Neurophotonics: non-invasive optical techniques for monitoring brain functions

Abstract

The aim of this review is to present the state of the art of neurophotonics, a recently founded discipline lying at the interface between optics and neuroscience. While neurophotonics also includes invasive techniques for animal studies, in this review we focus only on the non-invasive methods that use near infrared light to probe functional activity in the brain, namely the fast optical signal, diffuse correlation spectroscopy, and functional near infrared spectroscopy methods. We also present an overview of the physical principles of light propagation in biological tissues, and of the main physiological sources of signal. Finally, we discuss the open issues in models, instrumentation, data analysis and clinical approaches.

Figure 1

Assessment of brain activity by optical methods. Adapted from Villringer and Chance (1997)

Figure 2

Absorption spectra of main tissue components in the visible (400–700 nm, VIS) and near infrared (700–2000 nm, NIR) spectral regions. Oxygenated and deoxygenated hemoglobin (red and blue curve, respectively; axis on the left), water and lipid (green and yellow curve, respectively; axis on the right). The so-called therapeutic and diagnostic window is also shown.

Figure 3

Scheme of non-invasive CW DOS measurement in reflectance configuration. A long (>30 mm) source-detector distance generally allows penetration to cortical surface, while for short (<10 mm) distances light is confined mostly in extracerebral tissue. A similar scheme for the TD DOS can be found in Torricelli et al. (2014).