Study of Interaction of medical laser fibers

Keywords:surgical, fibers, medical, fiber,  Time:11-01-2016

The possibility of medical fibers- assisted lipoplasty was first reported in 1992 by Apfelberg. Apfelberg reported laser- assisted liposuction with the YAG laser beam enclosed in a cannula1. In conventional liposuction procedures, with the use of metal micro cannula in mechanical back and forth movements, the adipocytes in fat tissue were mechanically removed, causing many undesired effects while many of them were critical drawbacks. Scars, excessive blood loss, skin flaccidity and long recovery time were the mostly reported problems in conventional methods2. The application of the novel method of laser lipolysis has eliminated these problems. This technique uses an optical fiber inserted inside a 1 mm cannula, needing a smaller incision, resulting in less bleeding and scars. Interaction of laser radiation with skin tissue results in skin tightening and elimination of the former problems of skin flaccidity and laxity. There are fewer traumas in laser lipolysis procedure due to the small cannula size which results in faster recovery time. One of the most important advantages of this new method is the coagulation of small blood vessels by the laser light resulting in less blood loss during the procedure3. Laser lipolysis, also called laser lipoplasty, is now widely used in Europe and Latin America and has recently been introduced in Japan and the United States4. Laser lipolysis now is among the most popular cosmetic operations, with 400,000 operations in 2006 only in North America5. The interaction of the laser with the tissue is achieved through absorption of the laser energy by the receptive chromophores, thus producing sufficient heat to cause the desired thermal damage. The heat acts on fatty cell, the extracellular matrix and the microcirculation to produce cellular damages, which facilitate the liposuction through fewer traumas and bleeding6. The mechanism governing medical fibers lipolysis is selective tissue heating. The laser energy is delivered directly into treated tissue by using a flexible laser fiber. The laser energy converts into heat energy when absorbs by the target adipocytes causing their volume to expand and cell membrane damages. This results in liberating the cellular contents into the extracellular volume which can subsequently be removed via suction cannula7. Laser lipolysis has a wavelength dependent mechanism, with tissue heating and desired thermal effects caused by interaction of laser with tissue. However wavelength is the most predominant factor in this procedure. Selective laser heating can be achieved by utilizing an optical wavelength where the absorption by the target tissue is greater than the surrounding region. Specifically for fat treatments the absorption of lipids at vibration band near 915, 1210 and 1720 nm exceeds that of water8. Absorption of laser radiation by tissue cells depends extremely on wavelength. High absorption coefficient will conclude to large accumulation of heat and intense temperature rise at the target tissue. Absorption and scattering coefficients of various tissues, depend on wavelength, and are the most important factors in tissue heating, radiation diffusion and penetration depth. High absorption coefficient causes large quota of incident radiation absorb in shallow layers and this give rise to low penetration of radiation into deeper layers of target tissue. Nowadays there are many different wavelengths available for medical purposes, each of which has their own optical characteristics. Laser tissue interaction, as mentioned above, depends strongly on wavelength; therefor proper selection of wavelength is of vital importance. Each tissue has its own physical and optical properties that result in its dominant absorption for a specific wavelength. Thus proper wavelength selection results in selective treatment of that tissue.

Methods

Now many laser wavelengths have developed for medical purposes. Among them are 920, 980, 1064, 1320 and 1444 nm each of which has their own optical characteristics. Among these wavelengths 920nm has the smallest absorption coefficient in fat tissue and so penetrates the deeper layers of tissue. On the other hand 1320 and 1440nm have largest absorption coefficient in fat tissue causing smaller penetration depth inside fat tissue. These wavelengths are suitable for superficial treatment of such tissues. 1064 nm Neodymium-Doped Yttrium Aluminium Garnet (Nd:YAG) lasers is now widely used in laser lipolysis. Many researches have proved the efficacy of this wavelength in laser lipolysis and fat removal operations4,9. The absorption coefficient of fat tissue in this wavelength has a medium value of about 80µm-l, which results in good penetration depth into the fat tissue. In this paper we determine the penetration depth of this wavelength into the target tissue by using the famous Monte Carlo method and we also evaluate temperature rise of the fat tissue for this wavelength by using Comsol Multiphysics software. Description of the absorption and scattering characteristics of laser radiation in numerical way can be done by Monte Carlo method. This method is a stochastic one and relies on statistical procedures, its accuracy depends on the number of random numbers and so numerous quantities of photons have to be simulated10. However it is extensively used in the problems of laser- tissue interaction. Accuracy of this method has proved with experimental evidences10. In the Monte Carlo simulation, the laser beam is represented as a stream of a large number of laser “photons” each having a specific and well defined coordinate, direction and energy weight W3,11. In this method we randomly take sampling of variables from their probability distributions11. In this manner each random number represents one photon, perpendicularly incidents on the tissue surface. Initial positions of the photons are set to the origin of the coordinate system. We attribute an initial weight of w=1 for each photon. This weight regularly decreases once photon reaches a photon-tissue interaction site. In order to propagate photons inside the tissue we must determine the polar and azimuthal angles of photon direction. The polar angle of photon direction inside the tissue is obtained by random sampling from the probability distribution of the cosine of the deflection (polar) angle10,12,13. Azimuthal angle uniformly distributed in (0, 2) and sampled by using a random number. Photons interact with the tissue repeatedly until their weight fall below the predefined threshold value (e.g w< 0.001). Photons below this weight will terminate in an unbiased manner.

Results

In this article we use the reputed transport theory to numerically simulate photon-tissue interaction by using the Monte Carlo method. All of physical and optical properties of the 1064nm Nd: YAG laser have been used in this paper. The power of laser was set on 10 watt, absorption and scattering coefficients of this wavelength are 80µm-l and 1150 µm-l for fat tissue. The beam radius of the laser radiation sets on 200 µm. Program of this simulation was written in Matlab and has run for 1,000,000 of random numbers (photons). In order to have better understanding of the problem, we set up a grid system, with the typical dimensions of the fat cells, i.e. 100 µm, we recorded the absorbed weight of the photons in these grid elements.