Why do sub-dermal blood vessels look blue?

Tissue optics is complex. Sometimes it is more fascinating than first anticipated and the results are not always obvious. One such particular observation is that superficial sub-dermal blood vessels appear bluish and not reddish, as one might have suspected.

These types of superficial blood vessels plays and important role in certain laser treatments. Specifically, one of the preferred treatment modalities for hyper-vascularised lesions, such as port wine stains, is photo-coagulation using pulsed laser light.

The are two issues to be analysed in this project: a) Explain why subdermal vessels appear blue, and b) determine the optimal pulse length and wavelength for treating vessels with 200 mm diameters, 0.5 mm below the dermal-epidermal junction in the skin of a caucasian person. Here you should also motivate why pulsed light is preferable in the treatment of such lesions.

It is important to notice that the epidermal temperature in such a treatment is not allowed to exceed the damage threshold, to prevent resulting scar formation. The intention is that the project should include the following:

• You should try to motivate why the vessels appear blue. Include such a discussion in the introduction of the report and perform some modelling to support this discussion.
• You are supposed to briefly study the field of laser treatments of port-wine stains. Perform a literature search. What is the state-of-art treatment? Explain the advantages with this treatment. This background should be included in the introduction of the report.
• You may be able to measure the optical properties of muscle tissues at the wavelengths of interest (during the laboratory exercises). If you are not able to measure the properties of the wavelengths you are interested in, you have to find the values in the literature.  
• Optical properties for skin are difficult to measure and you should find that in the literature.
• You should calculate the light distribution for the wavelength of interest (during the computer exercises and by yourself).
• You should calculate the temperature distribution during and following such a treatment (during the thermal computer exercise and by yourself using the software used in this lab).
• Discuss how the pulse length and wavelength of the light alter the treatment effect. Also discuss the possibility to enhance the effect by cooling the tissue.

In the project at least these parameters should be considered: Source wavelength and pulse length, cooling, optical and thermal properties of the tissue. The diameter and depth of the blood vessel could also be considered (optional).

Suggested key-words: Laser, port-wine stains, vessel, skin, tissue optics