## 5 Applications
### In Vivo Characterization of Biological Tissues
Figure 15: (**a**) Absorption and (**b**) reduced scattering properties of bone measured at 6 locations (calcaneus, distal and proximal radius, trochanter, distal and proximal ulna), averaged over 17 volunteers [29]. These initial measurements reported in Figure 16 show that in the range of 600-1100 nm, the absorption properties at the thyroid location are dominated (around 600 nm) by the tail of hemoglobin and thyroglobulin absorption peaks at shorter wavelengths. Strong water absorption at 980 nm is the other dominant spectral feature. Finally, over the entire spectral range, the absorption is very high, spanning from 0.2 to 1.2 cm\({}^{-1}\), which may at least in part be attributed to thyroglobulin. Similar absorption properties are obtained at the two source-detector separations, with just slightly lower absorption below 800 nm for the shorter source-detector distance. In agreement with what expected based on thyroid physiology, the absorption at the reference location is on average lower over the entire spectral range, reflecting lower blood content and no thyroid-specific contributions. The measurements on the thyroid with \(d=1.5\) cm resulted in higher reduced scattering than other measurements. It is important to highlight that just a few subjects were analyzed and that the inter-subject variability is marked. Still, this preliminary study showed that the non-invasive characterization of the thyroid by optical means is feasible and could effectively be further pursued. Finally, it is interesting to consider adipose tissue. It has long been regarded just as an energy depot, while it is now becoming evident that it is an endocrine organ, deeply involved in the development of many pathologies, like obesity, diabetes, and metabolic diseases that are growing in impact on the quality of life and in burden on the healthcare systems [131; 140]. Visceral fat, located mainly within the abdominal cavity has an important link with pathologies. Thus, the non-invasive in vivo characterization of abdominal fat and the capacity to monitor its pathologic changes may certainly be of interest for pathophysiology studies. An interesting work on the optical assessment of abdominal adipose tissue was recently performed with a different approach to diffuse optics (a combination of frequency domain measurements at selected wavelengths and broadband CW measurements interpreted with a homogeneous model of the abdomen). That study suggested that DOS may be of help in monitoring the effects of a low-calorie diet on the obese subjects, and more generally in investigating the role of adipose tissue in metabolic disorders [141]. However, TD-DOS data collected at 610-1010 nm from on the abdomen of 3 healthy volunteers and interpreted with the diffusion approximation for a homogeneous semi-infinite medium showed that, depending on the subject, the absorption spectrum can be dominated by lipid absorption (as expected when the adipose tissue layer is properly probed) or by water and blood, when the subcutaneous adipose tissue is thin and the underlying muscle is reached [142]. That clearly evidenced the need to model the abdomen as a multi-layer structure, when aiming at the quantification of the optical  
Figure 16: Absorption (**a**) and reduced scattering (**b**) properties of thyroid at two source-detector separations \(d\) (red squares, d = 1.5 cm, green squares, d = 2.5 cm), and of a reference lateral location on the neck (violet square, d = 2.5 cm) [53]. properties and of the composition of adipose tissue. Further data in support of this need were recently obtained performing TD-DOS on 4 healthy volunteers at 3 different source-detector separations \(d\) (\(d\) = 1, 2, 3 cm), probing increasing depths. As evident from Figure 17, the measured absorption properties markedly depend on depth [143]. With \(d\) = 3 cm, when the adipose tissue layer is thick (>2 cm), it dominates the estimated absorption properties, which clearly resemble the spectral features of lipid.