In vivo study on NIR light propagation in the human head

Near-infrared spectroscopy (NIRS) based techniques in brain monitoring utilize the spectrum range approximately between 650 nm and 950 nm, where light attenuation is low enough to enable reaching the cerebral cortex of the brain. In these studies, particularly oxygenation changes in the cerebral cortex are of great interest since the concentrations of oxyhemoglobin (HbO) and deoxyhemoglobin (HbR) change due to coupling of hemodynamics to cortical neural activity. There are numerous simulation and phantom studies that show near-infrared (NIR) light can penetrate in the human head to a depth of approximately 1–2 cm, reaching the brain cortex. However, NIR propagation and light attenuation is also dependent on anatomy and size of the subject’s head. This related, we studied experimentally the effect of layer thicknesses of dura and cerebrospinal fluid (CSF), skull and skin to detected light intensity when measured in vivo from human heads with different layer thicknesses. We studied anatomy of 15 human heads in magnetic resonance imaging (MRI), particularly the thickness and morphology of the tissue layers of CSF, skull and skin. At the same time, we measured intensity and absorbance spectrum, at range of 600 nm to 1100 nm, from the forehead of these subjects when fibre detector was placed at distances of 1 cm and 3 cm from the fibre source. Our results show that each layer affects the detected NIR spectrum when layer thickness changes, particularly at 3 cm source-detector distance. However, these small spectral variations, caused by changes head anatomy, most likely do not have significant influence in quantifying cerebral hemodynamics.