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Background: Primate studies demonstrate that high-frequency electrical stimulation (HFS) of the caudate can enhance learning. Importantly, in these studies, stimulation was applied following the execution of behavior and the effect persisted into subsequent trials, suggesting a change in plasticity rather than a momentary facilitation of behavior., Objectives/methods: Although the mechanism of HFS-enhanced learning is not understood, evidence suggests that dopamine plays a critical role. Therefore, we used in vivo amperometry to evaluate the effects of HFS on striatal dopamine release in the anesthetized primate. While this does not directly examine dopamine during learning, it provides insight with relation to dopamine dynamics during electrical stimulation and specifically between different stimulation parameters and striatal compartments., Results: We demonstrate that HFS results in significantly more dopamine release in the striatum compared to low-frequency stimulation. In addition, electrical stimulation operates differentially on specific neuronal elements, as the parameters for dopamine release are different for the caudate, putamen and medial forebrain bundle., Conclusions: While not direct evidence, these data suggest that HFS evokes significant dopamine release which may play a role in stimulation-enhanced learning. Moreover, these data suggest a means to modulate extracellular dopamine with a high degree of temporal and spatial precision for either research or clinical applications.

Introduction: Fluorescence-guided resection (FGR) of brain tumors is an intuitive, practical and emerging technology for visually delineating neoplastic tissue exposed intraoperatively. Image guidance is the standard technique for producing 3-dimensional spatially coregistered information for surgical decision making. Both technologies together are synergistic: the former detects surface fluorescence as a biomarker of the current surgical margin while the latter shows coregistered volumetric neuroanatomy but can be degraded by intraoperative brain shift. We present the implementation of deformation modeling for brain shift compensation in protoporphyrin IX FGR, integrating these two sources of information for maximum surgical benefit., Methods: Two patients underwent FGR coregistered with conventional image guidance. Histopathological analysis, intraoperative fluorescence and image space coordinates were recorded for biopsy specimens acquired during surgery. A biomechanical brain deformation model driven by intraoperative ultrasound data was used to generate updated MR images., Results: Combined use of fluorescence signatures and updated MR image information showed substantially improved accuracy compared to fluorescence or the original (i.e., nonupdated) MR images, detecting only true positives and true negatives, and no instances of false positives or false negatives., Conclusion: Implementation of brain deformation modeling in FGR shows promise for increasing the accuracy of neurosurgical guidance in the delineation and resection of brain tumors., (2009 S. Karger AG, Basel.)

Background/aims: The precise mechanism of action of deep brain stimulation in the subthalamic nucleus (STN) for the treatment of Parkinson's disease and epilepsy is unknown. In the present study, the intracellular effects on STN neurons following high-frequency stimulation (HFS) of STN were examined to test the hypothesis that HFS results in either an increase or a decrease in neuronal action potential generation., Methods: Intracellular electrophysiological recordings were made in the rat STN neurons in in vitro slice preparations. A concentric bipolar stimulating electrode was placed in the STN, and electrical stimulation (duration, 100-2000 ms; amplitude, 10-500 microA, and frequency, 10-200 Hz) was delivered while simultaneously recording intracellularly from a STN neuron using a sharp electrode., Results: HFS of STN resulted in the generation of excitatory postsynaptic potentials and an increase in action potential firing during the stimulation period followed by a period of poststimulation inhibition of firing in STN neurons. The degree of increase in action potentials from HFS was critically dependent on the frequency of electrical stimulation, i.e. at approximately 100-140 Hz, maximal increase was obtained, but at 200 Hz, the activity was blocked. Interestingly, the duration of poststimulation inhibition of firing was dependent on the duration of stimulation, i.e. the longer the HFS, the longer the inhibition., Conclusions: These results suggest that the mechanism of action of deep brain stimulation involves initial excitation followed by later inhibition of STN neurons at a cellular level rather than primary inhibition, as previously hypothesized., (Copyright 2003 S. Karger AG, Basel)

Current neuronavigational systems face two primary challenges: (1) automatic and robust registration between preoperative images and the operating room space, and (2) compensation for brain deformations that compromise the accuracy of the initial registration. To contend with these difficulties, we firstly estimate the three-dimensional (3-D) structure of the cortical surface using a noncontacting 3-D digitizer. This 3-D structure is then used to establish the initial registration, and to update the preoperative MR volume as the brain deforms. We show that this approach improves the accuracy of registration in a phantom study, and demonstrate the ability to capture cortical motion in six clinical cases., (Copyright 2003 S. Karger AG, Basel)

Intraoperative brain shift and deformation pose challenges for image-guided surgery. One strategy to address these problems utilizes computational modeling coupled with intraoperatively acquired information from efficient and economical sources such as ultrasound and the optics of the operating microscope. Calibration algorithms for the accurate integration of these sparse data sources have been implemented. Assessment has been performed in both phantom and pig brain models, and accuracy better than 2 mm has been achieved. Methods of incorporating these data into the computational model are being developed., (Copyright 2002 S. Karger AG, Basel)

Medically intractable epilepsy of extra-temporal origin can represent a difficult therapeutic challenge. Our Epilepsy Service has managed these patients using standard investigative methods as well as ictal SPECT and intracranial electrode recording. In the present series of patients, image-guided surgery was used for all electrode implantation and resective surgery. Seizure localization and successful resection were achieved in 70-80% of 42 patients with follow-up of at least one year. Normal MRI and previous failed intracranial investigation were not associated with poorer outcome., (Copyright 2002 S. Karger AG, Basel)

Introduction: The long-term outcome of pediatric patients undergoing corpus callosotomy (CC) for palliative control of medically intractable seizures is presented., Methods: During a 27-year period, 43 patients, 20 years of age or younger, underwent CC for seizure palliation and had a minimum of 1 year follow-up. Seizure reduction and stability of that outcome for individual seizure types and for most disabling seizure were reviewed., Results: Overall, 63% of the seizures documented showed a good response. For the most disabling seizure, 56% of the patients had good outcomes. Changes in outcome status occurred within the first 6 months, and outcome was largely maintained after that point., Conclusion: Callosotomy achieves the goal of seizure palliation in more than half of the patients, with stable, good outcomes being maintained in the majority of patients., (Copyright 2000 S. Karger AG, Basel)

There is a strong rationale for investigation of the role of Gamma Knife radiosurgery in the treatment of medically intractable epilepsy. To explore this potential application, the well established and highly successful current outcomes associated with microsurgical treatment were reviewed, and include for temporal resections seizure-free results in 65-70% of patients, with permanent morbidity of less than 5% and mortality less than 1%. Advantages of open surgery include the opportunity to conduct electrocorticography and functional mapping, excellent visualization enabling an assured and discrete line of resection, freedom in general from volume constraints, and immediate efficacy. The preliminary evidence in support of radiosurgical treatment includes several series of patients with epilepsy associated with arteriovenous malformation, tumor, or cavernous anginoma in which results approaching those of surgery have been achieved. In a series of 15 patients with mesial temporal sclerosis, all but one patient with follow-up of at least one year are seizure-free; morbidity has been limited to one asymptomatic field defect. A series of ten patients with epilepsy associated with hypothalamic hamartoma has achieved seizure improvement in six of eight patients with at least one year follow-up, and no major morbidity has been seen. Radiosurgical treatment for intractable epilepsy appears to be effective in a majority of patients and can be performed safely and efficiently. Early experimental evidence suggests that seizure control might in some instances be achievable without ablation of the pathological target.

A strategy to update preoperative imaging for image-guided surgery using readily available intraoperative information has been developed and implemented. A patient-specific three-dimensional finite element model of the brain is generated from preoperative MRI and used to simulate deformation resulting from multiple surgical processes. Intraoperatively obtained sparse imaging data, such as from digital cameras or ultrasonography, is then used to prescribe the displacement of selected points within the model. Interpolation to the resolution of preoperative imaging may then be performed based upon the model. The algorithms for generation of the finite element model and for its subsequent deformation have been successfully validated using a pig brain model, and preliminary clinical application in the operating room has demonstrated feasibility., (Copyright 2000 S. Karger AG, Basel)

Introduction: The dynamic nature and three dimensionality of ultrasound data can be utilized to enhance the capabilities of image guidance systems., Methods: Coregistration of ultrasound data was done using an electromagnetic digitizer, and subsequent ultrasound images were correlated with preoperative MRI studies. Thirty-two patients undergoing craniotomy were investigated in this manner., Results: Phantom testing done with a rigid stylus and 3D ultrasound tracker demonstrated an accuracy of 1.36 +/- 1.67 mm in determining the location of a point. Thirty-two clinical cases were coregistered without difficulty., Conclusion: Coregistered ultrasound is a useful methodology that can aid in neuronavigation., (Copyright 2000 S. Karger AG, Basel)

Object: This study was performed to evaluate the reliability and utility of using monopolar impedance monitoring for estimation of the location of the thermocouple electrode during a pallidotomy procedure., Methods: In 26 patients, impedance profiles were measured at 2-mm intervals and anatomical boundaries were marked on postoperative MRI studies in 19 patients. The pattern of change in impedance as the electrode was towards the target coordinates in the globus pallidus was studied empirically in relation to MRI-defined anatomy and by electrical stimulations. The pattern of impedance change was of greater reliability than absolute impedance values. A drop in impedance was noted in 10 cases where the electrode entered the putamen or globus pallidus upon exiting the internal capsule, and in 9 of these this drop was measured before or at the distal boundary of the internal capsule. A variable pattern in the region of the base of the globus pallidus was observed, with 10 of 19 cases demonstrating a decrease and 8 cases an increase, consistent with the proximity of a CSF cistern in the former and the ansa lenticularis and optic tract in the latter. Simulations showed that the monopolar electrode was more sensitive to the approach of a boundary than the bipolar electrode., Conclusion: Our measurements and simulations suggest that a monopolar electrode could predict a nearby region of high or low impedance before penetrating it and a trend of decreasing impedance as the distal edge of the globus pallidus is approached., (Copyright 2000 S. Karger AG, Basel)

An anatomic variability study of the spatial relationship between the imaged globus pallidus, the optic tract, and the anterior and posterior commissures was performed using T1-weighted gradient echo three-dimensional volume acquisition MRI of 20 patients. Imaging data were resized, reformatted, and manually segmented for the above structures, and mean values and standard deviations for their medial, superior, lateral, and inferior limits determined. Findings demonstrate that the globus pallidus can be consistently and reliably imaged by MRI, that direct stereotactic targeting of this structure can account for normal anatomic variability which may be as much as several millimeters, and that the risk of injury to the optic tract may be reduced with this technique.

Implementation of a Vicom-VME image processing workstation for the frameless stereotactic operating microscope system has allowed an improved interpolation algorithm for more accurate reconstruction of three-dimensional data, the integration graphically of multimodality imaging information, an additional stereotactic graphics display outside of the microscope optics and optional three-dimensional displays through communication with a Convex mini-supercomputer. A precision-milled coupling for reproducible attachment of the spark gap bracket to the microscope has eliminated the need for that transformation calculation during each case. Clinical utility has been greatest in providing navigational guidance to small subcortical or deep lesions and in defining the extent of preoperatively planned resection of larger, infiltrating tumors. Mean accuracy in the past 17 cases has been 3.5-6.5 mm. The frameless methodology has also enabled extracranial stereotactic surgical procedures in 7 patients.