Retrospective fusion of radiographic and MR data for localization of subdural electrodes

Validating Patient-Specific Finite Element Models of Direct Electrocortical Stimulation

Direct electrocortical stimulation (DECS) with electrocorticography electrodes is an established therapy for epilepsy and an emerging application for stroke rehabilitation and brain-computer interfaces. However, the electrophysiological mechanisms that result in a therapeutic effect remain unclear. Patient-specific computational models are promising tools to predict the voltages in the brain and better understand the neural and clinical response to DECS, but the accuracy of such models has not been directly validated in humans. A key hurdle to modeling DECS is accurately locating the electrodes on the cortical surface due to brain shift after electrode implantation. Despite the inherent uncertainty introduced by brain shift, the effects of electrode localization parameters have not been investigated. The goal of this study was to validate patient-specific computational models of DECS against in vivo voltage recordings obtained during DECS and quantify the effects of electrode localization parameters on simulated voltages on the cortical surface. We measured intracranial voltages in six epilepsy patients during DECS and investigated the following electrode localization parameters: principal axis, Hermes, and Dykstra electrode projection methods combined with 0, 1, and 2 mm of cerebral spinal fluid (CSF) below the electrodes. Greater CSF depth between the electrode and cortical surface increased model errors and decreased predicted voltage accuracy. The electrode localization parameters that best estimated the recorded voltages across six patients with varying amounts of brain shift were the Hermes projection method and a CSF depth of 0 mm (r = 0.92 and linear regression slope = 1.21). These results are the first to quantify the effects of electrode localization parameters with in vivo intracranial recordings and may serve as the basis for future studies investigating the neuronal and clinical effects of DECS for epilepsy, stroke, and other emerging closed-loop applications.

Surface based electrode localization and standardized regions of interest for intracranial EEG

Intracranial recordings captured from subdural electrodes in patients with drug resistant epilepsy offer clinicians and researchers a powerful tool for examining neural activity in the human brain with high spatial and temporal precision. There are two major challenges, however, to interpreting these signals both within and across individuals. Anatomical distortions following implantation make accurately identifying the electrode locations difficult. In addition, because each implant involves a unique configuration, comparing neural activity across individuals in a standardized manner has been limited to broad anatomical regions such as cortical lobes or gyri. We address these challenges here by introducing a semi-automated method for localizing subdural electrode contacts to the unique surface anatomy of each individual, and by using a surface-based grid of regions of interest (ROIs) to aggregate electrode data from similar anatomical locations across individuals. Our localization algorithm, which uses only a postoperative CT and preoperative MRI, builds upon previous spring-based optimization approaches by introducing manually identified anchor points directly on the brain surface to constrain the final electrode locations. This algorithm yields an accuracy of 2 mm. Our surface-based ROI approach involves choosing a flexible number of ROIs with different spatial resolutions. ROIs are registered across individuals to represent identical anatomical locations while accounting for the unique curvature of each brain surface. This ROI based approach therefore enables group level statistical testing from spatially precise anatomical regions.

Registering imaged ECoG electrodes to human cortex: A geometry-based technique

BACKGROUND

The accurate localization of implanted ECoG electrodes over the brain is of critical importance to invasive diagnostic work-up for the surgical treatment of intractable epileptic seizures. The implantation of subdural electrodes is an invasive procedure which typically introduces non-uniform deformations of a subject's brain, increasing the difficulty of determining the precise location of the electrodes vis-à-vis cortex. Formalization of this problem is used to define a novel solution for the optimal localization of subdural electrodes.

NEW METHOD

We demonstrate that nonlinear transformation is required to accurately register the implanted electrodes to the non-deformed pre-surgical cortical surface, and that this problem is accommodated by utilizing known features of electrode geometry. Techniques to register chronically implanted subdural electrodes to the undistorted brain image are described and evaluated using simulated and clinical data.

RESULTS

Principal Axis, our novel analysis method that estimates an electrode's orientation by the moment of inertia of the solid electrode volume, proved to be the most reliable measure in both the simulated and clinical datasets.

COMPARISON WITH EXISTING METHODS

This method of electrode translation along its principal axis is an improvement over other techniques, such as the limited view provided by intraoperative photography, and the image degradation inherent in post-operative MRI.

CONCLUSIONS

This technique compensates for alterations due to post-operative brain edema, and translates subdural electrodes to their original location on pre-operative MRI 3D models. This is helpful in the correct localization of seizure foci and functional mapping of epilepsy patients.

Localizing ECoG electrodes on the cortical anatomy without post-implantation imaging

INTRODUCTION

Electrocorticographic (ECoG) grids are placed subdurally on the cortex in people undergoing cortical resection to delineate eloquent cortex. ECoG signals have high spatial and temporal resolution and thus can be valuable for neuroscientific research. The value of these data is highest when they can be related to the cortical anatomy. Existing methods that establish this relationship rely either on post-implantation imaging using computed tomography (CT), magnetic resonance imaging (MRI) or X-Rays, or on intra-operative photographs. For research purposes, it is desirable to localize ECoG electrodes on the brain anatomy even when post-operative imaging is not available or when intra-operative photographs do not readily identify anatomical landmarks.

METHODS

We developed a method to co-register ECoG electrodes to the underlying cortical anatomy using only a pre-operative MRI, a clinical neuronavigation device (such as BrainLab VectorVision), and fiducial markers. To validate our technique, we compared our results to data collected from six subjects who also had post-grid implantation imaging available. We compared the electrode coordinates obtained by our fiducial-based method to those obtained using existing methods, which are based on co-registering pre- and post-grid implantation images.

RESULTS

Our fiducial-based method agreed with the MRI-CT method to within an average of 8.24 mm (mean, median = 7.10 mm) across 6 subjects in 3 dimensions. It showed an average discrepancy of 2.7 mm when compared to the results of the intra-operative photograph method in a 2D coordinate system. As this method does not require post-operative imaging such as CTs, our technique should prove useful for research in intra-operative single-stage surgery scenarios. To demonstrate the use of our method, we applied our method during real-time mapping of eloquent cortex during a single-stage surgery. The results demonstrated that our method can be applied intra-operatively in the absence of post-operative imaging to acquire ECoG signals that can be valuable for neuroscientific investigations.

Localization of dense intracranial electrode arrays using magnetic resonance imaging

Intracranial electrode arrays are routinely used in the pre-surgical evaluation of patients with medically refractory epilepsy, and recordings from these electrodes have been increasingly employed in human cognitive neurophysiology due to their high spatial and temporal resolution. For both researchers and clinicians, it is critical to localize electrode positions relative to the subject-specific neuroanatomy. In many centers, a post-implantation MRI is utilized for electrode detection because of its higher sensitivity for surgical complications and the absence of radiation. However, magnetic susceptibility artifacts surrounding each electrode prohibit unambiguous detection of individual electrodes, especially those that are embedded within dense grid arrays. Here, we present an efficient method to accurately localize intracranial electrode arrays based on pre- and post-implantation MR images that incorporates array geometry and the individual's cortical surface. Electrodes are directly visualized relative to the underlying gyral anatomy of the reconstructed cortical surface of individual patients. Validation of this approach shows high spatial accuracy of the localized electrode positions (mean of 0.96 mm ± 0.81 mm for 271 electrodes across 8 patients). Minimal user input, short processing time, and utilization of radiation-free imaging are strong incentives to incorporate quantitatively accurate localization of intracranial electrode arrays with MRI for research and clinical purposes. Co-registration to a standard brain atlas further allows inter-subject comparisons and relation of intracranial EEG findings to the larger body of neuroimaging literature.

Individualized localization and cortical surface-based registration of intracranial electrodes

In addition to its widespread clinical use, the intracranial electroencephalogram (iEEG) is increasingly being employed as a tool to map the neural correlates of normal cognitive function as well as for developing neuroprosthetics. Despite recent advances, and unlike other established brain-mapping modalities (e.g. functional MRI, magneto- and electroencephalography), registering the iEEG with respect to neuroanatomy in individuals-and coregistering functional results across subjects-remains a significant challenge. Here we describe a method which coregisters high-resolution preoperative MRI with postoperative computerized tomography (CT) for the purpose of individualized functional mapping of both normal and pathological (e.g., interictal discharges and seizures) brain activity. Our method accurately (within 3mm, on average) localizes electrodes with respect to an individual's neuroanatomy. Furthermore, we outline a principled procedure for either volumetric or surface-based group analyses. We demonstrate our method in five patients with medically-intractable epilepsy undergoing invasive monitoring of the seizure focus prior to its surgical removal. The straight-forward application of this procedure to all types of intracranial electrodes, robustness to deformations in both skull and brain, and the ability to compare electrode locations across groups of patients makes this procedure an important tool for basic scientists as well as clinicians.

Three-dimensional visualization of subdural electrodes for presurgical planning

BACKGROUND

Accurate localization and visualization of subdural electrodes implanted for intracranial electroencephalography in cases of medically refractory epilepsy remains a challenging clinical problem.

OBJECTIVE

We introduce a technique for creating accurate 3-dimensional (3D) brain models with electrode overlays, ideal for resective surgical planning.

METHODS

Our procedure uses postimplantation magnetic resonance imaging (MRI) and computed tomographic (CT) imaging to create 3D models of compression-affected brain combined with intensity-thresholded CT-derived electrode models using freely available software. Footprints, or "shadows," beneath electrodes are also described for better visualization of sulcus-straddling electrodes. Electrode models were compared with intraoperative photography for validation.

RESULTS

Realistic representations of intracranial electrode positions on patient-specific postimplantation MRI brain renderings were reliably created and proved accurate when compared with photographs. Electrodes placed interhemispherically were also visible with our rendering technique. Electrode shadows were useful in locating electrodes that straddle sulci.

CONCLUSION

We present an accurate method for visualizing subdural electrodes on brain compression effected 3D models that serves as an ideal platform for surgical planning.

3D visualization of subdural electrode shift as measured at craniotomy reopening

PURPOSE

Subdural electrodes are implanted for recording intracranial EEG (iEEG) in cases of medically refractory epilepsy as a means to locate cortical regions of seizure onset amenable to surgical resection. Without the aid of imaging-derived 3D electrode models for surgical planning, surgeons have relied on electrodes remaining stationary from the time between placement and follow-up resection. This study quantifies electrode shift with respect to the cortical surface occurring between electrode placement and subsequent reopening.

METHODS

CT and structural MRI data were gathered following electrode placement on 10 patients undergoing surgical epilepsy treatment. MRI data were used to create patient specific post-grid 3D reconstructions of cortex, while CT data were co-registered to the MRI and thresholded to reveal electrodes only. At the time of resective surgery, the craniotomy was reopened and electrode positions were determined using intraoperative navigational equipment. Changes in position were then calculated between CT coordinates and intraoperative electrode coordinates.

RESULTS

Five out of ten patients showed statistically significant overall magnitude differences in electrode positions (mean: 7.2mm), while 4 exhibited significant decompression based shift (mean: 4.7mm), and 3 showed significant shear displacement along the surface of the brain (mean: 7.1mm).

DISCUSSION

Shift in electrode position with respect to the cortical surface has never been precisely measured. We show that in 50% of our cases statistically significant shift occurred. These observations demonstrate the potential utility of complimenting electrode position measures at the reopening of the craniotomy with 3D electrode and brain surface models derived from post-implantation CT and MR imaging for better definition of surgical boundaries.

A head phantom prototype to verify subdural electrode localization tools in epilepsy surgery

When planning epilepsy surgery, the position of subdural electrodes in relation to the cortex is crucial. Electrodes may dislocate after implantation. Neurosurgeons are highly interested in the accuracy of methods that visualize these electrodes. In order to determine the accuracy of an electrode visualization method, we have developed a physical head phantom and evaluated our new method of subdural electrode localization. This method projects automatically segmented electrodes of a preimplantation computed tomography (CT) data set onto the segmented brain surface of a postimplantation magnetic resonance imaging (MRI) data set within 2 to 5 min. The phantom consists of a skull, an adipose layer for skin replication, and a deformable brain. It further contains gyri and sulci structures, composed of gelatin and different additives used as phantom material for white matter, gray matter, and cerebrospinal fluid. The phantom allows a well-defined displacement of an "implanted" electrode grid perpendicular to the brain surface. By using the phantom data, we demonstrated that our electrode visualization tool did in fact function accurately. The image contrasts between different phantom materials in MRI and CT phantom data sets were similar to patient data sets. The phantom appears suitable for obtaining a more complex patient data replication, as well as for simulating different deformation scenarios.

Neuronavigation applied to epilepsy monitoring with subdural electrodes

Accurate localization of the epileptogenic zone is of paramount importance in epilepsy surgery. Despite the availability of noninvasive structural and functional neuroimaging techniques, invasive monitoring with subdural electrodes is still often indicated in the management of intractable epilepsy. Neuronavigation is widely used to enhance the accuracy of subdural grid placement. It allows accurate implantation of the subdural electrodes based on hypotheses formed as a result of the presurgical workup, and can serve as a helpful tool for resection of the epileptic focus at the time of grid explantation. The authors describe 2 additional simple and practical techniques that extend the usefulness of neuronavigation in patients with epilepsy undergoing monitoring with subdural electrodes. One technique involves using the neuronavigation workstation to merge preimplantation MR images with a postimplantation CT scan to create useful images for accurate localization of electrode locations after implantation. A second technique involves 4 holes drilled at the margins of the craniotomy at the time of grid implantation; these are used as fiducial markers to realign the navigation system to the original registration and allow navigation with the merged image sets at the time of reoperation for grid removal and resection of the epileptic focus. These techniques use widely available commercial navigation systems and do not require additional devices, software, or computer skills. The pitfalls and advantages of these techniques compared to alternatives are discussed.

Localization of neurosurgically implanted electrodes via photograph-MRI-radiograph coregistration

Intracranial electroencephalography (iEEG) is clinically indicated for medically refractory epilepsy and is a promising approach for developing neural prosthetics. These recordings also provide valuable data for cognitive neuroscience research. Accurate localization of iEEG electrodes is essential for evaluating specific brain regions underlying the electrodes that indicate normal or pathological activity, as well as for relating research findings to neuroimaging and lesion studies. However, electrodes are frequently tucked underneath the edge of a craniotomy, inserted via a burr hole, or placed deep within the brain, where their locations cannot be verified visually or with neuronavigational systems. We show that one existing method, registration of postimplant computed tomography (CT) with preoperative magnetic resonance imaging (MRI), can result in errors exceeding 1cm. We present a novel method for localizing iEEG electrodes using routinely acquired surgical photographs, X-ray radiographs, and magnetic resonance imaging scans. Known control points are used to compute projective transforms that link the different image sets, ultimately allowing hidden electrodes to be localized, in addition to refining the location of manually registered visible electrodes. As the technique does not require any calibration between the different image modalities, it can be applied to existing image databases. The final result is a set of electrode positions on the patient's rendered MRI yielding locations relative to sulcal and gyral landmarks on individual anatomy, as well as MNI coordinates. We demonstrate the results of our method in eight epilepsy patients implanted with electrode grids spanning the left hemisphere.

Frequency Domain Analysis of Human Subdural Recordings

SUMMARY

It is possible to localize many aspects of cortical function and dysfunction without the use of direct electrical stimulation of cortex. This study explores the degree to which information can be obtained about functional cortical organization relative to epileptogenic regions through analysis of electrocorticographic recordings in the frequency domain. Information about the extent of seizure regions and the location of the normal sensory and motor homunculus and some higher language and memory related areas can be obtained through the analysis of task-related power spectrum changes and changes in lateral interelectrode coherence patterns calculated from interictal and ictal recordings.

Locating chronically implanted subdural electrodes using surface reconstruction

OBJECTIVE

To determine the accuracy of locating subdural electrodes by means of 3-D surface rendering of CT scans.

METHODS

Open source software has been developed and posted on the web which segments the electrodes into 3-D surfaces and allows their 3-D locations to be exported to other EEG analysis programs. The accuracy of the technique was determined by studying 410 subdural electrodes implanted in four epilepsy patients. Accuracy was determined by comparing the locations from the rendering analysis to the locations of the same electrodes determined by conventional analysis of their appearance on individual CT slices.

RESULTS

The average accuracy of a study of 410 electrodes imaged in four patients repeated two times by three observers was 0.91 (+/- 0.41) mm, with a maximum error of 3.3 mm, about half of the diameter of an electrode.

CONCLUSIONS

The location of subdural electrodes can easily and quickly be determined within high-resolution CT scans through the use of 3-D rendering.

SIGNIFICANCE

This relatively fast and easy method for determining the location of subdural electrodes should facilitate their use in both clinical and research investigations.

Multimodality image-guided surgery for the treatment of medically refractory epilepsy

OBJECT

The aim of this study was to review seizure outcome, imaging modalities used, and complications following surgery in patients with epilepsy who had undergone multimodality image-guided surgery at our institution.

METHODS

Data from patients with epilepsy who had undergone surgery between April 1999 and October 2001 were reviewed. During this time period, 116 operations were performed in 109 patients with medically refractory epilepsy. Among these patients, 22 were selected to undergo multimodality image-guided surgery primarily on the basis of whether they had no lesion visible on conventional magnetic resonance (MR) imaging sequences, multiple lesions, or one very large lesion that could not be completely resected without the risk of significant postoperative morbidity. A fourth group of patients in whom there was a single lesion in the eloquent cortex, a location associated with a significant risk of postoperative morbidity, was also included in the analysis. This latter group was assessed with the aid of intracranial grid electrodes that were coregistered to the MR image and were used intraoperatively to minimize electrode position error. Other imaging modalities used included positron emission tomography (PET), fluid-attenuated inversion recovery (FLAIR) MR imaging, and subtracted ictal-interictal single-photon positron emission computerized tomography (SPECT) coregistered with MR imaging (SISCOM). After coregistration, images were then downloaded onto an image-guided surgical system and the epileptogenic area was then resected. The mean patient age was 33 years (range 17-46 years), and there was a mean follow up of 27 months (range 14-41 months). Multimodality coregistrations used were as follows: nine PET scans, seven subdural electrode grids, four SISCOM studies, one FLAIR MR image, and one combined PET/subdural grid. Seizure outcome was excellent in 17 patients (77%) and not excellent in five (23%), or favorable in 19 (86%) and unfavorable in three (14%). Six patients (27%) had a transient neurological deficit, one patient (5%) a permanent major deficit, and three patients (15%) a permanent minor deficit. Five patients (24%) had a transient psychiatric problem postoperatively.

CONCLUSIONS

Multimodality image-guided surgery offers a new perspective in surgery for epilepsy. Functional imaging modalities previously lateralized and often localized a seizure focus, but did not provide enough anatomical information to resect the epileptogenic zone confidently and safely. The coregistration of these modalities to a volumetric MR image and their incorporation into an image-guided system has allowed surgeons to offer surgery to patients who may not previously have been considered eligible, with outcomes comparable to those in patients with more straightforward lesional epilepsy.

Noninvasive identification of human central sulcus: a comparison of gyral morphology, functional MRI, dipole localization, and direct cortical mapping

The locations of the human primary hand cortical somatosensory and motor areas were estimated using structural and functional MRI, scalp-recorded somatosensory-evoked potential dipole localization, expert judgments based on cortical anatomy, and direct cortical stimulation and recording studies. The within-subject reliability of localization (across 3 separate days) was studied for eight normal subjects. Intraoperative validation was obtained from five neurosurgical patients. The mean discrepancy between the different noninvasive functional imaging methods ranged from 6 to 26 mm. Quantitative comparison of the noninvasive methods with direct intraoperative stimulation and recording studies did not reveal a significant mean difference in accuracy. However, the expert judgments of the location of the sensory hand areas were significantly more variable (maximum error, 39 mm) than the dipole or functional MRI techniques. It is concluded that because expert judgments are less reliable for identifying the cortical hand area, consideration of the findings of noninvasive functional MRI and dipole localization studies is desirable for preoperative surgical planning.

Digital photography and 3D MRI-based multimodal imaging for individualized planning of resective neocortical epilepsy surgery

PURPOSE

Invasive presurgical work up of pharmacoresistant epilepsies presumes integration of multiple diagnostic modalities into a comprehensive picture of seizure onset and eloquent brain areas. During resection, reliable transfer of evaluation results to the patient's individual anatomy must be made. We investigated the value of digital photography-based grid localization in combination with preoperative three-dimensional (3D) magnetic resonance imaging (MRI) for clinical routine.

METHODS

Digital photographs of the exposed cortex were taken before and after grid placement. Location of electrode contacts on the cortex was identified and schematically indicated on native cortex prints. Accordingly, transfer of contact positions to a 3D MRI brain-surface rendering was carried out manually by using the rendering software. Results of the electrophysiologic evaluation were transferred to either electrode contact reproduction and co-registered with imaging-based techniques such as single-photon emission computed tomography (SPECT), positron emission tomography (PET), and functional MRI (fMRI).

RESULTS

Digital photography allows precise and highly realistic documentation of electrode contact positions on the individual neocortical surface. Lesions underneath grids can be highlighted by semitransparent MRI surface rendering, and lobar boundaries can be identified. Because of integrating electrode contact positions into the postprocessed 3D MRI data set, imaging-based techniques can be codisplayed with the results of the electrophysiologic evaluation. Comparison with CT/MRI co-registration showed good accuracy of the method. However, grids not sewn to the dura at implantation can become subject to significant displacement.

CONCLUSIONS

Digital photography in combination with preimplantation 3D MRI allows the generation of reliable tailored resection plans in neocortical epilepsy surgery. The method enhances surgical safety and confidence.

Estimating tissue deformation between functional images induced by intracranial electrode implantation using anatomical MRI

This paper examines a solution to the general problem of accurately relating points within functional data acquired before and after subdural intracranial electrode implantation. We develop an approach based on nonrigid registration of high resolution anatomical MRI acquired together with the functional data. This makes use of a free-form B-Spline deformation model and registration is recovered by maximizing the normalized mutual information between the preimplant MRI and the postimplant MRI. We apply the approach to estimate the tissue deformation induced by the presence of intracranial electrodes over 15 patient studies. Maximum tissue displacements of 4 mm or greater were observed in all cases either in the cortex or around the ventricles due to CSF loss. In studies involving larger 4 x 4 grids, local tissue displacement estimates exceeded 10 mm from the preimplant brain shape. The key issue with this approach is whether the deformation estimates are contaminated by the presence of susceptibility-induced imaging artifacts. We therefore evaluate the deformation estimates in recovering alignment of essentially identical SPECT studies of eight patients acquired before and after electrode placement. An ROI-based analysis of the variance of resulting subtraction values between pre- and postimplant SPECT was carried out in regions of tissue below electrode grids. Results indicate for all cases a substantial reduction in residual SPECT subtraction artifacts to a level comparable to that in an equivalent region of undeformed tissue.

Usefulness of 3-D reconstructed images of the human cerebral cortex for localization of subdural electrodes in epilepsy surgery

Three-dimensional (3-D) images of 14 patients, in whom subdural electrodes were implanted for epilepsy surgery evaluation, were produced by fusing computerized tomography (CT) scans and magnetic resonance images (MRI) to determine the exact location of the electrodes. In 25% (198 of 806) of the subdural electrodes an exact location for resection strategy was not evident from the X-rays. The location of these electrodes ('doubtful location', DL) was compared to the 3-D images. Intraoperative inspection served as the gold standard. Concordance score was defined as good for 75-100% concordance, moderate for 50-75%, and poor for less than 50% concordance of the DL electrodes to the intraoperatively determined location. A comparative analysis of the 3-D images and X-ray films showed a highly significant difference (P < 0.0002) in favor of 3-D images. The concordance score for the DL electrodes in the 3-D images was good in 93% of the patients and moderate in 7%. In contrast, the concordance score was good in only 7% of the patients for the two-dimensional (2-D) skull films, moderate in 57%, and poor in 36%. Interobserver reliability was better for 3-D images (93%) than for the 2-D X-rays (43%). These findings suggest that 3-D images aid preoperative planning for resective epilepsy surgery.

An automatic registration method for frameless stereotaxy, image guided surgery, and enhanced reality visualization

There is a need for frameless guidance systems to help surgeons plan the exact location for incisions, to define the margins of tumors, and to precisely identify locations of neighboring critical structures. The authors have developed an automatic technique for registering clinical data, such as segmented magnetic resonance imaging (MRI) or computed tomography (CT) reconstructions, with any view of the patient on the operating table. The authors demonstrate on the specific example of neurosurgery. The method enables a visual mix of live video of the patient and the segmented three-dimensional (3-D) MRI or CT model. This supports enhanced reality techniques for planning and guiding neurosurgical procedures and allows us to interactively view extracranial or intracranial structures nonintrusively. Extensions of the method include image guided biopsies, focused therapeutic procedures, and clinical studies involving change detection over time sequences of images.

Displaying electrocorticographic findings on gyral anatomy

Human electrocorticographic findings recorded from subdural arrays of electrodes were topographically mapped directly onto magnetic resonance images of gyral anatomy. With this technique gyri involved in generating somatosensory evoked potentials and epileptic phenomena are easily identified. Regions of the cortex which exhibit local spectral changes associated with cognitive tasks can also be visualized. These composite images of structure and function can provide insight regarding the functional organization of human cortex in relation to gyral anatomy and localized pathologic rhythms.

Correlative image registration

Image registration in nuclear medicine and radiology refers to the spatial matching or merging of two or more images from the same or different imaging modalities. The coordinates of the corresponding picture elements (pixels) from different images are transformed to align and equate their positions and spatial coordinates. Correlative image registration is a more restrictive term that applies to the matching of spatial coordinates of images coming from different imaging modalities. The registration of correlative images provides a useful approach to combine the best sensitivities and specificities of complementary procedures to detect, locate, monitor, and measure pathological and other physical changes. Here we review the registration of images from nuclear medicine (single-photon emission computed tomography, positron emission tomography and planar imaging) with those from other imaging modalities (magnetic resonance imaging, computed tomography, digital subtraction angiography and ultrasound) to closely correlate changes in metabolism, blood flow, receptor density, and other functional measurements with regional anatomy and morphological changes. The types of image registration applications, techniques, and terminology associated with image registration and examples of application are presented.

A frameless stereotactic approach to neurosurgical planning based on retrospective patient-image registration. Technical note

A frameless stereotactic device interfacing an electromagnetic three-dimensional (3-D) digitizer to a computer workstation is described. The patient-image coordinate transformation was found by retrospectively registering a digitizer-derived model of the patient's scalp with a magnetic resonance (MR) imaging-derived model of the same surface. This procedure was performed with routine imaging data, eliminating the need to obtain special-purpose MR images with fiducial markers in place. After patient-image fusion was achieved, a hand-held digitizing stylus was moved over the scalp and tracked in real time on cross-sectional and 3-D brain images on the computer screen. This device was used for presurgical localization of lesions in 10 patients with meningeal and superficial brain tumors. The results suggest that the system is accurate enough (typical error range 3 to 8 mm) to enable the surgeon to reduce the craniotomy to one-half the size advisable with conventional qualitative presurgical planning.