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The Science of Lasers
Tissue Interaction and Selective Photothermolsysis
Selective photothermolysis refers to selective absorption of laser light pulses by a target chromophore. These pulses must be short in duration to sufficiently deposit enough laser energy into the target tissues before they can cool, thus achieving extreme and localized heating. As soon as the target is heated, it begins to dissipate heat through conduction and radiative transfer. The most selective target heating is achieved when the energy is deposited at a rate faster than the rate for cooling of the target structure. Therefore, the overall heating of the target chromophore is determined by active heating and passive cooling.
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| Target Chromophore Reference Chart |
| Wavelength (nm) | Laser | Penetration into skin (um) | Chromophore |
| 193 | Excimer | 0.5 | Protein |
| 355 | Tripled neodymium | 80 | Melanin |
| 488 | Argon-ion | 200 | Melanin, blood |
| 514 | Argon-ion, dye | 300 | Melanin, blood |
| 532 | Doubled Neodymium | 400 | Melanin, blood |
| 577 | Pulsed dye | 400 | Blood, melanin |
| 585 | Pulsed dye | 600 | Blood, melanin |
| 694 | Ruby | 1200 | Melanin |
| 760 | Alexandrite | 1300 | Melanin |
| 1060 | Nd:YAG | 1600 | Blood, melanin |
| 2100 | Holmium | 200 | Water |
| 2940 | Erbium | 1 | Water |
| 10,600 | CO2 | 20 | Water |
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The time required for significant cooling of the target chromophore is known as the thermal relaxation time. It is the time necessary for the target chromophore to cool down by 50 through transfer of its heat to surrounding tissues by thermal diffusion.
To limit the amount of thermal damage to the intended target, the pulse duration must be shorter than the thermal relaxation time of the target tissue. If the laser exposure is less than the thermal relaxation time, maximum thermal confinement will occur. The principle significantly decreases the potential damage to tissues surrounding the target chromophore.
Each target chromophore will readily absorb a specific laser wavelength. The spectrum of laser wavelengths is referred to as absorption spectra. However, it is the spectra of major skin chromophores that dominate most laser tissue interactions in cutaneous laser surgery. Each target chromophore has a variety of absorption peaks. To summarize the principle of selective photothermolysis three necessary elements are required:
- Sufficient fluence to damage the target chromophore without damaging surrounding tissue structures.
- An exposure or pulse duration less than the target thermal relaxation time.
- A wavelength that reaches and is preferentially absorbed by the desired target tissues.
How Lasers Light is Produced
A laser is a device that generates an intense beam of light. Light is part of a spectrum of electromagnetic energy. The emissions that make up this spectrum travel at the speed of light. Under certain conditions light exhibits the characteristics of a wave. The wavelength is defined as the distance between the crests of each wave and is what determines the functional properties of laser energy. Electromagnetic radiation with long wavelengths, measured in meters, are commonly used for radio and television broadcasting wavelengths in the 0.4-0.7um rang form the visible portion of the spectrum, Ultraviolet rays, x-rays, and gamma rays are forms of electromagnetic radiation with wavelengths shorter than visible light. Surgical lasers fall between the longest and shortest wavelengths in the infrared and visible as well as the ultraviolet portions of the electromagnetic spectrum. The most frequently used medical lasers are carbon dioxide, neodymium: YAG, frequency doubled neodymium: YAG, KTP, helium neon, visible dye, and excimer lasers.
All stable matter is composed of one or more types of atoms. Each atom contains a nucleus of protons (positive charge) and neurons (negative charge) ions. Negatively charged ions orbit the nucleus. Picture this as a tiny solar system. The number of positively charged protons in the nucleus is equal to the number of negatively charged electrons outside of the nucleus. The electrons exhibit different energy levels. Electrons are able to change orbits, but to do so they require an outside source of energy. Adding energy to an atom excites the atom. An excited electron then seeks a more stable orbit and in doing so releases the energy it gained in the form of a photon. This process is called spontaneous emission of radiation. It was Einstein who proposed the theory that an excited state is more likely to produce a photon of like energy and that two photons would most likely travel in the same direction. This process is called stimulated emission of radiation. A majority of excited atoms is called a population inversion. This condition is necessary to induce sufficient stimulated emission for laser action to occur. Many different substances can be made to emit laser light: solids such as doped glasses or crystals (alexandrite, ruby), gas mixtures such as carbon dioxide, liquid dyes, noble gases and many others. The energy added to the system to produce population invasion might be in the form of high voltage electricity, high intensity light or radio frequency emission. The medium used to create the laser energy is contained within the optical cavity or a tube with mirrors at each end. These mirrors reflect the emitted photons back and forth through the medium. The process of stimulated emission activates them further. One of the mirrors will generally deflect the medium back into the tube while another may reflect the light out for use as a laser beam. The laser light is then directed either by a series of mirrors in an articulated arm, through a flexible fiber, a hollow waveguide, a slit lamp or some other delivery device to be used on a target. This describes the basic physics of a laser.
Note: Usually, when the word "radiation" is mentioned, one tends to think of the more hazardous forms of ionizing radiation, such as x-rays and gamma rays. These types of ionizing radiation disrupt molecular structures and can increase the risk of cancer. Laser light, however, emits a non-ionizing form of radiation, which has been proven to be NOT HARMFUL to humans. This includes pregnant women and their babies.
Properties of Laser Light
Laser light has three distinct properties: collimated, monochromatic or coherent. The collimated beam travels in a single direction with very little divergence even over a long distance. This is unlike ordinary light waves, which lose intensity with distance i.e., sunlight. A collimated beam is a precise and powerful consistent tool. Monochromatic means that the light retains the properties of one spectrum of color and does not deviate. This is unlike sunlight, which contains a spectrum of color. Coherent light describes the movement of the light in one direction through time and space. Different lasers utilize one or more of these three properties of laser light.
Numerous lasers use light energy to create mechanical thermal effects on tissue. Most surgical lasers cause specific thermal effects on targeted tissue. They work on the premise of transforming light energy into thermal energy. Depending on the temperature within the tissue this thermal energy is capable of coagulating, vaporizing or ablating. Laser wavelength and power control absorption rates of the light into the tissue, pulse duration, optical sizes, and distance. Coefficients that are uncontrollable and sometimes variable are tissue density, overlying fluid and optical disbursement. Some of these factors can be adjusted by the technology itself and some of these properties of the laser define its use in medicine.
Power density or the effect of the laser beam on tissue depends on several parameters. The wavelength determines its absorption capabilities into certain chromophores. The diameter of the beam, the distance from the target and the amount of water influence its effect on tissue. The exposure time of the laser both on and off the target can produce significantly responses. If the laser beam were allowed to pass from the laser head to the target, its light would be large and diffuse. By passing the laser beam through a focusing lens, the intensity of the beam is increased dramatically. The laser beam is smallest at its focal point and diverges beyond this point. The power density is the power divided by the cross sectional area of the beam at the place where it meets the tissue. It is measured in watts per centimeter squared w/c2. The power density of a given spot determines the rate of tissue removal within that spot. Because Gaussian beams and most other laser beams do not have sharp edges, they do not have well defined diameters. The larger the spot size the lower the power density. Conversely, the smaller the spot size the higher the power density will be. The focusing lens controls the spot size. Power density varies inversely with the square of the effective diameter of the laser beam. Therefore, if the beam were halved, the power density would increase by four. If the power is doubled and the beam diameter remains the same, the power density is doubled. |
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