images.

Conclusions

This method is easily implemented and is useful for a wide variety of clinical and research applications. It is a straightforward process to extend it to additional image modalities that are emerging for surgical planning and image guidance.

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William J. Spire, Barbara C. Jobst, Vijay M. Thadani, Peter D. Williamson, Terrance M. Darcey and David W. Roberts

Object

The authors describe their experience with a technique for robotic implantation of depth electrodes in patients concurrently undergoing craniotomy and placement of subdural monitoring electrodes for the evaluation of intractable epilepsy.

Methods

Patients included in this study underwent evaluation in the Dartmouth Surgical Epilepsy Program and were recommended for invasive seizure monitoring with depth electrodes between 2006 and the present. In all cases an image-guided robotic system was used during craniotomy for concurrent subdural grid electrode placement. A total of 7 electrodes were placed in 4 patients within the time period.

Results

Three of 4 patients had successful localization of seizure onset, and 2 underwent subsequent resection. Of the patients who underwent resection, 1 is now seizure free, and the second has only auras. There was 1 complication after subpial grid placement but no complications related to the depth electrodes.

Conclusions

Robotic image-guided placement of depth electrodes with concurrent craniotomy is feasible, and the technique is safe, accurate, and efficient.

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Ann-Christine Duhaime, Andrew J. Saykin, Brenna C. McDonald, Carter P. Dodge, Clifford J. Eskey, Terrance M. Darcey, Loretta L. Grate and Paul Tomashosky

Object

The piglet is an excellent model for the developing human brain, and has been used increasingly in various centers for studies of traumatic brain injury and other insults. Unlike rodent or primate models, however, there are few behavioral scales for the piglet, and the available ones are used to test general responsiveness rather than specific functional outcome. The differing behavioral repertoires of animals of different ages provide an additional challenge when age-dependent injury responses are compared. To overcome these experimental limitations of piglets in brain injury research, the authors developed a functional magnetic resonance (fMR) imaging paradigm that can be used to track recovery in the somatosensory cortex over time in anesthetized animals of different ages.

Methods

Fifteen fMR imaging studies in eight piglets were performed before and after scaled cortical impact injury to the primary somatosensory cortex subserving snout sensation. Specific anesthetic and imaging protocols enabled visualization of cortical activation, and comparison with somatosensory evoked potentials obtained before and after injury was obtained. A piglet brain template for group-level analysis of these data was constructed, similar to the fMR imaging techniques used in humans, to allow for group comparisons and longitudinal change analysis over time.

Conclusions

Loss of function in a specifically traumatized cortical region and its subsequent recovery over time can now be demonstrated visually by fMR imaging in the piglet. Besides its value in understanding intrinsic recovery mechanisms and plasticity at different ages, this functional outcome measure will enable the use of the piglet model in treatment trials specifically designed for the immature brain.