Laser physics

Lasers are devices that generate light energy. A substance or medium is ‘pumped’ by an external energy source which moves electrons from a stable to an ‘excited’ state. Some of these excited electrons will spontaneously decay to a lower energy state. In the case of lasers, the excited molecule returns to a stable, lower energy state by emitting the extra energy as light in the form of a photon. A photon is a discrete bundle of energy with no mass. Once released, this photon cruises along in a population of like molecules which are also excited. When it strikes another excited molecule, the process of amplification occurs because the excited molecule is stimulated to release its extra energy as another photon of exactly the same wavelength and precisely in phase with the first. When all of this activity occurs in a tube with mirrors at both ends, the energy builds up as the light waves bounce back and forth. If one of the mirrors has a small central aperture, it can be opened in such a way as to release laser light.

Why ‘laser’? It’s an acronym formed from the first letters of the process described above; namely, Light Amplification by Stimulated Emission of Radiation. The concept of radiation in the form of wavelengths is derived from the dual nature of our understanding of light as being characterized by both photons and electromagnetic waves. There are unique features of laser light that make it useful for a variety of applications including medical ones. Laser light is collimated or highly directional which keeps the energy of the beam from dissipating over distance. Laser light is coherent with the peaks and troughs of the sinusoidal wave traveling through space in phase with one another. Coherence allows the energy within the beam to be concentrated with precision.

Finally, the light is monochromatic or emitted at a narrow band of the electromagnetic spectrum. It is this monochromatic property that is used in medicine to achieve specific goals in tissue. Once this laser light has been formed in the generator, it can be manipulated in several useful ways. The release can be controlled by a variety of gating mechanisms ranging from simple shutters to complex pulse generators. For medical uses, the beam is either released as a continuous beam or as a pulsed one. The power output is measured in watts or joules per second. The beam can be focused which is a useful property when cutting or resurfacing by scanning. It can be transmitted through mirrored conduction tubes, wave guides or fiber conductors.

If the beam is released in a pulse with the intent to conduct it to the target without significantly modifying its physical characteristics, the power of the laser must be high enough to produce an appropriate effect when it reaches the target. Once this emitted laser light strikes tissue, it can be reflected, scattered, absorbed or transmitted. While reflection is not a very useful property in medicine, there are some applications where scattering can be employed to achieve therapeutic goals. The most useful current medical applications are absorption and transmission.

Our current concept of what happens when laser light and tissue interact was articulated by Anderson and Parrish¹ in 1984 in their landmark paper describing selective photothermolysis. Specific components in tissue absorb light more intensely at some wavelengths than at others. These components are known as chromophores. For example, absorption by water peaks at 2900 nm and at 10600 nm. The erbium laser emits at 2940 nm and the carbon dioxide laser emits at 10600 nm. The incident energy from both of these lasers is rapidly absorbed by water in the most superficial layers of skin where it is converted to thermal energy. If the laser energy is sufficiently high, it will vaporize the water and in doing so will thereby remove or ablate the tissue. By controlling the incident energy and the duration of application, precise amounts of skin can be removed with limited collateral thermal damage.

Another example of this concept of absorption can be seen in the choice of an argon laser emitting at 514 nm (in the green part of the visible spectrum) where water absorption is very low and melanin absorption is relatively high. Energy emitted from this laser passes through the fluid medium of the eye to be absorbed by the chromophore melanin in the retinal pigment epithelium. The thermal energy generated by this effort can be useful in treating some pathology such as diabetic retinopathy. Figure 22-1 shows the different chromophores and their wavelengths of absorption. The goal of laser skin resurfacing is to vaporize or ablate surface tissue while minimizing thermal damage adjacent to the site of application. The obvious first step is to build the laser with enough power to reach the heat of vaporization, which in the case of water is 100°C. How efficiently this process of vaporizing tissue proceeds will be determined largely by the shape of the laser beam and the time of application. Since most medical lasers emit in either a true Gaussian shape or something similar to it, the most useful bell-shaped configuration will have the energy of the beam highly concentrated in the central portion of the beam. The objective in ablative resurfacing lasers is to exceed the energy of vaporization for the broadest area of the beam possible and to minimize the undesirable formation of thermally devitalized but intact skin on the margins of the beam. In prosaic terms, a beam configuration that looks more like a ‘high top hat’ than a ‘beret’ is desirable.

Figure 22-1 Chromophores and absorption wavelengths.

Duration of application of the laser beam can be a confusing parameter to understand. Conduction of any energy away from the site of origin/application requires time. When the water in tissue is the chromophore or target in tissue, the light energy is converted to thermal energy. If the laser energy is applied for a short enough time, thermal energy will be largely dissipated at the site. However if the energy is applied long enough, then the thermal energy will be conducted to the adjacent tissues. If this phenomenon occurs over a long enough time, significant thermal devitalization of the surrounding tissue is possible.² Thermal burns can be produced that can dramatically alter the healing process, to the extent of scarring. This period of time before significant conduction occurs is defined as the thermal relaxation time of tissue. For skin, that time is about 400 microseconds (400 μsec). The challenge is to build a laser with enough power to treat a fairly large area of skin with a ‘short burst.’

An alternative to a single pulse is a scanner that will trace a small focused beam over the target rapidly enough to produce a desirable effect and still not expose the target area for longer than the thermal relaxation time of the tissue. Of course at the sides and depths of the beam, some subablative energy will exist which will produce a residual of thermally devitalized tissue beyond the area of application. Actually, this thermal residual can be helpful because it produces cauterization of small vessels which otherwise might bleed thereby obscuring the field.³ The effect of ablative laser applications can be predicted. The common denominator, at least among lasers of the same wavelength, is fluence which is the amount of energy applied per unit area. Fluence is measured in joules/cm².

For the purpose of comparing lasers, both the carbon dioxide and the erbium laser can be considered to ablate 4 microns (μm) of skin/joule of fluence. Therefore, if the laser beam is applied at 20 J/cm², about 80 μm of skin will be ablated in one pass. 80 μm is about the thickness of eyelid skin. Remembering that the thickness of the papillary dermis is about that of the epidermis, a second pass at 20 J/cm² would place the thermal residual well within the reticular dermis. Thus, two passes over the eyelids at a fluence of 20 J/cm² would be aggressive treatment. For non-ablative devices, the laser or light energy is relatively poorly absorbed by chromophores in the skin. Therefore most of the energy from these devices will pass through the epidermis to be scattered by encounters with molecular structures deeper in the skin before being absorbed by the targeted chromophore. While thermal energy generated by this process is not sufficient to vaporize the tissue, there is enough thermal energy to devitalize the collagen and initiate a reparative process that will ‘renew’ the collagen in the papillary and upper reticular dermis. The entire process can be significantly enhanced by passive and dynamic cooling methods that are mainly designed to protect the epidermis.

The choice of wavelengths, duration of application and modification of effect by superficial cooling can be confusing. There are no simple guides. Furthermore, the results tend to vary considerably among patients, often in unpredictable ways. Perhaps the best approach is skepticism. Use conservative guidelines to gain familiarity. Most importantly, progress must be documented with pre- and postoperative photographs that can be readily accessed. We can all see what is, but our recollection of what was is poor. Remember, the improvement induced by non-ablative devices can take months to reach a maximum effect and that effect can be subtle.

Laser safety

Laser safety is a requirement for all physicians. There are many regulatory levels in laser medicine: federal, state, local, departmental, and institutional. Before offering laser resurfacing or using any laser tool in practice, be sure to check with each level for safety standards. Federal regulations are outlined by the Occupational Safety Health & Administration (OSHA, www.osha.gov) in the form of general industry standards, directives, and compliance letters. Laser safety standards are specifically further delineated by the American National Standards Institute (ANSI). ANSI Z136.1-2000 document covers the general safe use of lasers. ANSI Z136.3-1996 document provides standards for the safe use of lasers in health care facilities. This document is most often used by regulatory groups and in litigation. Most states also maintain regulations as adjuncts to OSHA and ANSI standards, which may be even more rigorous.