## 5 Applications
### In Vivo Characterization of Biological Tissues
Also, fast and accurate models of photon migration are needed to overcome the key limitations of the diffusion approximation when dealing with short (<200 ps) propagation times or thin (<2 mm) layers. In that direction, especially attractive is the proposal of analytical multi-layered models comparable with Monte Carlo in terms of accuracy, but quick enough to be used in multi-parameter inversion procedures [136; 137]. Figure 14: (**a**) Absorption and (**b**) reduced scattering spectra of the human forehead. Median values over 9 subjects are shown for data collected with different systems (CW, TD at 5 wavelengths, and broadband TD). A homogeneous model was applied to data from all systems. For the TD system at discrete wavelengths results are displayed also when the data are interpreted with a two-layer model [138]. In the last years, several other TD-DOS applications started to be considered, stemming from presently unmet clinical needs. One of them is the optical characterization of bone. The aging population suffers more and more from bone-related pathologies, but in several widespread cases (e.g., osteoporosis) effective non-invasive means are currently not available for early diagnosis, which instead is key to efficient treatment. An initial attempt towards the non-invasive optical characterization of bone tissue was made more than a decade ago, performing measurements in the 650-1000 nm range on the calcaneus of 7 female volunteers spanning over a broad age range (26-82 y): Measurement feasibility was shown and possible age dependence of the optical properties was suggested to reflect age-related changes in composition and structure of bone tissue [26]. More recently, data were collected from 17 healthy volunteers at 6 locations where the bone is rather superficial and thus accessible to optical probing, with the aim of identifying the best location for further systematic optical studies on bone [29; 138]. From Figure 15, it appears that, notwithstanding significant inter-subject variability (measured by the standard deviation), on average different locations are characterized by different optical properties. In particular, lower absorption, with smaller water contribution at 980 nm (calcaneus and trochanter) corresponds to higher scattering. Looking for an ideal location for clinical diagnostics of bone, three parameters were considered: the thickness of the superficial skin layer, the penetration depth of photons at the wavelength of operation, and the bone volume at the probed location. At the distal radius location, superficial tissue (skin) thickness is small, the high light penetration depth is foreseen based on the measured optical properties, and the bone volume is high, thus providing a strong contribution to the overall collected signal. All in all, these observations suggest the distal radius location as the best one for future diffuse optical studies of bone-related pathologies. Thyroid nodules are common, but presently their nature cannot be identified non-invasively. Fine needle aspiration followed by cytological analysis is the established standard to diagnose thyroid pathologies. However, besides being an invasive procedure, it may miss up to one-third of malignancies in palpable nodules [139]. A non-invasive optical diagnostics capable to estimate tissue composition and blood parameters may be of real interest. Thus, after the optical characterization of its specific constituents (tyrosine and thyroglobulin), described in Section 5.1, the in vivo characterization of the thyroid has recently started [53]. TD-DOS data were collected in reflectance geometry from 6 healthy volunteers on the thyroid area and at a reference lateral position on the neck. At the thyroid location, two source-detector distances \(d\) (\(d\) = 1.5 cm and 2.5 cm) were tested to identify best conditions for probing the gland (as a trade-off between signal level and minimum contribution from surrounding tissues).