Although the relatively low attenuation of near-infrared light in most tissues facilitates transmittance measurements over distances of several centimeters, the profound scatter of light renders those measurements sensitive to a much larger volume of tissue than that occupying a direct line between the source and the detector. e light traveling between two points on the surface of a uniform diusing medium separated by a few centimeters has typically explored a banana-shaped volume of tissue, oen referred to as the photon measurement density function (PMDF; see Figure 10.1) [2]. e breadth of the PMDF limits the spatial resolution achievable using the diuse optical

imaging (DOI) methods, although as discussed later, gains can be achieved by measuring more than just the intensity of the transmitted light, and by a judicious combination of multiple measurements with overlapping PMDFs. Note that due to the dominance of scatter, the terms “transmitted” and “reected” are somewhat arbitrary when applied to optical measurements on the head, although the latter term is generally used when the separation between the source and the detector is less than about one-quarter of the circumference.