The ballistic and single scattered photons can be used to acquire spatially localised information from the turbid medium. The multiple scattered or diffused photons have lost directional information, and they cannot be used in localisation of the structure of the medium. Ballistic and single scattered photons can be gated to obtain localised information, but only from the surface in the reflectance setup or from a thin layer in the transmission setup. If the measurement depth or layer thickness is larger than the mean free path length of photons in the medium, the number of single scattered or ballistic photons drop too low to convey enough information any more. Therefore, the depth of the pmf is called diffusion limit. If the medium needs to be probed deeper than the diffusion limit, the diffuse photons are also needed and localisation is poor.
Some methods exist to cope with optical signals deeper than the diffusion limit. One method is optical clearing, in which an optical clearing agent is used to change the optical properties of the medium, to enlarge thepmfof the medium. The optical clearing method does not require any special measurement device. The disadvantage is that optical clearing is not always possible. The clearing agent can only be injected into some media and many clearing agents are toxic, making it difficult to apply inin vivo tissue measurements.
Another method is photo-acoustic tomography (PAT). It is based on the photo-acoustic effect, where the light absorbed by the medium results in thermal expansion of the medium. The expansion causes a pressure wave, which can be observed as sound. The origin of the sound source can be located by triangulating with several microphones.
The maximum depth and the spatial resolution of PAT can be adjusted, extending the possible application areas. The disadvantages are that PAT technology needs rather complicated measurement devices, and the microphones need to have good acoustic contact with the medium, which may be difficult to arrange in some cases.
If accurate localisation is not needed, diffuse photons can be used, and the target medium can be probed much deeply than pmf. The diffuse spectroscopy and diffuse optical imaging are simple applications of diffuse photons. If time or frequency do-main measurement is also included, the diffuse methods can be sensitive to the depth of information, too. Diffuse optical tomography (DOT) is one of those kind of meth-ods. DOT can be used to obtain 3D information about a turbid medium, much deeper than the diffuse limit. However, the localisation is not very accurate in any of the three dimensions.
3 OPTICAL PROPERTIES OF SKIN
The previous section discussed the optical phenomena in turbid media, and methods for modelling them. Light propagation was modelled using general radiative transport the-ory, which was in turn used in deriving simpler methods for more specific cases. This section introduces how these optical properties affect light propagation and appearance of skin. The reflection spectra of skin chromophores and scattering spectra of skin are presented, at the end of the section.
Skin is the largest human organ; it is easily accessible, and its thickness is suitable for optical examination. Skin disorders, such as skin diseases, injuries, and cutaneous blood circulation regulation change the physical or chemical structure of the skin, changing the scattering or absorption properties, which in turn cause visible changes in the skin color. Many metabolic disorders, bilirubinea for example, can be observed as a skin colour change as well. Optical methods are effective for analysing many skin disorders and other abnormalities.
3.1 Skin structure
Human skin consists of two main layers,epidermisanddermis. Theepidermisis the topmost layer of the skin. Its purpose is to protect the skin from various physical and chemical risks, such as mechanical stress and dehydration. Theepidermiscan be fur-ther subdivided. The topmost layer of the epidermis is the stratum corneum, which consists mostly of dead cells, ceratinocytes. The thickness of theepidermisvaries be-tween 50µm to several hundreds of micrometers, and typical thickness is somewhere between 100-200µm (Igarashi, Nishino & Nayar 2007). Optically important character-istics of theepidermisare the strong scattering in the air–cell boundaries of thestratum corneumand the strong absorption of light of the melanin chromophore contained by the melanocytes. A collimated light beam may become nearly diffuse after transmitted through thestratum corneum. The melanin absorbs light strongly, especially UV, blue and green colours. The melanin is the most important chromophore for skin colour. A schematic diagram of the structure of the skin is shown in Figure 7.
Thedermisis situated immediately underneath theepidermis. It is much thicker than theepidermis, usually from 1 to 4 mm (Igarashiet al.2007). The main constituents of
Depth / mm Epidermis
Papillary plexus
Cutaneous plexus
Subcutis Papillary dermis
Reticular dermis
Figure 7. Schematic diagram of skin structure. The layers of the skin are depicted on the right hand side, and the depth is shown on the left.
thedermisare collagen and elastin fibers. The dermis contains a lot of arteries and veins as well as capillaries for blood circulation. The microvasculature are concentrated on the top layer of thedermis, calledpapillary plexusand the larger vessels are situated in a lower layer, called cutaneous plexus, see Figure 7 (Tuchin et al.1994; Reuss 2005;
Igarashiet al.2007). The layer between thepapillary plexusandcutaneous plexusmay contain fewer blood vessels. Often thedermisis optically modelled only by two layers, the upper part,papillary dermisand the lower partreticular dermis. The main absorber in thedermisis haemoglobin.
The layer beneath the dermis is calledsubcutis. It is not counted as part of the skin any more. The structure and thickness of thesubcutisvaries in different body locations and between persons, but often it contains a lot of fat cells to absorb shocks, and its thickness is 4 to 9 mm. The fat cells scatter light strongly and the haemoglobin in the subcutisis the most significant absorber.