LASERS IN MEDICINE

There are several types of lasers used to treat cutaneous disease

PHOTOTHERMOLYSIS.

Laser light is absorbed by chromophores, light-absorbing components of skin (melanin, hemoglobin, water, tattoo ink). The absorbed energy is converted to thermal energy with heating of the chromophore. If a targeted tissue (chromophore) strongly absorbs a selected wavelength, and the pulse duration is shorter than the thermal relaxation time (cooling time) of the tissue, then only selective thermal injury will occur. By limiting thermal damage to the target chromophore, less injury occurs to the surrounding tissue, with a reduced risk for scarring.

HOW LASERS WORK.

Lasers apply an electric charge that activates a medium, usually a gas such as carbon dioxide or a crystal such as a ruby rod. The activated medium releases light, which is focused by mirrors and released in a beam. The laser apparatus consists of three elements. The pumping system is the power supply. The flashlamp is one type of pumping system. The lasing medium supplies the electrons needed for stimulated emission of radiation. It can be gaseous (argon, carbon dioxide, He-Ne, copper vapor, excimer, krypton), liquid (tunable dye), or solid (alexandrite, ruby, Nd:YAG, Er:YAG, Ga:As) or comprised of free electrons. The optical cavity consists of two parallel mirrors (one partially reflective) enclosing the lasing medium that is excited by the pumping system.

The wavelength of light is determined by the laser medium present in the optical cavity.

LASER TYPES.

Continuous-wave lasers (argon, argon-pumped tunable dye, krypton) produce a laser beam that is emitted continually. Most of the skin is heated by conduction during the long exposure, regardless of the wavelength. Pulsed lasers produce a beam that is emitted in individual short pulses with a long period (0.1 to 1 second) between pulses.

Q-switching refers to a switch that allows the release of all of the laser energy in one powerful pulse. The target is heated at such a rapid rate that it shatters. Short-pulse (Q-switched ruby, alexandrite Nd:YAG) lasers are used for smaller structures (melanosomes, tattoo ink particles).

Carbon dioxide lasers have different lasing modes. In the continuous-wave mode, the laser beam coagulates, vaporizes, or cuts tissue. The “ultra-pulse” and scanned modes achieve complete tissue vaporization with little collateral damage; they are used for skin resurfacing to remove wrinkles and scars.

VASCULAR LESIONS.

Lasers are used to treat congenital and acquired vascular lesions, including port-wine stains, hemangiomas, facial telangiectasias, poikiloderma, cherry angiomas, venous lakes, and spider leg veins. The chromophore targeted is hemoglobin. Energy absorption by hemoglobin results in local thermal damage. Early continuous-wave lasers did not restrict damage to vascular structures and caused scarring. Pulsed laser systems, such as the 585-nm or 595-nm pulsed dye laser (PDL), minimize adverse effects.

The flashlamp-pumped pulsed dye laser revolutionized the treatment of port-wine stains in infants and in adults, where resolution of the port-wine stain is excellent, with a scarring rate of approximately 1% and only occasional temporary hyperpigmentation. The following lasers are used to treat vascular lesions: the argon at 488 to 514 nm; the frequency-doubled Nd:YAG at 532 nm; the krypton at 568 nm; the argon dye at 577 to 600 nm; the copper vapor at 578 nm; and the pulsed dye at 585, 595, and 600 nm. No one laser is ideal for all types of vascular lesions. Lasers with longer wavelengths penetrate more deeply.

Tattoos and pigmented lesions.

Treatment of benign pigmented lesions with a predominant epidermal component (solar lentigines, ephelides, melasma, nevus of Ota), and red, blue, black, and green tattoos are effectively treated with Q-switched lasers. Caf?-au-lait macules, Becker’s nevi, and melasma have a variable response to lasers. “Q-switching” lasers produce very high power and extremely short pulses of light. Q-switched lasers are the ruby at 695 nm, the Nd:YAG at 1064 nm in the near-infrared spectrum and 532 nm in the green light spectrum, and the alexandrite at 755 nm. The chromophores for these lasers are melanin and tattoo pigment. The wavelengths of these lasers are long enough to penetrate into the dermis where there is tattoo ink or dermal pigment.

The ultrashort pulse duration of these lasers produces photoacoustic waves that break tattoo particles into smaller pieces. Tattoo fragments are phagocytosed and removed. Blue-black tattoos respond best to the Q-switched ruby and 1064-nm Nd:YAG lasers; green tattoos respond to the alexandrite laser; and red tattoos are treated by the 532-nm Nd:YAG laser. Professional tattoos do not respond as quickly as amateur tattoos.

Non–Q-switched lasers are also used to treat pigmented lesions with the flashlamp-pumped pulsed dye with a green light at 510 nm and the krypton laser, which simultaneously produces a green light at 521 and 530 nm and a yellow light at 568 nm.

Skin resurfacing.

Skin resurfacing is used for the treatment of facial wrinkles and acne scarring. The lasers used for skin resurfacing are the CO2 at 10,600 nm and the Er:YAG at 2940 nm. Water is the chromophore targeted. The heat results in vaporization of the tissue. Peripheral thermal damage and scarring are minimized with the new super-pulsed lasers and computerized scanning. Darker skin patients may have unacceptable alterations in pigmentation.

Hair removal.

Lasers provide effective and possibly permanent hair removal, with minimal discomfort and low risk for scarring. Numerous devices are available.

Intense pulsed lightIntense pulsed light (IPL) systems are light sources that emit light over a spectrum of 550 to 1200 nm. Pulsed light machines use “cut off” filters to deliver the desired wavelengths. These wavelengths can be customized to reach the specific hair, blood vessels, or skin component being treated; and can be modified with each pulse. Longer (higher numbered) wavelengths penetrate deeper into the skin and are used to treat deeper targets and to avoid and protect superficial parts of the skin. Shorter wavelengths are used to treat more superficial targets and avoid damaging deeper skin parts.

IPL therapy is a nonablative resurfacing technique in that it targets the dermis without affecting the epidermis. Ablative laser resurfacing affects both the dermis and the epidermis. The advantage of IPL therapy is its minimal downtime. There are many different machines available.

They are used to treat vascular lesions (spider angioma, veins, port-wine stains) and pigmented lesions (lentigines, melasma, poikiloderma of Civatte) and for hair removal and acne treatment.

 

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