Functional near infrared brain imaging with a brush-fiber optode to improve optical contact on subjects with dense hair

A major obstacle in functional near infrared spectroscopy (fNIRS) and imaging is poor optical contact of the optodes with the scalp due to obstruction by hair. To overcome this problem a novel brush-fiber extension was made as an attachment to existing commercial flat-face fiber bundle optodes. The goal was that the brush-fiber extension would thread through hair and improve optical contact on subjects with dense hair. Simulations and experiments were performed to assess the magnitude of these improvements. Firstly, a Monte Carlo (MC) simulation of photon propagation through the scalp with hair and hair follicles was performed showing a drop in light transmission for different hair densities in the range of 3 to 9 dB. Similar levels of signal loss at individual optodes were determined experimentally in preliminary measurements on a human subject, when comparisons were made for finger tapping activation measurements with and without the brush-fiber extensions. The effects of such losses on reconstructed images were also simulated by using neutral density filters during tissue phantom measurements. As brush-fibers attenuated the signal by approximately 2.5 dB due to imperfect optical contact with the flat-face optode tips we explored the idea of using larger diameter brush-fibers to overcome that loss.