NeuroCaptain v2 - Interactive Three-Dimensional fNIRS Optode and Probe Montage Design Platform Based on Blender.

Significance: Accurate and reproducible optode placement is crucial for obtaining high-quality fNIRS data in both individual and group-level neuroimaging studies. Conventional optode/probe montage design tools usually transform a probe layout defined in 2D Cartesian space onto a 3D head surface using a mass-spring model. Such mechanical transformation, combined with the indirect mapping between the 2D probe definition and the 3D target space, can introduce placement variations across different head surfaces and subjects. Aim: We introduce NeuroCaptain v2, an open-source Blender-based add-on designed to enable interactive, anatomically guided optode design, registration, and cortical sensitivity visualization for fNIRS head-cap and probe creation. Approach: NeuroCaptain v2 enables researchers to add, move, and edit fNIRS sources and detectors directly over a 3D head surface mesh, defining anchored optode positions, as well as setting the stiffness of the springs between adjacent optodes. It then utilizes Blender's built-in physical simulation engine to relax the initial probe layout to satisfy the mechanical constraints. With the built-in mesh-based Monte Carlo (MMC) and diffusion-solver Redbird, NeuroCaptain v2 computes and renders 3D sensitivity maps to guide iterative optode adjustment. The resulting 3D optode layout is stored in the form of barycentric coordinates defined in a 10-20 landmark mesh, enabling consistent probe transfer across different head models. Results: We demonstrate interactive 3D montage design, cross-head-atlas probe registration, and cortical sensitivity visualization across multiple head geometries. Registering a probe across seven neurodevelopmental head atlases, the proposed anatomical-coordinate approach yields a mean per-optode standard deviation of 2.29 mm, a roughly 74% reduction in cross-subject placement variability compared to 8.68 mm using a conventional 2D-to-3D registration. Conclusions: NeuroCaptain v2 provides a reproducible, fully open-source workflow for fNIRS probe montage design that facilitates anatomically guided probe development and cross-subject registration directly in a three-dimensional anatomical environment.