karimi understanding lasers[1]

by Dr. Manouchehr Karimi MD. 1

UNDERSTANDING LASERS

An Introduction to Lasers and Laser –Tissue Interactions in Dermatology


Electromagnetic spectrum


LIGHT BULB VS. A LASER

Photons are the fundamental units of light, and they are the same whether produced by a light bulb or a laser

The laser and the light bulb differ in how their photons are organized

Light from a light bulb radiates in all directions, and there is a direct mathematical relationship between the loss of light intensity and the distance one moves away from the bulb. In the laser, photons are emitted parallel (or nearly parallel) to, and in phase with each other as they travel toward infinity.



Another feature of light from a light bulb is that it is white or yellow-white in color because it contains all the different colors and wavelengths in the visual portion of the electromagnetic spectrum and, hence, is polychromatic

A glass prism placed in front of a light bulb refracts the different wavelengths and allows the constituent colors to be seen .

With the laser, the prism produces light of one wavelength and one color. The light from the laser is therefore pure or monochromatic

This property, under some circumstances, allows for selective absorption of laser energy by a targeted chromophore within human skin—structures with high absorption at a laser wavelength can be selectively altered or destroyed


Another difference between the two light sources is their intensity. The number of photons per unit area of emission produced by the laser is much greater than for any other light source.

. In fact, millions more photons are emitted by a laser than by a comparable surface area of the sun. For example, peak powers of 1012 W can be obtained with certain short-pulsed lasers. Laser power can also be varied over a wide range to achieve quite different tissue effects.



For example, a low-power laser may be used to heat gently a tissue, with perhaps the only change being an increase in metabolic rate. A high-power pulsed laser may be used to achieve nonlinear optical effects (e.g., optical breakdown) causing explosions within the tissue



As a result of these three basic differences—coherence, monochromaticity, and intensity—the laser produces a highly unique form of electromagnetic radiation. Other wise, photons from the light bulb and the laser obey the same basic laws and principles governing their interaction with human skin



LASERS IN DERMATOLOGY

Different kinds of lasers are identified by the type of material that undergoes the lasing process (e.g., the argon laser has argon gas as the lasing medium) inside the device.

There are four types of lasing materials: solid, gas, liquid, or semiconductor


Examples of solid-state lasers are the ruby, alexandrite, and neodymium yttrium–aluminum–garnet (Nd:YAG) lasers

Examples of gas lasers are the excimer, argon, and carbon dioxide lasers

An example of a liquid laser is the pulsed dye laser. Liquid lasers employ complex organic dyes, such as rhodamine or coumarin, in solution or suspension

. Diode lasers use two layers of a semiconductor material, such as gallium arsenide. Some diode lasers are tuneable over a broad wavelength range. They can be operated on battery power and, due to their small size which provides for efficient packaging, are useful for remote operation.



Generally and by convention, a medical laser is referred to in terms of its wavelength in either nanometers (nm) or micrometers (micron).

there are a wide range of lasers available for clinical use.

The dye lasers, and in the future the free-electron laser, can be precisely tuned to emit photons at wavelengths that match absorption peaks of tissue chromophores, thus permitting their selective destruction and subsequent tissue ablation



CONTINUOUS WAVE vs. PULSED LASERS

In the CW mode, the laser delivers a continuous beam of light with little or no variation in power output over time In CW operation, laser output is controlled by the physician, typically by depressing a foot pedal.

A pulsed laser delivers its energy in the form of a single pulse or a train of pulses.

The frequency or pulse repetition rate is the number of pulses emitted in 1 s (Hertz).


From the physician’s point of view, superpulse is most useful for incisional surgery with a focused beam. High peak power maximizes tissue vaporization, and short pulse duration minimizes adjacent thermal injury.

CONTINUOUS WAVE vs. PULSED LASERS


Q-SWITCHING Q-switching of lasers is a mechanism often used to control

the light output by concentrat-ing all the energy into a single, intense pulse with a duration on the order of nanoseconds and an energy of 10 J or more. With Q-switching (the Q-factor stands for “quality factor,” used in electronics theory terminology), a fast electromagnetic switch (Pockel’s cell) in the laser cavity causes excitation of the active medium to buildup far in excess of the level of the medium when the shutter is open. In operation, the flashlamp is turned on and the population inversion gradually grows

The net result is an extremely high peak power (greater than 106 W) nanosecond duration pulse or series of pulses.


BASIC LASER–TISSUE INTERACTIONS: REFLECTION, SCATTERING, TRANSMISSION, AND ABSORPTION

It is important to recognize that these interactions are complex phenomena influenced not only by laser parameters, such as power, spot size, pulse duration, repetition rate, and wavelength, but also by properties of the tissue itself.

If a laser beam is directed at the skin, light may be reflected back to the source or to another undesired surface .

When light strikes the skin surface, approximately 5% is reflected. Since tissues reflect light, their reflectance properties are important consider ations. Instruments in the operative field may also reflect the light, creating potential health hazards for the patient and attending medical personnel.


BASIC LASER–TISSUE INTERACTIONS cont.

However, two things can happen other than absorption. Light can scatter from particles and structures in the skin to places where it is not wanted. Scattered energy is a major reason for the spread of tissue damage around the focused spot because it changes the direction of the incident beam .

In human skin, collagen fibers are particularly important scattering structures .

The degree of scatter is heavily dependent on the wavelength of the light and the composition of the tissue


Light scattering in human skin is greater and more random at shorter wavelengths .

Light scattering at longer wavelengths is less and more forward directed.

Finally, light might be transmitted through the tissue with only a minimal amount being absorbed.

Since every tissue has reflective, scattering, and transmissive properties, understanding these tissue characteristics is an important aspect of knowing how to use the laser.



CHROMOPHORES IN HUMAN SKIN Hemoglobin has significant light absorption in the violet,

blue/green, and yellow portions of the spectrum . This absorption starts to decline in the red region . . In the case of laser treatment of cutaneous blood vessels,

the wavelengths suitable for consideration are the hemoglobin Soret absorption band at 418 nm and the absorption bands at 542 and 577–595 nm

For these reasons, the pulsed dye laser (577–595 nm) has been the mainstay of treatment for multiple cutaneous vascular lesions.

The yellow light is absorbed by hemoglobin in these lesions causing thermal damage and thrombosis of targeted vessels.


Melanosomes are the fundamental sites for melanin synthesis and occur as organelles within melanocytes. Absorption is the dominant process by epidermal melanin.

Although highest in the ultraviolet portion of the spectrum, melanin absorption is also significant in the visible and near-infrared wavelengths. For these reasons, Q-switched green KTP), red (ruby, alexandrite), and near-infrared wavelengths have been utilized for the treatment of certain epidermal and dermal pigmented lesions.

CHROMOPHORES IN HUMAN SKIN


Water has no light energy absorption in the visible portion of the spectrum and very minimal absorption in the near-infrared portion. However, further out in the infrared (beyond 2 micron), water in the tissue has significant absorption.

The Er:YAG and carbon dioxide lasers are currently the most frequently used surgical lasers.

Since human skin contains water as a major constituent, the Er:YAG (2.94 micron) and carbondioxide (10.6 micron) lasers producing infrared photons that are specifically absorbed by water can have a direct effect on the epidermis and dermis.



If used properly, the Er:YAG and carbon dioxide lasers are clinically equivalent to dermabrasion because the laser gradually removes cell layer after cell layer through volatilization of water present in human skin.



Photons emitted by the Nd:YAG laser (at a wavelength of 1.06 mm), on the other hand, are very poorly absorbed by hemoglobin, melanin, water, and other chromophores in human skin. Light penetration into skin is very deep (4–6 mm) resulting in a large volume of coagulated tissue—substantially larger than that created by either the Er:YAG or the carbon dioxide laser .

Because of the extensive penetration of the Nd:YAG laser into skin, its use in the continuous mode is somewhat limited in cutaneous surgery.



For example, the Nd:YAG laser cannot be used to ablate superficial lesions like the Er:YAG or carbon dioxide laser. Recently, however, Nd:YAG lasers pulsed in the millisecond domain have been used to target telangiectasia and reticular veins of the legs.



Wavelengths suitable for nonablative laser skin rejuvenation must have low melanin absorption, which favors the choice of longer wavelengths (i.e., near- and mid-infrared) over visible light. Therefore, most attempts at nonablative laser skin regeneration have used wavelengths with low (650–1100 nm) or intermediate tissue water absorption to deposit laser energy nonselectively in the upper (100–400 micron) dermis.



Wavelengths from 1.3 to 1.8 micron meet this need, which currently limits the selection of light sources to the 1.32 and 1.44 micron Nd:YAG lasers, the 1.45 micron diode laser, and the 1.54 mm Er:glass laser.

. Because deposited heat is redistributed within the skin by heat diffusion, the ultimate depth of thermal injury is affected not only by the optical penetration of the selected wavelength but also by a multitude of other variables beyond the scope of this chapter.



HEAT Given that one goal of laser therapy is the precise

control of thermal energy, a thorough understanding of the sequelae of tissue heating is

required. As the tissue is heated and the temperature rises to

between 37 C and 60 C, the skin starts to retract and conformational changes occur. At a temperature above 60 C, there is protein denaturization and coagulation. From 90 C to 100 C, drying and shrinkage of the skin occur. Above 100 C, the skin is carbonized and subsequently vaporized and ablated


ABLATION OF HUMAN SKIN The Er:YAG and carbon dioxide lasers are

currently the lasers of choice for human skin ablation.

Since light is absorbed so intensely by water, penetration depths are only a few micrometers.

Absorption of many photons in such a small volume of tissue produces a rapid rise in skin temperature.

As energy is added, the temperature of water is raised to its boiling point.


The rapid expansion created by this excitation give rise to the actual ejection of microscopic tissue fragments at high velocities.

. Because vaporization is such a rapid process, thermal damage to adjacent tissue, although varying with the duration of exposure, is usually limited to a zone of 100–200 micron (carbon dioxide) or less (Er:YAG) in human skin.


SELECTIVE PHOTOTHERMOLYSIS

. Most important, from a clinical point of view, are the properties of emitted wavelength and pulse duration.

. If the clinical objective is to cause selective destruction of a specific chromophore, the wavelength chosen should match the highest

one goal of treatment is the precise control of thermal energy, the pulse duration of laser irradiation is just as important as optical and tissue factors.


One way to maximize the spatial confinement of heat is to use a laser with a pulse duration on the order of the thermal relaxation time (Tr) of the target chromophore (10).

Tr is defined as the time required for the heat generated by the absorbed light energy within the target chromophore to cool to half of the original value immediately after the laser pulse.


The target does not become appreciably warmer than its surroundings because the absorbed energy is invested almost uniformly in heating of the tissue during exposure.

As a result, longer pulse durations offer a more generalized heating and, therefore, less spatial selectivity resulting in nonspecific thermal damage to adjacent structures regardless of how carefully one has chosen a wavelength.


. However, if the laser pulse is suitably brief, its energy is invested in the target chromophore before much heat is lost by thermal diffusion out of the exposure field.

Therefore, selective target damage depends on delivering a pulse of light of shorter duration than Tr, which can be estimated because the latter is directly proportional to the square of the diameter of the target and inversely proportional to the thermal diffusivity of the tissue


A laser emitting at a selectively absorbed wavelength with a pulse duration less than Tr can be expected to cause highly selective target damage

This process, termed selective photothermolysis, was introduced in 1983 as a means of achieving target chromophore destruction by careful selection of wavelength and pulse duration.


SELECTIVE PHOTOTHERMOLYSIS OF TATTOOS

Amorphous carbon, graphite, India ink, and organometallic dyes, typically found in dark blue-black amateur and professional tattoos, have a broad absorption in the visible and near-infrared portions of the spectrum. At visible wavelengths longer than 600 nm, hemoglobin and melanin light absorption is minimized and tattoo dyes can be targeted selectively (13).


The pigment granules characteristically found in tattoos have diameters of 0.5–100 micron, which correspond to Tr of 20 ns to 3 ms.

. With the development of the Q-switched ruby (694 nm), alexandrite (755 nm), and Nd:YAG (1.06 micron) lasers, tattoo removal without scarring can be achieved. The frequency-doubled, Q-switched Nd:YAG laser (KTP laser) emits at a wavelength of 532 nm, which provides improved removal of red dye.


SELECTIVE PHOTOTHERMOLYSIS OF PIGMENTED LESIONS

The diameters of individual melanosomes (0.5–1.0 micron) and melanocytes (7 micron) correspond to Tr of 20–1000 ns.

Therefore, Q-switched green, red, and near-infrared wavelengths have been utilized for this indication.

Studies have shown that the effects of Q-switched lasers are melanosome dependent, and vacuolization of pigment-laden cells has been seen immediately after laser irradiation


Q-switched lasers have been used successfully to treat a wide variety of pigmented lesions including lentigines, ephelides, and nevus of Ota


SELECTIVE PHOTOTHERMOLYSIS AIR REMOVAL

The human hair follicle is a complex structure derived from both epidermal and dermal components.

The target chromophores, primarily melanin-rich hair shafts, are located deep in human skin (bulge around 1.5 mm and bulb at 2–7 mm).

). At this depth, only red and near-infrared wavelengths are useful (690–900 nm).


The follicular structure responsible for regeneration has not been conclusively identified and, therefore, current systems target the entire follicle.

As a result, long pulse widths on the order of milliseconds and high fluences capable of heating large volumes of tissue are required .

Millisecond-domain ruby, alexandrite, diode, and Nd:YAG lasers using high light doses can produce selective injury to human hair follicles resulting in prolonged growth delay and, in some cases, permanent hair loss after a single treatment.


The clinical objective of laser therapy is to maximize thermal damage to target chromophores while minimizing injury to overlying skin. Unfortunately, for many skin types, the threshold light dosage for epidermal injury due to melanin absorption can be very close to that for permanent removal of target chromophores.


Diode Lasers Diode lasers are small semiconductors. They have no

moving parts, and since they are extremely energy efficient, minimal cooling needs. Such devices are durable, and can function for very long periods.

There is some variability in the wavelength, frequency, and pulse parameters of diode lasers. Commonly used emission wavelengths include 800, 940, and 1450 nm.

Most devices in dermatologic use emit at approximately 800 nm, which is a physiologically useful output.

karimi understanding lasers[1]

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