Laser medicine is one of the rapidly developing medical fields today, and various laser treatment methods for hemangioma have emerged, which have become a common method for treating some children’s skin hemangiomas due to their remarkable efficacy and low side effects. 1, the characteristics of the laser Laser is very different from ordinary light, it is a beam of intense light produced by the excitation of material radiation. The brightness of the laser is high, the energy density is high, and it is the brightest source in the world today. The brightness of a strong laser can reach trillions of times the brightness of sunlight. When this light energy into heat, can instantly produce thousands of degrees to tens of thousands of degrees of high temperature. The laser is a beam of near-parallel light, it is shot in one direction, the divergence angle is very small, the range is far, by the lens focus can form a very small point of light. The high brightness, monochromaticity, directionality and coherence of laser light make laser energy highly concentrated in time, space and spectrum. These characteristics of the laser enable the laser beam to focus very high energy into a very small spot, which is ideal for precision cutting and vaporization surgery. 2, the effects of laser 2.1 thermal effects strong laser irradiation of biological tissue, instantaneous high temperature of several hundred degrees or even thousands of degrees, its thermal effects can make the biological tissue protein denaturation coagulation, and even tissue charring or vaporization. 2.2 mechanical effects generated by laser irradiation can be divided into two parts, that is, the radiation pressure of the laser itself on the pressure of biological tissue, that is, optical pressure, called primary pressure, because of the high energy density of the laser, thus generating a large radiation pressure, the pressure generated by the laser can reach 104-105 Pascal; biological tissue absorption of strong laser caused by thermal expansion and phase changes, as well as ultrasound, shock waves, electrostriction, etc. The pressure caused by the thermal expansion and phase change caused by the absorption of the laser, as well as the ultrasound, shock wave, electrostriction, etc., is called secondary pressure. In clinical practice, the use of laser-induced pressure can be used to treat a variety of diseases such as pressure perforation in ophthalmology. 2.3 light effect due to the absorption of biological tissue on the irradiation of laser reflection and heat transfer, color tissue (especially black) on the laser has a selective absorption effect, and therefore cause more destructive effect, the use of this effect, the need to destroy the tissue on the first tissue staining, and then laser tissue irradiation, you can get better results. 2. 4 electromagnetic field effect laser is an electromagnetic wave, so the laser produces electric field, magnetic 5 × 104W / cm2, the electric field strength can reach 4 × 1010V / m, in this strong electric field, biological tissue will produce ionization, so that the tissue cells are destroyed. 2.5 Photochemical effect is the chemical reaction of the molecules of a substance after absorbing the energy of foreign photons. As a highly concentrated and monochromatic light source, laser light can also cause some photochemical effects that cannot be caused by ordinary light. Photochemical reactions can be broadly divided into four main types: photodegradation, photoxidation, photopolymerization and photosensitization [ 2 ]. 2.6 Biostimulation effect When low power laser irradiates biological tissues, it does not cause irreversible damage to biological tissues directly, but produces some kind of effect similar to the biostimulation obtained by mechanical and thermal physical factors such as ultrasound, acupuncture, moxibustion, etc. It is called laser biostimulation effect. The properties of laser light and the specific patterns of its interaction with biological tissues can be used to study, diagnose and treat diseases. For example, since the 1970s, doctors have used laser interferometry, laser transillumination and laser polarization techniques to measure the composition and trace element content of blood, urine and other tissues of the body, as well as to identify and distinguish whether cells are diseased or cancerous; to coagulate, vaporize and cut lesions with strong laser beams, with little or no bleeding compared to traditional scalpels; to irradiate human tissues with weak laser beams to achieve physical and mental health effects. Laser irradiation of human tissue, can achieve the role of physical therapy irradiation treatment or light acupuncture treatment, and traditional physical therapy than the light therapy, the efficacy of the laser significantly improved, and a wider range of indications. 3, the basis of the application of laser in dermatology Anderson and other proposed selective photothermolysis theory is the theoretical basis for the development of many aspects of laser cosmetic medicine, laser energy will be selectively absorbed by certain specific tissue components, and through the thermal effect of this type of specific tissue components to be destroyed, while causing only minimal damage to the surrounding tissue. 3.1 Laser wavelength The laser wavelength should be able to act on the target tissue and be absorbed sufficiently effectively by the target tissue. For the same wavelength of laser, the absorption capacity of different skin tissues is different. When the laser is only absorbed by the target tissue, there will be no damage to the skin tissue around the target tissue. Under a certain wavelength, the penetration depth of the laser in the tissue is proportional to the wavelength of the laser, so the deeper the lesion, especially deep dermal lesions, the longer the laser wavelength should be chosen. 3.2 Laser pulse width The laser pulse width should be less than or equal to the thermal relaxation time of the target tissue. The laser light energy absorbed by the target tissue is converted into heat energy and will immediately start to diffuse. Normally, the laser irradiation time should be shorter than or equal to the thermal relaxation time (TRT) of the target tissue, which is the time for the temperature of the target tissue to drop from the maximum to half and it is replaced with the thermal damage time (TDT) of the target tissue.Woo et al [4] introduced the concept of vascular TDT, the time when laser energy is absorbed by oxygenated hemoglobin and delivered to the critical region of the vessel wall. This time is much longer than TRT, which is the basis for clinical treatment of vascular lesions with long pulse width or ultra-long pulse width laser. 3.3 Energy density The energy density should be able to bring the target tissue to a temperature sufficient to cause it to be damaged. A sufficient energy density is necessary to cause damage to the target tissue. This requires a relatively high laser energy density when the laser is absorbed by only a small amount of the target tissue, or when the target tissue contains very little pigment, or when the target tissue is located in the deeper layers of the skin. The choice of energy density is based primarily on vessel color and, to a lesser extent, on factors such as vessel size, depth and spot size. 3.4 Spot size The spot size not only affects the speed and efficiency of the treatment, but also the laser efficacy. It is generally believed that larger spots penetrate deeper than smaller spots. Smaller spots are effective for small superficial blood vessels, while larger spots can target deeper and thicker large blood vessels. 3.5 Epidermal Protection In addition to the pigment in the skin lesion, melanin in the epidermis also strongly absorbs laser light energy. Thus, when lasers with wavelengths in the strong absorption spectral range of melanin are used to treat dark skin, the risk of damage to the epidermis is greater. To reduce this risk, it is now common for laser medical systems to use skin cooling. Commonly used techniques for epidermal protection today include: application of cold gel, external application of ice packs, dynamic cooling technique with injected coolant (DCD), sapphire contact cooling, and cold air cooling with varying cooling effects. The understanding of the parameters and their handling is critical to the efficacy of vascular skin lesions, which currently rely on the operator’s judgment of vessel size, depth, color, and pressure, which is highly subjective. Future. Photoacoustic probe and some other measurement tools will help to strengthen the selection of parameters objectivity. 4.Dermal hemangioma Hemangioma is the most common benign skin tumor in infants and children. They usually occur at birth or shortly after birth and grow rapidly, with a proliferation period lasting 6 to 12 months, increasing to the maximum around 1 year of age. Generally, the growth is the fastest within 1 year old, and then grows slowly and stops at 5-10 years old. Usually, hemangiomas are divided into nevus, strawberry hemangioma, cavernous hemangioma and mixed hemangioma, and can also be divided into superficial hemangioma, deep hemangioma and mixed hemangioma according to the depth of involvement of the tumor. 60% of hemangiomas occur in the head and neck, which not only affect the aesthetic appearance, but also can have a variety of complications, such as ulceration, bleeding and infection. Hemangiomas in special areas (such as eyelids and trachea) can compress surrounding organs and even endanger life. Most strawberry hemangiomas fade slowly on their own, with the speed and extent of fading varying from person to person, with about 30% fading by age 3, 50% fading by age 5, and 70% fading by age 7. Hemangiomas of the eyelids, parotid glands, and tip of the nose may persist or only partially regress. Even if the hemangioma resolves on its own, 40% of them still have complications such as scarring, atrophy, pigment changes, and capillary dilatation after regression. Therefore, early treatment is necessary to control the growth and promote the regression of proliferating hemangiomas. There are many traditional treatment methods for hemangioma, including oral glucocorticoid, local injection of pinyamycin, hormone, freezing, arterial embolization, surgical resection, etc. However, the widely application of these methods is limited by the existence of different degrees of adverse effects. In the past 20 years, with the rapid development of laser medicine, there are various lasers that can be used for the treatment of hemangioma, and due to its remarkable efficacy and small adverse effects, laser has become the first choice for the treatment of cutaneous hemangioma. At present, the lasers used for the treatment of cutaneous hemangioma are 532nm frequency Nd:Y AG (neodymium-doped yttrium aluminum garnet) laser, pulsed dye laser, 1 064 nm Nd:YAG laser, photodynamic therapy and intense pulsed light system, etc. The target chromophore of laser treatment for hemangioma is oxyhemoglobin in blood. Oxyhemoglobin absorbs light energy and generates heat, which is transmitted to the surrounding blood vessel wall and causes damage to blood vessels. Oxyhemoglobin has three absorption peaks: 418 n m, 542 n m and 577 n m. Among them, 418 n m is the largest absorption peak, but the laser at this wavelength has poor penetrating power and is difficult to reach the dermal blood vessels, while melanin particles in the epidermis have strong absorption to it, which easily causes damage to the epidermis. 542 n m and 577 n m). 6.Diagnosis and classification of hemangioma and vascular malformation Reasonable classification and correct diagnosis are the basis for obtaining good therapeutic effect. In our country, Virehow classification is basically followed, and hemangioma is divided into capillary hemangioma, cavernous hemangioma, trabecular hemangioma and mixed hemangioma. in 1988, Muliken et al. proposed a biological classification method, based on medical history, clinical manifestations and biological characteristics of vascular endothelial cells, hemangioma in the traditional sense is divided into two categories: hemangioma and vascular malformation. in 1995 Warier and Suen [14] combined clinical practice and a large number of I clinicopathological studies to refine the Mullike and lowack classification and classified vascular malformations into: venous, small venous, capillary, lymphatic, arteriovenous and mixed malformations, classified PWS in Muilike’s classification into postcapillary microvenous malformations, and included arteriovenous malformations into capillary malformations types. In addition, hemangiomas located in some specific anatomical sites can compress the surrounding organs and even cause death, so early intervention is also needed. Therefore, the principle of treatment timing should be: early detection and early treatment. That is, hemangioma should be treated as soon as it is discovered. 7. Application of laser in children with cutaneous vascular crippling 7.1 Pulsed dye laser (PDL) Zuo Yagang et al. used 585 n m PDL for treatment of erythema nodosum, and the total effective rate was 84% after eight treatments [18]. Recently, 595 hill and 600 n m PDLs with a spot of 10-12 mm and a pulse width of 1.5-40 ms have emerged for the treatment of deep dermal hemangiomas and facial capillary dilation. mariwalla et al [20] applied a wavelength of 595 n m, a pulse width of 1.5 ms, an energy of 11-12 J/cm2, and a dynamic cooling device, to treat children under the age of l with erythema nodosum, and demonstrated that the 595 n m PDL had a higher cure rate than the previously used 585 n m PDL. Yang Chunjun et al. used the 595 n m adjustable pulse width dye laser to treat 76 cases of cutaneous hemangioma, among which the efficacy of the treatment was good in the case of erythema nodosum and strawberry hemangioma, with the efficiency rate reaching 83.2% and 86.67%, respectively. 7.2 KTP laser The KTP laser is a dual-frequency yttrium aluminum cudgel laser, which can emit green light with a wavelength of 532 n m. The 532 n m is very close to the peak absorption of hemoglobin, so it is very suitable for the treatment of superficial hemangiomas. It has a large variable pulse width, ranging from 1 to 100 ms, which can meet the need for long pulse widths and can slowly heat blood vessels without rupture of the vessel wall. In a recent comparative study, one group used the KTP laser with a 10 mm spot size and another group used the PDL to treat facial capillary dilation and diffuse erythema of the face. Spendel et al. treated spider nevi with a 532 n m multiplier Nd:Y AG laser at less than 0.7 mm with good results and less than 16 J/cm2 energy, with few adverse effects. The most common adverse reactions are erythema, edema and crusting. Compared with other long wavelength vascular lasers, the 532 n m Nd:Y AG laser has poor penetrating power and is less effective for deeper vascular areas, and is more suitable for superficial vascular damage, such as facial capillary dilation and spider nevus. In addition, the KTP laser often causes edema and crusting. In particular, the use of a small spot for the treatment of nasal capillary dilation can cause atrophic scars, which has been well documented. Furthermore, patients with dark skin types (whether racial or sun-induced) treated with the KTP laser are at risk of burns because the melanin in the skin is also a target of treatment. Like the IPL laser device, the KTP laser can be used to treat small skin moles for skin types I-III. 7.3 Infrared radiation Lasers infrared or wavelengths close to infrared have been used to treat hemangiomas, including the alexandrite laser (755 n m), diode (800-940 n m), and Nd:YAG laser (1064 n m). These wavelengths have been successfully used to treat reticulocytosis and mature wine nevi, which are challenging to treat because of the presence of hemoglobin-reduced hemoglobin in the deep macrovasculature. Kono et al. demonstrated that the alexandrite laser at 755 n m can significantly improve hypertrophic nevi. Yangetal et al. found that the Nd:YAG laser (1,064 nm) was effective in removing nevi because of the large penetration depth of the Nd:YAG laser (1,064 nm), so great care should be taken when treating nodular nevi. The Nd: Y AG laser (1 064 nm) has a large penetration depth, so care should be taken when treating nodular nevi to reduce the risk of depressed scars. The advantages of the Nd:YAG laser (1 064 nm) are the deeper penetration depth and the lower absorption coefficient of melanin. Because of the lower absorption coefficient of melanin, there is less damage to the epidermal appendages and the treatment is safer for patients with black skin. It also minimizes the chance of post-inflammatory hyperpigmentation. Additional protection is provided by the addition of an epidermal cooling device, which reduces the damage caused by melanin absorption. There are several ways to accomplish this cooling, one powerful device is to cool the skin by spraying refrigerant in the milliseconds before the laser, another device is to cool the epidermis by exposing the skin to cold sapphire glass or metal plates before the laser, while the cooling gas is blown out to protect the skin. The ideal epidermal cooling protection protects the epidermis without overcooling it. Chia et al. used the Nd:YAG laser to treat hemangiomas with good results and found that the method was less invasive and generally did not result in scarring. The Nd:Y AG (1,064 nm) laser is safe and effective in the treatment of nevus of wine, but it is best not to use high energy in the treatment of hemangioma in order to avoid major damage to the body. Angiero et al. used diode (800-940 nm) laser combined with photocoagulation technique to treat 136 patients with hemangioma, of which 134 patients were in complete remission and 2 patients were not effective. 7.4 Intense pulsed light (IPL) IPL can improve wrinkles and all photoaging phenomena including texture, irregular pigmentation and capillary dilation [34]. Treatment with shorter wavelengths (570 am filter) and smaller pulse widths is sufficient for smaller and superficial vascular lesions, while longer wavelengths (590 n m filter) and larger pulse widths are required for larger and deeper vascular lesions. 7.5 755 n m laser The mechanism of 755 n m laser treatment of vascular lesions is that the laser is selectively absorbed by oxygenated hemoglobin in the blood, generating heat and thus coagulating or destroying the blood vessels. 7.6 Photodynamic therapy (PDT) Gu Ying et al. have reported the basic research and clinical study of PDT for selective treatment of vivid nevus, and the results showed that PDT has a highly selective destructive effect on the superficial capillary network of the dermis, with high clinical efficacy and low side effects. 7.7 Multi-wavelength laser Since most of the vascular skin diseases are on the face, which can cause serious psychological trauma to the patients, the patients have very high requirements for the treatment, not only for the elimination of lesions, but also for the cosmetic effect (no scarring), i.e. the treatment aims at achieving normal skin color and texture. The Cynergy Vascular Workstation is equipped with advanced Multi-plex (multi-wavelength sequential emission) technology, which allows two different wavelengths of laser light (a high intensity pulsed dye laser and a Nd:YAG laser with a wavelength of 1,064 n m) to be emitted in a specified sequence under the same system. The 595 n m dye laser and the 1 064 nm long pulsed laser have made great progress in the treatment of vascular diseases, respectively, but it is still found that it is difficult to achieve the desired efficacy due to scar production or insufficient penetration depth due to excessive laser energy or too short laser wavelength. However, the pulsed dye laser in dual wavelength laser transforms oxyhemoglobin into methemoglobin, which increases the absorption of Nd:YAG laser by 3~5 times in target tissues, so that the 1,064 n m laser can achieve efficacy at low energy density, thus reducing the occurrence of side effects, increasing safety, reducing scarring, and improving efficacy. The combination of PDL and Nd: YAG laser treatment by multi-wavelength delivery technology can achieve better results compared to single-wavelength treatment. In contrast, PDL, long-pulse 1,064 n m Nd: YAG laser and KTP tunable pulse width 532 n m wavelength treatments usually require 2 to 3 treatments to achieve similar efficacy and low side effect rates. 7.8 Confirmation of nanoparticle location before laser irradiation Researchers at Wakefield University in the United States have made new advances in the use of lasers to treat tumors. They used MRI to successfully identify the location of carbon nanotubes after they entered tumor tissue, and irradiated and heated them with a laser, thereby destroying the tumor tissue. The use of laser to heat and destroy tumor tissue is not a new technique, and laser-induced thermal therapy (LITT) is already in use. There is a problem with LITT, however, because it is difficult to know exactly whether the nanoparticles have entered the tumor tissue because they cannot track their trajectory after they have been injected into the patient’s body and are able to absorb laser energy. If the nanoparticles enter normal tissues and are heated by the laser, they may cause unnecessary harm. To avoid the drawbacks of laser-induced thermal therapy, researchers at the University of Wakefield used iron-containing multi-walled carbon nanotubes (abbreviated as MWCNTs) instead of the commonly used nanoparticles and tracked the iron-containing multi-walled carbon nanotubes using MRI. In experiments completed on biological tissues with tumors from experimental rats, researchers identified the location of iron-containing multi-walled carbon nanotubes with the help of MRI. Although there are many treatment methods, most of them are invasive and non-specific, and none of them are aimed at the pathological mechanism of hemangioma, and they cannot inhibit the cell proliferation of the proliferating lesions after treatment. Early laser intervention can effectively control vascular proliferation, promote regression, shorten the course of treatment, have little or no side effects, and achieve medical aesthetic results. However, there are still problems such as more frequent treatments, pain during treatment, postoperative pigment changes and a few scar formation, which need to be further solved. With the continuous maturation and improvement of laser technology, new lasers have been introduced, such as excimer lasers, free electron lasers, and lasers in the CO2, EL:YAG, HF and X-ray wavelengths, with the aim of improving the efficacy, broadening the indications for laser treatment and reducing adverse effects. In the future, lasers used for medical treatment will develop in the direction of high power, miniaturization and intelligence. With the expansion of the wavelength range of semiconductor lasers and the increase in power, it will gradually replace gas and solid-state lasers and gain wide application in medicine. The combined use of multi-wavelength laser can produce medical effects better than a single wavelength. With the development of computer technology, the medical laser and electronic computers, optical fiber, image analysis, video, fluorescence spectroscopy and ultrasound technology and other new technologies and their new advances in the combination of results, so that the laser diagnosis and treatment level continues to improve. Laser in the medical field is increasingly widely used, showing its powerful life.