[Thermal effects of Nd:YAG and Co2 lasers on biological tissues]

The surgical use of CO2 c.w. laser as a knife and of Nd:YAG laser as photocoagulator is generally explained in terms of the different absorption coefficients of the water contained in high amount in the living tissues at the operating wavelengths of the two sources (alphaCO2 congruent to 10(2) cm(-1), alpha Nd congruent to 1cm(-1)). This approach can be shown to be incorrect once spectrophotometric measurements carried out on a mucous membrane and blood give the values gammaCO2 = 200 cm(-1) and gamma Nd = 10 cm(-1) for the effective absorption coefficient (the effective coefficient is gamma = alpha + beta where beta is the internal light scattering coefficient). It is worth noting that gammaCO2 congruent to alpha CO2 but gammaNd congruent to 10alphaNd. Then we have calculated the temperature distribution on tissues irradiated either by Co2 or by Nd laser. In the Co2 irradiation two energy transfer mechanisms are present: the surface optical absorption and the thermal diffusion in the bulk of the tissue. Under Nd irradiation the optical absorption of internally scattered radiation must also be taken into account. The calculation model is based on the classical heat equation, but assuming a "source" given by the expression S (r,z; t) = alpha [IB (r,z; t)+IS (r,z;t)], where IB and IS are respectively the direct and scattered components of a beam propagating along a z direction perpendicular to the tissue surface. The general expression of the temperature increases deltaT (r,z;t) have been obtained for both laser sources. The calculation show that even a few watts CO2 laser can induce a surface temperature rise of about 60 degrees C with negligible bulk effects, depending only on the thermal conductivity and on the irradiation time (knife effect). On the contrary a 40 w Nd:YAG laser induces a 50 degrees C temperature rise in the whole thickness of the tissue becoming in this way unable to "cut" even though it coagulates a region much larger than the beam dimensions.