Comparison of Brain Activity measured by Continuous Wave Near Infrared Spectroscopy and Time Resolved Near Infrared Spectroscopy

Abstract

There are two types of near infrared spectroscopy (NIRS) including Continuous Wave NIRS (CW-NIRS) and Time resolved NIRS (TR-NIRS); CW-NIRS employs CW light and the modified Beer-Lambert Law while TR-NIRS employs ultra short pulse light and the Photon Diffusion Equation. The NIRS evaluates brain function by measuring hemoglobin (Hb) concentration changes during activity. In order to clarify whether we can compare the brain activity measured by CW-NIRS and TR-NIRS, we measured Hb concentration changes in the bilateral prefrontal cortex (PFC) during mental arithmetic tasks in normal adults using CW-NIRS and TR-NIRS, and calculated Laterality Index (LI, i.e. [R-L]/[R+L] of total-Hb) for assessment of left-right asymmetry of PFC activity. Then, we evaluated the relation between LI of CW-NIRS and TR-NIRS. We observed significant positive correlation between LI of CW-NIRS and TR-NIRS (r=0.76, p<0.01). The results suggest that we can compare the brain activity measured by CW-NIRS and TR-NIRS.