Lowdown on lasers

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Lowdown on lasers
Dec 1, 2006
Madeliene E. Gainers, Joely Kaufman
Modern Medicine

Most of us use them Û or at least are fascinated by them Û but many of us may be blind to what really makes laser technology tick.

This week's column is devoted to the basic laser terms Û and the technologies that those terms describe Û that are critical to understanding, naming and operating any laser or light-based system. It is with these basic concepts that laser surgery has made such rapid advances in such a short period of time.

Power, energy and fluence are what many talk about, but are really just a few of the parameters of a laser. Adjusting the other parameters may also increase treatment efficacy and safety, so understanding them is critical.

Many of the devices that we commonly call lasers are not, in fact, true "lasers." For example, an intense pulsed light (IPL) device typically emits light in a range of wavelengths, and therefore does not meet the criteria of a true laser. A light source can qualify to be a laser (Light Amplification by Stimulated Emission of Radiation) by meeting three criteria:

MONOCHROMATIC Û The light must be of a single wavelength.
COLLIMATED OR UNIDIRECTIONAL Û The waves must move in one direction.
COHERENT Û The waves must all be "in sync" with each other.

Laser commonalities

Laser devices are similar in that all have three main components:

1. PUMP Û The device that generates the energy to power the laser. The pump excites the electrons, which are contained in the lasing medium.
2. LASING MEDIUM Û This medium can be gas (CO2), liquid (pulsed dye laser) or solid (Nd:YAG). The type of medium determines the wavelength of the light that passes through and, hence, the name of the laser.
3. OPTICAL CAVITY Û The chamber containing the lasing medium and two mirrors, one on each end of the cavity. Light amplification is created by reflection of the photons between these mirrors. Because the back mirror is 100 percent reflective and the front mirror is partially reflective, the amplified beam is released through the front mirror.

Pulse also considered

In addition to being named by the type of lasing medium they use, many lasers are named by the type of pulse they emit.

The pulse of a laser can be continuous, quasicontinuous or pulsed.

With the introduction of the theory of selective photothermolysis (Anderson and Parrish, 1983), the pulsed laser systems gained popularity over the others. This theory explains how we can use lasers to selectively destroy a target without damaging the surrounding tissues. A pulse is essentially the duration of time of active emission of light. Tissues react differently to different-length pulses.

The pulse duration plays a very important part in the clinical outcome. Practitioners can get completely different clinical results using the same wavelength and same power, but different pulses.

The lasers that emit the shortest pulses are termed "q-switched" lasers, with pulses in the nanosecond range. Because all of the energy is emitted in such a short period of time, high peak powers are attained, with intense tissue destruction. These lasers are mostly used for removal of pigment Û for instance, lentigos and tattoos.

With this high peak power also comes inflammation and high risk for postinflammatory pigment alteration. Longer-pulsed lasers (millisecond range), used for hair removal and treatment of vascular lesions, emit light in a "smoother" fashion and result in less inflammation. It is because of this safety that "super-long pulsed" lasers have gained popularity.

Wavelength plays role

Wavelength also plays a major role in the decision as to which laser system to use.

This choice is based primarily on the wavelength necessary for the chosen target chromophore (melanin, hemoglobin or water) to absorb the energy from the electromagnetic wave. In order for a laser to have any effect, the beam must be absorbed by some portion of the tissue. The chromophore is this light absorber.

Secondarily, longer wavelengths penetrate deeper into the skin, and this can also impact treatment results. For example, the beam of the original hair removal laser, the ruby (694 nm), was absorbed by melanin very well, but did not penetrate deep enough to permanently destroy the dividing cells of the follicle. Thus, longer-wavelength systems are now used (Alexandrite, 755 nm; Diode, 800 nm to 810 nm; and Nd:YAG, 1064 nm).

In addition to selecting the appropriate wavelength for optimal absorption by the target, the practitioner must be certain that the energy density (amount of energy absorbed, or fluence) must be of sufficient quantity to destroy the chromophore, and the pulse duration should be shorter than the thermal relaxation time of the chromophore. This thermal relaxation time is the time it takes for the target to release half of its heat, and is directly proportional to its physical size.

These principles allow for precise destruction with minimal dissipation of heat and, hence, minimal damage to surrounding tissue. Because earlier lasers such as the continuous wave (CW) and the first argon lasers did not comply with these concepts, they are no longer used due to the resultant widespread, nonselective tissue injury.

Cooling component

Cooling is another critical component of many laser systems.

Cooling allows for treatment of underlying structures with less damage to the epidermis. The most common types of cooling are contact, cryogen spray or air coolers. Contact cooling tips are placed directly onto the skin prior to and/or during the actual laser pulse. Cryogen sprays are outfitted directly on the laser system and emit liquid nitrogen immediately prior to the pulse of light. The cheapest and easiest form of cooling is an ice pack.

In addition to epidermal protection, cooling systems may provide some pain relief. Cooling is essential in systems aiming to treat only the dermis, such as the new nonablative-type lasers, and in cases where the epidermis could be an accidental target, such as with hair removal. Resurfacing and fractional resurfacing lasers do not use cooling systems, as the aim is heating of the epidermis.

Cooling times can be adjusted either manually (i.e., by leaving the contact in place for a longer period) or by changing the integrated cooling setting on the laser. This may add additional epidermal protection up to a certain point, and should be considered in patients at high risk for pigment alteration.

In every clinical setting, considering parameters in addition to the fluence will result in better outcomes. The pulse duration, type and length of cooling, and wavelength are all critical to the overall effectiveness and safety of each procedure.

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