Effect thermal of the lasers

The thermal effect of lasers, a review Andrea Ortiz * Juliana Perez * Adrian Rios   Third year dermatology residents Unisanitas Lasers used on the skin act by generating heat. Once the laser contacts the skin, the tissue molecules increase their speed, absorbing and dispersing its energy. The light will maintain optical phenomena in skin with little pigment such as reflection and diffraction. The set of phenomena that allow a laser to diffuse into the skin will depend on the type of light, its power, exposure time and we will call it selective photothermolysis. Since its clinical use, the laser has been indicated to better select tissues and the possibility of having   much shorter exposure times. Obtaining tropism towards hemoglobin, melanin and water in addition to being able to choose stable amounts of energy. However, after more than 60 years it has not been easy to understand and take advantage of all these advantages. The heat generated as a result of the combustion of tissues is produced by an exothermic chemical reaction, secondary to the rapid interaction of a fuel, our Carbon, excited by an oxidizer, our O oxygen, until reaching an ignition temperature (temperature to which the tissue must be brought for combustion to start and spread)Illustration 1 Combustion triangle http://el-trabajo-del-bombero.blogspot.com/2012/05/fuego-y-combustion.html?m=1  Illustration 2 Effect of fuel and oxidizer https://ocw.unican.es/pluginfile.php/1179/course/section/1440/T%2008%20OCW.pdf Water, the largest constituent of tissues, is a non-combustible compound, it absorbs heat by transforming into water vapor. Water vapor prevents the combustible element from coming into contact with more oxygen and carbon atoms in our tissues. Thermal phenomena in tissues: Various thermal effects are obtained according to the increase in temperature in the tissues:–              Vaporization: It is the change of an element from its liquid to gaseous state, secondary to an increase in temperature when it reaches close to 100ºC. It is used for tissue destruction of the ablation type or separation of tissue layers. –               Carbonization: It is evident when the tissue turns black due to the combustion of carbon, the necessary temperature reaches 350ºC. It is not an effect with therapeutic intent at the base of the lesions, where healthy tissue must be preserved. – Coagulation:It is visualized by a change in tissue color. It occurs when the temperature reaches 50ºC. Proteins are irreversibly denatured and cell necrosis occurs with very little damage to neighboring structures. Below 50 degrees Celsius, the coagulation process can be reversible. – Hyperthermia without coagulation (<50 degrees): This is a non-visual effect. It is perceived by means of a thermometer or by touch. It is reversible with cooling.  

Thermal tissue effect of lasers 

Depending on the type of laser, greater or lesser combustion effects will occur in the tissues. This depends on the laser’s affinity for atoms in our molecules that are similar to laser light or reflective.

Wavelengths (types of laser light):

The skin is heterogeneous and contains several elements that capture light: water for infrared lasers and hemoglobin and melanin pigments for the color of the laser. The purpose of the interaction between the laser and a tissue.

Visible spectrum laser (400-700 nanometers)

Lasers in the visible light spectrum (400-700 nanometers, nm) emit colored light and as such are absorbed by pigments such as melanin and hemoglobin. They do not interact with water. They exert their thermal effect on the tissues by being trapped by any dark pigment.

Lasers in the visible spectrum

1. 532 nm: The light generated is green. It is absorbed by melanin, hemoglobin and inks of a color other than green.
2. Dyes 585,590,595 nm: The light generated is yellow, it is absorbed by hemoglobin, melanin and inks of colors other than yellow.
3. Ruby 694 nm : The light generated is red and is absorbed by melanin and very little by hemoglobin since it has a similar color.

Figure 3 Visible and near-infrared lasers  https://www.sciencedirect.com/science/article/pii/B9780815515722500099  

Near and mid-infrared spectrum lasers:  

This group of lasers includes the near and mid-infrared spectrum (700-4000 nm). In this range we include two types of lasers, those that do not react with water due to their proximity to the visible spectrum and those with wavelengths above 1400 nm, which will show more and more interaction with water at longer wavelengths. In the infrared spectrum closest to visible light (700-1064 nm), called “near”, water, transparent to this type of wave, acts as a “dissipator” of the energy absorbed by the pigments, which makes it possible to preserve the tissue. All lasers in this range are applied to the skin with a gel to help protect the epidermis and papillary dermis. From 1200 nm onwards, the absorption of light by water is already greater.  Figure 4 Near and mid-infrared laser absorption curve in light skin https://studylib.es/doc/8275663/tissue-interactions-of-lasers

Near and mid-infrared spectrum lasers:

1. Alexandrite (755 nm) : It is absorbed by melanin and a little less by hemoglobin given its proximity to the red color. It is not absorbed by water.
2. Diodes 810-1064: They are absorbed by melanin and hemoglobin. They are not absorbed by water, very bright pigment or very saturated (intense) color.
3. Nd:YAG (Neodymium: Yttrium Aluminium Garnet 1064,1320 nm): They are absorbed by melanin and hemoglobin. They are not absorbed by water, very bright pigment or very intense color.
4. 1450-1540 nm: Absorption by hemoglobin over water is predominant depending on the amount of melanin present. There are fractional and surgical equipment from the manufacturers Candela (Ellipse) and Quanta (Youlaser)
5. 1540/1927 nm FRAXEL Fiber Optic Laser: This is a fractional laser from Solta. Fraxel 1550 is absorbed by water, hemoglobin and to a lesser extent by pigment in light skin. Fraxel 1927 (Tm) has lower skin penetration than 1550.
6. Er, Cr YSGG laser (Erbium, chromium, Ytrium, scandium, gallium, garnet) 2740 nm: Its dermatological version is the Pearl ® equipment with a wavelength of 2740 which is mainly absorbed by water, giving characteristics of less hemostasis than the CO ₂ laser and with pulse duration and frequency modulations, it manages to be a little more hemostatic than the Erbium-YAG (2940 nm). It is most commonly used for dental surgery, performing a “hydrokinetic” effect that is achieved by shooting water at the same time as the laser that absorbs the energy and thus achieves the cutting of hard and soft tissues without creating bone fissures or carbonization of the tissue (Biolase ®) 
7. Erbium:YAG laser (2940 nm): It is strongly absorbed by water with low carbonization and coagulation, a cooler evaporation predominates than that achieved with the CO ₂ laser .

 Figure 5 Visible and near- and mid-infrared lasers, the pigment curve is higher in dark skin http://idnps.com/clinical/latest-trends-in-burn-wound-dressing/1-3-laser-scar-revision-recent-trend-i/

Far infrared spectrum lasers: > 4000 nm

 CO2 laser 9400-10600:    It is absorbed by water or any surface in general given its size. Its predominant effects on tissue are evaporation, coagulation and carbonization. Coagulation can be achieved by time and power modulations. (Youlaser)  Figure 6 The place of the carbon dioxide laser https://link.springer.com/article/10.1007%2Fs41547-018-0047-y

Power and time.

The duration and amount of energy used determine the extent of thermal damage to tissue. For all lasers with wavelengths greater than 400 nm, the conversion of light into heat is the primary means by which tissue is destroyed. Time and power are always related to generate a vaporization, carbonization and/or coagulation effect. The accumulation of denatured material increases exponentially with temperature and proportionally with time. This explains why the ideal way to avoid carbonization is to first control the exposure time. It has been seen that the area affected by the CO2 laser _can reach up to 145ºC, but at its periphery, 500 microns from the treated area, the temperature only reaches around 45ºC. In continuous mode, i.e. without pulses, the surface temperature can reach between 120º-200ºC very quickly. Thermal coagulation will occur up to 1 mm deep secondary to tissue heat transfer, which is evident when the dermal bed changes color to a yellowish hue. This is why some robotic systems have replaced manual exposure, since they avoid overexposure and heterogeneous contact times. If the peak power density of the laser radiation is high enough, a localized microplasma is formed. The plasma eagerly absorbs the incoming laser, which expands explosively, creating shock waves that can break soft tissue and shatter hard materials such as bone and uroliths. The existence of a plasma is revealed by the bluish-white light it emits and by its characteristic dull clicking sound. The plasma shield also protects more distal structures from the laser beam because its high absorption contains scattering. This is impossible to achieve with a preformed plasma system or radiofrequency sources. Water has a high absorption coefficient at the emission wavelength of a CO _{2 laser and a TRT (thermal relaxation time) of about 326 microseconds μs. With these properties, if a CO 2 laser beam is applied to the skin, the water will absorb} the   water.When the laser hits the skin for less than 326 μs, most of the radiation is absorbed by water with almost no thermal diffusion. However, if the duration of incidence of the laser on the tissue is longer than 326 μs, heat is transmitted to the tissue and thermal injury occurs in the adjacent tissue. At 45° C, cultured human fibroblasts die after 20 minutes. However, they can withstand more than 100° C if the exposure time is only 1 millisecond, so it is not the temperature per se, but a combination of temperature and time that regulates the coagulative thermal damage. Thermal tissue coagulation has a well-defined character from a threshold. As the critical temperature is reached, coagulation occurs. This accounts for the histological limits of dermal coagulation in the laser. In contrast to the epidermis, connective tissue such as the dermis contains a large amount of extracellular matrix dominated by structural proteins such as collagen and elastin. Elastin is incredibly heat stable, able to survive boiling for hours without apparent change. However, type I collagen, which is the major component in the dermis, has a sharp melting transition to the ribbed form between 60-70º C. This transition is an absolute limitation to the elevation of the dermal bulk temperature above which loss of the collagen framework will occur. In contrast, in diffuse coagulative injury, there will be less risk. This is one of the reasons to explain why more predictable tissue remodeling effects are obtained with laser than with radiofrequency.

Ultrashort pulses by themselves do not cause thermal damage, but if we work at high frequencies, thermal damage is generated in the tissue.

The laser repetition rate must be high enough to allow time for the plasma to dissipate. In general, the mean plasma dissipation time can be said to be around 100 nanoseconds. Therefore, the repetition rate to avoid interactions between the pulse and the plasma should be less than one million pulses per second. However, this theoretical value does not take into account the individual thermal relaxation time (TRT) of each tissue. A more realistic value is proposed by the available equipment, with which a huge variety of pulse widths is possible. In general, the patient will not report intolerable pain at frequencies below 5 Hertz. Beyond this speed, if pain does not appear, it will be at very low powers.

Power exposure and pulse duration control systems:

Pulse Duration (Mode)

• Continuous: The laser emits as long as it takes to press the pedal.
• Single pulse: The shot is emitted for a preset time at the operator’s discretion.
• Repeated pulse: It is a sequence of shots with a duration and a predetermined interval
• Ultrapulsed (Candela®), Superpulsed, Pulser (Lumines®): High frequency and power pulse modulations

Availability of some lasers according to power, pulse ranges and applications

Handpieces, Scanner Systems and Fractional Modes:

Measurements of electromagnetic radiation have always been a bit confusing to non-physicists, particularly because several terms are often used to define the same measurement. The energy of light, measured in Joules, is directly proportional to the number of photons in the light beam. The rate at which this energy is released is defined as the laser output power, measured in Watts, where 1 Watt (W) = 1 Joule in one second. The power per unit area is the power density and is measured in W/cm ² . In general, increasing the power density increases the percentage of the surface area affected by the laser. This can be done with different powers; it may be equal in total energy density to use more powerful shots with fewer passes or to make more passes at lower power. At high power, fewer passes would be needed although the absorption effects of the laser could be very different and homogeneous results would not be achieved. The area of ​​the light spot is proportional to the square of its radius. Therefore, a threshold increase in spot diameter requires a 10-fold increase in power to achieve the same power density. A larger contact surface diameter will decrease the power density. Other commonly used terms are total energy and fluence. Total energy considers both power and time, where: Energy (J) = Power (J/cm ² ) times time (seconds) For example, if the output power is 20 W, and the emission time is 2 seconds, the output energy would be 40 Joules. Other terms are used interchangeably: Output power = Power (Watts, W) Power density = Irradiance = Intensity (W/cm ² ) Fluence = Energy density (J/cm2)

Lightning intensity profile

The beam or “spot” energy distribution over a cross-sectioned surface area is not uniform in intensity. The beam intensity profile, also called the transverse electromagnetic (TEM) mode, is determined by inherent properties of the laser cavity design. An ideal beam profile is one that has uniform intensity across the entire surface of the handpiece. The most common profile is a Gaussian energy distribution, known as the fundamental or TEM 00 mode , considered the lowest mode because it is the least complex. This type of beam is the least divergent, the easiest to focus, and therefore ideal for cutting. Because lasers have mono-uniform transverse power distributions, a simple diameter estimate cannot be used as a spot size. The arbitrary or “effective” spot size diameter is defined as the diameter that encompasses 86% of the output power. The effective spot size diameter can be estimated by measuring the diameter of a pattern burned in a 0.2 second exposure on any heat-sensitive paper or a wooden tongue depressor. (2) Scanners and/or fractional exposures offered today by optical means and robotic systems enable controllable homogeneous coverages. Current possibilities are multiple, including fractional double emission lasers.

The fabrics…

Vascular lesions…

They will respond according to their wavelength affinity in the spectrum from the visible to the near infrared. In illustration 5, it can be observed in Caucasian skin that there are two practically equal peaks in both the yellow segment and the near infrared, since in both the melanin is situated below the hemoglobin absorption curve. The final point sought is to achieve vessel spasm, before this, occasionally – in vessels of almost one millimetre in diameter – the change in colour of its blood content to an earthy colour can be observed. The presence of high flow in the vessel is a sign of inefficiency for the laser. The high flow will “wash away” the damage produced to the vessel wall by the irradiation of intravascular heat and the high thickness of the vessel will also make complete involvement impossible. On average, vessels with a diameter of less than 0.2 mm will not respond because they do not capture and vessels larger than 3 mm with palpable (thick) walls will not either. Fortunately, most visible vessels are between 0.3 and one millimetre in diameter. Depending on the skin colour, it may be possible to use dye lasers, but these will cause burns on dark skin as they are captured by melanin. The powers generally used by dye lasers are lower than those used with the Nd-YAG laser, and their pulse lengths are also shorter than the very long pulses more easily obtained with the Nd-YAG laser.    Figure 7 Wavelengths and penetration possibilities of some lasers in light skin

The pigment melanin

Melanin acts as a chromophore with a continuous absorption coefficient curve, with progressive decrease from the ultraviolet spectrum to the far infrared, which makes it a target compound for various wavelengths, with absorption capacity from 250 nm to more than 1200 nm. In pigmented lesions, the target is the intracellular melanosomes. To ensure that the photons are absorbed, it is sufficient to deliver energy in nanoseconds. These extra-short pulses generate selective damage to the melanosomes but not so much to the nucleus of the melanocyte. The immediate effect on the tissues is the rupture of melanosomes, with the formation of gas bubbles. Therefore, no changes occur in the normal pigment coding, which makes it possible to obtain a selective destruction of the pigment accumulated in the pigment-laden melanocytes of an Ota nevus deposited in the dermis without generating epidermal damage in normal melanocytes.

It is very difficult to destroy melanin with heat, in fact, it is practically impossible to hydrolyze it to analyze its components.

In contrast, café-au-lait spots will always be in competition with a genetically encoded pigmentation that is more pronounced than the adjacent skin, and what is achieved with pigment-selective lasers is often dyschromia, whereas in ephelides, the pigment component is in a shorter segment, which makes it possible to use ablative lasers to depopulate melanocytes. In white skin, light transmission increases steadily from 400 nm (50%), reaching 90% at 1200 nm. In dark skin, on the other hand, light transmission within the visible spectrum is lower (20%), but also increases to 90% at 1200 nm.

The thermal effects are summarized in that melanin acts as a “cushion” of light but at the same time it will transmit heat and electrons that will ionize surrounding tissues when receiving high energies such as those of the laser.

Laser hair removal… 

In hair removal, the most commonly used lasers are in the millisecond range. Available powers range from 10 to 120 j/cm ² . There are short-duration high-frequency pulse modalities (Primelase) and others with definitely long pulses (Soprano). Since the Soprano laser, the equipment has changed from seeking effectiveness per pulse to the accumulation of energy in a target area. This, however, has had some setbacks, especially for treating hair with little melanin and has left lasers such as Alexandrite still in use. In general, hairs with a smaller diameter respond to short pulses in milliseconds (ms) and follicles with diameters greater than 0.1 mm respond better to longer pulses (>100 ms). The real difference in the offer between the different wavelengths available is given by the hair color. The higher the frequency of the laser wave, the better the option to treat fine hair, and the longer wavelengths and pulses will have a better effect on thick follicles. Likewise, high wavelength frequencies (those with a shorter wavelength) more easily cause burns on tanned skin, while the longer wavelengths can be used on dark skin.

Tattoos…

Tattoos include metal salts, oxide, mercury, iron, copper, carbon, with variability in stability and durability according to the technique and combination of components. For tattoo removal, the most commonly used lasers are Q-switched lasers, which release high energy in a very short time (nanoseconds ns); the energy is absorbed by the tattoo pigment, thermal expansion occurs and a mechanical-acoustic effect fragments the pigment particles, which facilitates phagocytosis by macrophages. This has two visual characteristics: the first is an immediate lightening, which occurs when there is little ink, while in professional tattoos where the amount of ink is greater, the color does not change for more than a few minutes. The second effect will occur months later once the macrophages move the fractured pigment to the lymph nodes. For clinical purposes, wavelength considerations take priority over skin color, making it completely impossible to select colors on darker skin, since this will lead to depigmentation by competing with melanin. On light skin, selective effects occur that resemble antagonistic colors on the chromatic color scale.    Illustration 8 Only two almost pure color sources are available: green and red, both antagonistic. With modulations on the handpieces, frequencies of 630 (red) and 585 (yellow) can be obtained. Frequencies are not available for other colors. References  

1. https://studylib.es/doc/5783227/interacciones-l%C3%A1ser-tejido
2.  https://www.academia.edu/28709994/Los_laser_en_Cirugia_cutanea_Dermatologia_y_CIrugia_Plastica_Estetica._244_casos.Lasers_in_Cutaneous_Surgery_Dermatology_and_Plastic_Aesthtetic_surgery._244_cases_1_?email_work_card=view-paper