NIR laser irradiation - how does the heating depend on illumination sizeand pulse length

The most frequent use of lasers in medicine is to cut and destroy tissue, as performed in, for instance, laser surgery. Depending on the application several different types of lasers are used. Diode and Nd:YAG lasers are the most frequently used for treatment of larger volumes due to the deep light penetration in tissue for those sources, while CO₂- and Er:YAG lasers are preferable and widely used when very shallow light penetration is preferable. In this project you should concentrate on NIR sources offering deep light penetration, for most tissues. Once the tissue is heated beyond the threshold for carbonisation, also light in this wavelength region is absorbed in a shallow layer. This effect is specifically investigated in a separate project, and you may or may not choose to ignore this effect. Here you should focus on the application for open surgery to destroy and remove tissue bulks, especially for oncological applications.

In this project you should analyse the volume destroyed during laser surgery and for several different beam sizes, several total fluences and several fluence rates. You should also discuss the advantages and risks associated with such a treatment.

This can be addressed by including the following:

• You are supposed to briefly study the field of NIR laser surgery. You should make a literature search and based on the findings discuss clinical applications and risks associated with the treatment. This background should be included in the introduction of the report. Focus the presentation on oncology, but also exemplify and cite literature for other applications.
• You should measure the optical properties of muscle tissue at the wavelengths of interest (during the laboratory exercises). Compare the obtained values with data from literature. It is difficult to measure values of carbonized tissue.
• You should calculate the light distribution for the wavelength of interest (during the computer exercises and by yourself).
• You should calculate the temperature distribution and the destroyed tissue volume as a function of spot size and fluence rate for such a treatment (during the FEMLAB exercise and by yourself). Investigate this both when the delivered energy, and when the treatment time is kept constant.  What happens if you instead keep the power density or energy density constant? It is probably a good idea to produce one plot for each problem.
• Discuss the pros and cons with various beam sizes and fluence rates in laser surgery and compare with usual settings during surgery.

In the project at least these parameters should be considered: The illuminated area, the pulse length, the temperature threshold for tissue coagulation, carbonisation, source wavelength and power, treatment time, optical and thermal properties of the tissue type chosen.

Suggested key-words: laser surgery, laser thermotherapy, hyperthermia, lasers, tissue optics, diode lasers