The pulsed dye laser delivers energy at a wavelength and duration that has been optimized for the selective treatment of vascular lesions. It is used to treat port wine stains, hemangiomas, hypertrophic scars, telangiectasias and warts. The Food and Drug Administration (FDA) has cleared pulsed-dye lasers to be used in these treatments. These lasers are used as an alternative to surgical excision or carbon dioxide lasers.
How a Pulsed Dye Laser Works
A pulsed dye laser is composed of an organic dye mixed with a solvent, which may be circulated through a dye cell, or streamed through open air using a dye jet. A high energy source of light is needed to “pump” the liquid beyond its lasing threshold. A fast discharge flashlamp or an external laser is usually used for this purpose. Mirrors are also needed to oscillate the light produced by the dye’s fluorescence, which is amplified with each pass through the liquid. The output mirror is normally around 80% reflective, while all other mirrors are usually more than 99% reflective. The dye solution is usually circulated at high speeds, to help avoid triplet absorption and to decrease degradation of the dye. A prism or diffraction grating is usually mounted in the beam path, to allow tuning of the beam.
Dyes
The dyes used in these lasers contain rather large organic molecules which fluoresce when exposed to the proper frequency of light. Dyes will emit stimulated radiation when the molecules are in their singlet state. In this state, the molecules emit light via fluorescence, and the dye is quite clear to the lasing wavelength. Within a microsecond, or less, the molecules will change to their triplet state. In the triplet state, light is emitted via phosphorescence, and the molecules begin to absorb the lasing wavelength, making the dye opaque. Liquid dyes also have an extremely high lasing threshold. Flashlamp pumped lasers need a flash with an extremely short duration, to deliver the large amounts of energy necessary to bring the dye past threshold before triplet absorption overcomes singlet emission. Dye lasers with an external pump laser can direct enough energy of the proper wavelength into the dye with a relatively small amount of input energy, but the dye must be circulated at high speeds to keep the triplet molecules out of the beam path.
Chemicals Used in the Dyes
Some of the dyes are rhodamine, fluorescein, coumarin, stilbene, umbelliferone, tetracene, malachite green, and others. While some dyes are actually used in food coloring, most dyes are very toxic, and often carcinogenic. Many dyes, such as rhodamine 6G, (in its chloride form), can be very corrosive to all metals except stainless steel.
A wide variety of solvents can be used, although some dyes will dissolve better in some solvents than in others. Some of the solvents used are water, glycol, ethanol, methanol, hexane, cyclohexane, cyclodextrin, and many others. Solvents are often highly toxic, and can sometimes be absorbed directly through the skin, or through inhaled vapors. Many solvents are also extremely flammable.
Adamantane is added to some dyes to prolong their life.
Cycloheptatriene and cyclooctatetraene (COT) can be added as triplet quenchers for rhodamine G, increasing the laser output power. Output power of 1.4 kilowatt at 585 nm was achieved using Rhodamine 6G with COT in methanol-water solution.
Applications
- Warts
- Port wine stains and other hemangiomas when lesions are located on the face and neck
- Keloids or other hypertrophic scars
- Multiple, superficially located glomangiomas in the face and neck
- Pyogenic granuloma in the face and neck
- Psoriasis
- Genital warts
- Granuloma faciale.