Cortical electrode localization from X-rays and simple mapping for electrocorticographic research: The “Location on Cortex” (LOC) package for MATLAB

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Abstract

Medically refractory epilepsy accounts for more than 30% of the epilepsy population. Scalp EEG electrodes have limited ability to localize seizure onset from deep structures and implantation of subdural electrodes with long term monitoring provides additional information. Apart from clinical application, this patient population provides a unique opportunity for acquiring electrocorticography data in research paradigms. We present a method for rapid localization of electrodes using lateral and anterior–posterior X-rays. Skull landmarks and proportions are used for co-registration with the standardized Talairach coordinate system. This MATLAB-based “Location on Cortex” (LOC) package facilitates rapid visualization of clinical and experimental data in a user-friendly manner.

Introduction

Medically refractory epilepsy affects more than 30% of the epilepsy population (Kwan and Brodie, 2000). One significant clinical challenge is the accurate localization of seizure onset. Scalp EEG electrodes are limited in their ability to localize seizure onset from deep structures, and implantation of subdural electrodes with long term monitoring provides additional seizure localization information (Lesser et al., 1991). In addition to the clinical application of subdural electrodes, long term electrocorticographic monitoring provides a unique opportunity for acquiring data in research paradigms. In order to accurately interpret the seizure onset and to interpret electrocorticography data for research paradigms, the three-dimensional location of the electrodes must be known. Although intra-operative photographs of subdural grids provide locations for electrodes on exposed cortex (see supplemental figure), they do not provide information for electrodes passed beyond the limits of the cranial exposure. These electrodes may be located with post-operative CT or MRI (Kovalev et al., 2005, Wellmer et al., 2002), but this process requires sophisticated image acquisition preoperatively and postoperatively and these image studies are often unavailable. Post-operative X-rays, however, are ubiquitous and readily accessible. The method and the associated package presented here address the need for a fast, reliable co-localization technique to locate and standardize radio-opaque electrodes from anterior–posterior (AP, also called ‘coronal’) and lateral X-rays.

This MATLAB (The MathWorks, Inc.) based “Location on Cortex” (LOC) package uses skull landmarks and proportions for co-registration with the standardized Talairach coordinate system. The Talairach coordinate system defines the origin as the anterior commisure (AC) and the anterior–posterior (y) axis as the line connecting the anterior and posterior commisures (AC/PC line). The x axis is perpendicular to the y axis in the axial plane, and the z axis is normal to the xy plane(Talairach and Tournoux, 1988).

Skull landmarks visible on the lateral X-ray may be used to approximate the Talairach axes. The line segment drawn from the glabella to the inion (GI line) is parallel to the AC/PC line (Fox et al., 1985, Friston et al., 1989). The y axis is defined as the AC/PC line. The x axis is the direction of left–right symmetry and the z axis is perpendicular to the GI line, and thus normal to the xy plane. The origin is defined in the x direction as the center of the skull on AP X-rays. In the z direction, it is defined as 21% along the perpendicular line segment corresponding to the maximum distance between the GI line and the inner table of the superior surface of the skull. In the y direction, the distance from the posterior inner table of the skull to the anterior inner table along the y axis is normalized to 173 mm, and the origin is taken at 115 mm anterior to the midpoint of this line. The y axis is positive anterior to the origin, the x axis is positive on the patient's right, and the z axis is positive superior to the origin.

To interpret the location of subdural electrodes for clinical analysis and inter-patient comparison, the patient's brain is normalized to standard dimensions. By making the assumption that the inner table of the skull matches the brain surface, the maximum x, y, and z coordinates of the inner table of the skull can be normalized to a template brain volume. Furthermore, the position of any radio-opaque objects within this volume can also be normalized to the template brain. The AFNI template brain is routinely used in imaging research and is available in Talairach coordinates (Brett et al., 2002, Collins et al., 1994, Holmes et al., 1998; http://afni.nimh.nih.gov/). Its maximum x, y, and z dimensions are 138 mm, 173 mm, and 116 mm, respectively. Normalization to this template brain allows clinicians and researchers to interpret electrocorticographic data with reference to this standardized brain atlas.

Section snippets

Methods

This technique was implemented on patients enrolled in an electrocortigraphic research study at the University of Washington Regional Epilepsy Center. As part of standard clinical practice, subdural platinum electrode arrays with 1 cm center-to-center separation were implanted for seizure localization (AdTech, Racine, WI). Digital skull X-rays were obtained for clinical purposes to localize the subdural electrodes. These X-rays were stored and analyzed as part of a pre-approved research protocol

Results

This technique was used on 17 patients enrolled in an electrocortigraphic research study at the University of Washington Regional Epilepsy Center. We present the results for 4 patients as a demonstration of the technique. Fig. 1, Fig. 2, Fig. 3 demonstrate how the LOC program may be used to localize electrodes and visualize electrocorticographic data.

To examine how reproducible the calculation of electrode positions was with this method, two of the authors independently calculated positions in

Discussion

This Location on Cortex (LOC) package facilitates the rapid localization of subdural electrode arrays onto a standardized template brain volume. Once familiar with the procedure, the average time to localize a 64-contact grid using lateral X-ray only, or a 20 contact subtemporal strip array using AP and lateral X-ray is 5–10 min. Co-registration of electrode locations by individual calculation takes an hour or more for a 64-contact grid using a lateral X-ray. Calculation of Talaraich axes using

Conclusion

The Location on Cortex (LOC) package implements a rapid and reliable method of estimating the coordinates of radio-opaque markers such as subdural electrocorticography electrodes. The open source LOC package can also be used to visualize clinical and experimental data in a quantitative fashion with reference to a standardized brain template. This tool can be readily implemented in clinical research settings in which invasive electrophysiology is recorded during the treatment of patients with

Acknowledgements

KJM is funded by the Poncin award, the NIH MSTP grant, a Neurosurgical Training Grant through the University of Washington and the Packard award via RPNR. AOH is funded by the Epilepsy Foundation through the generous support of Abbott Laboratories. JGO is funded by NS41272. SM is funded in part by the Swartz Foundation. The National Institute for Mental Health (RO1-NS497293) supports the continued development of the open source EEGLAB environment.

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