High-density diffuse optical tomography (HD-DOT)

High-density diffuse optical tomography is the imaging-grade descendant of sparse fNIRS. Instead of a few source-detector pairs it uses overlapping arrays with multiple source-detector separations, so that many light paths cross each brain voxel and a tomographic reconstruction can localise haemodynamic changes in three dimensions with resolution approaching that of fMRI over superficial cortex.

Multi-distance measurement also addresses fNIRS's core confound: near-infrared light traverses scalp and skull before reaching cortex, so short separations sample mostly extracerebral tissue while long separations sample deeper. Combining them lets short-channel regression remove systemic and scalp haemodynamics (heartbeat, blood pressure, skin blood flow) that would otherwise masquerade as brain activation. HD-DOT still sees only outer cortex, is limited by photon scattering to roughly centimetre-scale depth, and inherits the seconds-slow, indirect nature of the haemodynamic response.