Laser Therapy for Cosmetic Purposes: Laser Basics

Laser Treatment of Skin
Laser Therapy for Cosmetic Purposes

Laser basics for Skin Treatments

Light is made of bundles of energy called photons. Electrons within atoms will absorb photons and store the energy associated with them. The electrons can release the energy by emitting the photons as a form of radiation. Photons from like atoms have the same energy.


Emitted photons move in waves. A wavelength is measured from the top of one wave to the next. These lengths are reported in nanometers (nm) or in billionths of a meter. Atoms of each material emit photons in specific wavelengths that identify them like fingerprints. Some emitted wavelengths are visible and some are invisible. Humans can see a certain set or band of wavelengths called the visable spectrum. Ultraviolet and infrared energy wavelengths are not visible to the human eye. But they are all part of the Electromagnetic spectrum.

There are basically two ways of making light for clinical uses. They are lasers and pulsed light devices.

The laser emits light energy of one single wavelength and the beam can be focused. Broad band light is emitted as light energy containing multiple wavelengths and cannot be focused.

Pulsed light devices utilize a light source such as a flashlamp that emits energy in a band or multitude of wavelengths from the base of the visible spectrum (400nm) to the border between the near and mid infrared wavelengths (1300nm). The light is directed often through a crystal to the tissue. Some pulsed light devices use filters to limit energy transmission to protect the tissue.

Blocking portions of the spectrum can protect the nontarget tissue to avoid complications. Most filters are high pass devices which block light at and below a specific wavelength. Only light generated above the specified wavelength is emitted to tissue. An example would be a filter to block wavelengths that are absorbed by red blood vessels and allowing through only wavelengths treating specifically brown melanin based lesions like age spots. In this way the age spots can be removed without damage to the surrounding blood vessels.

LASER stands for Light Amplification by Stimulated Emission of Radiation. A laser harnesses an excitation source, often a flashlamp, to drive photons from the laser medium. The light generated has very distinct properties and a single wavelength.

The types of lasers are classified by the type of medium used and the wavelengths of light energy they emit.

Lasers will emit light energy in different ways. It can transmit energy as one continuous beam or break up the transmissions into numerous short pulses.

Light from either device is often delivered to tissue through a hand piece. It may be attached to a fiber optic cord or a hollow arm with directional mirrors within the joints. They commonly use optics to focus the beam to a small diameter or spot size, at a specific distance before tissue contact. When the light comes in contact with tissue there are predictable and controlled reactions.

When light energy is directed at the skin it can be reflected, transmitted, or scattered, none of which cause any clinical effects. However, when light energy is absorbed then significant laser light interaction can take place.

The most common reaction that occurs when laser light energy is absorbed is heating of the tissue. The depth of tissue heated is determined partly by the absorption wavelength.

To be clinically beneficial, enough of the laser energy must be absorbed by tissue to cause thermal damage. This results in ridding tissue of certain unwanted elements.

In tissue there are several elements or structures called chromophores. Each chromophore will selectively absorb light of only certain wavelengths.

Chromophores that are targets for laser light therapies include hemoglobin in red blood vessels (to destroy vascular lesions), melanin in skin pigment (to remove pigmented lesions or unwanted hair), water (to ablate certain levels of skin), and exogenous substances (as ink in tattoo removal).

This slide illustrates the different lasers and their emitted wavelengths and which targets they will treat.

Different types of lasers emitting their different wavelengths will also penetrate skin tissue to different depths.

Erbium and CO2 lasers provide light wave lengths that are strongly absorbed in the epidermis (the outside layer of skin). Therefore, they are heavily utilized for destroying this layer where the wrinkles are as a part of resurfacing procedures.

Visible light or intense pulsed light (IPL), or broadband therapies can penetrate to the deeper dermis for more aggressive resurfacing or treatments that get rid of undesirable skin pigment like age spots.

Basic Laser Priniciples

Basic laser settings, depending on the device, include fluence, power, pulse width, and spot size. The amount of energy delivered to the tissue is measured in joules. The amount of energy delivered per unit surface area is called the fluence, or joules per centimeter squared. Some laser settings are displayed in watts. One watt is one joule of energy delivered over one second. For lasers that deliver energy in pulses rather than continuously, the amount of time the laser is on during a pulse is called the pulse width, measured in seconds, milliseconds, even nanoseconds. The spot size is the size of the target area hit by the laser.

The interaction of laser energy with tissue
The amount of energy that is within a laser beam is related to the wavelength. There is more energy in the photons of a laser beam of shorter wavelength than of longer wavelengths. In fact a beam with a wavelength of 532nm delivers twice as much energy per unit than a wavelength of 1064nm. Laser photons can be relected or absorbed. When absorbed by a target molecule called a chromophore, the molecule will undergo a change which could be damaging. Each chromophore will tend to absorb light energy of only certain wavelengths. When treating skin, there are three main chromophores that are being targeted; water, hemoglobin (blood), and melanin (skin pigment). Water will absorb wavelengths at primarily 980nm, 1480nm, and 1060nm, with a maximum at 2940nm. Hemoglobin will absorb at 415, 540, 577, and 940nm. Melanin absorbs wavelengths <800nm.>

Exposing wide areas of tissue to laser energy but damaging only selected targets like blood vessels or hair relies on a principle called selective photothermolysis. In this way laser energy is delivered with a wavelength that is absorbed only by the target chromophore say melanin. If enough energy is delivered, the target lesion containing melanin (say hair follicles) will be destroyed without harming surrounding skin that do not absorb that particular wavelength. This would be like launching smart missiles that hit only the desired targets.

Preventing damage to tissue around the target chromophore relies on the principle of the thermal relaxation time. The thermal relaxation time is the amount of time it takes for the target to cool a certain amount from the peak temperature achieved. For a lasered target it is the amount of time it takes for the target to cool off after the laser pulse is given. So, if the pulse is too long, the target can cool off during the pulse. Just like trying to slowly fill a bucket with a big leak. The bucket will never fill. In the same way if cooling is taking place during laser heating, the target may never reach the temperature high enough to destroy it. Therefore, the pulse width or duration of a laser pulse is set at less than the thermal relaxation time to assure all of the destructive energy is delivered to damage the target before it has a chance to cool off. If the pulse width is too short, the pulses can vaporize the target, a more explosive result that yields more damage to target blood vessels with more bleeding and unsightly bruising. Longer pulses can more gradually heat damage targets without blowing up the vessels.

Lasers can treat skin lesions in other ways other than with heat damage. Lasers can be used to stimulate chemical reactions to destroy tissue. One example is with treatment of acne. A chemical called aminolevulinic acid is spread on the skin where cells in sebaceous glands convert it to PPIX. When PPIX is hit with photons, it is converted to an oxygen radical that destroys acne causing gland cells. Light therapy can also cause biostimulation. LED light can supposedly increase collagen tissue formation in the skin to tighten and firm up aging facial skin.

The skin has two basic layers, the epidermis or outer layer and the dermis or underlying layer. The epidermis absorbs most of the light exposure because most of the melanin giving skin its pigment is located there. The dermis contains several of the target chromophores including blood vessels and hair follicles. It also contains collagen tissue that supports the skin and gives it its tone. There are several different kinds of lasers with various applications. The violet intense pulsed light and LED lamps can treat acne. The 940nm and 1064nm Nd:YAG lasers are used for resurfacing to treat wrinkles. Wrinkles and scars are also addressed with CO2 or Er:YAG lasers. Spider veins and telangiectasias can be treated with 1064nm lasers or intense pulsed light (IPL). 1064nm lasers can be used for unwanted hair removal from the face and back as well.

Cooling
When deeper targets in the dermis are approached with a laser, especially hair or blood vessels, the outer layer of skin, the epidermis, may be damaged because melanin in this layer absorbs laser light as well. This can result in burning, crusting and scarring of the epidermis. To avoid this, the epidermis is cooled to protect it from heat injury while laser energy passes on through to targets in the dermis. Surface cooling can be achieved with ice, cooled gels, and cold glass or sapphire plates the laser can pass through. Devices like the Zimmer chiller blows cold air on the site to keep it cool. Cooling can also result in pain reduction as well with laser treatments.