PENETRATION OF THE HOLMIUM-YAG LASER THROUGH FLUID

PURPOSE: The 2.1-mu m pulsed holmium:yttrium-aluminum-garnet (Ho:YAG) laser combines the properties of transmissibility down a flexible sili ca fiber, enabling percutaneous or endoscopic use, with high water abs orption, suggesting good safety characteristics. Laser attenuation in practice, however, is an extremely complex process, The authors studie d its effective penetration through blood, bile, urine, saline, and co ntrast media. MATERIALS AND METHODS: With use of a fiberoptically dire cted beam and a laser power meter, penetration was measured in vitro w ith the fiber tip separated from the medium by 5 cm (noncontact mode) and with the fiber tip immersed (contact mode), Logarithm of energy fa lloff was measured against fluid thickness. Attenuation coefficients ( mu) and half value layer (HVL) distances (estimated thickness of fluid needed to halve power) were measured. RESULTS: In noncontact mode, po wer falloff was exponential, Non-sanguinous media had similar values f or mu and HVL (mu = 2.24-2.70 mm(-1) and HVL = 0.26-0.31 mm) close to theoretical predictions. Blood caused significantly (P <.05) more atte nuation (mu = 5.15 mm(-1), HVL = 0.13 mm). In contact mode, attenuatio n was much more complex with ''plateau'' distances of up to 1.2 mm, be low which attenuation was negligible. The HVL distances ranged from 0. 9 to 1.8 mm and were up to 14 times higher, The main reason is probabl y the formation of microcavities around the fiber tip. CONCLUSION: The effective penetration of this laser when immersed may be several time s that predicted, with important clinical implications.