Moreover, this method requires the purchase of a 3D digitizer, which is also relatively expensive (cost ∼£3000). However, electromagnetic tracking methods are significantly affected by the presence of metal objects in the surrounding environment, 9 whereas the acquisition process is also time-consuming. The stylus contains a tri-axis magnetic field sensor that can detect the AC magnetic fields at its location and thus its position relative to the magnetic field source can be determined. Most electromagnetic digitizers rely on AC magnetic tracking, in which a source produces orthogonal alternating magnetic fields along all three axes. 6 – 8 The electromagnetic digitizer method is usually implemented using a stylus to touch the optodes on the device to detect and record their positions. The traditional methods for the spatial registration of fNIRS/DOT devices include electromagnetic digitizers 3 – 5 and photogrammetry. 1 When applying the fNIRS/DOT devices in practice, a three-dimensional (3D) model of the head surface containing the position of the optodes on the scalp is required to ensure proper spatial inferences, a process called “spatial registration.” 2 Functional near-infrared spectroscopy (fNIRS) and its extension diffuse optical tomography (DOT) have become noninvasive neuroimaging methods that offer affordable, accessible, and user-friendly solutions for functional brain imaging.
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