Single Camera Photogrammetry System for EEG Electrode Identification and Localization

Abstract

In this study, photogrammetric coordinate measurement and color-based identification of EEG electrode positions on the human head are simultaneously implemented. A rotating, 2MP digital camera about 20 cm above the subject's head is used and the images are acquired at predefined stop points separated azimuthally at equal angular displacements. In order to realize full automation, the electrodes have been labeled by colored circular markers and an electrode recognition algorithm has been developed. The proposed method has been tested by using a plastic head phantom carrying 25 electrode markers. Electrode locations have been determined while incorporating three different methods: (i) the proposed photogrammetric method, (ii) conventional 3D radiofrequency (RF) digitizer, and (iii) coordinate measurement machine having about 6.5 mum accuracy. It is found that the proposed system automatically identifies electrodes and localizes them with a maximum error of 0.77 mm. It is suggested that this method may be used in EEG source localization applications in the human brain.

EEG–MRI Co-registration and Sensor Labeling Using a 3D Laser Scanner

“…This short time must be compared to a mean of about 8 min to acquire 67 points with the electromagnetic digitizer, 21 to around 10 min for 61 measurements with an ultrasound apparatus, 12 or around 4 min 30 s for 22 sensors with a photogrammetric system. 4 This time saving can be explained by the technology used and, for the stylus-based digitization, by the identification algorithm that overcomes the acquisition of positions in a given order. In a preliminary study, we also digitized the position of sensors taped directly on the scalp.…”

“…4,20 More recently, Gouws and Woods suggested using a 3D camera to locate the MEG coil 8 and we also presented a similar study which used a handheld 3D laser scanner to digitize an EEG cap. 17 Fiducial landmarks (e.g., nasion and pre-auricular points) identified on the subject's head and on the MR images could be used to guide co-registration without any markers.…”

“…This can be achieved during the digitization step by acquiring the sensors' coordinates in a given order. As this is a time costly and operator dependent procedure, several automatic techniques have also been developed: the combinatorial optimization based algorithm, 27 recognition of colored stickers in photogrammetric images, 4 and our method that uses a priori information on the relative positions of sensors. 15 In this work, we focus on the automatic co-registration of EEG sensors (included in a cap) with MRI data in a clinical context of high resolution EEG.…”

A Single Camera Photogrammetry System for Multi-angle Fast Localization of EEG Electrodes

“…With this method, the time of picture acquisition is very short, but 11 cameras are still not cost-effective and the system must be used with the associated product, the Geodesic Sensor Net, which increases the complexity in both system design and operation. Baysal et al 5 have proposed a single camera photogrammetry system. In this system, a rotating digital camera above the subject's head is utilized, and the images are acquired at predefined stop points separated azimuthally at equal angular displacements.…”

“…Currently, five different methods for the localization of EEG electrodes are described in literature: manual methods, 7,11 electromagnetic digitization, 19,25 ultrasound digitization, 31 magnetic resonance imaging (MRI)-assisted approach, 8,15,21,30,35 and photogrammetric method. 4,5,29 Among these methods, photogrammetric method is accurate, easy to operate, patient specific, repeatable, and less prone to human errors. Such measurement requires each EEG electrode on a head to be visible in at least two views to determine its position in the threedimensional (3D) space.…”