6 - Near-Infrared Spectroscopy and Imaging

Abstract

This chapter describes the approaches based on light in the near-infrared (NIR) allowing gathering data on the human brain through the intact skull. By comparison with other noninvasive functional neuroimaging methods such as positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and magnetoencephalography (MEG), NIR spectroscopy (NIRS) surpasses these techniques in flexibility, biochemical specificity, and high sensitivity to detect small substance concentrations. NIRS may also extend beyond the imaging changes in hemodynamics, potentially allowing for the imaging of mitochondrial metabolism and even neuronal activity. The most significant disadvantages of NIRS are the relatively poor spatial resolution (in the order of centimeters) and the limited depth penetration. Technically, NIRS has an exquisite temporal resolution, theoretically limited but by the velocity of light. The choice of the sampling rate thereby essentially depends on the estimation of a signal-to-noise ratio appropriate to the study's aim. Spatial resolution is the major shortcoming of the method. This limits its capability to address questions of functional anatomy of the cerebral cortex in the adult.