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ORIGINAL ARTICLE
Year : 2007  |  Volume : 52  |  Issue : 3  |  Page : 145-149
Evaluation of litcit software for thermal simulation of hair removal lasers


1 From the University of Tehran, Research Center for Science and Technology in Medicine (RCSTIM), Tehran, Iran
2 Medical Physics Department, Medical Sciences/University of Tehran, and Research Center for Science and Technology in Medicine (RCSTIM), Medical Sciences/University of Tehran, Tehran, Iran
3 Medical Physics Department, Medical Sciences/University of Tehran, Tehran, Iran
4 Dermatology and Academic Center for Education, Culture and Research (ACECR), Tehran, Iran
5 Noor Medical Imaging Center, Tehran, Iran

Correspondence Address:
S Sarkar
RCSTIM, Imam Khomeini Hospital Complex, Keshavarz Blvd, Tehran
Iran
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0019-5154.35094

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   Abstract 

Background and Objectives : In this study, we evaluate LITCIT software for its application as a thermal simulation software for superficial hair removal laser systems. Materials and Methods: Two articles were used as our references. Complete information regarding the tissues, such as optical/thermal properties and geometrical modeling and also the laser systems such as wavelength, spot size, pulse duration and fluence were extracted from these texts. Then, this information regarding the tissues and systems was entered into the LITCIT simulation software. Further, we ran the program and saved the results. Finally, we compared our results with the results in references and evaluated the. Results : Output results of the LITCIT show that they are consistent with the results of references that were calculated with a different thermal modeling. Such a small average error shows the accuracy of the software for simulation and calculating the temperature. Conclusions : This simulating software has a good ability to be used as a treatment planning software for superficial lasers. Thus, it can be used for the optimization of treatment parameters and protocols.


Keywords: Hair removal laser, LITCIT software, simulation


How to cite this article:
Shirkavand A, Sarkar S, Hejazi M, Ataie-Fashtami L, Alinaghizadeh M R. Evaluation of litcit software for thermal simulation of hair removal lasers. Indian J Dermatol 2007;52:145-9

How to cite this URL:
Shirkavand A, Sarkar S, Hejazi M, Ataie-Fashtami L, Alinaghizadeh M R. Evaluation of litcit software for thermal simulation of hair removal lasers. Indian J Dermatol [serial online] 2007 [cited 2019 Oct 22];52:145-9. Available from: http://www.e-ijd.org/text.asp?2007/52/3/145/35094



   Introduction Top


The use of laser for the treatment of pathological tissues is becoming significant. Laser irradiation is often used interstitially or superficially to coagulate tissues in a selected target region. However, the expected tissue response can not be directly assessed by physicians. Therefore, accurate radiation planning plays an important role not only in the routine clinical treatment but also for the development of new application systems or therapy modalities.

The computer simulation LITCIT software (laser in medizin thechnologie Gmbh, Berlin, Germany, version 1.31) calculates and displays the temperature distribution in biological tissues during superficial and interstitial thermal treatments. It also calculates the resulting damage of the tissues. [1]

To date, there have been some studies for treatment planning and optimization of the treatment parameters. For example, we referred to the study in which Olsrude et al. used the LITCIT software to calculate the temperature distribution precisely for the treatment planning of human liver tumors. [2] In another example, Vogl et al. used LITCIT for treatment planning and controlling before and after LITT in a study on MR-guided laser-induced thermotherapy of liver tumors. [3]

To date, there are no applications of this software in the treatment planning of superficial applications of lasers. This is the first study that uses this simulating software for modeling the temperature distribution of diode laser for hair removal in skin tissues. If the accuracy of this software is demonstrated with respect to other standard methods such as numerical solutions for heat transfer, then it can be used for other future simulation studies in the field of dermatology for optimizing the treatment parameters.

The purpose of this study is the evaluation of the LITCIT computerized software for using it in thermal simulation before treating unwanted hairs using laser systems so that it can be used as a treatment planning software before clinical trails.


   Methods and Materials Top


In this study, LITCIT (laser-induced-temperature-calculation-in-tissue) simulation software version 1.3 (laser in medizin thechnologie Gmbh, Berlin, Germany) was used. This simulation program consists of three main components:

  1. A Monte Carlo technique for simulating the light distribution in tissue.
  2. A finite difference solution for the heat conduction equation for simulating heat distribution.
  3. An arrhenius rate process integral to calculate the thermal damage of irradiated tissue.


The LITCIT program consists of an algorithm, which is shown in [Figure - 1], in order to calculate the absorption and heat distribution produced due to laser irradiation in the tissues.

To perform the calculation of energy distribution, heat transport and tissue reaction, a calculation space (region of interest) should be defined. The biological tissue should be geometrically modeled. Tissue should be divided into small voxels (volume elements) in order to resolve the temperature gradients in tissue.

The LITCIT program uses the Monte Carlo technique for modeling the light transport in tissue. It calculates the



in the treatment process.

Moreover, it calculates the heat produced in tissue volume. In fact, at each time interval, the heat deposition due to laser absorption in target tissue is individually calculated for each voxel of tissue. The increase in temperature, ∆ T(x,y,z) , is determined from the amount of converted energy, ∆ E(x,y,z) , as follows:



In this equation, ρ is the density, c p is the heat capacity of the tissue and V voxel is the volume of each tissue element. (1)

Further, the equation of heat conduction is solved by finite difference method in order to model the heat diffusion in the tissues during and after laser irradiation and to calculate the time-dependent temperature increase in the irradiated tissues. The temperature of each voxel T(x,y,z) at a time t + ∆ t depends on the temperature of six adjacent voxels at time t .

LITCIT also calculates the thermal damage of the irradiated tissue using the arrhenius integral. The integral calculates the state of the protein denaturation in terms of a rate equation.



Here, A and E are the arrhenius constants, R is the universal gas constant and T is the absolute temperature at time t . (1)

For performing a simulation study using this software, one should model the geometry of the irradiated tissue. The optical and thermal properties of each component of the tissue should be defined. In addition, the laser irradiation parameters should be defined as input parameters for this software.

Since this was the first study that used LITCIT software for the thermal simulation of hair removal lasers in skin tissue, it was necessary that this software validate against other standard techniques such as the MCML technique or the numerical solution for the heat transfer equation. In other words, the reliability of the LITCIT results should be evaluated under different conditions.

We used two referenced papers. [4],[5] After that, we performed simulations using LITCIT software similar to those that had been conducted. For achieving this purpose, we conducted a step-by-step study of their methods and then extracted the crucial details such as geometrical tissue properties (number of structures, thickness of each structure and the size of tissue voxels) and the optothermal properties (refractive index, absorption and scattering coefficients, anisotropy factor, specific heat capacity, thermal conductivity and density) of each tissue structure.

Subsequently, we conducted the simulations based on the similar conditions and saved the results.

Finally, we compared the LITCIT results with the referenced results in order to determine the reliability of the results. For achieving this purpose, we performed the evaluation studies in different steps. These steps are described as follows.

First step: We modeled the study conducted by Klavuhn et al. Further, we developed a theoretical analysis using the Monte Carlo model and solved the heat transfer equation in order to construct a more complete role of the sapphire cooling disk during the laser treatment of hair removal. [4]

In this study, a three-layer model of the skin tissue consisting of 0.080-mm-thick epidermis, 0.020-mm-thick basal layer and 4.9-mm-thick dermis has been used. They assumed a cylindrical hair shaft with 0.200 mm in diameter at the central part of the model. [Figure - 2] shows the geometrical view of the modeled tissue.

In this study, two different skin boundaries were assumed. First, the laser beam was applied through the external medium, i.e., air ( n = 1) with an approximate initial temperature of 20C; second, it was applied through the sapphire contact cooling ( n = 1.76) with different initial temperatures (5C and 20C).

Laser beam has been modeled as a square pulse of constant irradiance, E 0 = 2000 w/cm 2 , over a pulse duration of 30ms that results in a total fluence of 60 J/cm 2 .

The optical properties of each tissue component in this study are listed in [Table - 1]. The thermal properties for the skin components were calculated using the empirical relations (Takata 1977) based on the water content of the tissue. (A water content of 0.5g/cm 3 was used for the epidermis, basal layer and hair and 0.75g/cm 3 was used for the dermis). [4]

Second step: The study conducted by Elie Jane Fiskerstrand et al. was modeled using LITCIT. They modeled the thermal distribution in and around hair follicle during and after laser irradiation using the exact solution of heat diffusion.

They modeled a laser beam with a wavelength of 800nm as a single pulse with 30ms pulse duration and a fluence of 35 J/cm 2 .

They also modeled a laser beam with a wavelength of 810nm as a double pulse with 45ms (245) pulse duration that is separated by an interval of 40ms and a fluence of 35 J/cm 2 .

They used a two-layer model of the skin tissue. The epidermal and dermal thicknesses were 0.1 and 3 mm, respectively. The follicle diameter was 300 . [5]

We used the same geometry of tissue that was used in the referenced studies. Furthermore, the irradiation parameters of laser systems (fluence, spot size and pulse duration) and optical and thermal properties and initial temperature of boundaries were defined as the input parameters for LITCIT. Further, the simulations were conducted, and the results were saved and analyzed.


   Results Top


In this section, we describe the results that were obtained from LITCIT simulations. Then, we compare them with the results that were published in the referenced papers.

At first, we present the LITCIT results and compare them with the referenced results of article. [4]

Simulations were run 15 times. Further, the results were evaluated with the statistical t-test

[Figure - 3],[Figure - 4] show the calculated temperature in the pigmented basal layer at a point with following dimensions: radial distance=1.17mm and depth=0.090mm).

[Figure - 3] is the resulted temperature profile obtained from a fluence of 60 J/cm 2 and a pulse duration of 30ms at 800nm wavelength, while using sapphire contact cooling with an initial temperature of 5C.

This figure shows a considerable decrease in temperature in the basal layer of skin during the precooling period of 250m from 30 to 16C. As shown in this figure, this type of precooling period with such a low temperature for sapphire enabled approximately 56% of heat deposit onto the sapphire and it precooled the adjacent tissues. Further, a significant reduction in the maximum temperature is experienced by the epidermis.

The standard deviation of the LITCIT results and reference results for the temperature at the end time of 250ms precooling period was calculated to be equal to 1.455. It was evaluated by α<0.05, and no significant difference was observed.

This temperature in our simulation was 16.75C. In all the time intervals, the mean error between the LITCIT results and referenced results was approximately 0.046.

[Figure - 4] shows the temperature profile obtained from a fluence of 60J/cm 2 fluence and a pulse duration of 30ms at a wavelength of 800nm while using sapphire contact cooling with an initial temperature of 20C. It shows a reduction in the temperature of basal layer from 30 to 25C at the end of 250-ms precooling period. The mean of this temperature in LITCIT simulation was calculated to be 25.6C. The standard deviation of LITCIT results and reference results for this temperature was calculated to be 1.455. It was evaluated by α<0.05, and there has not been any significant difference. The mean error in all the time intervals between temperatures calculated using LITCIT and referenced ones was approximately 0.0456. [4]

In the second step, the temperatures of the center and outer sheath of the follicle after the end of the period of radiation of the laser were calculated. For this part, we run the simulations 15 times in the first stage and 8 times in the second stage. Then, the results were evaluated with the statistical t-test. [5]

We have summarized the results of the first and second stages in [Table - 2],[Table - 3], respectively.


   Conclusion Top


Recently, the use of laser systems in different treatment fields is becoming significant. Accurate radiation planning plays an important role not only in the routine clinical treatment but also for the development of new application systems or therapy modalities.

In dermatology, laser systems have been considered as effective tools for treating unwanted hairs. Although they are useful tools, if the treatment parameters are not selected properly, then there will be side effects after the treatment process.

Therefore, it appears that an appropriate simulating software is useful for predicting the thermal reaction of skin tissues.

In this study, we used LITCIT software, which is one of the useful softwares for the simulation of heat and damage patterns of laser-tissue interactions. It can be used for the simulation of heat distribution and also the thermal damage of different kinds of tissues or different kinds of laser irradiation parameters.

In this study, we modeled heat distribution of lasers systems with different irradiation parameters that have been previously modeled with different standard methods. The standard deviation in all the cases of comparison was evaluated by α <0.05. No significant difference was observed between the LITCIT results and results of the standard methods. It was also proved that there were no significant differences between the LITCIT results and the results of other standard methods. These results indicate the reliability of the LITCIT software in conducting simulation studies. This amount of error obtained may have resulted from different algorithms that were applied in different methods.

According to the results calculated using the LITCIT software and their accuracy, it appears that this software has a high ability for the simulation of the treatment process of unwanted hairs using lasers.

It is presumed that this LITCIT software has the ability to be used as a relevant treatment planning software. If this computerized program is used and biological tissues are modeled in practice and the input parameters correctly defined, then the treatment parameters and protocols can be optimized before setting a real treatment in a clinic for hair removal using lasers. It can assist in selecting an appropriate laser system according to the tissue conditions of the patients, thereby avoiding unwanted side effects of treatment process.


   Acknowledgements Top


We would like to thank Dr. Shahram Akhlaghpoor for his invaluable cooperation and support us with providing LITCIT software in Noor medical imaging center during the performance of this study.

 
   References Top

1.LMTB. LITCIT for windows: Therapy planning system for thermal laser application. 1.31. Berlin; 2000.  Back to cited text no. 1      
2.Olsrud J, Wirestam R, Persson BR, Tranberg KG. Simplified treatment planning for interstitial laser treatment thermotherapy by disregarding light transport: A numerical study. Laser Surg Med 1999;25:304-14.  Back to cited text no. 2      
3.Vogl TH, Straub R, Zangos S, Mack MG, Eichler K. MR-guided laser induced thermotherapy (LITT) of liver tumors experimental and clinical data. Int J Hyperthermia 2004;20:713-24.  Back to cited text no. 3      
4.Klavuhn KG, Green D. Importance of cutaneaous cooling during photo thermal epilation: Theoretical and practical considerations. Laser Surg Med 2002;31:97-105.  Back to cited text no. 4      
5.Fiskerstrand EJ, Svaasand LO, Nelson JO. Hair removal with long pulsed diode lasers: A comparison between two systems with different pulsed structures. Laser Surg Med 2004;32:399-404.  Back to cited text no. 5      


    Figures

  [Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4]
 
 
    Tables

  [Table - 1], [Table - 2], [Table - 3]

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