Role of adipose tissue and bone tissue

Near-Infrared Spectroscopy (NIRS) has emerged as a valuable tool in the non-invasive assessment of skeletal muscle hemodynamics, oxygenation, and metabolism. However, since biological tissue is not homogeneous, non-invasive optical studies of skeletal muscle need to take into account potential contributions from tissue other than the targeted muscle tissue. A particularly important case is that of superficial tissue, such as the adipose tissue layer, which has attracted a significant attention from researchers. For example, approaches to correct NIRS muscle measurements for adipose tissue thickness (ATT) include calibration measurements based on a population of subjects that cover a wide range of ATT values, simulation studies on two-layered media that mimic adipose tissue and muscle tissue, and exploiting the temporal information content of time-domain NIRS (TD-NIRS). Another possible source of confounding contributions is bone tissue. While bone tissue has been specifically targeted by NIRS measurements at locations where bone is closer to the tissue surface, not much attention has been devoted to the potential impact of bone tissue in NIRS study that target muscle measurements. In typical NIRS muscle measurements, bone is deeper than muscle but may still contribute to the measurements for relatively thin muscle and relatively large source-detector separations.

The dual-slope (DS) method in frequency-domain NIRS (FD-NIRS) provides access to different data types (intensity and phase measured in single-distance and dual-slope configurations) that feature different distributions of spatial sensitivity within tissue. Therefore, DS FD-NIRS can be used to investigate the spatial distribution of hemodynamics in heterogeneous tissue, which is especially relevant in NIRS muscle studies.

Fig. 1. Blood flow measurements in the human forearm with the venous occlusion protocol. Different data types were collected with dual-slope FD-NIRS on two subjects with bone depth of 22 mm (panel (a)) and 15 mm (panel (b)).

Figure 1 reports blood flow measurements in the human forearm obtained with the venous occlusion protocol using single-distance (SD) and dual-slope (DS) FD-NIRS measurements. The subject reported in Fig.1(a) has deeper bone tissue (22 mm) than the subject reported in Fig. 1(b) (15 mm). The data types that feature deeper sensitivity (namely, SD-long vs. SD-short, DS vs. SD, phase vs. intensity) provide indications on deeper vs. superficial blood flow. Since muscle blood flow is greater than blood flow in adipose tissue and bone, bone tissue does not appear to significantly contribute in the case of Fig. 1(a), whereas it appears to impact the data in the case of Fig. 1(b). DS FD-NIRS performed at a range of source-detector separations may allow for the selection of optimal data types for muscle measurements in subjects with different anatomical characteristics.