Chapter 54 Lasers in dermatology
2. What does “stimulated emission of radiation” mean?
Stimulated emission is a complicated phenomenon of physics, first described by Albert Einstein. Atoms must be in an excited state (an electron is in an elevated orbit). Normally, adding a photon with energy equal to the energy between orbits would raise this electron to a higher orbit. Instead, in special circumstances found in laser systems, two photons are released from the atom, and the electron returns to its lower, resting state. These two photons are then able to enter two other atoms with excited electrons, allowing rapid multiplication of photons in a process similar to a chain reaction. This process accounts for the stimulated emission of radiation used in the acronym laser.
3. How is the light amplified in the laser system?
The laser system uses an optical resonator to amplify and orient the light. This is a cylindrical chamber filled with the laser medium. There are mirrors on each end and an absorptive lining. The photons of light are reflected between the mirrors. The lining will absorb any light that is not perfectly parallel. These parallel photons continue to enter additional atoms, producing more photons by stimulated emission. By this process, the laser light is amplified. One end of the optical resonator has a mechanism to release the light periodically from the chamber.
4. What types of medium are used in laser systems?
The ability of an atom to be used in a laser system is a complicated function of quantum mechanics and the physical characteristics of the lattice in which it is constructed. The basic requirement of any medium is to be able to support a population inversion so that there may be stimulated emission. Some types of lasers include the following:
• Solids: Ruby crystal: composed of aluminum oxide (Al3O2) with scattered atoms of chromium (Cr) replacing some aluminum atoms in the crystal lattice
5. What are the special features of laser light?
• Coherence: Coherence is the property that represents a uniform wave front, that is, the peaks and valleys of the waves are aligned as the light exits the laser, which allows the light to be in phase and focused to very small areas. This also allows the energies to be additive.
• Monochromaticity: This means all light waves have the same wavelength. Some lasers produce more than one wavelength of light, but these are predictable and the laser still produces only those specific wavelengths of light expected by the laser medium used.
6. Why is monochromatic light useful?
Many lasers target specific chromophores, which are biologic structures with a specific absorption spectrum. Two common chromophores are hemoglobin within the red blood cell and melanin within melanosomes. The absorption spectrum is the amount of light absorbed at various wavelengths. The idea is to match a peak absorption wavelength with the wavelength of the laser.
7. What is selective photothermolysis?
The theory of selective photothermolysis assumes the laser light will pass through tissue until it targets a specific chromophore with an absorption spectrum corresponding to the wavelength of the laser. The target then absorbs the light, generating heat in the target tissue. The spread of heat is determined by the thermal relaxation time (TRT) of the tissue. This is the amount of time necessary for 50% of the peak heat to diffuse out of the target. It is important for the laser pulse duration not to exceed the thermal relaxation time of the target or the heat will diffuse into surrounding tissue, causing damage and possibly scarring. Thus, the TRT depends on the actual size of the target. The smaller the target, the shorter the TRT, and thus the need for shorter laser pulse durations.
8. What is an ablative laser?
An ablative laser vaporizes tissue. Different lasers cause different types of ablative reactions. Er:YAG lasers, due to their very high water absorption, cause almost pure ablation with very little collateral thermal heating. CO2 lasers have less water absorption and therefore deliver less pure ablation and have more collateral heating. This heat causes thermal damage in a positive and negative manner. An example of a positive effect is collagen contraction and new collagen stimulation. Other positive effects of CO2 lasers are sealing of blood vessels leading to less bleeding and sealing of nerve endings leading to less pain. An example of a negative effect is hypopigmentation due to inadvertant melanocyte damage. Scarring from laser procedures is usually due to excessive collateral heating.
9. What is a nonablative laser?
Most lasers are capable of delivering laser energy in a nonablative or nonvaporizing manner. Pulsed dye lasers, long-pulsed alexandrite, and Nd:YAG and many diode lasers deliver energy so as not to ablate tissue. The only lasers that truly ablate are those listed above and are used to remove tissue in a physical manner. The entire concept of selective photothermolysis is the ability to deliver energy in a nonablative manner.
11. What is a fractional laser?
Fractional lasers deliver short pulses of laser that are separated by space on the surface of the skin. These lasers may either by nonablative, leaving a column of heat-damaged skin, or ablative, leaving a column of true ablation. This will be discussed in detail in a later question. The concept of fractional lasers was first proposed by Dr. Rox Anderson.
13. What lasers have historic interest but are seldom used?
The argon laser (488 nm, 514 nm), was one of the first dermatologic lasers. This is seldom used because of scarring issues. The krypton laser generates dual wavelengths: 520-nm (green) and 568-nm (yellow) light. The continuous-wave thermal nature of the laser caused too much surface heating. Copper vapor lasers generate dual wavelengths of 511 nm (green light) and 578 nm (yellow light). These lasers are not currently used. There was a pulsed dye pigment lesion laser with a wavelength of 504 nm. This laser effectively treated surface pigmentation but was mechanically unsound and is no longer available (Table 54-2).
14. What are the basic features of the carbon dioxide (CO2) laser?
The CO2 laser emits radiation at 10,600 nm, in the far-infrared region. All water in tissue absorbs this wavelength of light, and this absorption is not dependent on selective absorption by any biologic tissue. As the water absorbs energy, the temperature rapidly rises, vaporizing the tissue. The amount of tissue damage is related to the energy setting and the amount of time the laser impacts on the target. There is some true ablation of tissue and this is surrounded by a zone of thermal damage. This area of thermal damage is used in resurfacing by causing immediate collagen contraction and later collagen remodeling.
Table 54-2. Lasers of Historic Interest Only
| LASER MEDIUM | WAVELENGTH | PROBLEMS |
|---|---|---|
| Argon | 488 nm, 512 nm | Scarring |
| Krypton | 520 nm, 568 nm | Scarring |
| Copper vapor | 511 nm, 578 nm | Ineffective |
| Pulsed pigment | 504 nm | Mechanical nightmare |
The new superpulsed and ultrapulsed CO2 lasers have powers up to 60 W and pulse duration in the range of 250 μsec to 1 msec. There are now many fractional CO2 lasers designed to reduce the side effects of CO2 laser ablation (Table 54-3).
15. What are some uses for the standard carbon dioxide laser?
Treatment of warts has been the hallmark of CO2 laser therapy. Large plantar and periungual warts are effectively treated. The main advantage of CO2 laser treatment in these larger warts is the ability to decrease the bleeding during the laser excision and a slight decrease in scarring. Any benign lesion may be removed using CO2 laser but this is not a common method used in dermatology. CO2 lasers in a high-power focused mode are very effective at surgical cutting, especially in patients on anticoagulants or those with other bleeding disorders.
Table 54-3. Carbon Dioxide Lasers
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