Sources of error in comparing functional magnetic resonance imaging and invasive electrophysiological recordings

Object. Several authors have recently reported studies in which they aim to validate functional magnetic resonance (fMR) imaging against the accepted gold standard of invasive electrophysiological monitoring. The authors have conducted a similar study, and in this paper they identify and quantify tw o characteristics of these data that can make such a comparison problematic . Methods. Eight patients in whom surgery for epilepsy was performed and five healthy volunteers underwent fMR imaging to localize the part of the senso rimotor cortex responsible for hand movement. In the patient group subdural electrode mats were subsequently implanted to identify eloquent regions of the brain and the epileptogenic zone. The fMR imaging data were processed to correct for motion during the study and then registered with a postimpla ntation computerized tomography (CT) scan on which the electrodes were visi ble. The motion during imaging in the two groups studied, and the deformati on of the brain between the preoperative images and postoperative scans wer e measured. The patients who underwent epilepsy surgery moved significantly moro during fMR imaging experiments than healthy volunteers performing the same motor task. This motion had a particularly increased out-of-plane component and w as significantly more correlated with the stimulus than in the volunteers. This motion was especially increased when the patients were performing a ta sk on the side affected by the lesion. The additional motion is hard to cor rect and substantially degrades the quality of the resulting fMR images, ma kings it a much less reliable technique for use in these patients than in o thers. Also, the authors found that after electrode implantation, the brain surface can shift more than 10 nun relative to the skull compared with its preoperative location, substantially degrading the accuracy of the compari son of electrophysiological measurements made in the deformed brain and fMR studies obtained preoperatively. Conclusions. These two findings indicate that studies of this sort are curr ently of limited use for validating fMR imaging and should be interpreted w ith care. Additional image analysis research is necessary to solve the prob lems caused by patients' motion and brain deformation.