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.
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.
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.
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.