Transcranial brain atlas based on photon measurement density function in a triple-parameter standard channel space.

Functional near-infrared spectroscopy (fNIRS) is a widely-used transcranial brain imaging technique in neuroscience research. Nevertheless, the lack of anatomical information from recordings poses challenges for designing appropriate optode montages and for localizing fNIRS signals to underlying anatomical regions. The photon measurement density function (PMDF) is often employed to address these issues, as it accurately measures the sensitivity of an fNIRS channel to perturbations of absorption coefficients at any brain location. However, existing PMDF-based localization methods have two limitations: (1) limited channel space, and (2) estimation based on a single standard head model, which usually differ anatomically from individuals. To overcome these limitations, this study proposes a continuous standard channel space for fNIRS and constructs a PMDF-based transcranial brain atlas (PMDF-TBA) by calculating PMDFs using MRI data from 48 adults. The PMDF-TBA contains group-averaged sensitivities of channels to gray matter and brain regions as defined in 3 atlases: Brodmann, AAL2, and LPBA40. We evaluated the prediction ability of PMDF-TBA for sensitivity of unseen individuals. The results show that it outperformed PMDFs based on single standard head models, making PMDF-TBA a more generalizable fNIRS spatial localization tool. Therefore, in the absence of individual sMRI data, PMDF-TBA can optimize optode montage design, enhance channel sensitivity in target brain regions, and assist in source localization for fNIRS data, thereby facilitating the application of fNIRS in neuroscience research.