This pilot study is intended to investigate possibilities of skin nevus imaging using digital still image camera. The main objective is to develop method of dermatology images interpretation, which enables the looking on the skin lesions and nevus from the optical background of skin coloration. Kubelka-Munk calculation method for light transport and reflection from multilayered complex media is applied in modeling of light reflection spectra of skin. Calculation of model shows that red, green, blue and infrared colors lighting is satisfactory to access distribution of comparative estimates of the following skin parameters: volume fraction of melanin in epidermal layer, volume fraction of hemoglobin in dermal layer, presence of dermal melanin and thickness of papillary layer. Performance of image processing method on fourteen samples of images of common melanocytic nevi, dysphasic melanocytic nevi, Spitz nevus, thrombotic hemangioma and surrounding healthy skin were made.
Skin spectral properties
Understanding how light interacts with skin, can assist in designing physics based dermatological image processing. The key is understanding how light interacts with skin tissue. Skin consists of different layers with different spectral properties.
Fig 1. Skin model and its physical view
Whent incident light is applied to skin layer, the part of it absorbed and other part is scattered. The main layers of skin are as as follows: Stratum cornea it practically doesn’t absorb light, but diffuses it; Epidermis consists of cels producing pigment melanin. Melanin strongly absorbs strongly absorbs light wavelengths towards ultraviolet part; Dermis is next skin layer which consists of collagen fibetrs. It can be split in to two sublayers: Papilary dermis and dermis itself. Papillary dermis consists of high dence of collagen fibbers who is strong scatterer of light.
Implementation of skin spectral model
Skin is modeled as two light fluxes through three layer media. As Kubelka Munk theory says, Incident light fluxe is resoved into two fluxes: one is directed in to deeper layers, and other is opposite directed because of back scattering.
Because of layered skin structure, there are multiple reflections. Thei can be solved as infinit sum:
For N layer system, R1,2…n and T1,2..n is expressed as recursive equations:
The main model requirement is that light has to be scattered. Stratum cornea is supposed as scattering filter. According to earlier studies skin can be characterized as follows:
1) Epidermis, depending on wavelength can be characterized with melanin absorbtion coefficient μam(λ) and melanin concentration cm;
2) Papillary dermis can be described with hemoglobin absorbtion coefficient μah(λ), hemoglobin concentration ch, collagen scattering coefficient μspd and collagen layer thickness dpd;
3) Dermis can be described with scattering coefficient μsrd and thickness of layer drd.
Using those parameters the model of skin was calculated which shows reflected light R(λ) dependency on skin parameters and wavelengths of light:
Ranges of volume fraction of melanin cm –are in range for normal healthy skin: 0,01 – 0,5; ch – hemoglobin volume fraction coefficient, in model: 0,001 – 0,05.
RR(λ), RG(λ), RB(λ) are reflectance spectras for red, green and blue illumination. SLEDR, SLEDG, SLEDB – light source spectral charecteristics; SCCDR, SCCDG, SCCDB – CCD sensor sensitivity to light wavelength.
After these calculations, we get one RGB vector pointed to one point in RGB space for one independent set of parameters:
All vector values is displayed in RGB space drawing a color surface of all available healthy skin colors
This model is valid for all healthy skin, where is no melanin presence in papillary dermis. This model does not depend on race or even on sunburn degree.