Prior photon measurement density functions constructed in standard channel space for functional near-infrared spectroscopy.

Photon measurement density function (PMDF) determines the measurement locations and their associated sensitivity to the hemodynamic changes for an fNIRS channel. It is therefore essential for pre-experimental optode arrangement and post-experimental signal analysis and interpretation in fNIRS studies. However, accurate PMDF estimation requires subject-specific MRI data, which are unavailable in common fNIRS studies. Previous studies often rely on PMDFs derived from a single head template (template-PMDF), ignoring inter-subject variability in anatomy and optode placement, and are limited to sparse predefined scalp locations. Here, we propose a methodological framework to construct prior PMDFs (prior-PMDF) by synthesizing PMDFs derived from multiple subjects in a structural MRI database. This approach incorporates inter-subject variability and enables estimation of PMDFs in the standard brain space for arbitrary scalp locations. We demonstrate the utility of the prior-PMDF in estimating regional sensitivities, including voxel-wise sensitivity, sensitivity to intra- and extracerebral tissues, and sensitivity to atlas-defined brain regions. The estimation accuracy of the prior-PMDF is evaluated by comparing it with subject-specific PMDFs derived from MRI data that are not included in the construction database. Furthermore, comparison with template-PMDF showed that the proposed method significantly improves accuracy, thereby supporting more reliable PMDF estimation in MRI-free fNIRS studies.