Figure 1, the absorption versus penetration depth for the two wavelengths is presented. The findings of our simulation confirmed the expected differences between the penetration depth of the two wavelengths as the result of having different absorption and scattering coefficients. The outcomes showed that penetration depths for 980 nm diode lasers and 1064 nm Nd:YAG lasers were 2.65 and 3.1 mm, respectively. The results implied that absorption and scattering coefficient were the key factors in laser lipolysis, since they determined the penetration depth of laser radiations inside the tissue ( 1- 7). Deeper penetration in 1064 nm Nd: YAG offered the preference of this wavelength to 980 nm, as the former propagated deeper in the fat tissues. Absorption coefficient of 1064 nm wavelength was shorter than that of 980 nm, consequently direct heating volume in 1064 nm was larger. Furthermore, skin retraction and tissue tightening with 1064 nm wavelength were obtained more efficiently in superficial treatment ( 1) due to the penetration of radiation to dermal layers and initiation of collagen gel retraction (CGR). The latter is one of the most important features of laser lipolysis causing skin retraction which leads to the decrease in skin laxity following fat removal operations.
Fluence is another major factor in laser-tissue interaction. It is defined as energy conveyed per unit area and the dimension is Joule/m
2. Fluence can affect laser lipolysis in different ways. Given that, the effects of increasing and decreasing fluence on penetration depth and radiation absorption in the tissue were studied. First, powers of the lasers were increased to 10, 20 and 30 watt for both wavelengths, then achieved results of the simulation were compared. The outcomes again revealed that 1064 nm wavelength could penetrate to deeper layers of fat tissue. Figure 2 and Figure 3 depicted the absorption versus the penetration depth for 1064 nm and 980 nm, respectively. It appeared that the deepest penetration for both wavelengths occurred in power of 30 watt which was equal to 3.2 mm for 1064 nm and 2.8 mm for 980 nm. Accordingly, increasing laser fluence for having deeper penetration was obtained more efficiently in 1064 nm wavelength.
This time, the effect of decreasing laser fluence on radiation penetration was scrutinized. To this end, radius of the laser beams was increased up to a fixed power of 10 watt, which caused fluence reduction. The beam radius was increased from the initial value of 200 µm to 282.84 and 346.41 µm for both wavelengths. We apply the influence of increasing of beam radius as increasing of the FWHM (full width at half maximum). In
Figure 4 and Figure 5, the absorption versus the penetration depth for 1064 and 980 nm wavelengths are illustrated, respectively. The results indicated slight difference between the two wavelengths. Maximum penetration depth for 1064 nm wavelength occurred in 200 µm beam radius which was equal to 3 mm. Similarly, the deepest penetration for 980 nm happened in 200 µm; equal to 2.5 mm due to the influence of absorption coefficient of fat tissues. As the radius of laser beam increased, radiation spread over wider areas in tissue and was immediately absorbed by tissue cells, led to reduction in penetration depth for both 1064 and 980 nm wavelengths.
As mentioned above, 980 nm wavelength of diode laser and 1064 nm of Nd:YAG laser had almost different absorption and penetration depths, hence these wavelengths showed different impacts on laser-tissue interaction. Results of the simulation verified the hypotheses; the wavelengths had different behaviors towards fat tissues. This is why nowadays 1064 nm wavelength is widely used in laser lipolysis instead of 980 nm diode lasers (
1- 3). Having had more penetration depth in tissue, 1064 nm wavelength had larger directly-heated volume. Its absorption coefficient was smaller than 980 nm, as the result laser beam perforated deeper to the tissue and heated larger . As the laser beam for 980 nm wavelength had greater absorption; the radiation therefore was accumulated in smaller volume, caused significant increase in the temperature for low volume tissues and resulted in unwanted damages to tissue such as bruise and hyperthermia.
In this stage, simulation of the tissue heating and temperature rise of the fat tissues following the irradiation of both 980 nm and 1064 nm wavelengths was analyzed. The results of the tissue heating for 1064 and 980 nm wavelengths are presented in
Figure 6 and Figure 7, respectively. The issue was heated up to 339.687 k for irradiation of 1064 nm Nd:YAG laser. The simulation of the tissue heating for the 980 nm diode laser showed that tissue temperature rose to 343.952 k. To better comprehend the results of tissue heating, the following figures of cannula insertion to the tissue surface were designed. In Figure 8, the temperature rise is seen for the location of laser radiation of 1064 nm Nd:YAG in the tissue; 1 cm below the tissue surface. It clearly shows the temperature rises up to 66℃ in the site of laser radiation. This temperature was sufficient for achieving the desired reversible and irreversible effects of fat tissues. The results of this simulation indicated different temperature rise for the two wavelengths. Our findings were in agreement with the literature as well as experimental studies ( 1, 2). These studies confirmed that 1064 nm wavelength was suitable for efficient laser lipolysis and had less unwanted effects on tissue. Lukac affirmed that radiation of 1064 nm could produce the authorized temperatures needed for lipolysis ( 3). Most researches agreed that the temperature range of 50-65℃ was the clinical endpoint for the applicability of laser lipolysis. In other words, the temperature range were essential to obtain the desired tissue damages. Besides, they were also sufficient to trigger the favorable thermal effects which could have the desired reversible and irreversible damages. Temperatures below 50-65℃ were not enough to create the desired damages and temperatures higher than that would result in unfavorable effects such as bruise and hyperthermia ( 2, 3). The findings of the simulation proved the preference of 1064 nm over the 980 nm wavelength in many ways. The penetration depth of 1064 nm was deeper and it was also absorbed in larger volume of the fat tissues. This wavelength also resulted in better skin tightening and fat reduction. In contrast, as 980 nm had greater absorption coefficient, it caused less penetration and higher temperature rise in shallow parts of the tissue, possibly caused side effects such as necrosis, bruise and hyperthermia.
Figure 1. Penetration Depth for Wavelengths Under Study
Figure 2. Absorption Versus Penetration Depth for Various Powers of 1064 nm Wavelength and Fixed Beam Radius of 200 µm
Figure 3. Absorption Versus Penetration Depth for Various Powers of 980 nm Wavelength and Fixed Beam Radius of 200 µm
Figure 4. Absorption Versus Penetration Depth for Various Radiuses of 1064 nm at Fixd Power of 10 Watt
Figure 5. Absorption Versus Penetration Depth for Various Beam Radiuses of 980 nm at Fixed Power of 10 Watt
Figure 6. Temperature Rise Following the Irradiation of 1064 nm Wavelength at Power of 15 Watt
Figure 7. Temperature Rise Following the Irradiation of 980 nm Wavelength at Power of 15 Watt
Figure 8. Temperature Rise of Fat Tissue After Radiation of 1064 nm Wavelength at the Depth of 1 cm Below the Tissue Surface