Excimer Lasers in Medicine

Introduction Excimer lasers emit light energy, short optical pulses at ultraviolet wavelengths, that results in a unique laser tissue interaction. This has led to an increasing number of studies into medical applications of these lasers in fields such as ophthalmology, urology, cardiology and neurology. The typical excimer laser is operated with a combination of a halogen (fluorine or chlorine), and a rare gas (argon, krypton,of xenon). Each combination results in emission of a specific wavelength. These wavelengths are listed in Table 1, together with their dimer combinations. Also in Table 1, excimer lasers are compared to other laser systems that are used in medicine. Table 2 shows the relevant output characteristics of a typical commercial excimer laser system. For a certain medical application, the output parameters can be optimized, and incorporated in the overall design of a medical excimer laser. Each medical application may have a different set of parameters, such as wavelength, energy, pulse repetition rate, and pulse duration. One aspect that has hindered the use of excimer lasers in medicine has been an optimum delivery system of the excimer radiation to the tissue. Currently, the fibers that are most efficient are pure silica fibers. These are often cladded by other materials such as plastic or doped silica. For XeF, XeCl, and KrF there is a relatively low damage threshold at the input surface which limits the amount of power that can be directed into the fiber for transmission. For ArF however, the critical damage threshold is subsurface [1]. In an attempt to overcome this threshold , fibers with flared input tips have been designed, which increases the input surface area. Furthermore, it has been recognized that with long pulse (200 ns) XeC1 lasers, one is able to transmit more energy through a fiber than one is able to with standard short pulse (20 ns) lasers. Immersion of the input tip of the fiber in a high pressure electronegative gas environment, or in a vacuum, increases the damage threshold [2], as does careful polishing of the input face. At the output end, higher energy densities can be delivered by forming a lens at the output tip. Fig. 1 shows this along with other possible possible contoured shapes of optical fibers. Fig. 2 (a), shows the energy distribution for a flat ended fiber (Fig. la), while Fig. 2 (b) shows the energy distribution for a rounded tip fiber (Fig. lb). Another area which needs to be explored before the excimer laser is accepted more widely in medicine is the potential mutagenic effects of excimer laser radiation. Studies have shown that KrF excimer laser radiation causes mutations in the cells DNA proportionate to the dose of the excimer light. ArF excimer light, however, does not induce mutations above the level observed in dark controls [3]. It is believed that this could be due to 'cytoplasmic shielding', that is strong absorption of the laser light by cellular components surrounding the nucleus. The mutagenic potential of XeC1 and XeF excimer lasers is thought to be lower than that of either the ArF and KrF excimer, and these longer wavelength ultraviolet sources are believed to be relatively safe.