## 4 Results and Discussion
### Effect of Vessel Depth on the Selection of Laser Fluence  
Deep PWS vessels are usually resistant to laser surgery. In this study, the effect of vessel depth on the PDL treatment of PWS nevus was investigated. Figure 19 shows \(S_{\mathit{rput}}\) of a vessel buried in the superficial layer (0.25 mm) or the deep layer (0.5 mm) when \(f_{m}\) is 3%. A shallow buried blood vessel leads to rupture and bleeding at low fluence. When \(H\) decreases from 0.5 mm to 0.25 mm, the bleeding threshold decreases significantly from 8 J/cm\({}^{2}\) to 2 J/cm\({}^{2}\) because the deeper the buried vessel is, the thicker the dermis will be for light propagation to reach the vessel. In this regard, more photon energy is absorbed and scattered by the dermis, so the energy required for vessel rupture consequently increases. At the same burial depth, the rupture thresholds and \(E_{\mathit{eff}}\) of the vessels with different stdGC vary. For the vessel depth of \(H=0.25\) mm and \(H=0.5\) mm, the rupture fluence of the vessels with a large stdGC (64.5) is lower than that with a medium or small stdGC, whereas \(E_{\mathit{eff}}\) is 1-2 J/cm\({}^{2}\) higher. Figure 19: Purpura rate of blood vessels with melanin content \(f_{m}=3\%\). (**a**) Vessel burial depth \(H=0.25\) mm (**b**) Vessel burial depth \(H=0.5\) mm  
Figure 18: \(E_{\mathit{eff}}\) of laser at different melanin contents with the vessel buried at a depth of 0.375 mm (marked by solid rectangles). As the laser fluence increases, the vessels with a large stdGC initially rupture, subsequently experience pur pur pur pura, and finally reach \(E_{\mathit{eff}}\). The occurrence of pur pur pur pura does not mean that the vessel has been properly treated. The optimal treatment parameters can only be determined by the appropriate \(S_{\mathit{pru}}\). Simply taking the occurrence of pur pur purura as the TEP leads to an inadequate treatment and a high recurrence rate of vessels with a large stdGC. This observation reveals one of the reasons why clinically inexperienced physicians fail to treat many patients with PWS. Therefore, in the treatment of vessels with a large stdGC, the therapeutic effect should be determined on the basis of appropriate \(S_{\mathit{pru}}\) (higher than \(S_{\mathit{pru}}\) of marked effect and lower than \(S_{\mathit{pru}}\) of side effects), and the required laser fluence should be higher than that of vessels with a small stdGC. Figures 21 and 22 depict \(E_{\mathit{eff}}\) of laser at different melanin contents and vessel depths of 0.25 and 0.5 mm. \(E_{\mathit{eff}}\) differs by 1-2 J/cm\({}^{2}\) with different values of stdGC. For white people (\(f_{m}\) = 1%), superficial vessels (\(H\) = 0.25 mm) can be treated reasonably without CSC, whereas deep vessels (\(H\) = 0.5 mm) need CSC. As shown in Figure 20, when the melanin content is equal to 7%, \(E_{e,R134a}\) in the superficial vessel (\(H\) = 0.25 mm) is lower than \(E_{\mathit{eff}}\). Therefore, the cooling effect of the epidermis should be improved. For median (\(H\) = 0.375 mm) and deep (\(H\) = 0.5 mm) vessels, insufficient epidermal cooling occurs when the melanin contents are 5% and 3%, respectively. Therefore, for darkly pigmented skin, R134a cannot meet the requirement for treating median and deep blood vessels. [PERSON] et al. (2019) found that an R134a spray can be substituted with low-boiling-point cryogen R404A (with a standard boiling point of \(-\)46.1 \({}^{\circ}\)C) to improve the surface cooling effect. Therefore, R404A can be used to cool the epidermis in the treatment of PWS lesions in darkly pigmented people and protect the epidermis from damage while removing vascular lesions. The depth of blood vessels influences the selection range of \(E_{\mathit{eff}}\). For a large-stdGC vessel (64.5), when the vessel depth is 0.25 or 0.375 mm, \(E_{\mathit{eff}}\) is 2 J/cm\({}^{2}\) (shown as the height of the red rectangle in Figures 19 and 21), whereas it reduces mostly to 1 J/cm\({}^{2}\) when the vessel depth is 0.5 mm (shown as the height of the red rectangle in Figure 21).