A low-cost and robust photogrammetry method for determining electrode positions

NICOLÁS VON ELLENRIEDER; LEANDRO BELTRACHINI; ALEJANDRO BLENKMANN; MARIANO FERNÁNDEZ CORAZZA; SILVIA KOCHEN; CARLOS MURAVCHIK

Montreal

Congreso; 30th International Epilepsy Congress; 2013

The International League Against Epilepsy

Purpose: The position of the electroencephalography electrodes shouldbe taken into account to perform high quality EEG source localization.We describe a photogrammetry method for determining these positionswith nothing but a camera. No special requirements are needed for theimages, so the method does not take more than a minute of the patient′stime, and the pictures can be taken without moving the patient to a dedicatedzone.Method: Based on images taken from different and arbitrary angles, wesolve a nonlinear problem to reconstruct the 3D position of the electrodes.The algorithm inputs are the 2D positions of the electrodes on thepictures and it solves a large nonlinear problem iteratively to estimate theelectrode and camera positions, taking into account the projective geometryof the images.Results: The method was tested with different subjects, cameras, andoperators. The results were very repeatable in terms of the dispersion ofthe electrode positions among different reconstructions. The mean dispersionvalue was 0.7 mm, and in 95% of the cases the dispersion wasbelow 1.6 mm. The same experiments with an electromagnetic positioningpencil, with an additional sensor devoted to head tracking, yielded ahigher dispersion with a mean value of 1.7 mm and a 95% percentile of3.3 mm.Conclusion: The proposed method is completely non-invasive, portable,fast for the patient, and low cost, since a simple cellphone camera is sufficientto achieve the reported results. Software to aid in the labeling of theelectrodes in the pictures is under development.