A close look at best use, technique, history of the ruby

Apr 01, 2007, 4:00am

Literature review, best use, techniques for the ruby laser.

The first laser invented was the ruby. T.H. Maiman constructed his version of the current ruby laser in 1960 while employed at the Howard Hughes Aircraft Co. Maiman's ruby laser emitted coherent light at 694 nm, similar to the current ruby lasers on the market. His laser was constructed using a chromium-doped ruby rod surrounded by a flash lamp. The chromium gives the ruby rod its red color by absorbing green and blue and reflecting red. It emitted a pulse of light at 693.7 nm with a pulse duration of 1 ms.

The ruby laser was first used in medicine for retinal photocoagulation. It later lost favor for this indication due to the complication of retinal hemorrhage. It next became popular in dentistry, neurosurgery and cardiovascular medicine, but it was not until 1967 that Leon Goldman used the ruby laser on skin.

He reported the use of the normal-mode ruby laser for the removal of tattoos. His normal-mode ruby laser was also used to treat vascular lesions, malignant melanoma and other pigmented lesions. In 1981, Reid and colleagues used the first Q-switched ruby laser for the treatment of tattoos. This later became the most popular way of removing tattoos, essentially replacing the CO2 laser, due to the ruby laser's non-scarring nature. With the widespread commercialization of the Q-switched ruby came the removal of pigmented lesions including nevus of Ota, lentigines, nevus spilus and other nevi.

Years later, in 1996, the ruby laser was first used for hair removal. Grossman successfully used the normal-mode ruby laser for photoepilation, and in 1997 the ruby laser became the first laser approved by the Food and Drug Administration for hair removal.

Photothermolysis theory

As described by the theory of photothermolysis, each laser has a specific target chromophore determined by its wavelength. The three main chromophores of the skin are water, hemoglobin and melanin. At a wavelength of 694 nm, melanin is the primary chromophore for the ruby laser. When popular, the ruby laser was found to be successful in treating melanocytic lesions, as well as other pigments absorbing at this wavelength, including tattoo pigment. The pigment of the hair and possibly of the "bulge region" also became a target used for the purpose of hair removal.

Three types

Three types of ruby lasers made it to the mass market. Each laser varies by the duration of the pulse of light emitted (pulse width). The original ruby was a normal-mode laser with a pulse duration of approximately 0.2 ms to 1 ms. The Q-switched has a pulse duration of 20 ns to 80 ns, and the long-pulsed ruby laser used for hair removal has a pulse duration of approximately 3 ms.

As with any laser, varying the pulse duration will affect the target and depth of the laser light. In general, the longer the pulse, the better possibility of penetration of the laser light to deeper tissue. The thermal relaxation time of the target also plays a critical role in determining which mode of laser to use. The thermal relaxation time (TRT) is the time it takes for the target tissue to dissipate one-half of the heat attained by the laser pulse.

In general, this is proportional to the square of the diameter of the target. To achieve effective ablation of the target, the laser pulse must be equal to or shorter than the thermal relaxation time of that target. If the pulse duration is longer than the TRT, the target has sufficient time to dissipate heat to the surrounding tissue, thereby reducing the effectiveness of the ablation and increasing the risk of complications.

Histologic studies with the ruby laser show that at least 50 percent of the laser energy is lost in the tissue at a depth of 1 mm. This means that high fluences are needed to heat the follicle sufficiently to cause permanent damage at a depth of greater than 2 mm, where the bulb of the follicle resides. With this higher energy comes an increased risk of epidermal damage and hence the complications of pigment alteration frequently seen with the ruby laser.