Accuracy of the electrodes location method for simultaneous SPECT and EEG examinations

A simultaneous SPECT and EEG examination allows for a combined analysis of brain structural and functional changes. The examinations can be visualized as 3D maps of overlapping SPECT (radiopharmaceutical concentration) and EEG (bioelectric potential) data. Synchronization of both maps is difficult, as SPECT shows neither the skull outline nor the EEG electrodes. Thus a technique to reflect electrodes placement in SPECT data was needed. Earlier we devised a method to make a small number of electrodes visible in SPECT without compromising SPECT accuracy. We also proposed a procedure approximating coordinates of the 10–20 system EEG electrodes in a 3D space using only 5 electrodes coordinates, while assuming that all electrodes are placed on 9 intersecting ellipses. Here we used 20 phantoms of real heads from the BrainWeb project and the Oostenveld calculation of electrodes canonical placement in an averaged head model. We divided the electrodes placement error into an easy-to-assess “distance error” (distance from the head surface) and a difficult-to-assess “angular error” (a wrong direction in relation to the symbolic head center). Applying our procedure to the Oostenveld data set, we assessed the ratio between the distance and the angular error and showed that a majority part of the entire approximation error results from the distance error. Our approximation procedure was applied to the BrainWeb phantoms and the distance error was computed allowing estimation of the entire error of electrodes placement. The estimated average error of the electrodes coordinates’ approximation procedure was 4.2 mm and the maximum error was 15.4 mm.