Overview of Medical Lasers

Medical Lasers

Medical lasers use the principle of Selective Photothermolysis to deliver the right amount of energy to the target tissue in a mannner that spares injury to adjacent structures. Some medical lasers are application-specific; others are versatile devices suitable for a variety of applications. All are designed to deliver the correct wavelength at the right energy and pulse width for the task at hand. No one laser or light based device is suitable for all medical indications.

Wavelength: Most medical lasers are designed to deliver a single wavelength; some can be frequency-doubled, others offer multiple wavelengths in a single "box". Configurable "platforms", essentially a common power supply offering a choice of attachments for specific applications, are available.

Energy: All medical lasers offer a way to adjust the energy output and pulse width, and some are capable of multiple modes, for example "long" pulse (millisecond) or Q-switched (nanosecond) operation.

Delivery devices: A delivery device gets the laser energy to the target. Typically, laser energy is delivered through a fiberoptic cable or articulating arm through an output device, such as a handpiece (for surgical or dermatologic use), an operating microscope (for ENT, GYN, and Neurosurgery), insulated fibers for use with endoscopes in gastrointestinal and bronchial surgeryor a slit lamp (for use in some types of eye surgery).

Laser or light energy may be delivered directly from the resonator or light source to the target:

Some lasers use scanning devices, which allow the beam to be delivered in a preset pattern to:

limit the dwell time on tissue (ie, a CW laser beam scanned rapidly across a target)
reduce operator fatigue when treating large areas;
to create a treatment-dependent distribution of energy (ie, fractional resurfacing).

Cooling devices: Because most medical laser applications involve the addition of heat to a target tissue, many medical lasers incorporate some sort of cooling device to protect the epidermis, and/or minimize patient discomfort. Cooling can applied before, during or after application of laser energy. Cooling methods include:

Contact cooling using chilled plates or rollers, "shoot through" chilled sapphire windows, or something as simple as cooled transparent gel or ice packs;
Cryogen cooling using R-134a refrigerant delivered through a device incorporated in the laser handpiece, applied before, during or after the laser pulse
Forced cold air delivered continuously to the treatment area from a separate or incorporated refrigerating device

Medical Lasers: a Review

The CO2 Laser is the original "surgical" laser. Its effect on tissue most resembles the traditional surgical techniques of cutting, vaporizing, and coagulation.

The CO2 laser is a gas laser emitting a mid infrared beam at 10,600nm, which is strongly absorbed by water. Because water constitutes 80% or more of soft tissue, the CO2 laser is used as a cutting or vaporizing tool to incise or ablate soft tissue in pulsed mode, or in CW mode with a scanning device to precisely control the depth and area of ablation. Typically, delivery is through an articulating arm for higher powers, although waveguide or fiberoptic cables may be used by some devices. Some uses include:

Removal of skin lesions such as moles, warts, keratoses;
As a "laser scalpel" in patients or body areas prone to bleeding;
"Non-contact" removal of tumors in neurosurgery
Shaving, dermabrading and resurfacing scars, rhinophyma, and skin irregularities;
Cosmetic ablative laser resurfacing for photoaging and wrinkles.

The Argon Laser was one of the first lasers to be used clinically. A gas laser operating in CW mode, the Argon laser emits a blue green light at 488 and 515 nm which is strongly absorbed by both hemoglobin and water.

Although the CW beam can be timed by a mechanical shutter, the Argon Laser can cause significant non-selective heating with collateral thermal injury compared to pulsed vascular lasers. Bulky and ineffiecient, the Argon laser is currently is rarely used for dermatologic and surgical applications, althought it still find utility for retinal and inner ear surgery. Beam delivery is though a fiberoptic cable to a handpiece, slit lamp or operating microscope.

YAG Lasers are a family of solid-state "bulk" lasers that use a Yttrium-Aluminum-Garnet crystal doped with a rare earth element as a gain medium. YAG lasers can be operated in CW, pulsed, or Q-Switched mode, although most YAG medical lasers operating in a single mode for a given application.

CW and pulsed output is typically delivered via fiberoptic cable; Q-switched output is delivered via an articulating arm.

Nd:YAG Laser: A true "workhorse", the Neodymium:YAG laser emits a near infrared beam at 1064 nm, coinciding with an absorption minimum for all tissue chromophores. CW delivery through a fiberoptic cable to a sapphire tip can be used to cut or coagulate tissue; "long pulsed" (millisecond domain) operation is effective for laser hair removal and treatment of vascular lesions including leg veins. The Q-Switched Nd:YAG is effective for the treatment of tattoos and certain pigmented lesions. Nd:YAG has a second transition at 1320 nm, and this wavelength is used for tissue heating, soft tissue coagulation, and endovascular treatment of varicose veins.
KTP Laser: When Nd:YAG laser light at 1064 nm is passed through a non-linear crystal such as potassium-titanyl-phosphate (KTiOPO4), the wavelength is halved to 532 nm, a brilliant green light well absorbed by hemoglobin and melanin used in CW mode to cut tissue, in pulsed mode for vascular lesions including facial and leg veins, and in Q-Switched mode for red tattoo pigment. Delivery is through fiberoptic cable to a handpiece, scanner, or microscope for CW/pulsed mode, and articulating arm for Q-Switched mode.
Er:YAG Laser: Often referred to as the "Erbium" laser, it emits a mid-infrared beam at 2940 nm, which coincides with the absorption peak for water. Its principal use is to ablate tissue for cosmetic laser resurfacing for wrinkles. The Erbium laser has been advertised to offer advantages of reduced redness, decreased side effects and rapid healing compared to the pulsed or scanned CO2 laser because of its limited penetration into tissue; however, results are limited compared to the more versatile CO2 laser. It has also been used in dental practice to prepare cavities for restoration, and more recently, for fractional laser resurfacing.
Ho:YAG Laser: Relatively new to the medical/dental fields, the Holmium:YAG laser emits a mid-infrared beam at 2070 nm. It's principal use is to precisely ablate bone and cartilage in pulsed mode, with other applications in orthopedics for arthroscopy, urology for lithotripsy (removal of kidney stones), ENT for endoscopic sinus surgery, and spine surgery for endoscopic disc removal. The Ho:YAG laser was recently approved for TURP (prostate removal).

YSGG Lasers: The Erbium:Chromium:Yttrium-Scandium-Gallium-Garnet (Er:Cr:YSSG) laser is the newest medical laser. With an output wavelength of 2790 nm, the Er:Cr: YSSG has somewhat less water absorption than the Er:YAG, and in pulsed mode finds application in superficial facing resurfacing and dentistry.

A solid-state bulk laser similar to YAG lasers, the YSSG crystal can be doped with various rare earth and/or transition metal ions to manufacture gain media with various wavelengths.

Ruby Laser: The Ruby laser emits red light with a wavelength of 694 nm. The lasing medium is a synthetic ruby crystal of aluminum oxide and chromium atoms, which is excited by flashlamps.

The first laser system to be built by T. H. Maiman in 1960, early ruby laser systems were used for retinal surgery, but weren't used widely for dermatologic work until the development of Q-Switching technology in the mid 1980's for tattoo treatments. Ruby laser light is strongly absorbed by blue and black pigment, and by melanin in skin and hair. Modern ruby laser systems are available in Q-Switched mode, with an articulating arm, "free running" (millisecond range) mode with a fiber optic cable delivery, or as dual mode lasers. Current uses include:

Treatment of tattoos (Q-Switched mode)
Treatment of pigmented lesions including freckles, liver spots, Nevus of Ota, cafe-au-lait spots (Q-Switched mode)
Laser Hair Removal (free-running mode)

Alexandrite Laser: Similar to the Ruby Laser, the Alexandrite Laser is a flashlamp pumped solid state bulk laser using a rod of synthetic Chromium doped Beryllium Aluminum Oxide (chrysoberyl, BeAl₂O₄) a gemstone said to have been discovered in Russia in 1830 on the future Czar Alexander II's 13th birthday.

It emits a deep red light at 755 nm, and has properties similar to the ruby laser, but its somewhat longer wavelengthwith slightly less absorption by melanin permits deeper penetration into skin. This epidermal sparing compared to the ruby laser make the Alexandrite laser the most popular and versatile device for Laser Hair Reduction. Prinicipal dermatologic applications include laser hair removal in millisecond-range pulsed mode, and tattoo treatment and pigmented lesion removal in Q-Switched mode.

Pulsed Dye Laser: Because the yellow light at 577-585 nm coincides with the peak absorption of hemoglobin in blood, the Pulsed Dye Laser (PDL) is especially useful for the treatment of vascular lesions.

A gain medium of rhodamine dye in a liquid solvent is excited by flashlamps, emitting a pulse in the range of 300-450 microseconds , just less than the thermal relaxation time of minute blood vessels (the so-called "long-pulsed" pulsed dye lasers emit a "train" these shorter pulses). Originally developed for medical use in the late 1980's, the Pulsed Dye Laser became the preferred laser for the treatment of vascular lesions, including spider veins, strawberry birthmarks and port wine stains, replacing the Argon Laser for the treatment of vascular lesions because of the PDL's pulsed output and minimal thermal effect. However, the PDL's short pulse and high absorption ruptures the targeted blood vessels, causing unsightly purpura (black and blue marks) which can last up to 2 weeks. Other vascular lasers can treat various facial and leg veins effectively without purpura, but the Pulsed Dye Laser remains the treatment of choice for:

Port Wine Stains, especially in infants and children
Laser treatment of red or hyperemic scars

Copper Vapor Laser: Vaporized copper bromide is the lasing medium in the Copper Vapor Laser (CVL), which emits yellow light at 577 nm and green light at 511 nm, delivered through a fiberoptic cable. Unlike the PDL, there is no purpura because of the longer pulse duration. However, a long warm up time and short laser tube life make the CVL a less popular choice than the PDL for vascular lesions.

Diode Lasers: Diode lasers are solid state devices similar in construction to LED's. The familiar "laser pointers" are in fact diode lasers.

Diode lasers used clinically emit near-infrared light in the 800-900 nm range. Currently their prinicipal application is in millisecond-range pulsed mode for laser hair removal and for periodontal surgery. Other applications include treatment of leg and facial veins. Diode bars are commonly used to excite or "pump" more traditional laser media such as YAG or Vanadate (YVO4) rods. Because of their relative simplicity and low maintenance requirements, Diode lasers, diode-pumped solid state lasers and other diode pumped lasers will be used more in the near future as their cost decreases.

Fiber lasers: The newest entry to the medical laser armamentarium is the fiber laser. Diode pumped, rugged, simple in design and very low maintenance, fiber lasers are capable of high energies and highly focusable beams in the near infrared spectrum.

Fiber lasers can be Q-switched and frequency doubled. Currently there is only one medical fiber laser on the market, an Er:Fiber unit emitting 1550 nm through a specially designed scanner for fractional laser resurfacing.

Excimer Lasers: When excited by high voltage electrical discharge, noble gases such as neon, argon, and krypton can combine with highly reactive halogens such as fluorine, chlorine and bromine to form to form "excited dimers" capable of lasing in the Ultraviolet (UV) spectrum.

Covalent bonds in protein absorb the UV laser energy and dissociate non-thermally, allowing "cold" ablation of soft tissue. The short wavelength allows focusing of the beam to very small spot (nanometer) spot sizes, and coupled with a scanning device, allow precise ablation or "shaving" of soft tissues. The primary medical application is for Laser in-situ Keratomileusis (LASIK).

Intensed Pulse Light: Although not a true laser, Intense Pulsed Light (IPL) takes advantage of the high intensity and large spot size available with these devices.

Basically, an IPL device is a flashlamp attached to a power source. Pulses of broadband light are applied through filters which can be adjusted to "cut off" the shorter wavelengths depending on the application and patient's skin type. Cost of the devices (and of the procedures!) is similar to that of comparable lasers. Although fast and versatile, IPL devices are as a rule less effective at a given task than a laser dedicated to the purpose.

As a photonic "Swiss Army Knife", Intensed Pulsed light (IPL) devices have become very popular over the recent past. They are much less costly to manufacture than lasers-they're basically a pulsed laser without the costly laser components such as the laser rod, pump chamber, optics, and delivery device. Broadband light has the ability to target multiple chromophores, including blood and melanin, and although cutoff filters can modify the emitted broadband spectrum to some extent, it cannot match the specificity of monochromatic laser light. Any photons not absorbed by the target chromophore will be absorbed by other chromophores and then dissipated as heat. This "collateral damage" has actually been touted as an advantage of IPL over lasers by some proponents, but in reality the amount of energy delivered to the intended target may be compromised by undesireable effects on adjacent structures.

IPL devices have been agressively marketed to physicians as a less expensive, "one box" alternative to multiple dedicated lasers. In addition to versatility, IPL devices have the advantage of larger spot sizes than lasers, which is useful for treating large areas quickly. Their lack of specificity limits their use, especially in darker skinned patients. As a rule IPL devices are less effective than a dedicated laser for a given task.

IPL devices are used to treat a variety of skin conditions including tattoos, telangiectasia (spider veins), leg veins, as well as for hair removal and photorejuvenation.

Electro-optical Devices: Marketed under the brand "ELOS", (Electro-optical synergy), these devices use diode laser or broadband light in combination with bipolar radiofrequency (RF). Optical energy preheats the target area followed by RF so that the thermal effect is primarily delivered by the RF, rather than the optical energy as with traditional lasers. The optical energy provides a low impedance path for the flow of electrons from the RF, thus sparing the epidermis from exposures to high optical energies when the target structure is subepidermal (ie, hair follicle, collagen, etc.)

Plasma Devices: Although not a true optical device, high energy nitrogen plasma delivered to the skin in millisecond pulses can be used for facial rejuvenation and superficial "ablation".

The target area is continuously flushed with nitrogen during treatment, so there is no actual oxidation of tissue. The intense heat dessicates epidermis and upper dermis, leaving it in place as a biologic dressing until regeneration takes place, minimizing the chance of infection or scarring.

Next: Medical Laser Applications

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Page last updated March 06 2008