Prior photon measurement density function constructed in a standard channel space for functional near-infrared spectroscopy

Photon measurement density function (PMDF) determines the measurement locations and their associated sensitivity to the hemodynamic fluctuations of an fNIRS channel, which is essential for pre-experiment optode arrangement and post-experiment signal analysis/results interpretation in fNIRS studies. However, obtaining the exact PMDF relies on individual MR images, which are unavailable in common fNIRS studies. Previous studies alternatively adopt the PMDF derived from single head templates, i.e., template-PMDF, which ignores the anatomical difference between the template and the participants, as well as the anatomical variability across participants. Here, we propose a methodological framework using a standard channel space to construct a prior-PMDF from a structural MRI database for arbitrarily given channel locations. We demonstrate the utility of the prior-PMDF in estimating channel-wise fNIRS imaging parameters, such as subject-specific PMDF, sensitivity in intracerebral and extracerebral tissue compartments, and sensitivity to regions defined by brain atlas. The performance of the prior-PMDF tested on an independent dataset was compared with the template-PMDF. The results show that the prior-PMDF significantly surpasses the template-PMDF.