EasyTopo: A toolbox for rapid diffuse optical topography based on a standard template of brain atlas

Diffuse optical topography remains a valid tool in functional near infrared spectroscopy (fNIRS) since it avoids solving the forward and inverse computational problems, which are encountered in diffuse optical tomography. Topography is particularly useful when a sparse array of optodes is used and depth specificity is not the primary interest. We have developed an easy toolbox for diffuse optical topography (“EasyTopo”) based on a standard template of brain atlas. EasyTopo approximates the cortical layer of the brain as a hemispherical surface. Therefore, the stereotaxic coordinates of the brain surface and the co-registered fNIRS measurements (channels) are converted into the spherical coordinates, where 2D angular interpolation of the channel-wise data is implemented to obtain a topographic image of brain activation in the latitude-longitude space. Then, the interpolated image is projected back onto the brain surface in the original 3D stereotaxic coordinates. Compared with the existing 3D topography methods, EasyTopo is more computationally efficient and does not require any data extrapolation. Another advantage of EasyTopo is that the data between two spatially adjacent channels are interpolated along their included angles (i.e., along the angular direction) rather than along a straight line going under the brain surface. The former geometry in principle matches better with the realistic brain structure than the latter one. EasyTopo has been validated with both simulation and human experiments. Now this toolbox is publically available.