From going bananas to going nuts

In addition to providing self-calibrated measurement of the optical properties of tissue, dual-slope measurements also achieve preferential depth sensitivity in tissue. This feature is illustrated in Fig. 1, which reports the normalized sensitivity of FD-NIRS data to a 0.7 cm³ absorber that is scanned within a highly scattering medium. It is well-known that single-distance NIRS measurements probe a banana-shaped tissue volume that extends from the light source to the optical detector and thus interrogates superficial as well as deeper tissue. This is shown in Fig. 1(a) for the case of data collected at a single source-detector distance of 35 mm (top: amplitude; bottom: phase). In the case of single slopes measured from multi-distance FD-NIRS data, as shown in Fig. 1(b) for one source and two detectors at distances of 20 and 35 mm, the region of sensitivity splits into two asymmetrical bananas, still reaching the tissue surface on the side of the two detectors. 

Fig. 1. Normalized absorption sensitivity for FD-NIRS data collected with (a) single distance (banana-shaped), (b) single slope, or (c) dual slope (nut-shaped). S, D are the source and detector locations on the tissue surface.

The approaches of Fig. 1(a) (single-distance) and Fig. 1(b) (single slope based on a single source and two detectors) are commonly used in the NIRS field. Measurements of dual slopes, which require a symmetrical arrangement of two sources and two detectors, achieve a unique region of sensitivity that is more localized than the single-distance banana-shaped sensitivity and show preferential sensitivity to deeper tissue. The dual-slope region of sensitivity resembles a nut shape, as illustrated in Fig. 1(c). The transition from a banana-shaped to a nut-shaped region of sensitivity affords a greater localization of non-invasive optical measurements in tissue. More importantly, it allows for a greater relative sensitivity of deeper vs. superficial tissue, which is highly desirable in non-invasive applications to the human brain, skeletal muscle, and other deep tissues.