Tissue Heating With A Pulsed Nd-YAG Laser

Leonard I Grossweiner; Anan M Al-Karmi

doi:10.1117/12.945358

3 June 1988 Tissue Heating With A Pulsed Nd-YAG Laser

Leonard I Grossweiner, Anan M Al-Karmi

Proceedings Volume 0908, Laser Interaction with Tissue; (1988) https://doi.org/10.1117/12.945358
Event: 1988 Los Angeles Symposium: O-E/LASE '88, 1988, Los Angeles, CA, United States

Abstract

Neodymium-yttrium aluminum garnet (Nd-YAG) lasers are finding increasing appli-cations in laser surgery of vascular tissues because of their good hemostatic properties. Heat penetration is deeper than the carbon dioxide laser, because the 1064 nm Nd-YAG emission is located in a "window" between the strong absorptions of oxyhemoglobin and tissue water. The basic physics of laser-tissue interactions suggests that damage to peripheral tissues can be confined by using sufficiently short pulses. In continuous mode (CW) operation, heat flow driven by temperature gradients leads to tissue heating external to the optical absorption profile. When the energy is delivered in pulses, however, conductive heat flow is minimized if the pulse duration (tn) is shorter than the thermal relaxation time constant (t ). Pulsed operation should be especially useful for the Nd-YAG laser, where the 1/e optical penetration depth (5) at 1064 nm is the order of 0.3 to 0.5 cm. Taking t" =2/2a, where a is the thermal diffusivity (the order of 0.001 cm2/s for tissues), typical values of t* for heat conduction are the order of 1-2 min. Heat removal by blood flow augments thermal conduction in vascularized tissues. The rate of this process is characterized by 1/Q, where Q is the volume blood perfusion rate. Values 1/Q range from the order of 15 s for human kidney and thyroid to more than 15 min for muscle.1 Accordingly, heat removal by conduction and blood flow during the pulse duration can be neglected for many tissues exposed to Nd-YAG laser pulses. This paper describes an analytical solution to the two dimensional laser bioheat equation applicable to pulsed operation. The theory was applied to measur-ements on potato tuber heated by low-power pulses from a clinical Nd-YAG laser. The initial temperature elevations are in satisfactory agreement with the analysis, but thermal relaxation was faster than predicted. The suggested explanation for the discrepancy involves evaporative heat transfer to internal water.

Citation Download Citation

Leonard I Grossweiner and Anan M Al-Karmi "Tissue Heating With A Pulsed Nd-YAG Laser", Proc. SPIE 0908, Laser Interaction with Tissue, (3 June 1988); https://doi.org/10.1117/12.945358

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